Article

Accurate Source Depths and Focal Mechanisms of Shallow Earthquakes in Western South America and in the New Hebrides Island Arc

Tectonics

December 1983
2(6):529-563

DOI:10.1029/TC002i006p00529

Authors:

Bryan L. Isacks

Cornell University

Synthetic seismograms are matched to long-period P waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms. Results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc. The depths, the orientations of the earthquake nodal planes, and the close relationships to Quaternary tectonics imply basement-involved crustal shortening in much of the E Andes and sub-Andean zones of Peru and Argentina.-from Authors

Persistent seismic activity in the epicentral region of the 1977 double earthquake, Sierra de Pie de Palo, SAN JUAN, Argentina

Article

Oct 2022
J S AM EARTH SCI

This work analyzes the modern seismic activity in the crust around the Sierra de Pie de Palo, where have occurred the largest shallow earthquake (M = 7.5) ever recorded in Argentina. Hypocentral parameters have been quantitatively characterized from seismicity recorded during 2008 and 2009 by two temporary seismological experiments, in the most active portion of the continental crust of the Andean back-arc of the southern central Andes. Total of 127 events have been relocated using a local velocity model. Epicenters from these 127 events are heterogeneously distributed beneath the Sierra de Pie de Palo, and occurring mainly at middle-to-lower crustal levels with depth between 10.9 and 39 km. Local magnitudes 0.6 ≤ ML ≤ 3.7 and moment magnitudes 1.5 ≤ MW ≤ 3.7 have been estimated. The focal mechanisms determined from P-wave first motion polarities are predominantly reverse and, in general, the azimuths follow a north-south trend. It has been possible to observe that the orientation of the average stress field σ1 is consistent with the direction of convergence of the Nazca plate in this sector of the Andean back-arc. The pattern of hypocenters determined in the present work is consistent with that observed for the aftershocks of the 1977 double earthquake, recorded by a portable network during 10 days in December 1977, and with the depth distribution of the seismicity recorded during 1988 and 1989 by the PANDA temporary network. At present, the hypocentral zones of the 1941 earthquake and 1977 double earthquake are still active with small-to-moderate seismicity.

Neotectónica de la Zona de Falla Liquiñe-Ofqui Entre los 39º S y los 40.5º S: Implicancias para la Evaluación de la Amenaza Sísmica (Neotectonics of the Liquiñe-Ofqui Fault System, between 39°S and 40.5°S: Implications for Seismic Hazard Assessment)

Thesis

Full-text available

May 2022

Luis Astudillo-Sotomayor

At convergent margins, deformation partitioning between subduction fault and transcurrent fault systems on the upper plate (cortical faults) is a common phenomenon. These fault systems have proven to be capable of producing moderate to high seismicity with a great impact on society. The Liquiñe-Ofqui Fault System corresponds to a dextral transpressional fault system, developed in the arc of the Patagonian Andes, which accommodates part of the deformation induced by the oblique convergence between the Nazca and South American plates. The intense post-glacial volcanic activity, together with the dense vegetation cover developed along the Patagonian Andes make it difficult to identify evidence of deformation at the scale of thousands of years. Thus, the lack of knowledge about the thousand-year scale behavior of this system, between 39°S and 40.5°S, as well as the relationship between its evolution and the subduction earthquake cycle motivate the development of this thesis. Through morphometric analysis of digital topography, it was established that the glacial relief forms preserved in the Patagonian Andes represent the accumulation of the various glacial cycles that have occurred during the Quaternary, and that the Liquiñe-Ofqui Fault System controlled their development, attested by a continuous activity during this period. The drainage network is also affected by the presence of the Liquiñe-Ofqui Fault System, allowing the identification of zones with recent tectonic activity throughout the study area, despite the marked and strong glacial imprint of the landscape. The low uplift rates would be responsible for the preservation of the glacial relief. The reported ground evidence allowed us to define 5 active faults during the last 14 ka accommodating deformation according to different kinematics and orientations. Displacements of hundreds of meters of geomorphological markers suggest that the activity of these structures was sustained in time. Meanwhile, metric displacements measured in outcrops suggest the occurrence of Mw~6.5 earthquakes on the studied faults. Coulomb stress change models suggest that the dextral structures of the Liquiñe-Ofqui Fault System accumulate stress during the seismic and interseismic phases of the subduction earthquake cycle. On the other hand, the studied arc-oblique structures do not seem to be compatible with the modeled phases of the subduction earthquake cycle. The study of a specific trace of the Liquiñe-Ofqui Fault System allowed estimating a dextral slip rate of 18.8+2 mm/yr for the Liquiñe Fault for the last 9 ka. This slip rate implies that deformation partitioning was high during that period, with the Liquiñe Fault accommodating 82% of the component parallel to the convergence vector margin, and that part of this is aseismically accommodated. However, the latter needs to be verified by a GPS data inversion experiment. The segmentation of the interplate zone, together with the occurrence of inherited NW faults in the upper plate define a first order segmentation in the Liquiñe-Ofqui Fault System. In this respect, the Liquiñe Segment was defined. It extends between the Mocha Villarrica Fault Zone (39.5°S) and the Valdivia Futrono Lineament (40.5°S). The main trace of the Liquiñe-Ofqui Fault System within this segment reaches an extension of ca. 100 km and would be capable of producing Mw 7.0 earthquakes. A model is proposed to explain the observed behavior along the Liquiñe Segment. In this model, the activity of the Liquiñe-Ofqui Fault System would be controlled by the presence of fluids (meteoric waters and magma) in the fault planes, weakening the structures and facilitating aseismic creep. Discrete blocked patches would be responsible for the micro-seismicity reported for the area, while, the sudden injection of magma in critically stressed faults would have the capacity to trigger seismic events of Mw>6, similar to those of the Aysén seismic crisis in 2007.

Seismotectonic characterization of the 1948 (MW 6.9) Anta earthquake, Santa Bárbara System, central Andes broken foreland of northwestern Argentina

Article

Apr 2022
J S AM EARTH SCI

The region of the Andean back-arc of northwestern Argentina has been struck by several magnitude ≥6 crustal earthquakes since the first historically recorded event in 1692. One of these events corresponds to the Anta earthquake on 25 August 1948, with epicenter in the Santa Bárbara System causing three deaths and severe damage in Salta and Jujuy provinces with maximum Modified Mercalli seismic intensities (MMI) of IX. We collected and digitized analog seismograms of this earthquake from worldwide seismic observatories in order to perform first-motion analysis and modeling of long-period teleseismic P-waveforms. Our results indicate a simple seismic source of M0 = 2.85 × 1019 N m consistent with a moment magnitude Mw = 6.9. We have also tested for the focal depth determining a shallow source at 8 km with a reverse focal mechanism solution with a minor dextral strike-slip component (strike 20°, dip 30°, rake 120°) from the best fit of waveforms. Using magnitude-size empirical relationships, the comparison of the obtained Mw 6.9 magnitude value and the ca. 10,000 km2 area of MMI ≥ IX from our seismic intensity map, which was obtained from newspaper and many historical reports, indicates a rupture length of 42 ± 8 km for the Anta earthquake. We show our results in a 3D geological model around the epicentral area, which integrates modern seismicity, geological data, and information of a previously studied east-west cross section located a few kilometers south of the 1948 epicenter. The integration of all available information provides evidence of the re-activation of the Pie de la Sierra del Gallo fault during the 1948 Mw 6.9 shallow earthquake; this thrust fault bounds the Santa Bárbara System along its western foothill.

Crustal structure of the northern Andean Precordillera, Argentina, based on seismological and gravity data

Article

Jul 2021
J S AM EARTH SCI

This paper aims to better comprehend the crustal structure of the Precordillera and the Iglesia Valley. Located in the northern sector of the Pampean flat-slab (San Juan, Argentina), this region was the epicenter of the historical 1894, M > 7 earthquake (intensity IX on the modified Mercalli scale) and thus considered as a region of particularly high seismic hazard. The structure and deformation style associated with such events remain poorly understood. In this study, we analyze 4 months of regional seismic records from national and international stations in order to characterize a sequence of 74 crustal events that followed the occurrence of an intermediate magnitude (MW = 4.8 and ML = 5.2) earthquake on October 28th, 2019 for which we constrain the source parameters using regional moment tensor inversion. The analysis also includes the determination of 11 focal mechanisms from P-wave polarities and P–S waves amplitude ratios. Most of the focal mechanisms indicate reverse motion solutions. Seismological results are then integrated with other geophysics (gravity and seismic reflection profiles) as well as geological data, to build a 3D structural model that allows a better understanding of the crustal deformation mechanisms of the study area. Our results suggest that the seismic deformation is mostly concentrated in the basement of the Western Precordillera, at the transition with the Frontal Cordillera. This deformation seems respond to compressional stresses. The proposed model provides evidence of ongoing seismic activity related to the interaction between the thick-skinned deformation of the Frontal Cordillera with the thin-skinned fold and thrust belt of the Western Precordillera.

The La Laja spring system in the Argentine Precordillera: A conceptual model based on geochemical and isotopic data

Article

Feb 2021
J S AM EARTH SCI

The La Laja Spring System (LLSS) (31.34°S; 68.48°W) in the Argentine Precordillera is well-known for its low enthalpy thermal baths since pre-Columbian times and its surroundings full of travertine fields. This work focuses on geochemical and isotopic analysis of the thermal waters and their hydrological implications. We found a system with partially equilibrated waters with dominant Cl−–Na+ and subordinated HCO3−Ca2+(Mg2+) and SO42−Ca2+compositions, which seem affected to the basin fill composed of thick Cenozoic synorogenic deposits overlying early Paleozoic carbonate successions. Several cationic geothermometers suggest a deep reservoir with average temperatures reaching ∼170 °C, while the silica geothermometers suggest a shallow reservoir at 91 °C. The average temperature of the springs is 28 °C and the low-geothermal gradients (∼27 °C/km) in the region indicate a relatively deeper (∼5500 m) reservoir and another shallower (∼2500 m). The source of heat is related to the orogenic belt because this region is on top of the flat-slab subduction of the South-Central Andes, with an absence of active volcanism. Hence, this spring system is mainly affected by the relatively low-geothermal gradient within the Eastern Precordillera. The geochemical and isotopic characteristics O18/O16 as well as H2/H1 ratios of waters indicate meteoric source associated with evaporation in relation to strong aridity and secondary processes modifying the chemistry of waters is likely to be erased by any thermal end-member signature. Our conceptual model for the LLSS indicates the N–S trending Villicum-Zonda regional thrust and the minor accommodation faults within the Eastern Precordillera domain seem to behave as the preferential zone for recharging and latter thermal water rising.

Orogen‐Parallel Transition From a Decoupled Fore‐Arc Sliver to Andean‐Type Mountain Chain: Paleomagnetic and Geologic Evidence From Southern Chile (37–39°S)

Article

Full-text available

Feb 2020
TECTONICS

The Chiloé fore-arc sliver is a ~N-S elongated crust block detached from South America along the dextral intra-arc Liquiñe-Ofqui fault zone (LOFZ). The sliver is internally dissected by active NW-SE sinistral faults whose relations with the LOFZ are speculative, also due to widespread fluvio-glacial and volcanic blanket hiding the substratum. We focus on the northern LOFZ end and on the Biobio fault, supposedly the northernmost of the sinistral fault set, reporting on the results from field investigation and paleomagnetism of 48 (mostly Oligo-Miocene) volcanic sites. We find that the Biobio fault is an old inherited crust discontinuity that did not yield significant block rotation and deformation during the Cenozoic, thus testifying the end of sinistral shear at about 38°S. At the same latitudes, a northward transition from pure strike-slip to transpressive LOFZ deformation occurs. Intense tectonic deformation and >90° clockwise rotations characterize the main LOFZ strand. Conversely, a supposedly western LOFZ strand display counterclockwise rotations, similar to the pattern previously documented in the fore-arc, thus it does not represent a LOFZ segment. LOFZ and sinistral fault kinematics must be related, and we suggest that crust pushed northward west of the LOFZ escapes laterally towards the trench along the sinistral faults. We also speculate that the northward increasing age of the subducting Nazca plate implies a concomitant decrease of heat transfer on the upper plate, thus an increasing crust rigidity that eventually inhibits strain partitioning and sliver decoupling from 38°S.

Tectonic role of margin-parallel and margin-transverse faults during oblique subduction in the Southern Volcanic Zone of the Andes: Insights from Boundary Element Modeling: SLIP PARTITIONING SOUTHERN ANDES

Article

Full-text available

Aug 2016

Obliquely convergent subduction margins develop trench-parallel faults shaping the regional architecture of orogenic belts and partitioning intra-plate deformation. However, transverse faults also are common along most orogenic belts and have been largely neglected in slip partitioning analysis. Here, we constrain the sense of slip and slip rates of differently oriented faults to assess whether and how transverse faults accommodate plate-margin slip arising from oblique subduction. We implement a forward 3D boundary element method (BEM) model of subduction at the Chilean margin evaluating the elastic response of intra-arc faults during different stages of the Andean subduction seismic cycle (SSC). Our model results show that the margin-parallel, NNE-striking Liquiñe-Ofqui Fault System (LOFS) accommodates dextral-reverse slip during the interseismic period of the SSC, with oblique slip rates ranging between 1-7 mm/yr. NW-striking faults exhibit sinistral-reverse slip during the interseismic phase of the SSC, displaying a maximum oblique slip of 1.4 mm/yr. ENE-striking faults display dextral strike-slip, with a slip rate of 0.85 mm/yr. During the SSC coseismic phase, all modeled faults switch their kinematics: NE-striking fault become sinistral whereas NW-striking faults are normal-dextral. Because coseismic tensile stress changes on NW faults reach 0.6 MPa at 10-15 km depth, it is likely that they can serve as transient magma pathways during this phase of the SSC. Our model challenges the existing paradigm wherein only margin-parallel faults account for slip partitioning: transverse faults are also capable of accommodating a significant amount of plate-boundary slip arising from oblique convergence.

Late Cenozoic Extensional Formation of the Antofalla Depression, Southern Puna Plateau, Argentina: An Effect of Lithospheric Foundering?

Article

Full-text available

Mar 2022
TECTONICS

Deformation within orogenic plateaus functions to establish a dynamic equilibrium between tectonic boundary stresses and plateau gravitational potential energy. Temporal changes in deformation kinematics record perturbations to boundary stresses or internal plateau processes, such as lithospheric foundering. We integrate new mapping, field observations, and geochronologic ages with published data to document complex late Cenozoic upper crustal deformation in the region of the Antofalla depression, a ∼125 km long, sublinear basin within the southern Puna orogenic plateau, Argentina. The juxtaposition of stratal ages across the depression requires >900 m vertical offset on a surface‐breaking fault. Regional geologic structure, basin geomorphology, and our observation of a breccia paralleling the depression margin suggest formation of the depression by normal faulting. We interpret published stratigraphic logs to suggest that the depression formed between ca. 16 and 11 Ma following Andean shortening. Folded Late Miocene to Quaternary strata on the eastern depression margin indicate that extension ended and shortening resumed before present, revealing toggling between extensional and contractional kinematic regimes. The kinematic evolution of the Antofalla depression contrasts with the rest of the southern Puna plateau, which underwent shortening until latest Miocene to Quaternary time, followed by extension and strike‐slip deformation. Taken together, the spatial and temporal variations in late Cenozoic deformation of the southern Puna plateau are inconsistent with mechanisms that would affect the entire orogen, such as slowing convergence, but are compatible with lithospheric foundering.

Weak, Seismogenic Faults Inherited From Mesozoic Rifts Control Mountain Building in the Andean Foreland

Article

Full-text available

Mar 2022
GEOCHEM GEOPHY GEOSY

Sam Wimpenny

New earthquake focal mechanism and centroid depth estimates show that the deformation style in the forelands of the Andes is spatially correlated with rift systems that stretched the South American lithosphere in the Mesozoic. Where the rifts trend sub‐parallel to the Andean range front, normal faults inherited from the rifts are being reactivated as reverse faults, causing the 30–45 km thick seismogenic layer to break up. Where the rift systems are absent from beneath the range front, the seismogenic layer is bending and being thrust beneath the Andes like a rigid plate. Force‐balance calculations show that the faults inerhited from former rift zones have an effective coefficient of static friction μ′ < 0.2. In order for these frictionally weak faults to remain seismogenic in the lower crust, their wall rocks are likely to be formed of dry granulite. Xenolith data support this view, and suggest that parts of the lower crust are now mostly metastable, having experienced temperatures at least 75–250°C hotter than present. The conditions in the lower crust make it unlikely that highly pressurized free water, or networks of intrinsically weak phyllosilicate minerals, are the cause of their low effective friction, as, at such high temperatures, both mechanisms would cause the faults to deform through viscous creep and not frictional slip. Therefore pre‐existing faults in the Andean forelands have remained weak and seismogenic after reactivation, and have influenced the style of mountain building in South America. However, the controls on their mechanical properties in the lower crust remain unclear.

