Tectonophysics

Published by Elsevier
Online ISSN: 0040-1951
Publications
Article
Qβ for shear-waves is determined for the inner part of the Hellenic arc, the back-arc area, as a function of frequency in the range 0.6–16 Hz. We used 314 digital records from 32 earthquakes with magnitudes (Mw) ranging from 3.9 to 5.1. Epicentral distances ranged from 65 to 515 km. The data were obtained in 1997 during a 6-month operation of a digital portable network in Greece. The Qβ estimates were made for five frequency bands centred at 0.8, 1.5, 3.0, 6.0 and 12.0 Hz and the Qβ values obtained were 47, 79, 143, 271 and 553, respectively. The results show that Qβ for S-waves increases with frequency taking the form Qβ=55f0.91 (or Qβ−1≃0.018f−0.91). The high attenuation and the strong frequency dependence found, which is close to the frequency dependence of coda Q for Greece, are characteristic of an area with high seismicity, rapid extension, and in agreement with other similar studies in Greece.
 
Article
The most damaging earthquake to occur in New Zealand in 1982 was the Hawke's Bay earthquake of September 02 (mb = 5.6). An aftershock study using portable microearthquake recorders has established that this event occurred some 20 km south of Hastings, at a depth of about 47 km. This depth places the event in the mantle of the subducted Pacific plate.This earthquake is a member of a recently recognized class of intraplate shocks occurring along the strike of the subducted plate near the east coast of the North Island. Such events are characterized by normal-faulting mechanisms. When taken together with activity in the crust of the subducted plate, this subcrustal activity resembles a double-planed seismic zone, restricted in the down-dip direction of the subducted plate. Several lines of evidence suggest that deformation in the mantle of the subducted plate is controlled by the state of coupling of the plate interface at shallow depth: 1.(1) the subcrustal activity occurs directly beneath the source region of major interplate earthquakes along the east coast of the North Island, and there appears to be a pairing of interplate and subcrustal events;2.(2) stress directions determined for the subcrustal events are concordant with geodetic strain directions; and3.(3) the amount of subcrustal activity appears to be related to the state of coupling of the plate interface.
 
Article
This paper discusses the surface rupture of the Cariaco July 09, 1997 Ms 6.8 earthquake in northeastern Venezuela – located at 10.545°N and 63.515°W and about 10 km deep. The field reconnaissance of the ground breaks confirms that this event took place on the ENE–WSW trending onshore portion of the dextral El Pilar fault (between the Gulfs of Cariaco and Paria), which is part of the major wrenching system within the Caribbean–South America plate boundary zone. Dextral slip along this fault was further supported by the structural style of this rupture (en echelon right-lateral R shears connected by mole tracks at restraining stepovers) and by larger geometric complexities (pop-ups at Las Manoas and Guarapiche), as well as by the focal mechanism solutions determined for the event by several authors. This 1997 surface ruptre comprised two distinct sections, from west to east: (a) a main very conspicuous, continuous, 30-km-long, rather straight, 075°N-trending alignment of en echelon surface breaks, with a rather constant, purely dextral coseismic slip of about 25 cm, but reaching a maximum value of 40 cm slightly northwest of Pantoño; and (b) a secondary discontinuous, 10-km-long, boomerang-shaped rupture, with a maximum coseismic slip of 20 cm at Guarapiche. The onshore extent of the surface rupture totalled 36 km, but may continue westward underwater, as suggested by the very shallow aftershock seismicity. This aftershock activity also clearly defined the steep north dip of the fault plane along the western rupture, suggesting tectonic inheritance on this major fault.
 
Article
Seven accretionary sutures, formed between 1.16 and 1.03 Ga, have been identified by different authors in the Ontario–Quebec–Adirondack (OQA) segment of the Mesoproterozoic Grenville orogen in Canada. With one exception, the inferred accretionary terrane boundaries lie within, or at the margins of the Central Metasedimentary Belt (CMB), located between the Central Gneiss Belt and the Adirondack Highlands (Central Granulite Terrane). However, geological, geochronological, and petrological data suggest that the Grenville orogen on both sides of the proposed terrane boundaries (sutures) preserves a common 1.4–1.03 Ga tectonomagmatic history, inconsistent with its origin as a post-1.4 Ga collage of exotic tectonic blocks. Features which straddle the proposed 1.16–1.03 Ga ‘sutures’, from the Central Gneiss Belt, via the Adirondack Highlands, to the Mauricie area, include: (1) Mesoproterozoic continental crust (1.5–1.4 Ga) forming the host and/or basement to younger magmatic and supracrustal suites. (2) A 1.35–1.3 Ga continental arc, remnants of which occur from the CMB boundary zone (CMBBZ) in Ontario to the Appalachians in the United States, built on the 1.5–1.4 Ga continental crust. (3) Intrusions of 1.17–1.13 Ga age in the Central Gneiss Belt (mafic suite), and the Adirondack Highlands and their Quebec extension (AMCG suite, i.e. anorthosite massifs and related granitoids). (4) Relics of 1.18–1.14 Ga sedimentary basins in the northwestern CMB and the Mauricie area.
 