Extension and Dynamics of the Andes Inferred From the 2016 Parina (Huarichancara) Earthquake

Article

Aug 2018

The Mw 6.1 2016 Parina earthquake led to extension of the south Peruvian Andes along a normal fault with evidence of Holocene slip. We use interferometric synthetic aperture radar, seismology, and field mapping to determine a source model for this event and show that extension at Parina is oriented NE-SW, which is parallel to the shortening direction in the adjacent sub-Andean lowlands. In addition, we use earthquake source models and GPS data to demonstrate that shortening within the sub-Andes is parallel to topographic gradients. Both observations imply that forces resulting from spatial variations in gravitational potential energy are important in controlling the geometry of the deformation in the Andes. We calculate the horizontal forces per unit length acting between the Andes and South America due to these potential energy contrasts to be 4–8 ×10¹² N/m along strike of the mountain range. Normal faulting at Parina implies that the Andes in south Peru have reached the maximum elevation that can be supported by the forces transmitted across the adjacent foreland, which requires that the foreland faults have an effective coefficient of friction (Formula presented.) 0.2. Additionally, the onset of extension in parts of the central Andes following orogen-wide compression in the late Miocene suggests that there has been a change in the force balance within the mountains. We propose that shortening on weak detachment faults within the Andean foreland since ∼5–9 Ma reduced the shear tractions acting along the base of the upper crust in the eastern Andes, leading to extension in the highest parts of the range.

An M_w = 7.7 slow earthquake in 1960 near the Aysén Fjord region, Chile

Article

Oct 2017

We re-examine the source characteristics of an M_s = 6.9 earthquake in Chile which occurred on 1960 June 6, near the Aysén Fjord region where a remarkable earthquake swarm occurred in 2007 with more than 7000 earthquakes and hundreds of landslides. The June 6 event occurred during the aftershock activity of the 1960 May 22, M_w = 9.5 great Chilean earthquake. A recently found well-calibrated strain seismogram of the June 6 event recorded at Isabella (ISA), California, is the impetus to this study. We confirm that this event is a slow earthquake caused by a source process extending at least 190 s with a seismic moment of M_0 = 4.5 × 10^(20) N⋅m (M_w = 7.7). Although the mechanism cannot be uniquely determined, an NS trending right-lateral strike-slip mechanism is consistent with the ISA record. The depth cannot be constrained well, but the slowness of the event suggests that it may have occurred in a somewhat deeper high-temperature ductile environment caused by nearby subduction of the Chile Rise. The mechanism and the proximity of this event to the Liquiñe-Ofqui fault (LOF) which extends north–south over 1000 km along the Chilean coast suggest that the June 6 event represents a slip on this fault. The large moment of the June 6 event indicates strong interaction between the Nazca-South American plate boundary and the LOF with significant slip partitioning.

Structural and lithological controls on large quaternary bedrock landsliedes in NW-Argentina

Article

Full-text available

Jan 1999

Understanding kinematics of intra-arc transcurrent deformation: Paleomagnetic evidence from the Liquiñe-Ofqui fault zone (Chile, 38-41ºS)

Conference Paper

Dec 2015

The Liquiñe-Ofqui fault zone (LOFZ) is a major ~1000 km long dextral shear zone of Southern Chile (38°-47°S), likely related to strain partitioning of Nazca Plate oblique convergence with South America. To understand the block rotation pattern along the LOFZ, we paleomagnetically sampled 55 sites (553 samples) of Oligocene to Pleistocene volcanics and Miocene granites at a maximum distance of 20 km from the LOFZ and at both sides of it between 38° and 41°S. Rotations with respect to the South America plate show that the crust around the LOFZ is fragmented into small blocks (~1 to10 km). Equidimensional blocks can undergo very large rotations (150º-170º) while elongated slivers subparallel to the LOFZ do not rotate and simply are translated following LOFZ kinematics. Rotation pattern across the LOFZ is markedly asymmetric. East of the fault and adjacent to it, rotations are up to 170º clockwise and fade out ~10 km east to fault. These data support a quasi-continuous crust kinematics (small rigid blocks drag by the underlying ductile crust flow) and imply 120 km of total fault offset. Conversly, crust west of the LOFZ is cut by seismically active NW-SE sinistral faults and it yields counterclockwise rotations up to 170º at 8-10 km from LOFZ, besides the unrotated blocks.

Paleomagnetic rotation pattern of the southern Chile fore-arc sliver (38°S–42°S): A new tool to evaluate plate locking along subduction zones

Article

Feb 2016

The Chile fore arc at 37°S–47°S represents the coseismic deformation zone of the 1960 Mw 9.5 Valdivia earthquake. Here we report on the paleomagnetism of 43 Oligocene-Pleistocene volcanic sites from the fore-arc sliver between 38°S and 42°S. Sites were gathered west of the 1000km long Liquiñe-Ofqui dextral fault zone (LOFZ) that represents the eastern fore-arc sliver boundary. Nineteen reliable sites reveal that the fore arc is characterized by counterclockwise (CCW) rotations of variable magnitude, except at 40°S–41°S, where ultrafast (>50°/Myr) clockwise (CW) rotations occur within a 30km wide zone adjacent to the LOFZ. CCW rotation variability (even at close sites) and rapidity (>10°/Myr) suggest that the observed block rotation pattern is related to NW-SE seismically active sinistral faults crosscutting the whole fore arc. According to previously published data, CW rotations up to 170° also occur east of the LOFZ and have been related to ongoing LOFZ shear. We suggest that the occurrence and width of the eastern fore-arc sliver undergoing CW rotations is a function of plate coupling along the subduction zone interface. Zones of high coupling enhance stress normal to the LOFZ, induce high LOFZ strength, and yield a wide deformation zone characterizedbyCWrotations.Conversely,lowcouplingimplyaweakLOFZ,alackofCWrotations,andafore arc entirely dominated by CCW rotations related to sinistral fault kinematics. Our locking inferences are in good agreement with those recently derived by GPS analysis and indicate that seismotectonic segment coupling has remained virtually unchanged during the last 5Ma.

Seismicity and geodynamics in the central Vanuatu subduction zone

Thesis

Full-text available

Jun 2014

Christian Baillard

Plio-Quaternary kinematics and geometry of the Calama-Olacapato-El Toro fault zone across the Puna Plateau, Argentina

Article

Jan 2010

The Andean structure of the Sierras Pampeanas based on earthquake focal mechanisms in their Northwestern region

Article

Full-text available

Dec 2010

Modeling of broadband seismic waveforms recorded by global and Chilean networks for two moderate crustal earthquakes of the northwestern Sierras Pampeanas shows their focal mechanisms, depths and seismotectonic features. The magnitude Mw 5.8 earthquake on 28 May 2002, located in the eastern flank of the sierra de Velasco and the Mw 6.2 earthquake on 7 September 2004, with epicenter in the southwestern part of the sierra de Ambato, have shallow focal depths of 10 and 8 km, respectively. These results combined with the historical seismicity of the region allow us to estimate the deep structure of the Sierras Pampeanas in the study region. The seismic analyses together with interpretations of the surface structure and previous neotectonic studies ruled out extensional or strike slip deformation as the main responsible mechanism of the Present structure of this sector of the sierras de Ambato and Velasco in the northwestern Sierras Pampeanas. The comparison between the Nazca-South America plate convergence orientation as well as GPS velocities in the upper plate with the summation of the seismic moment tensor for the largest seismic energy released by crustal earthquakes of this region in the last 30 years, shows a clockwise rotation of 50° of the average P-axis orientation from the convergence orientation to the northeast suggesting important strain partition. This partition is controlled by the Eopaleozoic basement fabric, which has guided the orientation and vergence of the Andean faults.

Neotectonic faults in the Southern Chile intra-arc (38°S–40.5°S): Insights about their seismic potential and the link with the megathrust earthquake cycle

Article

Dec 2022
TECTONOPHYSICS

Stress transmission along mid-crustal faults highlighted by the 2021 Mw 6.5 San Juan (Argentina) earthquake

Article

Full-text available

Oct 2022

Understanding the mechanisms of crustal deformation along convergent margins is critical to identifying seismogenic structures and assessing earthquake hazards for nearby urban centers. In the southern central Andes (28–33◦S), differences in the style of middle to upper‐crustal deformation and associated seismicity are highlighted by the January 19th, 2021 (Mw 6.5) San Juan earthquake. We integrate waveforms recorded at regional and teleseismic distances with co‐seismic displacements calculated from local Global Navigation Satellite System time series, to re‐estimate the source parameters of the 2021 San Juan earthquake, confirming a mid‐crustal nucleation depth (21 ± 2 km) and right‐lateral transpressional mechanism. Considered alongside decades of seismic observations and geological data, this event provides evidence for retroarc deformation partitioning among inherited basement faults and upper‐crustal structures in response to oblique convergence of the Nazca and South American plates. As they may transfer shortening to active upper‐crustal faults associated with historically devastating shallower earthquakes, a better understanding of seismogenic basement faults such as the mid‐crustal structure activated during the 2021 San Juan earthquake earthquake could help future re‐assessment of the seismic risk in western Argentina.

Neotectonic Faults in the Southern Chile Intra-Arc (38°S-40.5°S): Insights About Their Seismic Potential and the Link with the Megathrust Earthquake Cycle

Article

Jan 2022

Weak, Seismogenic Faults Inherited From Mesozoic Rifts Control Mountain Building in the Andean Foreland

Preprint

Full-text available

Nov 2021

Sam Wimpenny

New earthquake focal mechanism and centroid depth estimates show that the deformation style in the forelands of the Andes is spatially correlated with rift systems that stretched the South American lithosphere in the Mesozoic. Where the rifts trend sub-parallel to the Andean range front, normal faults inherited from the rifts are being reactivated as reverse faults, causing the 30--45 km thick seismogenic layer to break up. Where the rift systems are absent from beneath the range front, the seismogenic layer is bending and being thrust beneath the Andes like a rigid plate. Force-balance calculations show that the faults inerhited from former rift zones have an effective coefficient of static friction < 0.2. In order for these frictionally-weak faults to remain seismogenic in the lower crust, their wall rocks are likely to be formed of dry granulite. Xenolith data support this view, and suggest that parts of the lower crust are now mostly metastable, having experienced temperatures at least 75--250 degrees hotter than present. The conditions in the lower crust make it unlikely that highly-pressurised free water, or networks of intrinsically-weak phyllosilicate minerals, are the cause of their low effective friction, as, at such high temperatures, both mechanisms would cause the faults to deform through viscous creep and not frictional slip. Therefore pre-existing faults in the Andean forelands have remained weak and seismogenic after reactivation, and have influenced the style of mountain building in South America. However, the controls on their mechanical properties in the lower crust remain unclear.

Seismic evidence of the active regional tectonic faults and the Copahue volcano, at Caviahue Caldera, Argentina

Article

Full-text available

Mar 2021

Understanding interactions between tectonic faults and a nearby active volcano is often realized by combining seismic and field observations. A good example of such an interaction is the Caviahue caldera. It is located in an intra-arc extensional pull-apart basin, within a transition zone joining the northern part of the right-lateral strike-slip Liquiñe-Ofqui Fault System and the thrust-fault Antiñir-Copahue fault zone. Most of the active volcanoes in South Chile are related to the Liquiñe-Ofqui Fault System. Some faults located inside the Caviahue caldera were described with reverse mechanisms by some studies whereas they were found to be normal by others. In order to discriminate the actual focal mechanisms, two seismic clusters that occurred in 2017 and 2018 inside the Caviahue rectangular caldera, close to the active Copahue volcano, were studied. Earthquakes (520) were located; focal mechanisms (56) were determined from which an average seismic moment tensor was calculated. The locations and focal mechanisms of the earthquakes allow splitting the seismicity into two main regions, one of tectonic origin (with strike-slip faults) and another one of volcanic origin (with normal faults). The first seismic cluster is located close to Caviahue village, with strike-slip focal mechanisms, in an NNE direction as the nearby Liquiñe-Ofqui Fault strikes. The other part of the seismicity is located close to the northeastern structures of Copahue volcano, in the hydrothermal zone of Anfiteatro, Termas de Copahue, and Maquinitas. It is oriented in an NE direction and is composed of earthquakes with normal focal mechanisms, not reverse as postulated in past studies. The active Copahue volcano lies in the SW prolongation of these normal faults, in agreement with the tectonics of the Caviahue caldera. Then, the two nearby seismic clusters reveal both a tectonic origin, with strike-slip focal mechanisms compatible with the Liquiñe-Ofqui Fault System, and a hydrothermal origin with normal focal mechanisms, compatible with the hydrothermal system of the Copahue active volcano.

Fast Holocene slip and localized strain along the Liquiñe-Ofqui strike-slip fault system, Chile

Article

Full-text available

Mar 2021

In active tectonic settings dominated by strike-slip kinematics, slip partitioning across subparallel faults is a common feature; therefore, assessing the degree of partitioning and strain localization is paramount for seismic hazard assessments. Here, we estimate a slip rate of 18.8 ± 2.0 mm/year over the past 9.0 ± 0.1 ka for a single strand of the Liquiñe-Ofqui Fault System, which straddles the Main Cordillera in Southern Chile. This Holocene rate accounts for ~ 82% of the trench-parallel component of oblique plate convergence and is similar to million-year estimates integrated over the entire fault system. Our results imply that strain localizes on a single fault at millennial time scale but over longer time scales strain localization is not sustained. The fast millennial slip rate in the absence of historical Mw > 6.5 earthquakes along the Liquiñe-Ofqui Fault System implies either a component of aseismic slip or Mw ~ 7 earthquakes involving multi-trace ruptures and > 150-year repeat times. Our results have implications for the understanding of strike-slip fault system dynamics within volcanic arcs and seismic hazard assessments.

Neotectonic evidence for Late Quaternary reverse faulting in the northern Chile outer forearc (22.5°S-23°S): Implications for seismic hazard

Article

Feb 2021
J S AM EARTH SCI

In the northern Chile outer forearc (19°S to 23°S), extension is the dominant deformation style conducted by crustal faults during the Neogene-Quaternary. Most of this extension has been produced by normal faulting along submeridian branches of the Atacama Fault System (AFS). During the Late Quaternary, several of these faults have built conspicuous metric-scale normal faulting scarps in alluvial deposits; the construction of these scarps has been related, in some cases, to M~7 normal faulting paleoearthquakes. On the contrary, shortening during this time span has only been locally reported. Until now, no metric-scale reverse faulting scarps have been documented along submeridian strands of the AFS. This study presents novel evidence corresponding to reverse fault scarps produced during the Late Quaternary by three NNE and NNW trending faults in the outer forearc between 22.5°S and 23°S: the El Toro (ETF), Caliche (CF), and Sierra Valenzuela (SVF) faults. These scarps, with up to 2.5 m height, developed in basement rocks, pre-Quaternary, and/or Quaternary alluvial deposits. Based on their characteristics and the evidence suggesting that similar fault scarps in the Andean forearc have a seismogenic origin, we interpret that these neotectonic landforms were produced by single reverse faulting paleoearthquakes. Considering the maximum scarp height as a proxy for the coseismic slip and fault length as a proxy for surface rupture length, we have estimated magnitudes between M6.2 and M6.9 for the causative rupture events. To discuss how reverse crustal faulting may be related to the interseismic and/or coseismic stages of the subduction cycle, we perform elastic dislocation models to calculate the horizontal displacement fields (at 0 and 20 km depth) induced by the interplate locking distribution in the area (over a seismic cycle) and three megathrust earthquakes (Antofagasta 1995, Tocopilla 2007, and a synthetic rupture). Modeling results suggest that the studied faults may experience reverse slip during both stages of the subduction cycle. We propose that the ETF, CF and SVF are faults capable to produce moderate-to-large earthquakes in the future. Finally, we reinforce the need to address the earthquake potential of the studied faults, and others similar to them, to acquire a complete assessment of the seismic hazard in the northern Chile outer forearc.

Coseismic and Postseismic Deformation of the 2016 M W 6.2 Lampa Earthquake, Southern Peru, Constrained by Interferometric Synthetic Aperture Radar

Article

Full-text available

Apr 2019

We use Sentinel‐1 radar imagery to explore the coseismic and postseismic surface displacements associated with the 2016 MW 6.2 Lampa earthquake in southern Peru. Based on coseismic interferograms, the preferred slip model links to a blind south southeast striking, south southwest dipping normal fault with a shallow dip (45.2°) and a peak slip of 0.71 m at depth ~5.3 km, which is consistent with seismic solutions. Postseismic interferograms, derived from two tracks of the Sentinel‐1A/B satellites using a small baseline subset method, show subsidence up to ~3 cm in the first year after the mainshock. The kinematic inversions of interferometric synthetic aperture radar (InSAR) observations imply that the postseismic surface displacements observed in 1 year after the earthquake are governed by afterslip occurring along the updip extension of the coseismic slip patches. To further improve the data fitting, we generate a fault with variable strike to refine the kinematic afterslip model. The stress‐driven afterslip forward modeling shows that the postseismic deformation is controlled by afterslip distributed at the edge of the compact coseismic slip area. The surface displacement predictions of the poroelastic rebound show subsidence of the hanging wall, but the magnitude of the displacements is small compared to the observed signal. We as well test a collection of viscoelastic relaxation models and find that the predicted surface displacements are not consistent with the observations. The InSAR results show that the strike of the seismogenic fault is quasi‐parallel to the Vilcanota normal fault system and both the fault associated with earthquake and Vilcanota normal fault dip in the same direction. Therefore, we suspect that the causative fault of this 2016 event may be a normal fault belonging to a “domino” faulting system.

Regional subsurface mapping and 3D flexural modeling of the obliquely converging Putumayo foreland basin, southern Colombia

Article

Aug 2020

The Putumayo foreland basin (PFB) is an underexplored, hydrocarbon-bearing basin located in southernmost Colombia. The PFB forms a 250-km long segment of the 7000-km-long corridor of Late Cretaceous-Cenozoic foreland basins produced by eastward thrusting of the Andean mountain chain over Precambrian rocks of the South American craton. We use ∼4000 km of 2D seismic data tied to 28 exploratory wells to describe the basin-wide structure and stratigraphy of an underexplored hydrocarbon basin. Based on seismic interpretation and comparison with published works from the southward continuation of the PFB into Peru and Ecuador, three main across-strike, structural zones include: 1) the 20-km-wide, Western structural zone closest to the Andean mountain front characterized by inversion of older, Jurassic half-grabens during the late Miocene; 2) the 45-km-wide, Central structural zone characterized by moderately-inverted Jurassic half-grabens; and 3) the 120-km-wide, Eastern structural zone characterized by the 40-km-wide, N-S trending Caquetá arch. The five mainly clastic tectonosequences of the PFB include: 1) Lower Cretaceous pre-foreland basin deposits; 2) Upper Cretaceous-Paleocene foreland basin deposits; 3) Eocene foreland basin deposits related to the early uplift of the Eastern Cordillera; 4) Oligocene-Miocene underfilled, foreland basin deposits; and 5) Plio-Pleistocene overfilled, foreland basin deposits. We used 3D flexural modeling to identify the elastic thickness (T e ) of the lithosphere below the PFB, in order to model the location of the sedimentary-related and tectonically-related forebulges of Cretaceous to Oligocene age. Flexural analysis shows two pulses of rapid, foreland-related subsidence first during the Late Cretaceous-early Paleocene and later during the Oligocene-Miocene. Despite the present-day oblique thrusting of the mountain front, flexure of the PFB basement has produced a tectonic forebulge now located in the Eastern structural zone and controls a basement high that forms the eastern, updip limit for most hydrocarbons found in the PFB.

The Mechanics of Active Faulting and Mountain Building

Thesis

Full-text available

Jun 2019

Sam Wimpenny

The aims of this thesis are to place new constraints on the rheological properties of active faults, and to investigate the influence of fault rheology on mountain building. The first chapter studies the postseismic deformation following the 2003 Bam earthquake in south-eastern Iran, with the intention of probing the material properties of the fault zone. Measurements of ground deformation following the earthquake made using InSAR suggest there was an unusually small amount of postseismic afterslip at Bam compared to similar earthquakes. I use numerical calculations of stress-driven afterslip to show that, in order to account for the limited postseismic afterslip at Bam, most of the fault zone must remain frictionally locked. I also find that, to explain the postseismic InSAR measurements and the long-term growth of topography at Bam, there must be either an E-W deviatoric stress of 2-10 MPa acting across the fault zone, or some component of fabric on the fault zone controlling the rake of fault slip. The second chapter expands in scope, investigating the orogen-scale deformation of the high Andes in south Peru. First I present a source model for a Mw 6.1 normal-faulting earthquake that occurred in the shallow crust of the south Peruvian Altiplano, which indicates that the Andes are extending parallel to the direction of shortening in the adjacent sub-Andean forelands. I then discuss a compilation of earthquake source models from across the Andes, using the earthquake depth distribution and slip vectors to infer that buoyancy forces are important in controlling the pattern of deformation. Calculations of the buoyancy forces indicate that faults cutting through the foreland support forces ~4-8 TN/m and have effective coefficients of static friction <0.2. Finally, I speculate that the recent normal faulting in the Andes is a result of a reduction in the shear stresses transmitted across faults on the eastern margin of the range, causing the high plateau to extend and thrust eastwards over the adjacent South American foreland. The final chapter remains focused on the normal faulting in south Peru. I use field observations and remote sensing to show that a 20 km-wide band of normal faults between Cusco and Lake Titicaca are extending NNE-SSW to NE-SW at ~1-3 mm/yr. The normal faults became active in the past ~5 Myrs and re-activate pre-existing reverse faults. To account for the extension rates across the normal faults, I calculate that the average shear stresses transmitted across the sub-Andean detachment may have decreased by 0.02-3 MPa, which is 0.1-30\% of their absolute value. I conclude that within most mountain belts the rate and style of faulting is sensitive to small spatial and temporal variations in the material properties of faults in their forelands.

Structure et dynamique de la subduction centrale du Vanuatu

Thesis

Dec 2018

Océane Foix

The 1,400 km long Vanuatu subduction zone is located in the southeast Pacific Ocean. It is part of the "Ring of Fire" and one of the most seismically and volcanically active subduction zones in the world. The Vanuatu arc delineates the boundary between the Australian plate and the North-Fijian microplate. The former subducts beneath the latter with a convergence rate ranging from 170 mm/yr in the north to 130 mm/yr in the south. This rapid convergence rate is abruptly decreases to 35 mm/yr, at the central segment. This slowing is attributed to the subduction/collision of the d’Entrecasteaux fracture zone. The slowing is accompanied by segmentation of the forearc and uplift of the islands close to the trench by up to 6 mm/year. The purpose of this thesis is to understand the deformation generated in the forearc of the Vanuatu central segment due to the interaction of the trench with the seafloor reliefs. We use seismological data from the ARC-VANUATU program (2008-2009), which included a temporary (10 month) network of 30 seismic stations (20 on land and 10 at sea). The network recorded more than 40,000 earthquakes. We calculated the first 3-D P and S velocity models of the region, using local passive tomography. We then used these models to locate 11,703 earthquakes beneath the network during the ARCVANUATU experiment, and 58,303 further earthquakes using a 5-station network left in place after the experiment (from 2009-2014). The 2009-2014 indicates a change in the seismic dynamic from the 2008-2009 data and illuminates areas not seen in the ARC-VANUATU catalogue. The tomographic inversion highlighted a strong P and S waves heterogeneity in the first 40 km below the surface [Foix et al., 2019] : 1) a trench-parallel alignment of low velocity anomalies is observed west of the large forearc islands (Santo and Malekula) and directly correlated with subducted reliefs ; 2) a trench-parallel alignment of high velocity anomalies further to the east and between 5 and 15 km deep, which could play the role of a backstop ; 3) a relatively thick upper plate crust (29 ± 3 km), compatible with recent indications of a continental origin. 4) a seismogenic zone with an average 15° dip, observed by aligned seismicity in the area not facing seafloor reliefs. A receiver function study of the central Vanuatu subduction is currently underway. The data is very complex, probably related to both the complexity of the area and noise from the ocean-island setting, and may be difficult to interpret.

Intra-Arc Crustal Seismicity: Seismotectonic Implications for the Southern Andes Volcanic Zone, Chile

Article

Full-text available

Feb 2019
TECTONICS

We examine the intra-arc crustal seismicity of the Andean Southern Volcanic Zone. Our aim is to resolve interseismic deformation in an active magmatic arc dominated by both margin-parallel (Liquiñe-Ofqui fault system, LOFS) and Andean transverse faults. Crustal seismicity provides information about the schizosphere tectonic state, delineating the geometry and kinematics of high strain domains driven by oblique-subduction. Here, we present local seismicity based on 16-month data collected from 34 seismometers monitoring a ~200-km-long section of the Southern Volcanic Zone, including the Lonquimay and Villarrica volcanoes. We located 356 crustal events with magnitudes between M w 0.6 and M w 3.6. Local seismicity occurs at depths down to 40 km in the forearc and consistently shallower than 12 km beneath the volcanic chain, suggesting a convex shape of the crustal seismogenic layer bottom. Focal mechanisms indicate strike-slip faulting consistent with ENE-WSW shortening in line with the long-term deformation history revealed by structural geology studies. However, we find regional to local-scale variations in the shortening axes orientation as revealed by the nature and spatial distribution of microseismicity, within three distinctive latitudinal domains. In the northernmost domain, seismicity is consistent with splay faulting at the northern termination of the LOFS; in the central domain, seismicity distributes along ENE- and WNW-striking discrete faults, spatially associated with, hitherto seismic Andean transverse faults. The southernmost domain, in turn, is characterized by activity focused along a N15°E striking master branch of the LOFS. These observations indicate a complex strain compartmentalization pattern within the intra-arc crust, where variable strike-slip faulting dominates over dip-slip movements.

Subsecretaria de Eenrgía y Minería de la Nación SEGEMAR-IGRM DIRECCIÓN DE GEOLOGÍA AMBIENTAL Y APLICADA Programa Nacional de Cartas Geológicas y Temáticas de la República Argentina 1:250.000 Carta de Peligrosidad Geológica 3369-II Boletín N° 324 Dirección de Geología Ambiental y Aplicada

Technical Report

Full-text available

Nov 2018

Technical report on the geologic hazards of the Mendoza city zone

Tectonic control of erosion in the southern Central Andes

Article

Jan 2018
EARTH PLANET SC LETT

Deformación cuaternaria en América del Sur

Article

Full-text available

Dec 2006

Carlos H. Costa

Una perspectiva sobre las principales deformaciones cuaternarias de América del Sur

Article

Full-text available

Dec 2006

Las deformaciones que han afectado al sector continental de Sudamérica durante el Cuaternario aparecen vinculadas con los procesos geodinámicos dominantes durante el Neógeno. Las mismas están principalmente controladas por las anisotropías heredadas de una prolongada y compleja historia evolutiva y por las características cinemáticas y geométricas que caracterizan a la interacción actual de placas. Las principales características de la tectónica cuaternaria en los extremos norte y sur de Sudamérica, derivan en forma directa de las interacciones entre bordes de placas, constituyendo muchos de estos rasgos estructurales los límites entre las mismas. A lo largo de la costa Caribe las principales estructuras con actividad durante el cuaternario exhiben principalmente una orientación E-O y régimen transcurrente. Entre los Andes venezolanos y el golfo de Guayaquil predominan estructuras con orientación NE y una cinemática variable entre regimenes transcurrentes, transpresivos y compresivos. En los Andes Centrales (4ºS-46º30'S) la mayoría de las deformaciones cuaternarias resulta de una compleja distribución y partición de esfuerzos en el interior de la placa Sudamericana, reactivando discontinuidades preexistentes. La expresión superficial de este tipo de deformaciones está mejor representada en la pendiente oriental andina y sectores adyacentes del antepaís. Aquí, la geometría actual de la subducción de la placa de Nazca representa el principal control respecto a la distribución y características de las deformaciones cuaternarias. La principal estructura con actividad cuaternaria en el sector andino austral, está representada por un borde transformante con componente sinestral en Tierra del Fuego, resultante de la interacción entre las placas de Sudamerica y de Scotia.

Full moment tensors for small events (Mw < 3) at Uturuncu volcano, Bolivia

Article

Full-text available

Jul 2016

We present a catalog of full seismic moment tensors for 63 events from Uturuncu volcano in Bolivia. The events were recorded during 2011-2012 in the PLUTONS seismic array of 24 broadband stations. Most events had magnitudes between 0.5 and 2.0 and did not generate discernible surface waves; the largest event was Mw 2.8. For each event we computed the misfit between observed and synthetic waveforms, and we used first-motion polarity measurements to reduce the number of possible solutions. Each moment tensor solution was obtained using a grid search over the six-dimensional space of moment tensors. For each event we show the misfit function in eigenvalue space, represented by a lune. We identify three subsets of the catalog: (1) 6 isotropic events, (2) 5 tensional crack events, and (3) a swarm of 14 events southeast of the volcanic center that appear to be double couples. The occurrence of positively isotropic events is consistent with other published results from volcanic and geothermal regions. Several of these previous results, as well as our results, cannot be interpreted within the context of either an oblique opening crack or a crack-plus-double-couple model. Proper characterization of uncertainties for full moment tensors is critical for distinguishing among physical models of source processes.

Field trip guide: Frontal and Main Andean Cordilleras near the southern boundary of the Pampean shallow subduction zone

Chapter

Full-text available

Jan 2008

Victor A. Ramos

This one-day field trip provides a sampling of the main components of the Andean deformation front in the Precordillera, and the Frontal and Main Cordilleras of the central Andes east of the drainage divide, which at these latitudes coincide with the political boundary of Argentina and Chile. The absence of Pliocene to Recent volcanic rocks in this transect over the southern hinge of the modern shallow subduction allows the older Andean rocks and their structures to be well seen. The structural consequences of shallow subduction are also well seen. The field trip stops provide a view of the Late Paleozoic sedimentary and magmatic rocks of the Frontal Cordillera, the Triassic volcanic and plutonic sequences and associated sedimentary rift sequences east of the Main Cordillera, the Mesozoic to Miocene arc magmatic and sedimentary basin sequences of the high Cordillera, and the Miocene foreland basin deposits to the east. The structure of the Triassic Cuyo rift and inverted normal faults is contrasted with both the Miocene thick-skinned contractional structures affecting the dominantly magmatic rocks of the Frontal Cordillera and the thin-skinned folds and thrusts of the Aconcagua belt affecting the Jurassic to Miocene sedimentary and volcanic rocks of the Principal Cordillera. Depending on climatic conditions, Cerro Aconcagua (6967 m above sea level), the highest peak in the Western and Southern hemispheres and the top of the Backbone of the Americas, can be viewed.

Active Tectonics and Its Interactions with Copahue Volcano

Chapter

Jan 2016

The aim of this review is to describe the state-of-art of the neotectonic setting of this area as well as to present new data resulting from a recent structural field survey. The integrated analysis of literature and new structural data shows incongruities mainly in some regional structures and in the ENE-WSW striking fault system that affects and controls the feeding system of Copahue volcano. In addition, taking into account the very recent volcanic activity, the structural constraints and the earthquakes occurred in the area close to the volcano, a static stress numerical model was applied to simulate the variations of the local stress perturbing the normal activity of the volcanic plumbing system favouring magma ascent and consequent eruptions. At present, a comprehensive structural model is lacking and more in-depth studies can furnish a complete tectonic framework of the area, which can provide fundamental information to assess volcanic hazard, forecast future volcanic activity, and to enhance the development of the associated geothermal field.