Article
The tectonic evolution of the ca. 2.0-1.75 Ga old Svecokarelian fold belt is reviewed, and evidence is presented for large-scale intraplate strike-slip movements along ductile megashears. After the formation of the Kola collision suture and the neighbouring Granulite-Tanaelv thrust belt around 1.9 Ga ago, dextral shearing was initiated along N-S trending megashears. Subsequent anticlockwise rotation of the initially NNE-SSW oriented principal compressive stress caused dextral shearing along a NW-SE trending megashear and reversal in the sense of shearing in the N-S trending ones. Further anticlockwise stress rotation (to a total of about 120°) brought an end to sinistral shearing along the N-S megashears around 1.8 Ga ago and caused reversal to sinistral slip along the NW-SE megashear. Both the older (1.9-1.85 Ga) and younger (1.84-1.8 Ga) parts of this evolution are recorded within the Karelian province and its southwestern margin, where consolidation of the lithosphere took place shortly after 1.9 Ga ago. In the Svecofennian province, where crustal accretion did not start until around 1.9 Ga ago, the older movements may have caused synaccretional crustal folding, but with increasing consolidation, the deformation was concentrated along megashears. Although it is still not possible to interrelate the function of active subduction zones and intraplate megashears. the evolution traced so far provides support for plate tectonic interpretations of the Early Proterozoic geodynamics of the Baltic Shield.
 
Article
Eighty-two palaeomagnetic samples of calcareous and jaspilitic grainstones (iron-formation or ‘taconite’) and chert carbonate were collected from the 1.88-Ga Gunflint Formation at 22 sites in the Thunder Bay area, Ontario. Twenty clasts of Gunflint taconite also were sampled from the basal conglomerate of the overlying Mesoproterozoic Sibley Group. Anisotropy of magnetic susceptibility measurements indicate the Gunflint Formation in the sampling area has not experienced regional dynamic metamorphism. Analyses by variable-field translation balance and X-ray diffraction show that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in some samples. Altogether, 213 Gunflint specimens and 59 Sibley conglomerate specimens were subjected to stepwise thermal demagnetisation and 74 Gunflint specimens to stepwise alternating-frequency demagnetisation. The following components were isolated for the taconites:
 
Article
The work by Koglin et al. (Koglin, N., Kostopoulos, D., Reichmann, T., 2009. Geochemistry, petrogenesis and tectonic setting of the Samothraki mafic suite, NE Greece: Trace-element, isotopic and zircon age constraints. Tectonophysics 473, 53–68. doi:10.1016/j.tecto.2008.10.028), where the authors have proposed to nullify the scenario presented by Bonev and Stampfli (Bonev, N., Stampfli, G., 2008. Petrology, geochemistry and geodynamic implications of Jurassic island arc magmatism as revealed by mafic volcanic rocks in the Mesozoic low-grade sequence, eastern Rhodope, Bulgaria. Lithos 100, 210–233) is here put under discussion. The arguments for this proposal are reviewed in the light of available stratigraphic and radiometric age constraints, geochemical signature and tectonics of highly relevant Jurassic ophiolitic suites occurring immediately north of the Samothraki mafic suite. Our conclusion is that the weak arguments and the lack of knowledge on the relevant constraints from the regional geologic information make inconsistent the proposal and the model of these authors.
 
Article
The traditional catalogues of historical seismicity mention a destructive earthquake that occurred on March 29th of the year 1000. It should have been felt over a large part of Europe (from Belgium to Northern Italy and Poland), with its epicentre (intensity VIII) situated at Ljubljana in Slovenia. In reality, a new critical study of the sources shows that this earthquake was only felt in the region between Southern Flanders (St. Amand), Picardy and the Mosan area (Liège). This is an example of a process by which since the 15th century compilers have been led to distort the original data relative to this event and to give it an excessive extension.
 
Article
Torsion deformation experiments were performed on solid cylinders of Carrara marble at high temperature (1000 K) and constant twist rate (about 4×10⁻⁴ rad s⁻¹) to large twist angles (between 80° and 840°). These conditions correspond to simple shear deformation at constant shear strain rate (3×10⁻⁴ s⁻¹) and variable amounts of shear strain γ of 1, 2, 5 and 11 at the outer mantle of the sample cylinders. Lattice preferred orientations (LPO) were measured on polished thin sections using automated electron backscatter diffraction (EBSD) to analyze the microtextures in orientation imaging micrographs (OIM).
 