Carta de peligrosidad geológica 3369-II, Mendoza

Article

Full-text available

Apr 2004

Se presenta la Carta de Peligrosidad Geológica de la Hoja Mendoza (3369-II), realizada según la normativa del SEGEMAR para este tipo de cartografía temática. La información que integra este trabajo es básica y de escala regional. Presenta datos generales de las características climáticas, geográficas y geológicas, y estudios más detallados de los procesos geológicos que actúan en la zona. La representación gráfica consta de esquemas a escala 1:1.000.000 de las características climáticas, densidad de población, y un esquema geológico regional a escala 1:2.500.000. Para la realización de la Carta final de zonación de Peligrosidad, se realizaron cartas a escala 1:250.000 de las Unidades litológicas, la geomorfología, los procesos geológicos, y por la importancia en la zona, una carta de estructuras cuaternarias. Los procesos geológicos actuantes que se analizaron son erosión, salinización, piping, inundación, remoción en masa, sedimentación, sismos, licuefacción y volcanismo. En relación con la cantidad de procesos, recurrencia, distribución y magnitud de los mismos, así como las características intrínsecas del terreno (factor condicionante) se dividió la superficie de estudio en seis zonas de diferente peligrosidad geológica: I. Área con ocurrencia de hasta cinco procesos potencialmente perjudiciales y por lo menos uno de ellos presenta ocurrencia estacional. II. Área con ocurrencia de hasta tres procesos activos de perjuicio moderado y un proceso potencial de perjuicio alto. III. Área con ocurrencia de hasta dos procesos generalizados, con perjuicio moderado. IV. Área de ocurrencia de dos procesos activos y uno de ellos con perjuicio moderado. V. Área de ocurrencia de hasta dos procesos activos con perjuicio leve. VI. Área donde se han detectado procesos activos que constituyan perjuicio. Las zonas de mayor peligrosidad geológica no abarcan grandes superficies, no obstante se recuerda que este análisis es de carácter regional, y por lo tanto es de consideración para la etapa de reconocimiento.

Reverse faulting in the Isthmus of Tehuantepec: Backarc deformation induced by the subduction of the Tehuantepec ridge

Article

Full-text available

Jan 2000

Gerardo Suárez

The seismicity in the Gulf of Mexico is sparse. There is, however, a cluster of shallow crustal seismicity in the northern part of the Isthmus of Tehuantepec. Two earthquakes occurring in this band of seismic activity are large enough to be studied in detail using a formal inversion algorithm that gleans the best-fitting centroidal focal depth and source mechanism from the radiated P and SH waves. The resulting mechanisms indicate reverse faulting with axes of maximum compression oriented northeast-southwest, subparallel to the direction of relative plate motion in the subduction zone to the south. The presence of these reverse-faulting earthquakes suggests deformation of the backarc in the northern Isthmus of Tehuantepec. The horizontal compressive stress transmitted to the upper continental plate by the subduction of the Cocos plate is apparently enhanced here by the presence of the Tehuantepec ridge, a feature of high bathymetric relief on the subducting Cocos plate. The collision of the ridge with the North American plate apparently produces not only the backarc deformation reflected by this seismicity, but also a northward deflection of the trench and of the coastline on the Pacific Ocean.

Field trip guide: Evolution of the Pampean flat-slab region over the shallowly subducting Nazca plate

Chapter

Full-text available

Jan 2008

Victor A. Ramos

This field guide provides an opportunity to examine the central Andes between 31° and 32°S latitude in a segment characterized by flat-slab subduction. The field trip road was chosen to observe the westernmost contact between the basement uplift of Sierras Pampeanas and Precordillera, the early Paleozoic stratigraphy, and the Andean structure of the Precordillera, as well as a complete section of the Frontal and Principal Cordilleras in Argentina and Chile. The trip ends in the Coastal Cordillera along the Pacific margin. This road log discusses a complete early and late Paleozoic history of the central Andes with their typical Famatinian and Gondwanan orogenic rocks and the accretionary evolution of the Pacific margin at these latitudes. Superimposed on this framework, the structure of the Andes is viewed through the examination of the Precordillera and the Aconcagua fold-and-thrust belts, together with the observation of the Andean volcanic history, will allow reconstructing the shallowing of the subduction zone through the Neogene and the final formation of the Pampean flat-slab.

Geología del Área Quehuita-Chela

Technical Report

Jan 2019

Javier Álvarez Amado

A Tribute to “Analog” Seismologists

Article

Jun 2022

Domenico Di Giacomo

Granulometría, morfología, geoquímica y geocronología de los productos Holocenos del volcán Hudson, región de Aysén, Chile

Thesis

Full-text available

Jan 2014

Lizette Bertin

El Monte Hudson (45°54’S, 72°58’W) es el volcán con mayor frecuencia de grandes erupciones explosivas (VEI>5), en los Andes del Sur. Está ubicado en la Región de Aysén a 30 km al este de la Zona de Falla Liquiñe-Ofqui (ZFLO) y a 280 km al este del punto triple, donde se subducta la Dorsal de Chile, y se sitúa en la posición más austral de la Zona Volcánica Sur (ZVS). El Volcán Hudson corresponde a una caldera de forma circular de 10 km de diámetro que anida en su interior un casquete de hielo desde la cual se desprende una importante lengua glacial hacia el Río Huemules. La presencia de dichas masas de hielo, hacen del Hudson un volcán muy susceptible a producir lahares en los ríos colindantes, en particular hacia el Río Huemules, sin la necesidad de un evento eruptivo de gran magnitud. Además, favorece la interacción agua-roca que causa erupciones freatomagmáticas. Durante tiempos recientes el Hudson ha experimentado 3 episodios eruptivos (1971, 1991 y 2011) con generación de lahares hacia los valles que descienden del macizo (Huemules, Cupquelán, Sorpresas e Ibáñez) y con una periodicidad regular de 20 años entre cada erupción. Éstos 3 episodios, sumados a otros 6 episodios eruptivos holocenos identificados en el registro tefrocronológico en la dirección de los vientos predominantes, son analizados mediante parámetros sedimentológicos (mediante tamizaje mecánico, granulómetro láser y densidad), químicos (analizando los elementos mayores mediante XRF, los elementos trazas con ICP-MS y fracciones vítreas mediante EDS), morfología de partículas (SEM), petrográficos (mediante lupa binocular y microscopio petrográfico) y geocronológicos (dataciones 14C). A partir de éstos, es posible reconstruir en parte la historia eruptiva holocena del Volcán Hudson. El Hudson, se distingue químicamente de los volcanes de la Zona Volcánica Sur sur (ZVSS) y Zona Volcánica Austral (ZVA), por poseer productos calcoalcalinos con una amplia variación composicional que va desde basaltos a traqui-dacitas, de medio a alto-K, enriquecido en elementos incompatibles. Los minerales frecuentes en el material juvenil son plagioclasa, augita, hipersteno, titanomagnetita, ilmenita, olivino y epidota en forma subordinada. El evento eruptivo más antiguo estudiado en este trabajo posee 7.900 años de antigüedad, donde la ceniza eyectada posee composición traqui-dacítica. En forma posterior, ocurren dos erupciones plinianas de gran tamaño (VEI>5) con generación de lapilli acrecionario y posibles flujos piroclásticos. La más antigua posee una edad de 7.300 años y composición traqui-dacítica a traqui-andesítica, mientras que la más joven posee 4.000 años de antigüedad y composición traqui-dacítica. Hace 1.700 y 1.000 años atrás, ocurren dos episodios de composición andesítico-basáltico y basáltica, respectivamente, donde el último es acompañado de eyección de lava hacia el Valle Huemules. Durante el siglo XIX habría ocurrido un sexto evento eruptivo de magnitud menor y composición traqui-andesítica, no documentado en el registro histórico. El evento pliniano más reciente ocurrió en agosto de 1991, el cual expulsó 2,7 km3 (Naranjo y otros, 1993) de tefra traqui-basáltica y traqui-andesítica, distribuido en 2 fases eruptivas. Gran parte de los eventos eruptivos poseen características morfológicas de sus partículas, como blocky y superficies hidratadas, que indican una importante componente freatomagmática causada probablemente por fusión parcial del casquete glacial ubicado sobre la caldera del Hudson, que interacciona con el reservorio magmático, produciendo explosiones que fragmentan parte del conducto volcánico, eyectando hacia la atmósfera fragmentos líticos accidentales que constituyen parte de los depósitos.

Caracterización geológica, geoquímica y estructural de la Veta Los Viscos del Complejo Volcánico Farallón Negro, Catamarca, Argentina.

Thesis

Full-text available

May 2020

Maria Micaela Ninni

Este proyecto de investigación se realizó como parte de los requisitos para la obtención del título universitario de Licenciada en Ciencias Geológicas de la Universidad de Buenos Aires. En él se presenta una caracterización geologíca, geoquímica y estructural de una de las vetas mineralizadas del Complejo Volcánico Farallón Negro ubicado en la Provincia de Catamarca, Argentina. El CVFN corresponde a una antigua caldera Miocena, consta de numerosos centros eruptivos muchos de ellos mineralizados, entre los que se encuentran el pórfiro cuprífero de Bajo de la Alumbrera y las venas epitermales (Au-Ag) de sulfuración intermedia Agua de Dionisio. La tesina se concentró en el estudio de un área de 6000 km2 en la porción NO de la veta Los Viscos. Durante el estudio de campo se realizó un muestreo sistematico de las vetas y la roca de caja, así como también la medición de diferentes estructuras (diaclasas, y diques). Las muestras fueron analizadas al microscopio, petrográfico y calcográfico (29 en total), y a través del método de roca total en forma geoquímica (11 muestras). Una vez obtenidos los resultados se confeccionó el mapa geológico, litologíco y de alteraciones hidrotermales de la zona. Las especies primarias de las vetas son sílice, carbonatos, óxidos e hidróxidos de manganeso, acompañados por elementos nativos como oro y plata, aleaciones, sulfosales de plata, óxidos de hierro y sulfuros. Entre las especies secundarias distinguidas preponderan los óxidos e hidróxidos de manganeso y de hierro. La secuencia definida para la zona de estudio evidencia que la depositación de los principales minerales de mena, sulfosales de plata, oro y plata nativos, fue en asociación con geles de sílice y carbonato. A través de un estudio de rosetas en las vetas y vetillas se determinaron los esfuerzos principales a la hora de depositación con una dirección principal NO-SE y una inclinación promedio de 60°. El estudio de las diaclasas y el dique arrojó una variación en los esfuerzos posterior al magmatismo. Con respecto a la interacción del fluido hidrotermal con la roca de caja ígnea, a partir del análisis petrográfico y geoquímico de muestras provenientes de estas rocas se identificaron una alteración propilítica- fílica de carácter regional y alteraciones argílicas próximas a las vetas y vetillas. Dichas alteraciones evidencian fluidos con un rango de temperatura entre los 100 y 300°C, con pH neutros a levemente ácidos. Por último, con los datos de roca total se determinaron los factores de enriquecimiento de la veta a través de dos métodos: MacLean (1990, 1993) y Gressen y Grant (1967, 1986) arrojando una concentración de metales preciosos de un 100%. El origen de los minerales de manganeso fueron estudiados a a través del método de Nicholson (1992) para determinar la presencia de dubbhitas. La veta de los Viscos fue comparada con Farallón Negro Rama Norte- Encuentro para determinar si el mismo fluido hidrotermal fue el que dio origen a ambos depósitos.

The influence of sediment blanketing on subduction-zone seismicity

Article

Dec 2020
EARTH PLANET SC LETT

Subduction of oceanic lithosphere is now occurring beneath the Aegean and central Caspian Seas, and beneath the Indo-Burman Ranges in NE India, generating zones of earthquakes that reach depths of more than 150 km. Their existence is surprising. In general, where the temperature of the mantle part of the oceanic lithosphere can be estimated, earthquakes are confined to material whose temperature is below 600C∘. In these three regions the oceanic lithosphere is overlain by sediments with low thermal conductivity whose thickness is probably 10–25 km. The temperature of the oceanic crust and upper half of the lithosphere should be considerably increased by such a thick sedimentary cover, yet there is no obvious difference between these subduction zones and those where there is little sediment on the subducting plate. Detailed thermal modelling of the temperature shows that, for the first 40 Ma, the deposition of even a thick sedimentary layer has little effect on the thickness of the region of lithosphere whose temperature is below 600C∘, principally because little heat is conducted across the Moho. The modest effect of sediment blanketing over a period as long as 40 Ma was quite unexpected. Over the next 200 Ma the lithospheric temperature increases, until the Moho temperature reaches ∼600C∘. This behaviour has important implications for oil and gas generation in the sediments, and for the existence of large earthquakes at depths of 150 km or more beneath the Hindu Kush, Pamirs and Romania.

Deformación cuaternaria

Chapter

Full-text available

Dec 2017

Northwestern Argentina is part of the Central Andes, an active subduction orogen that enhances reactivation of older structures and growth of new ones that trigger shallow seismic activity giving to the region the status of high seismic risk. The seismic record (historical and instrumental) shows frequent events including destructive ones. However, the analysis of Quaternary structures deforming the surface, which should be related to paleo earthquakes with M ≥ 6.5, shows an apparent lack of correlation between seismicity and deformation. An interseismic interval much longer than that covered by historical-instrumental records (~325 years) is probably the cause of this lack of correlation. This contribution introduces an updated synthesis on the knowledge of Quaternary structures and highlights the strategic necessity of improving the investigation on Quaternary deformation in order to include this key data into seismic hazard assessment.

State of In Situ Stress in Northern Chile and in Northwestern Argentina

Chapter

Jan 1994

In situ stresses have been determined at rock surfaces by the overcoring method at 45 locations in northern Chile and in northwestern Argentina between 22° and 26° S. From the Pacific coast to the Subandean Ranges we recorded the actual state of stress of all prominent morphotectonic units. There are four distinct regional stress fields: (1) The west Chilean stress field comprises the complete Coastal Cordillera between the Pacific coast and the western margin of the Longitudinal Valley. While horizontal tensile stresses oriented E-W prevail on the Mejillones Peninsula, along the coast north of Antofagasta and in the area of Taltal, horizontal compressive stresses (σHmax = σ1) directed NE-SW predominate in the entire Coastal Cordillera. σHmax reveals mean values of 9 MP, a whereas σHmin (= σ2) oriented NW-SE yields — 1 MPa on average. In situ stresses of the west Chilean stress field signify an actual uplift of the Coastal Cordillera bordered on the west (Mejillones Peninsula) and on the east (Atacama Fault) by active normal faults striking N-S. (2) The Central Chilean stress field includes the Longitudinal Valley and the western part of the Chilean Precordillera. From west to east compressive aHmax directed NW-SE to N-S decreases from 40 to 7 MPa, whereas tensile σHmin oriented NE-SW to E-W increases from 0 to -7 MPa. Generally aHmax trending NW-SE corresponds to σ2 in the west and to al in the east of the Central Chilean stress field, indicating actual normal faulting in the west (Atacama Fault, Longitudinal Valley) and strike-slip or thrust faulting in the east (Calama). (3) The Central Andean stress field extends from the Preandean Cordillera Domeyko across the Preandean Depression, Western Cordillera, and Puna, and includes the western area of the Eastern Cordillera. Maximum horizontal in situ stresses are generally oriented E-W yielding average values of 14 MPa (N-S, σHmin = 0.7 MPa) in the western part of the Central Andean stress field (Cordillera Domeyko, Preandean Depression, Western Cordillera), whereas in the Puna lower E-W compressive stresses only occasionally exceed 10 MPa. Horizontal tensile stresses oriented NW-SE and of -5 to -15 MPa along the Olacapato — El Toro lineament indicate its probable recent strike-slip movement. (4) The east Andean stress field is defined by the eastern part of the Eastern Cordillera or may extend to the east into the Subandean Ranges. Maximum horizontal stresses are compressive in the NE-SW direction and vary between 2 and 17 MPa. In situ horizontal stresses across the uppermost crust of the Andes between 22° and 26° S are dependent on the thickness of the rigid Andean crust rather than on topographic elevation.

Geología del área Collacagua-Rinconada, Región de Tarapacá

Article

Full-text available

Jan 2013

The Collacagua-Rinconada Area Sheet, scale 1:100,000, lies between parallels 20°30’ and 21°00’S and between meridian 69°00’W and the border with Bolivia, covering a surface area of 1,750 km2. It is located in the high mountains of the Tarapacá Region and comprises, from west to east, the eastern portion of the Altos de Pica ridge (up to 5,220 m a.s.l.), the intermountain depression formed by the Huasco salt flat and the Collacagua river (average 3,800 m as.l.), and the Main Cordillera (up to 5,100 m a.s.l.). The geology of the Collacagua - Rinconada area is dominated by extensive outcrops of the Early Miocene large-volume, welded rhyolitic tuffs Tambillo (Miit; ~20 Ma) and Huasco (Miih; ~16 Ma) ignimbrites. These ignimbrites largely cover the western and southern portions of the study area, leaving therefore few small windows where underlying units are exposed. However, along the north-western border of the map, in the Columtucsa and Japu hills, Late Cretaceous to Eocene stratified and intrusive units tower above the pyroclastic cover, and constitute basement highs that surpass 5,200 m a.s.l. These basement units include the Late Cretaceous Cerro Empexa Formation (Ksce; ca. 79-68 Ma), consisting of breccias and conglomerates of mainly andesitic clasts, folded and faulted during the ‘K-T’ tectonic event, overlain by gently folded sedimentary, pyroclastic and lava beds of the late Lower to early Middle Eocene (47.5 Ma) Icanche Formation (Ei). Both units are intruded by variably altered Middle Eocene (43-40 Ma) intrusive rocks (Japu-Columtucsa Granitoids; Eg), commonly porphyritic, of diorite, quartz-diorite and quartz-monzodiorite composition. Toward the northern end of the sheet, a volcanic and epiclastic sequence, also of Early Miocene age (20-18 Ma; Miv) is interstratified between the two large ignimbrites. Lower Miocene units are gently tilted and folded, and cut by ~NS striking reverse faults (Huasco and Guañada faults) associated to the Oligocene-lower Late Miocene compressional event. The reverse Huasco and Guañada faults define the Huasco salt flat-Collacagua river basin. Additionally, the Early Miocene ignimbrites are deformed by a system of NE normal faults that originate narrow grabens, probably formed during the Late Miocene, associated to a decrease of compressive deformation in the Altiplano. Among these structures the Diablo Marca fault stands out, a dextral strike slip fault with a normal component.

Compressional Pliocene and comoressional-transoressional Pliocene states of stress, southern Chilean Andes (38-42°30'S)

Article

Full-text available

Jul 1999
REV GEOL CHILE

Miocene-Pliocene and Quaternary intrusive rocks and sedimentary deposits from the Central Depression and the Main Cordillera, between 38 and 42°30'S, have been affected by local and regional brittle deformation. Microfault geometry and kinematic analysis along with calculation of deviatoric tensors allowed to determine regional-scale states of stress. Two tectonic events were identified. A Pliocene event, prior to the Quaternary, affects the entire zone of study, and is characterized by a maximum compressional stress a, roughly oriented in an E-W direction. A Pleistocene event corresponds to an overall deformation partitioned into two coeval distinctive states of stress: a compressional stress a, oriented in a N-S to NNE-SSW direction in the fore arc zone, and a dextral transpressional state of stress with 0, striking NE-SW, in the intra-arc zone.

Shallow Seismicity in the North Fiji Basin

Chapter

Jan 1994

This paper presents a new compilation of shallow seismicity and focal mechanism data that help constrain the model of extension in the North Fiji Basin. Earthquakes are broadly distributed throughout the basin, in marked contrast to the narrow earthquake zones observed near mid-ocean ridge spreading centers. Areas of relatively high activity include the Fiji Fracture Zone, the Hazel Holme Ridge, the western Hunter Fracture Zone, and the proposed spreading center immediately west of Fiji. Areas of deep water in the northern and western North Fiji Basin are notably aseismic and may represent older, presently undeforming portions of the basin. Twenty-three focal mechanism solutions for the basin indicate that the region is dominated by strike-slip deformation. We observe no simple basin-wide system of stress distribution, but consistent stress orientations for groups of mechanisms provide evidence for (1) transcurrent faulting along the Fiji Fracture Zone; (2) hinge faulting of the Indo-Australian Plate near the Hunter Fracture Zone; (3) strike-slip faulting near the Hazel Holme Ridge; (4) strike-slip faulting within the central North Fiji Basin; and (5) normal faulting in the western North Fiji Basin. The orientations of the normal faulting events’ tension axe, and the strike-slip events’ fault planes are at odds with the configuration of the major spreading centers proposed by previous investigators. A model of “diffuse extension” in the North Fiji Basin provides an explanation for the broadly distributed shallow seismicity, the paucity of normal faulting mechanisms, the obliquity of earthquake fault planes to the strike of proposed spreading centers, and the lack of a uniform basin-wide pattern of stress orientation.

Tectonic influence in Cenozoic vulcanism in north-western Argentina

Article

Full-text available

Jan 1999

A model postulating that the differences in the volcanic evolution between North and South Puna are related to strong variations of the orogenic front development in the foreland is presented. Subandean Belt-Northern part-, Santa Bárbara System -Central part-and Pampean Ranges -Southern part-show different tectonic behaviors due to the mechanic nature of their pre-Andean basement. A thin-skin fault-fold belt developed in the Subandean Belt is a consequence of the presence of a thick Paleozoic sedimentary cover. Thrust progressive migration east implicted the displacement of inner orogenic parts on low detachment levels, which sealed magmatic chambers. So, volcanic activity finished or suddenly decreased when the fault-fold belt began to develop in the foreland. On the other hand, mechanic anisotropies of the pre-Andean basement in the Santa Bárbara Systems and Pampean Ranges controlled and stopped the migration of the orogenic wedge toward the foreland, therefore deformation increased in the inner part of the orogen. Tectonic stacking and the consequent elevation of the back zone of the orogenic wedge would generated an extensional field for equilibrating the wedge geometry. Normal faults are widespread at South Puna and were the conduits by which big ignimbritic deposits have been extruded at 4 Ma (Laguna Amarga) and 2 Ma (Galán) from medium crust levels and also basalts of less than 1 Ma from upper mantle.

6: Seismicity, crustal structure, and intraplate tectonics of the interior of the western Cordillera

Chapter

Full-text available

Jan 1978

Robert B. Smith

Seismicity, fault-plane solutions, and Cenozoic geology are used to infer contemporary west to northwest extension and components of lateral slip between subplates of the western North American plate. Seismicity of the western interior of the Cordillera is characterized by earthquakes that occur in broad zones, up to 150 km wide, in the Nevada and the Intermountain seismic zones. Epicenters are scattered and when accurately located many do not coincide with mapped faults. Focal depths are shallow and seldom exceed 20 km. Secondary seismicity occurs around the margins of the Colorado Plateau, in central Idaho, and in eastern Oregon and Washington. The contemporary strain pattern of the Western United States, as interpreted from fault-plane solutions, suggests that three intraplate lithospheric blocks—the Sierra Nevada, the Great Basin-High Lava Plains, and the Northern Rocky Mountains-Columbia Plateau are moving west to northwest as “slivers” between the obliquely converging Pacific and North American plates. Intraplate extension within the Great Basin is primarily accommodated by north-south normal faulting in north-central Nevada and along the Wasatch Front, but significant components of strike-slip faulting occur along active seismic zones in southwestern Nevada and along the northern Intermountain seismic belt in Montana. A thin crust, ~25 km, characterizes the east margin of the Great Basin, with average Pn-velocities of ~7.5 km /s. A crustal low-velocity layer, at 5 to 15 km depth, coincides with the eastern margin of the Great Basin. A thin crust, with an apparent Pn-velocity of ~7.7 km /s, occurs on the northwest margin of the Great Basin and beneath parts of the Oregon-High Lava Plains. The central part of the Great Basin has a thicker crust, ~30 km, and higher Pn-velocities, 7.7 to 7.9 km /s. The Colorado Plateau and the Rocky Mountains have thicker crusts, 40 to 50 km, and Pn-velocities of ~7.8 km/s. An upper-mantle diapir or an upwelling thermal mechanism, facilitated by stress relaxation above the now-truncated Farallon subducting plate, is postulated to have uplifted, extended, and heated the crust of the Great Basin beginning at ~20 m.y. Late Cenozoic centers of volcanism appear to have progressed outward from a northern Great Basin thermal center in two divergent directions—northwesterly along a combined extensional-strike-slip zone of deformation in the High Lava Plains of southeastern Oregon and northeasterly along an extensional zone marked by the Snake River Plain. Lithospheric heating is thought to have produced a broad uplift of the Great Basin with concomitant crustal thinning and diminishing of upper-mantle Pn-velocities. The thermal mechanisms are hypothesized to have produced laterally divergent mantle flow to form symmetric zones of crustal thinning and low Pn-velocities that presently mark the east and west margins of the Great Basin.

Andean tectonics related to geometry of subducted Nazca plate

Article

Full-text available

Jan 1983

Seismological and geological data show that tectonic segmentation of the Andes coincides with segmentation of the subducted Nazca plate, which has nearly horizontal segments and 30o E-dipping segments. Characterisitcs of Andean tectonics. Early Cenozoic tectonics of W N. America were quite similar to the Neogene Andes. However, duration of segmentation was longer and the width of deformation was greater in the W US. Patterns of crustal seismicity are systematically related to Plio-Quaternary structural provinces, implying that current deformational processes have persisted since at least the Pliocene. Strain patterns in the forearc region are complex and perhaps extensional and a broad region of the Altiplano-Puna and Eastern Cordillera appears to be aseismic.

Oceanic intraplate earthquakes: Implications for local and regional intraplate stress

Article

Full-text available

Jan 1980

Focal mechanisms of intraplate earthquakes provide the only means at present by which to characterize the long-wavelength tectonic stress field in oceanic lithosphere. Stress orientations inferred from focal mechanisms may not accurately reflect the state of stress in the epicentral area, however, or the measured stresses may be dominated by local rather than regional sources. To establish a data set with which to study these possibilities, a comprehensive catalog of 159 oceanic intraplate earthquakes has been complied for events since 1963 with m/sub b/ 4.7 or larger. Focal mechanisms are available for approximately one quarter of the events, and several new mechanisms are presented here. For a representative subset of this catalog (83 events), the bathymetry and tectonic history of the epicentral areas have been assembled, and the earthquakes have been rated according to their association with (1) a preexisting fault zone, which might decouple the P axis of the focal mechanism from the true orientation of maximum compressive stress, and (2) large bathymetric relief, which might be a source of large local stresses. Oceanic intraplate earthquakes are commonly found in association with zones of previous weakness (usually fracture zones), but they do not show any particular association with large bathymetric features. In the central Indian Ocean there are enough focal mechanisms available to establish a well-defined NW-SE orientation for P axes and presumably for the direction of greatest compressive stress. The consistency of the P axes of these widely varying mechanisms in the presence of the Ninetyeast Ridge, a site of major intraplate deformation and large bathymetric relief, is remarkable. A possible explanation is that in the presence of a large number of preexisting faults with a range of orientations, slip occurs on those faults which have large resolved shear stresses from the regional stress field.

Total energy and energy spectral density of elastic wave radiation from propagating faults

Article

Dec 1964

N. A. Haskell

Starting with a Green's function representation of the solution of the elastic field equations for the case of a prescribed displacement discontinuity on a fault surface, it is shown that a shear fault (relative displacement parallel to the fault plane) is rigorously equivalent to a distribution of double-couple point sources over the fault plane. In the case of a tensile fault (relative displacement normal to the fault plane) the equivalent point source distribution is composed of force dipoles normal to the fault plane with a superimposed purely compressional component. Assuming that the fault break propagates in one direction along the long axis of the fault plane and that the relative displacement at a given point has the form of a ramp time function of finite duration, T, the total radiated P and S wave energies and the total energy spectral densities are evaluated in closed form in terms of the fault plane dimensions, final fault displacement, the time constant T, and the fault propagation velocity. Using fault parameters derived principally from the work of Ben-Menahem and Toksöz on the Kamchatka earthquake of November 4, 1952, the calculated total energy appears to be somewhat low and the calculated energy spectrum appears to be deficient at short periods. It is suggested that these discrepancies are due to over-simplification of the assumed model, and that they may be corrected by (1) assuming a somewhat roughened ramp for the fault displacement time function to correspond to a stick-slip type of motion, and (2) assuming that the short period components of the fault displacement wave are coherent only over distances considerably smaller than the total fault length.

Site 286

Article

Jan 1975

J.E. Andrews

The February 9, 1971 San Fernando earthquake: A study of source finiteness in teleseismic body waves

Article

Feb 1978

Charles Langston

Teleseismic P, SV, and SH waves recorded by the WWSS and Canadian networks from the 1971 San Fernando, California earthquake (ML = 6.6) are modeled in the time domain to determine detailed features of the source as a prelude to studying the near and local field strong-motion observations. Synthetic seismograms are computed from the model of a propagating finite dislocation line source embedded in layered elastic media. The effects of source geometry and directivity are shown to be important features of the long-period observations. The most dramatic feature of the model is the requirement that the fault, which initially ruptured at a depth of 13 km as determined from pP-P times, continuously propagated toward the free surface, first on a plane dipping 53°NE, then broke over to a 29°NE dipping fault segment. This effect is clearly shown in the azimuthal variation of both long period P- and SH-wave forms. Although attenuation and interference with radiation from the remainder of the fault are possible complications, comparison of long- and short-period P and short-period pP and P waves suggest that rupture was initially bilateral, or, possibly, strongly unilateral downward, propagating to about 15 km depth. The average rupture velocity of 1.8 km/sec is well constrained from the shape of the long-period wave forms. Total seismic moment is 0.86 × 10^(26) dyne-cm. Implications for near-field modeling are drawn from these results.

Focal mechanism of the August 1, 1975 Oroville earthquake

Article

Aug 1976

Long-period teleseismic P and S waves from the WWSS and Canadian networks are modeled to determine the focal parameters for the main shock in the Oroville earthquake series. Using the techniques of P first motions, wave-form synthesis, and phase identification, the focal parameters are determined as follows: dip 65°; rake −70°; strike 180°; depth 5.5 ± 1.5km; moment 5.7 ± 2.0 × 10^(24) dyne-cm; and a symmetric triangular time function 3 sec in duration. This is a north-south striking, westward dipping, normal fault with a small component of left-lateral motion. The time separation between the small foreshock and mainshock appears to be 6.5 sec at teleseismic distances, rather than 8.1 sec as observed at short distances.

Spectral and magnitude characteristics of anomalous Eurasian earthquakes

Article

Apr 1977

A number of main shock-aftershock sequences in the Eurasian interior contain some aftershocks whose mb:MS values are close to those of underground explosions. This paper is concerned with a study of the amplitude spectra of the P waves and Rayleigh waves for earthquakes of those main shock-aftershock sequences. It is found that for any given sequence studied, there is little if any variation in focal depth or focal mechanism. This rules out variations in these quantities as being the cause of anomalous mb:MS values. A study of the P-wave spectra establishes that one or both of the corner periods of anomalous earthquakes are smaller than those of non-anomalous earthquakes of the same moment. Thus the cause of anomalous mb:MS values of the earthquakes studied is a relative enrichment of the short-period portion of the spectrum of the anomalous events, which cannot be attributed to focal depth or focal mechanism.

The corner frequency shift, earthquake source models, and Q

Article

Jun 1981

Thomas C. Hanks

The very common seismological observation that the P waveform is enriched in high-frequency motion relative to the S waveforms of the same earthquake manifests itself, in spectral studies of the earthquake mechanism, as the “corner frequency shift,” the general although not ubiquitous tendency for the P-wave corner frequency fo(P) to be greater than the S-wave corner frequency fo(S). In point source, time-domain modeling studies of the earthquake mechanism which follow the recipe of D. V. Helmberger and C. A. Langston that explicitly suppresses the corner frequency shift, it is an equally common result that the synthetic S waves are systematically enriched in high-frequency motion relative to the observations. Almost three dozen spectral analysis and time-domain modeling studies are recapitulated in this one to conclude: (1) the corner frequency shift is a very common condition of the far-field body waves of earthquakes, with no discernible dependence on earthquake source strength, hypocentral distance or depth, or recording device; and (2) the corner frequency shift is the manifestation of an intrinsic property of earthquakes, source finiteness. Anelastic attenuation estimates for shear waves determined on the basis of a source model derived from P waves are likely to be strongly biased to high values of Q-1, if the shear excitation is estimated directly from the equivalent far-field compressional excitation, i.e., with no allowance for source finiteness. The corner frequency shift, moreover, places strong constraints on admissible earthquake source models; point-source and Haskell-type dislocation models will not be among them.

Seismicity and plate subduction in the central Aleutians

Article

Jan 1977

Eric Robert Engdahl

Seismic and aseismic slip along subduction zones and their tectonic implications

Article

Jan 1977

Hiroo Kanamori

Results of detailed mechanism studies of great earthquakes are used together with their repeat times to determine the amount of seismic slip along various subduction zones. Comparison of the seismic slip with the rate of plate motion suggests that, in Chile, and possibly Alaska, the seismic slip rate is comparable to the rate of plate motion while, in the Kuriles and Northern Japan, the seismic slip constitutes only a very small portion, approximately 1/4, of the total slip. In the Sanriku region, and to the south of it, the relative amount of seismic slip is even smaller. These results suggest that in Chile and Alaska the coupling and interaction between the oceanic and continental lithosphere are very strong, resulting in great earthquakes with a very large rupture zone, and in break-off of the undergoing lithosphere at shallow depths. In the Kuriles and Northern Japan, the oceanic and continental lithosphere are largely decoupled, so that the slip becomes largely aseismic, and the rupture length of earthquakes reduced. The reduced interaction at the inter-plate boundary may allow the oceanic lithosphere to subduct more easily and to form a continuous Benioff zone extending to depths. It may also facilitate ridge subduction beneath island arcs, which may play an important role in the formation of marginal seas such as the Japan Sea. The decoupling is also evidenced by silent or tsunami earthquakes [e.g., the 1896 Sanriku earthquake], great intra-plate normal-fault earthquakes [e.g., the 1933 Sanriku earthquake], and crustal deformation. A natural extension of this concept of inter-plate decoupling is the spontaneous sinking of the oceanic lithosphere with a consequent retreating subduction. Retreating subduction may be an important mechanism in the format ion of marginal seas such as the Philippine Sea, and explains the complete lack of major shallow earthquake activity along some subduction zones such as the Izu-Bonin-Mariana arc.

The Caracas, Venezuela, earthquake of July 1967: A multiple-source event

Article

Jan 1978

Jose A. Rial

Determination of source parameters of mid-plate earthquakes from the waveforms of body waves

Article

Dec 1980

The source parameters and stress drops of five mid-plate earthquakes were determined by matching the synthetic and observed far-field P waveforms in the time domain. The stress drops estimated from these mid-plate events are on the order of a hundred to a few hundred bars. These values are significantly higher than those for interplate (30 bars) and intraplate (100 bars) earthquakes which occurred near the plate boundaries. The difference of the earthquake stress levels between mid-plate and plate boundary events may suggest a lateral variation of stress level in the lithosphere and provide important constraints on the driving mechanism of plate motion.

Focal depth determination from the signal character of long-period P waves

Article

Aug 1976

Robert Herrmann

The shape of long-period teleseismic P-wave signals is a function of many factors, among which are focal depth, focal mechanism, the source time function, and the earth structures at both the source and receiver. The effect of focal depth is quite pronounced, so much so, that focal depths should be able to be determined to within 10 km on the basis of the long-period P-wave character. This resolution capability is demonstrated for events occurring in continental and oceanic crust as observed by seismographs in the 30° to 80° epicentral distance range.

Fault plane solutions using relative amplitudes of P and pP

Article

Jan 1980

RG PEARCE

The method of computing fault plane solutions for small shallow earthquakes using relative amplitudes of P and pP, as described in an earlier paper, is extended to include sP. It is shown theoretically that even a single relative amplitude observation can impose a severe constraint on the orientation of an assumed double couple source, and earthquakes studied in the earlier paper are reprocessed with the inclusion of sP information. The method is also extended to deep earthquakes using long period seismograms, and to undersea earthquakes by allowing for the effect of a sea layer on the surface-reflected phases. Also described are options to search only a restricted range of source orientations, and to identify those orientations which are incompatible with one or more relative amplitudes. These options are applied to several earthquakes, to demonstrate the scope of the relative amplitude method. Results are illustrated using the seismic modelling method of Hudson (1969a,b) and Douglas, Hudson & Blamey (1972).-Author

A large, deep Hawaiian earthquake—The Honomu, Hawaii event of April 26, 1973

Article

Dec 1979

The April 29, 1965, Puget Sound earthquake and the crustal and upper mantle structure of western Washington

Article

Jun 1977

Simultaneous modeling of source parameters and local layered earth structure for the April 29, 1965, Puget Sound earthquake was done using both ray and layer matrix formulations for point dislocations imbedded in layered media. The source parameters obtained are. dip 70 ° to the east, strike 344 °, rake --75 °, 63 km depth, average moment of 1.4 -1-0.6 X 10 ~6 dyne<m, and a triangular time function with a rise time of 0.5 sec and falloff of 2.5 sec. An upper mantle and crustal model for southern Puget Sound was determined from inferred reflections from interfaces above the source. The main features of the model include a distinct 15-km-thick low-velocity zone with a 2.5-km/sec P-wave-velocity contrast lower boundary situated at approximately 56-km depth. Ray calculations which allow for sources in dipping structure indicate that the inferred high contrast value can trade off significantly with interface dip provided the structure dips eastward. The ef-fective crustal model is less than 15 km thick with a substantial sediment section near the surface. A stacking technique using the instantaneous amplitude of the analytic signal is developed for interpreting short-period teleseismic observations. The in-ferred reflection from the base of the low-velocity zone is recovered from short-period P and S waves. An apparent attenuation is also observed for pP from com-parisons between the short-and long-period data sets. This correlates with the local surface structure of Puget Sound and yields an effective Q of approximately 65 for the crust and upper mantle.

Post-Eocene Plate Tectonics of the Eastern Pacific

Article

Jan 1976

David Handschumacher

A Miocene reorientation of spreading patterns in the eastern Pacific is proposed based on post-Eocene tectonic reconstructions using principles of plate theory together with observed magnetic anomaly patterns. The reconstruction shows that the Pacific-Farallon ridge was, prior to 26 m.y.B.P., intact and eastwardly migrating relative to the Americas plate. This ridge, exceeding 11,000 km in overall length, had an equatorial spreading rate of ˜ 13.5 cm/yr. At ˜ 26 m.y.B.P., a section of the ridge collided with the Farallon-Americas trench off the west coast of North America, stopping all spreading and subduction activity at the point of collision and initiating a direct dynamic coupling between the Pacific and the Americas plates. The coupling, presumably unstable to earlier spreading kinematics, required a major reorientation of sea-floor spreading patterns in the eastern Pacific, and included the following correlative events: a clockwise rotation of the southern portion of the ridge; initial N-S spreading along the Galapagos rift zone; break-up of the Farallon plate into the Gorda, Cocos, and Nazca plates; and dual spreading from the Pacific-Antarctic ridge and the Chile ridge. Between ˜26 m.y.B.P. and ˜4 m.y.B.P., there were further concurrent adjustments to the spreading regime involving the continuing overriding of the ridge off the west coast of North America. Included were numerous sectional jumps of the East Pacific rise, northward migration of the Chile triple junction, westward migration of the Galapagos triple junction, and development of the Gorda-Juan de Fuca-San Andreas-Gulf of California spreading system. It is proposed that a reorientation of sea-floor spreading kinematics on the order of that described for 26 m.y.B.P. would have produced dramatic changes on circum-Pacific plate boundaries and that present circum-Pacific trench-continent and trench-island arc-marginal basin regimes were either created or significantly altered by this early Miocene reorientation of sea-floor spreading kinematics in the eastern Pacific.

The Bellingshausen Sea earthquake of February 5, 1977: Evidence for ridge-generated compression in the Antarctic plate

Article

Jan 1980
EARTH PLANET SC LETT

Emile Okal

Body- and surface-wave data from a magnitude Ms = 6.4 event in the Bellingshausen Sea are used to infer a thrust fault mechanism, with compressional stress directed parallel to the vector of relative motion between Antarctica and the Pacific plate at the nearby ridge. The solution would not, on the other hand, be compatible with stress fields involving absolute plate motion, such as asthenospheric drag.

Ringing P waves and submarine faulting

Article

Jan 1979

Steven Ward

Long-period teleseismic records from shallow focus oceanic events sometimes exhibit 'ringing' P waves. The ringing portion of these prolonged wave groups characteristically has a narrow band frequency content, an amplitude which monotonically decreases from values comparable to the primary P pulse, and a high station to station coherence. These traits are suggestive of leaking compressional modes of the oceanic water layer. This layer is an efficient wave guide for elastic body waves once their energy is effectively introduced into the system. Through the modeling of point and extended sources interior to oceanic crust it is found that to account for the large excitation of these modes a particular type of faulting is required. Specifically, the fault must originate in a setting such that a substantial fraction of the dislocation surface is situated above the Moho and be oriented so that a significant amount of compressional energy is propagated upward. These provisions are met by steeply dipping or thrust faults in continental margins and subduction zones where thickened oceanic crustal and sedimentary layers are commonly found. Events of this type are likely to generate perturbations in submarine topography. Hence real-time observation of ringing P waves from large earthquakes in these geographical provinces could be diagnostic of shallow faulting with high tsunamigenic potential.

The Caracas, Venezuela, earthquake of July 1967: A multiple-source event

Article

Jan 1978

Jose Rial

A general study of Caribbean plate tectonics is first focused on the determination of fault parameters and source processes of the Caracas (Venezuela) earthquake of July 29, 1967 (mb=6.5,M8=6.7). Synthetic seismograms which closely reproduce the observed P, SH, and Love wave seismograms were generated using generalized ray and mode theories. The results indicate a complicated faulting process, consisting of at least three separated sources aligned along a N10°W trending 'en echelton' vertical left lateral strike slip system of three faults that ruptured from north to south, at three discrete plates with an extreme separaton of 90 km. The process of rupture progressed southward with a mean velocity of 3 km/s. The focal depths of the individual sources varied between 8 and 27.5 km. The total dislocation was calculated as 120 cm along the direction N10°W, and the total average moment as 4×1026 dyn cm. The multiple character of the event severely constrains the number of suitable source models that can be inferred, thus facilitating the process of inversion. Tectonic implications are briefly discussed, and local geology is successfully invoked to support the source model.

Crustal Structure of the Melanesian Area

Article

Apr 1971

Seismic refraction studies in the Melanesian Borderland (the area between Australia on the west and New Zealand and Tonga on the east) show extreme diversity of crustal structure. The Lord Howe rise and the Norfolk ridge are topped by thick sediments and have deep crustal roots and thick layers of material with the same compressional wave velocity as the Australian continental crust. The Kermadec and Lau ridges, on the other hand, have structure and velocities typical of normal island arcs. The South Fiji basin structurally resembles an oceanic area with additional sedimentary fill. The western part of the Fiji plateau has thin sediments and has the structure of an area of normal deep-ocean basin that has been uplifted two kilometers. All the features are compatible with the hypothesis that the area has been disrupted and fragments of continental material have been separated from the Australian mass.

Alaskan Earthquake of 1964 and Chilean Earthquake Implications for Arc Tectonics of 1960

Article

Feb 1972

George Plafker

The 1964 Alaskan earthquake (Ms ≈ 8.4) involved a segment of the eastern Aleutian arc 800 gm long; the 1960 Chilean earthquake sequence (Ms ≈ 8.5) affected roughly 100 km of the southern Peru-Chile arc. These two major events are strikingly similar in that (1) seismicity was shallow (<70 km), the earthquake focal regions and most of the associated tectonic deformation being between the oceanic trenches and volcanic chains of the two arcs; (2) regional vertical displacements were characterized by broad asymmetric downwarps elongate parallel to the arcs with flanking zones of marked uplift on the seaward sides and minor, possibly local, uplift on the landward sides; and (3) horizontal displacements, where determined by retriangulation, involved systematic shifts in a generally seaward direction and transverse tensile strains across the zones of subsidence. Surface displacements and seismicity for both events are compatible with dislocation models involving predominantly dip-slip movement of 20 meters or more on major complex thrust faults (megathrusts) inclined at average angles of about 9° beneath the eastern Aleutian arc and perhaps 20° beneath the Peru-Chile arc. The thrust-fault mechanism deduced for both the Alaskan and Chilean earthquakes is broadly consistent with the concept that the sectors of the Pacific rim in which they occurred are major zones of convergence along which the oceanic plates progressively underthrust the less mobile America plate. Directions of convergence between lithospheric plates at these arcs as deduced primarily from paleomagnetic data are in reasonably good agreement with the observed earthquake-related deformation; the deduced rates of convergence, however, appear to be too high in the eastern Aleutian arc and too low in the southern Peru-Chile arc. Despite gross similarities in tectonic setting and the present style of earthquake-related deformation, the geologies of the continental margins in the eastern Aleutian arc and southern Peru-Chile arc differ significantly. This difference suggests that Mesozoic and Cenozoic sediments and volcanic rocks conveyed into the eastern Aleutian trench have progressively accreted to the Alaskan continental margin, whereas most or all of the material carried into the southern Peru-Chile trench has disappeared beneath the Chilean continental margin.

Shallow Structure of the New Hebrides Island Arc

Article

Jan 1974

Marine geological and geophysical studies of the New Hebrides island arc have been made to study (a) the present development of lithospheric plate boundaries, (b) evidence for creation of oceanic crust behind the frontal arc in interarc basins, and (c) evidence for reversal of the arc from east-facing to the present-day west-facing orientation. The arc system is bisected between 13° and 15° S. by the east-west Hazel Holme Fracture Zone which connects the trench and a north-south-trending spreading center (Nova Rise) on the Fiji Plateau near 174° E. The crust on the plateau south of the fracture zone is very young. Narrow interarc basins are present but youthful, south of about 18° S. North of the Hazel Holme Fracture Zone, interarc basins are less well developed and apparently even younger. Most of the Fiji Plateau has apparently been formed by spreading from the Nova Rise rather than within interarc basins associated with the New Hebrides. The tectonics of the central region of the arc system, immediately south of 15° S., appears to be governed by the transform section of the Hazel Holme Fracture Zone and by subduction of the D'Entrecasteaux Fracture Zone into the trench rather than by interarc spreading. In this region, the western and eastern chains of the New Hebrides group have been recently uplifted and tilted toward one another, creating a sedimentary basin. Most data do not support the idea that the eastern island belt in this region, including Maewo and Pentecost islands, is an ancient remnant of an east-facing arc system. These islands have been uplifted only very recently and later than the western islands. Therefore, any east-facing subduction phase must have ceased recently and occurred after subduction beneath the western islands. We suggest instead that the eastern island belt represents an interarc basin floor or a frontal arc uplifted behind the volcanic line.

Part 2. Younger Pacifics arcs and the eastern marginal seas: Arc reversals, and a tectonic model for the North Fiji Basin

Article

Jun 1975

David Falvey

The North Fiji Basin (or "Plateau") is a morphologically complex marginal basin lying between the New Hebrides arc and Fiji. According to the normally accepted view of basin-island arc polarity, it is a reversed marginal basin. The North Fiji Basin is a part of the Pacific plate which overrides the India plate along the New Hebrides arc. Conversely, the adjacent South Fiji Basin (Packham, this volume) is part of the India (Australia) plate which overrides the Pacific plate along the Tonga-Kermadec arc.

Magnetic anomalies and tectonic fabric of marginal basins North of New Zealand

Article

Jan 1982

Detailed airborne magnetic studies conducted over the region of the S. W. Pacific marginal basins extending from the Solomon Islands to New Zealand suggest that three major phases of basin formation and island arc development have occurred in this region. Development of the Tasman Sea took place during the Late Cretaceous-Paleocene. Development of the basins to the east of the Tasman Sea occurred predominantly during the Oligocene as well as during the Upper Miocene to Recent. The South Fuji Basin, consisting of the Kupe and Minerva Abyssal Plains, is marked by the presence of possibly two RRR triple junction spreading centers that were active between the times of anomalies 13 to 7 (36--25.5 m.y.). The Kupe Abyssal Plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center. The western limb appears to have been subducted beneath the present site of the Three Kings Rise. This seafloor spreading phase (calculated half-spreading rate of 35 mm/yr) was coincident with the overthrusting phase of the New Caledonia ultramafic rocks. During that period, active volcanism along the then continuous Solomons-New Hebrides-Fiji-Lau Island arc was taking place. Magnetic anomalies from 1 to 4 (0--8 m.y. B. P.) are seen to extend along a clearly defined lineation pattern over the North Fuji Basin.

Seismicity of the forearc marginal wedge (Accretionary prism)

Article

Jan 1982

Three different types of seismic data have been examined for seismic events occurring within the zone called the accreted wedge or forearc marginal wedge that underlies the inner wall of some arcs. These types of data are 1) teleseismically recorded earthquakes that have been reported in the literature as occurring in major arc-trench regions; these events fail to demonstrate that earthquakes occur within the accreted wedge because the uncertainty of focal depth usually exceeds the depth dimension of the accreted wedge; these data include many tsunamigenic earthquakes, 2) local earthquakes located by combined ocean bottom seismograph and land networks in the arc-trench region in the New Hebrides and the central and E Aleutian Trench; none of the more reliable of these hypocenters lies within the accreted wedge; 3) S-P intervals measured at stations on islands located on the outer ridge or at ocean bottom seismograph stations on the forearc marginal wedge; these data do not show the existence of events occurring within the accreted wedge; e.g. from 18 ocean bottom seismograph stations with a cumulative operation time of approx 1yr, the smallest S-P time is approx 2.5s for events in the New Hebrides and approx 4s for events in the Adak and Kodiak regions. We found no S-P time <2s from 6yr of seismograms recorded at Middleton Island, Alaska, and no S-P time <4s from 25yr of seismograms recorded on Barbados. -Authors

Fine structure of the dipping seismic zone and subduction mechanics in the Shumagin Islands, Alaska

Article

Jan 1982

Microearthquake data from a permanent, telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the Shumagin Islands, Alaska. The shallow microseismicity is characterized by active interplate thrusting in the 25-45 km depth range. Arcward of this interplate activity, microearthquakes in the overlying plate have a strike slip mechanism. The near-horizontal P axis of this mechanism is oriented in the direction of plate convergence, suggesting that the plate interface at shallow depths is currently efficiently coupled. Below 45 km depth, the dipping seismic zone appears to be double-planed. The upper plane dips at 32° from 45 to 100 km depth, where it exhibts a kneelike bend below the volcanic front. The lower plane begins at about 65 km depth, where it is separated from the upper plane by some 25 km, and appears to converge with the upper plane at about 120 km depth. The inferred geometry of the plate interface suggests that the double-planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate. A composite fault plane solution for events in the upper plane indicates, however, that downdip tension is currently present rather than the downdip compression that would be expected from unbending. It is proposed that because of locking of the plate interface at shallower depths, slab pull is currenlty overprinting the unbending stresses which predominate at times when the plates are unlocked. This model is consistent with previous interpretations that there is a high probability of a great earthquake in the Shumagin Islands seismic gap within the next decade or two.

Comparison of Mechanical Models of the Oceanic Lithosphere

Article

Nov 1980

Donald Forsyth

A realistic mechanical model of the oceanic lithosphere must be able to explain more than just the topography of a typical trench and outer rise system. A successful model should also be consistent with our knowledge of rock mechanics from laboratory studies, with the distribution of earthquakes associated with bending of the plate before subduction, with the amount of strain within the plates as indicated by the seismic moment of the earthquakes, and with the topography associated with other loads, such as seamounts and trenches where plate convergence is very slow. These observational tests are applied to six mechanical models of the lithosphere: models in which the regional, horizontal compressive stresses are of the order of several kilobars are the least successful.

Characterization of barriers on a seismic fault

Article

Jan 1979

Keiiti Aki

Recently, Bouchon (1979a) reinterpreted strong motion seismograms obtained during the Parkfield earthquake of 1066 using a new method applicable to a finite propagating dislocation source in a layered medium. His results and other pertinent data, interpreted in terms of the barrier model of Das and Aki (1977), suggest that the rupture may be stopped by a barrier with the specific fracture energy of about 109 erg cm-2. Using the formulas of Ida (1973b), we estimated parameters of the barrier as follows: breaking slip of about 20 cm, cohesive stress of about 100 bars, and length of end zone (nonelastic zone) of a few hundred meters. The barrier parameters for the great 1857 earthquake were also obtained from the description of surface fault breaks by Wallace (1968). The result led to the estimation of maximum acceleration of about 1.5g near the fault, under the assumption that the end zone length is proportional to the diameter of individual crack of the barrier model. Barriers for other earthquakes are discussed, and they are classified into geometrical barriers such as fault bend and corner and inhomogeneous barriers such as the high velocity anomaly straddling the San Andreas fault near San Juan Bautista. The barriers act not only as a stopper of rupture but also as an initiator of rupture, as well as a stress concentrator, causing twin earthquakes and migration or progression of major earthquakes along the plate boundary.

Precise relocations of earthquakes and seismotectonics of the New Hebrides Island Arc

Article

Oct 1978

Earthquakes for the period 1962–1973 are relocated by the method of joint hypocenter determination in order to resolve better the configuration and structure of the inclined seismic zone in the New Hebrides island arc. Twelve new focal mechanism solutions are reported and, together with previously published solutions, are integrated with the new information on the spatial distribution of hypocenters. At intermediate depths the seismic zone has a uniformly steep dip of about 70° and exhibits no resolvable contortions or disruptions along at least 700 km of the subduction zone. The thickness of the zone, about 20 km, may be in part due to seismically active fault zones which cut across a portion of the descending lithosphere. Features associated with the anomalous central region of the arc, where the d'Entrecasteaux fracture zone is being subducted and where the islands of Santo and Malekula appear to be in positions normally occupied by the oceanic trench, include the following: (1) an inclined zone of shallow earthquakes with very much smaller dip than is found elsewhere in the arc, (2) a pronounced gap in seismic activity at depths between about 50 and 120 km, (3) evidence for features in the upper or overthrust plate with trends transverse to the arc and parallel to the east‐west trends of the topographic feature being subducted, including nodal planes of shallow focal mechanism solutions, and (4) two features which appear to coincide with the downdip projection of the northern scarp of the d'Entrecasteaux fracture zone, including a well‐defined boundary between two adjacent zones of plate slippage along the main plate boundary and a faultlike feature in the intermediate depth seismic zone which also has a strike parallel to the fracture zone. Appendix table and entire article are available on microfiche.Order from American Geophysical Union, 1909 K Street, N.W.,Washington, D. C., 20006. Document J78‐006; $1.00. Payment mustaccompany order.

Fault plane with barriers: A versatile earthquake model

Article

Dec 1977

Shear cracks with finite cohesive forces can propagate by skipping past barriers. The barriers left behind may remain unbroken or may eventually break because of subsequent increase in dynamic stress depending on the ratio of barrier strength to tectonic stress. This model can explain a variety of observations on rupture in the earth, including (1) segmentation of the fault or ruptured zone in earthquakes and rock bursts, (2) ripples in seismograms which cannot be explained by path effect, and (3) departure of the scaling law of the seismic spectrum from that based upon the similarity assumption. The model also explains why the simple uniform dislocation model sometimes works better than the crack model without barriers. It also predicts, contrary to common belief, that an earthquake with low average stress drop may generate relatively greater amounts of high-frequency waves than an earthquake with high average stress drop. One important consequence of of our barrier model is the possibility of predicting the occurence of aftershocks by analyzing the source spectrum of the main shock.

Is the Oceanic Lithosphere Elastic or Viscous?

Article

May 1977

Jean-Claude de Bremaecker

The topography of the ocean bottom near the trenches can be explained as the result of a point load fixed in space and resting on a moving lithosphere of high viscosity hydrostatically supported. The best agreement is obtained for a lithosphere approximately 120 km thick and of a viscosity approximately 1023 N s/m2(1024 P). The tensile stress does not exceed 600 bars, which is 15 times lower than that in the elastic case.

Why and Where Great Thrust Earthquakes Occur Along Island Arcs

Article

Nov 1974

For many subducting margins of the Pacific there appears to be a characteristic maximum for the source dimensions of large shallow earthquakes. The major conclusion of this investigation is that this characteristic maximum for a particular island arc or segment of an arc is strongly influenced by the geometry of the interface zone, particularly the width of the interface between underthrust and overthrust slabs of lithosphere. By width of interface we refer to the downdip dimension of the contact zone between the two abutting slabs of lithosphere. Maps displaying inferred variations in interface geometry along five major island arcs agree generally with the known locations and extent of rupture during large shallow earthquakes of this century. Ruptures of extraordinary length (>400 km) occur near gently dipping slabs of lithosphere that abut over a broad contact zone against the overthrust slab. Moderately large earthquakes (maximum rupture length, < 150 km) occur along slabs that dip more abruptly and have a thin zone of contact with the overthrust lithosphere. If the major conclusion of this study is valid, then detailed knowledge of interface geometry may provide a technique for estimating the extent of the greatest shallow earthquakes likely to occur in an existing seismic gap. That is, a narrow width of interface apparently places severe restrictions on the maximum extent of rupture that can occur during an earthquake. The extent and the location of ruptures along large strike-slip faults may also be influenced principally by the width of interface (vertical extent of hypocenters for strike-slip faults). Along the two great transform fault systems near western North America (that starting near spreading centers in the Gulf of California and extending northward into the San Andreas system and that starting near the Juan de Fuca spreading center and extending northward into the Queen Charlotte Islands-Fairweather fault system) both the width of interface and the typical dimensions of large earthquakes tend to increase in size with distance from the spreading center. Most large shallow earthquakes along island arcs result from fault motion across the zone of contact between the overthrust and the underthrust plates of lithosphere. The dimensions of rupture among such large earthquakes vary widely, and during an extraordinarily large earthquake the rupture zone may extend as much as 1000 km laterally along an arc. Our observations indicate that the extent of these large ruptures and their locations correlate for many island arcs with the downdip width of the zone of contact, or interface, between the two abutting plates. That is, by using seismic data, such as the width of the zone of shallow earthquakes, estimates of variations in the downdip width along the interface correlate, in some cases in some detail, with the locations and source dimensions for many of the large earthquakes of this century. Rupture zones of extraordinary length (>400 km) tend to occur only along those portions of arcs where underthrust and overthrust slabs interact across a broad interface, or zone of contact. Along those island arcs where there appears to be a narrow interface between plates the maximum dimension of known rupture zones is limited, possibly to 150 km or less, for all earthquakes along an entire arc. Thus the fundamental conclusion of this study is that the geometry of the interface area is a key parameter in influencing the location and extent of rupture during large earthquakes along island arcs. A relationship between the length of rupture and the contact area of two opposing lithospheric plates has been suggested by previous investigators. Isacks et al. [1968] suggested that upper limits to the sizes of earthquakes may be controlled by the area of lithospheric contact. Kanamori [197 la] examined the zones of interaction between the oceanic and the continental lithospheres and the occurrence of large earthquakes along the

Mechanism of Spatial Distribution of Chilean Earthquakes with Relation to Subduction of the Oceanic Plate

Article

Aug 1973

William Stauder

The focal mechanisms for 61 earthquakes occurring in northern and central Chile during the years 1962-1970 indicate underthrusting of the oceanic plate for earthquakes with focal depth 30-60 km. The axis of tension for intermediate-depth earthquakes is parallel to the direction of dip of the plate. For deep-focus earthquakes the axis of compression is parallel to the axis of the plate. Together with the seismicity of the region, the focal mechanisms indicate that subduction of the oceanic plate under this part of the coast of South America takes place in discrete and localized episodes and that the lithospheric slab itself is broken into a series of tongues that are absorbed independently and quite differently from one latitude zone to the next or even at one depth as opposed to another. Near the Chile-Peru corner, 18°S-25°S, the more principal present day activity is at intermediate focal depths. The motion of the plate is in the azimuth N85°E at shallow depths, veering to N65°E at intermediate depths. The zone 25°S-27°S is at present a silent zone at intermediate depths. The zone 27°S-34°S corresponds to underthrust of the oceanic plate but such that at depths of about 120 km this segment of the plate moves horizontally under the continent. The deep-focus zone, 19°S-28°S, overlaps the three zones just mentioned and is discontinuous with them. It more probably corresponds to an independent and earlier epoch of plate absorption. Under central Chile the plate motion appears to correspond to a current episode of subduction of relatively recent initiation. The motion is in the azimuth N80°E.

Source Parameters of Southern California Earthquakes

Article

Dec 1973

The source characteristics of southern California earthquakes with local magnitudes ML between 2 and 7 have been estimated from the gross spectral properties of the locally recorded S waves written on broad band torsion seismograph systems that have operated in California since 1932. The seismic moments M0 are consistent with the magnitude-moment relation obtained by M. Wyss and J. N. Brune from surface wave amplitudes, although a single straight line given by log M0 = 1.5 ML + 16.0 fits the data equally well. Source dimensions 2r vary from about 0.6 to 25 km, and stress drops Δσ lie between 0.3 and 200 bars. Neither parameter is a well-defined function of magnitude, although source size roughly increases with increasing ML. A theoretical relation between ML and source parameters is developed by using Brune's source model and the displacement response of the Wood-Anderson seismograph. The result is ML = log M0 - 32 log r - 17.8, where 3 < ML < 7, M0 is in dynes centimeters, and r is in kilometers. The ML may be accurately obtained by this equation, and the result is used to construct the following relations: log M0 = 2.0 ML + 14.2 - log Δσ, where Δσ is in bars, and log 2r = ⅔ ML + 2.9 - ⅔ log Δσ. Single-valued relations are not expected unless Δσ is constant. However, all available data are bracketed by these equations if stress drops vary over the range observed in this study. Radiated energies computed by spectrum integration agree with a theoretical result, log Es = 2.0 ML + 8.1, where Es is in ergs. Both are consistent with B. Gutenberg and C. F. Richter's energy-magnitude relation above ML = 4.5 but depart considerably from their result at lower magnitudes. The apparent stress parameter can provide source information that is independent of stress drop only if the high-frequency spectral falloff is a well-determined source property, which is unlikely for most of the data examined in this and other recent spectrum studies. Regional differences in source dimensions and stress drops within the southern California area are suggested by the spectral observations. However, the pattern is complex, and there are significant variations in these source parameters within any one region. Close-in seismic measurements are needed to substantiate the suggested regional differences, and determination of fault zone properties at depth is required to more fully understand the regional and local variations in seismic source parameters suggested by this study.

Andean Crustal and Upper Mantle Structure

Article

May 1971

David E. James

Phase and group velocities of Rayleigh and Love waves have been used to derive models of the structure of the crust and upper mantle beneath southern Peru, Bolivia, and northern Chile. A three-dimensional model of crustal structure has been obtained that shows crustal thickness varying from about 11 km (including water layer) in the ocean basin to 30 km along the coast to more than 70 km beneath the western cordillera and western part of the altiplano. The crust thins eastward and beneath the eastern cordillera is only 50–55 km thick. The crust beneath the crest of the Andes appears to thin to the north and south of the altiplano region, and the maximum crustal thickness in those parts of the Andes not associated with the altiplano may be on the order of 55–60 km. Mean crustal velocities within this region of the Andes are characteristically low; typical values of mean P- and S-wave velocities are ∼6.2 km/sec and 3.45 km/sec, respectively. No significant low-velocity zone for shear waves has been found in the upper 150 km of the mantle beneath the continental area studied, although subcrustal velocities for the entire region are somewhat low. The lowest uppermost mantle velocities measured are for the region between La Paz, Bolivia, and Huancayo, Peru. Here the upper mantle shear velocity corresponds to the axis of the high electrical conductivity anomaly that has been found in the Andean region. Beneath the oceanic areas, a major shear-wave low-velocity zone is required to satisfy phase- and group-velocity observational data. The top of this zone is at a depth of 50–60 km, and the low velocities extend to a depth probably in excess of 200 km. There is a slight suggestion that the upper boundary of the low-velocity zone may be dipping toward the continent.

Focal process of the great Chilean Earthquake, May 22, 1960

Article

Dec 1974
PHYS EARTH PLANET IN

Long-period strain seismogram recorded at Pasadena is used to determine the focal process of the 1960 Chilean earthquake. Synthetic seismograms computed for various fault models are matched with the observed strain seismogram to determine the fault parameters. A low-angle (~ 10°) thrust model with rupture length of 800 km and rupture velocity of 3.5 km/sec is consistent with the observed Rayleigh/Love wave ratio and the radiation asymmetry. A seismic moment of 2.7 . 1030 dyn . cm is obtained for the main shock. This value, together with the estimated fault area of 1.6 . 105 km2, gives an average dislocation of 24 m. The strain seismogram clearly shows unusually long-period (300-600 sec) wave arriving at the P time of a large foreshock which occurred about 15 minutes before the main shock, suggesting a large slow deformation in the epicentral area prior to the major failure. A simple dislocation model shows that a dislocation of 30 m, having a time constant of 300-600 sec, over a fault plane of 800 × 200 km2 is required to explain this precursory displacement. The entire focal process may be envisaged in terms of a large-scale deformation which started rather gradually and eventually triggered the foreshocks and the ``main'' shock. This mechanism may explain the large premonitory deformations documented, but not recorded instrumentally, for several Japanese earthquakes. The moments of the main shock and the precursor add to 6 . 1030 dyn . cm which is large enough to affect the earth's polar motion.

Spatial distribution of earthquakes and subduction of the Nazca plate beneath South America

Article

Jan 1976
GEOLOGY

A detailed study of the spatial distribution of precisely located hypocenters of South American earthquakes that occurred between lat 0° and 45°S shows that the data can be explained by the simple model of a descending oceanic plate beneath a continental plate and that the following conditions obtain: (1) The hypocenters clearly define five segments of inclined seismic zones, in each of which the zones have relatively uniform dips. The segments beneath northern and central Peru (about lat 2° to 15° S) and beneath central Chile (about lat 27° to 33° S) have very small dips (about 10°), whereas the three segments beneath southern Ecuador (about lat 0° to 2°S), beneath southern Peru and northern Chile (about lat 15° to 27°S), and beneath southern Chile (about lat 33° to 45°S) have steeper dips (25° to 30°). No clear evidence exists for further segmentation of the descending Nazca plate beneath South America. If the two flat segments are in contact with the lower boundary of the continental plate, the thickness of that plate is less than approximately 130 km. This is in marked contrast to the reports of thicknesses exceeding 300 km for the South American continental plate. (2) There is considerable seismic activity within the upper 50 km of the overriding South American plate. This seismic activity is well separated from the inclined seismic zones and probably occurs in the crustal part of the South American plate. Thus, hypocenters in South America are not evenly distributed through about a 300-km-thick zone as previously described. (3) A remarkable correlation exists between the two flat segments of the subducted Nazca plate and the absence of Quaternary volcanism on the South American plate. (4) The transition from the flat Peru segment to the steeper Chile segment is abrupt and is interpreted as a tear in the descending Nazca plate. The tear is located approximately beneath the northern limit of the Altiplano (a high plateau in the Andes), and about 200 km south of the projection of the oceanic Nazca ridge down the subduction zone. (5) A gap in seismic activity exists between depths of 320 and 525 km. *Present address: Department of Geology, University of Petroleum and Minerals, Dhahran, Saudi Arabia

Geometry of Benioff Zones: Lateral segmentation and downwards bending of the subducted lithosphere

Article

Jan 1977

Earthquakes and Bending of Plates at Trenches

Article

Nov 1979

The mechanisms, distribution, and total moment of earthquakes within the bending oceanic plate seaward of trenches constrain possible mechanical models of the lithosphere. The average annual horizontal slip in normal fault earthquakes, as estimated from the cumulative seismic moment, exceeds the total extension predicted by an elastic plate model. Thus bending is not predominantly an elastic process but must include a large amount of permanent deformation. Normal-faulting earthquakes are associated with every major trench system in the Pacific and Indian oceans, but events indicating horizontal compression are rare. The predominance of normal faults over thrust faults, normal events as deep as 25 km within the lithosphere (well constrained by pP), and the existence of large (Ms more than 7.5) normal fault earthquakes suggest that the average oceanic lithosphere is not under regional horizontal compressive stress which is a large fraction of the bending stresses.-Authors

An analysis of the v-ariation of Ocean floor bathymetry and heat flow with age

Article

Feb 1977

Two models, a simple cooling model and the plate model, have been advanced to account for the variation in depth and heat flow with increasing age of the ocean floor. The simple cooling model predicts a linear relation between depth and t1/2, and heat flow and 1/t1/2, where T is the age of the ocean floor. We show that the same T1/2 dependence is implicit in the solutions for the plate model for sufficiently young ocean floor. For larger ages these relations break down, and depth and heat flow decay exponentially to constant values. The two forms of the solution are developed to provide a simple method of inverting the data to give the model parameters. The empirical depth versus age relation for the North Pacific and North Atlantic has been extended out to 160 m.y. B.P. The depth initially increases as t1/2, but between 60 and 80 m.y. B.P. the variation of depth with age departs from this simple relation. For older ocean floor the depth decays exponentially with age toward a constant asymptotic value. Such characteristics would be produced by a thermal structure close to that of the plate model. Inverting the data gives a plate thickness of 125+/-10 km, a bottom boundary temperature of 1350°+/-275°C, and a thermal expansion coefficient of (3.2+/-1.1) ×10-5°C-1. Between 0 and 70 m.y. B.P. the depth can be represented by the relation d (t) =2500+350t1/2 m, with t in m.y. B.P., and for regions older than 20 m.y. B.P. by the relation d (t) =6400-3200 exp(-t/62.8) m. The heat flow data were treated in a similar, but less extensive manner. Although the data are compatible with the same model that accounts for the topography, their scatter prevents their use in the same quantitative fashion. Our analysis shows that the heat flow only reponds to the bottom boundary at approximately twice the age at which the depth does. Within the scatter of the data, from 0 to 120 m.y. B. P., the heat flow can be represented by the relation q (t) =11.3/T1/2 mucal cm-2 s-1. The previously accepted view that the heat flow observations approach a

Moment magnitude scale

Article

May 1979

The nearly conincident forms of the relations between seismic moment Mo and the magnitudes ML, Ms, and Mw imply a moment magnitude scale M=2/3 log Mo-10.7 which is uniformly valid for 3

A body wave inversion of the Koyna, India, earthquake of December 10, 1967, and some implications for body wave focal mechanisms

Article

May 1976

Charles Langston

With a generalized inverse technique, WWSSN (World-Wide Standard Seismograph Network) long-period P and SH wave forms are analyzed from the Koyna earthquake. The effects of local plane-layered earth structure near an imbedded point dislocation source are put in by using a modified plane-wave ray theory which includes the standard reflection and transmission coefficients plus source corrections for radiation pattern and geometrical spreading. The generalized inverse compares synthetic seismograms to the observed ones in the time domain through the use of a correlation function. By using published crustal models of the Koyna region and primarily by modelling the crustal phases P, pP, and sP, the first 25 s of the long-period wave forms is synthesized for 17 stations, and a focal mechanism is obtained for the Koyna earthquake which is significantly different from previous mechanisms. The fault orientation is 67° dip to the east, -29° rake plunging to the northeast, and N16°E strike, all angles being +/-6°. This is an eastward dipping, left lateral oblique slip fault which agrees favorably with the trend of fissures in the meizoseismal area. The source time duration is estimated to be 6.5+/-1.5 s from a triangular time pulse which has a rise time of 2.5 s, a tail-off of 3.9 s source depth of 4.5+/-1.5 km, and seismic moment of 3.2+/-1.4×1025 dyn cm. Some short-period complexity in the time function is indicated by modellling short-period WWSSN records but is complicated by crustal phases. The long-period P wave forms exhibit complicated behavior due to intense crustal phase interference caused by the shallow source depth and radiation pattern effects. These structure effects can explain much of the apparent multiplicity of the Koyna source. An interpretation of the Koyna dam accelerograms has yielded an S-P time which can be used along with IMD (Indian Meteorological Department) epicenter and present depth determination to place the epicenter directly on the meizoseismal area.

Shallow seismicity in subduction zones

Article

Nov 1978

During 1965 the University of San Agustin operated a net of 9 high-sensitivity seismographs in southern Peru. Stable hypocenters for local earthquakes were determined by an interactive method utilizing both the P and the S arrival times. The hypocenters delineate a 30° dipping Benioff zone but also indicate considerable intraplate activity. Of particular interest is the existence of an aseismic wedge between the shallower earthquakes in the subducting lithosphere and those in the continental lithosphere. Investigations of other regions, e.g., the Aleutians and Japan, suggest that this may be a global phenomenon. We suggest that this aseismic wedge occurs because the compressive stress due to the interaction of the oceanic and continental plates is counterbalanced by tension caused by the downwarping of the continental lithosphere due to the drag of subduction. The unstressed zone (at depths less than 40 km) includes the coastal areas and explains the absence of large shallow earthquakes in that region. Further inland, beyond the region of downwarping, stresses are again adequate to cause large shallow earthquakes.

The November 29, 1978, Oaxaca, Mexico, earthquake: A large simple event

Article

Jun 1981

The recent Oaxaca Mexico, earthquake November 29, 1978 (M/sub W/ = 7.6, M/sub s/ = 7.8, seismic moment, Mâ = 3.2 x 10Â²â· dyne cm), is of special interest because of its location within a predetermined seismic gap. The event excited long-period (100-200 s) multiple Rayleigh and Love waves which were well-recorded by the WWSSN. These data along with P wave first-motion data and P waveforms were used to constrain the source mechanism. The results indicate an oblique thrust mechanism consistent with subduction of the Cocos plate to the northeast beneath Mexico (dip = 14Â°, strike = N90 Â°W, rake = +54Â°); hence this event is indeed of the type anticipated by Ohtake et al. (1977). A local network of stations, installed in a joint University of Mexico-California Institute of Technology program, began operation 20 days prior to the mainshock (Gonzalez, 1979; McNally et al., 1979; McNally, 1981; Ponce et al., 1980). The 43 foreshocks of magnitude M> or =2.8 were recorded by the network in a period of 20 days prior to the mainshock. These events show an interesting spatial and temporal pattern, which culminates in the last 1.8 days of the sequence with an apparent migration of activity toward the epicenter of the impending earthquake. This pattern can be interpreted as a buildup of stress or migration of stress toward a fault zone asperity. With supplemental stations, the network continued operation until December 12, 1978 (Singh et al., 1980) and provided good hypocentral control for the more than 169 aftershocks of magnitude M> or =2.8. The area of the aftershock zone determined from these events is 5525 kmÂ² (85 km by 65 km). In spite of the large size of the event, P waves for the Oaxaca event indicate an extremely simple source, at the period range of the WWSSN long-period seismographs. The simplicity suggests that the P waves were generated by a limited portion of the rupture surface, perhaps by the breaking of a fault zone asperity.

Global study of seismic wave attenuation in the upper mantle behind Island Arcs using pP wave

Article

Mar 1975

Observations of striking and consistent differences in the attenuation of pP produced by mantle earthquakes and recorded by the World-Wide Standard Seismograph Network (WWSSN) provide data for mapping variations in the attenuation of high-frequency (0.5- to 2-Hz) compressional waves in the wedge of mantle above nearly all of the inclined seismic zones on earth. The data reveal several zones of high attenuation that in nearly all cases correspond to major presently active tectonic features above or near the inclined seismic zones. Zones of high attenuation behind the Tonga, New Hebrides, Mariana, and Japanese island arcs coincide with zones of presently or recently active crustal extension and creation of marginal basins. IN western South America zone of high attenuation underlies most of the broad uplifted terrain of the Andean Altiplano. In contrast to the zones beneath active marginal basins this zone is not related to crustal spreading. A high-attenuation zone exists beneath the Sea of Okhotsk that, although it is near, is offset from the Kuril-Kamchatka arc and thus may not be directly related to that system. This zone may instead be related to possible offshore continuations of the Baykal-Aldan rift system or the postulated plate boundary between Asia and North America. Where they are determined, the high-attenuation zones seem to exist in the upper 200-300 km of the mantle. Wherever data are available, they show that the zones of high compressional wave attenuation correspond to zones of high attenuation for shear waves, low seismic wave velocities, and high heat flow. These properties, in addition to the geological evidence of crustal extension or uplift, suggest that the compressional wave attenuation results from high temperatures and/or partial melting of the upper mantle material. Although all the high attenuation zones occur near inclined seismic zones that reach depths greater than 300 km, the absence of such zones in the Indonesian, Philippine, Izu-Bonin, and New Britian-Solomon regions shows that lithospheric subduction to great depths is not a sufficient condition for their occurrence. The zones of high attenuation are also not systematically related to the distributions of active volcanos: this condition, together with other data, indicates that large bodies of partially melted material probably do not exist beneath the volcanos of island arcs.

Subduction of the Nazca Plate under Peru as evidenced by focal mechanisms and by seismicity

Article

Mar 1975

William Stauder

The focal mechanisms of 40 earthquakes in Peru and Ecuador, together with the seismicity of the region, indicate particular features of the subduction of the oceanic plate beneath this portion of South America. At shallow depths near the coast and at foci along the contact between the subduction zone and the continental plate the focal mechanisms indicate an underthrust of the continent by the oceanic plate on a thrust plane dipping 10°-15° beneath the continent. Near this same depth but at foci within the oceanic plate, normal faults occur that correspond both to flexure of the plate and to downdip axial tension. At intermediate depths the plate continues to act as a stress guide, the axis of tension being down about 30° from the horizontal and trending to the ENE. The dip of the Benioff zone steepens notably in southern Peru near the Peru-Chile corner, and the motion of the descending slab relative to the continental plate is in a direction N40°E. Deep-focus earthquakes indicate a vertical segment of plate under axial compression at depths of 550-600 km. Numerous earthquakes also occur interior to the continent and within the continental lithosphere at depths down to 90 km. Both strike slip and reverse-type faults are found, but in either case the stress system corresponds to an E-W horizontal compression. Comparison woth the seismicity is consistent with the model of an oceanic plate moving almost horizontally under the continental lithosphere in northern and central Peru and a separate, more steeply plunging a segment of plate moving normal to the coast under southern Peru.

Focal Mechanisms and Plate Tectonics of the South-West Pacific

Article

Sep 1972

Ninety-six new focal mechanisms were determined for earthquakes on the belt of seismic activity separating the Pacific and Australian plates. The direction of convergence of these plates varies from Ntg-SW to E-W. The Australian plate underthrusts the Pacific plate to the ENE under the Solomon and New Hebrides islands and overthrusts the Pacific to the east along the Tonga-Kermadec arc and the North Island of New Zealand. The data for the Macquarie ridge concur with the idea that. the pole of rotation for the Pacific and Australian pla.tes is nearby and to the east of this feature. The data also suggest a. NNE-SSW convergence of the Pacific and Australian plates in northwestern New Guinea. The relative motions of the plates near the Bismarck Archipelago are complex because of the presence of at least three additional small plates. The south Bismarck plate, the best defined, underlies the southern part of the Bismarck Sea. It is bounded on the north by a.n E-W belt of seismicity at about 3øS defining a left-lateral strike-slip fault. The New Britain arc forms the southern boundary, where the Solomon Sea floor underthrusts the south Bismarck plate to the NNW. There is some evidence for SW convergence of the south Bismarck and Australian plates in northeastern New Guinea. Small plates, less well-defined seismically, are also proposed under the northern part of the Bismarck Sea and under the Solomon Sea. The plate under}ying the Solomon Sea floor is bounded by the Solomon and New Britain arcs and by eastern New Guinea. The southern boundary is not sharply defined by seismic data. The Solomon Sea plate is moving approximately NW with respect to the Australian plate and underthrusting the Pacific plate to the NE along the Solomon arc. The consistent pattern of relative motions of these three small plates allows quantitative estimates of relative rates of motion between them. These data demonstrate that plate tectonics is applicable even for regions with dimensions of only a few hundred kilometers. Geologic data from New Guinea are used to speculate about earlier plate motions in that area. Many recent studies have demonstrated that most shallow earthquakes, especially in oceanic areas, indicate relative motion of two lithospheric plates [e.g., Isacks et al., 1968; McKenzie and Parker, 1967]. Data from 96 new focal mechanisms for shallow earthquakes and 29 published mechanisms from other studies are used to analyze relative plate motions in the southwest Pacific. This region is important because it includes a long boundary between two of the larger plates of the world. In general, the seismic data as well as other geologic data in this region are consistent with convergence of the Pacific and Australian plates about a pole of rotation determined to be south of New

A magnitude scale for very large earthquakes

Article

Sep 1978
TECTONOPHYSICS

Accurate Source Depths and Focal Mechanisms of Shallow Earthquakes in Western South America and in the New Hebrides Island Arc

Abstract

No full-text available

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