Article
Intraplate compressional features, such as inverted extensional basins, upthrust basement blocks and whole lithospheric folds, play an important role in the structural framework of many cratons. Although compressional intraplate deformation can occur in a number of dynamic settings, stresses related to collisional plate coupling appear to be responsible for the development of the most important compressional intraplate structures. These can occur at distances of up to ±1600 km from a collision front, both in the fore-arc (foreland) and back-arc (hinterland) positions with respect to the subduction system controlling the evolution of the corresponding orogen. Back-arc compression associated with island arcs and Andean-type orogens occurs during periods of increased convergence rates between the subducting and overriding plates. For the build-up of intraplate compressional stresses in fore-arc and foreland domains, four collision-related scenarios are envisaged: (1) during the initiation of a subduction zone along a passive margin or within an oceanic basin; (2) during subduction impediment caused by the arrival of more buoyant crust, such as an oceanic plateau or a microcontinent at a subduction zone; (3) during the initial collision of an orogenic wedge with a passive margin, depending on the lithospheric and crustal configuration of the latter, the presence or absence of a thick passive margin sedimentary prism, and convergence rates and directions; (4) during post-collisional over-thickening and uplift of an orogenic wedge. The build-up of collision-related compressional intraplate stresses is indicative for mechanical coupling between an orogenic wedge and its fore- and/or hinterland. Crustal-scale intraplate deformation reflects mechanical coupling at crustal levels whereas lithosphere-scale deformation indicates mechanical coupling at the level of the mantle-lithosphere, probably in response to collisional lithospheric over-thickening of the orogen, slab detachment and the development of a mantle back-stop. The intensity of collisional coupling between an orogen and its fore- and hinterland is temporally and spatially variable. This can be a function of oblique collision. However, the build-up of high pore fluid pressures in subducted sediments may also account for mechanical decoupling of an orogen and its fore- and/or hinterland. Processes governing mechanical coupling/decoupling of orogens and fore- and hinterlands are still poorly understood and require further research. Localization of collision-related compressional intraplate deformations is controlled by spatial and temporal strength variations of the lithosphere in which the thermal regime, the crustal thickness, the pattern of pre-existing crustal and mantle discontinuities, as well as sedimentary loads and their thermal blanketing effect play an important role. The stratigraphic record of collision-related intraplate compressional deformation can contribute to dating of orogenic activity affecting the respective plate margin.
 
Article
In the Western Alps, some recent scarps were previously interpreted as surface ruptures of tectonic reverse and normal faults that agree with microseismicity and GPS measurements. Our analysis shows that in fact there are hundreds of recent scarps, up to 30 m high and 2.1 km long, with only pure normal motions. They share the same characteristics as typical sackung scarps. The scarps are mainly uphill facing, parallel to the ridge crests and the contour lines. They are relatively short (less than 2.1 km) with respect to tectonic fault ruptures, and organized in swarms. They cut screes and relict rock glaciers with a slow (commonly 1 mm/year) average slip rate. In the Aiguilles Grives massif these sackung scarps clearly express the gravitational toppling of sub-vertical bedding planes in hard rocks. In contrast, the Belledonne Outer Crystalline Massif exhibits scarps that stem from the gravitational reactivation of conjugate tectonic faults. The recent faults extend to about 1600 m beneath the Rognier ridge crest, but are always above the valley floor. The main scarp swarm is 9.2 km long and constitutes the largest sackung ever described in the Western Alps. 10Be dating of a scarp and offset surfaces shows that > 4 m slip may have occurred rapidly (in less than 3800 years) sometimes between the end of the glaciation and 8800 ± 1900 years ago. This dating, together with the location of some faults far from the deep glacial valleys, suggests that sagging might have been triggered by strong earthquakes during a post-glacial period of probably enhanced seismicity. The Belledonne and Synclinal Median faults (just beneath the Rognier sackung) could have been the sources of this seismicity.
 
Seismic stations in countries surrounding the Gulf of Aqaba that were used in this study.
Seismicity as a function of time and location in the Gulf of Aqaba, where the dashed lines delineate the Eilat and Aragonese basins. Open circles represent events with magnitude less than 4.0, while small and large solid circles represent events with magnitudes 4.0 to 4.9, and 5.0 to 5.9, respectively. MS represents the location of the main shock.
Cumulative frequency -magnitude relationship for earthquakes in the Gulf of Aqaba during November 1995 to December 1996, for the whole Gulf, and Eilat and Aragonese basins.
Article
We analyzed the aftershock sequence of the Gulf of Aqaba earthquake recorded by short period and broad band stations between November 1995 and December 1996. The seismicity pattern reveals significant activity often confined to known surface traces of active faults or within local grabens. Most of the aftershock activity is concentrated north of the main shock in the Aragonese and Eilat basins, while the activity is diffused south of the main shock in the Dakar and Tiran basins. Most of the moderate to strong aftershocks of the whole aftershock sequence occurred in the Aragonese basin, mainly in the first 100 days following the main shock. The Eilat basin is characterized by a larger number of aftershocks composed mainly of small-magnitude implying a slower decay rate. Assuming a detection level of ML=3.2 we obtain b-values of 1.01, 1.15, and 0.89 for the whole Gulf of Aqaba, and Eilat and Aragonese basins, respectively, in good agreement with recent studies of the Dead Sea fault. Following Omori's law, we obtained a decay parameter of 0.94, 0.88, and 0.96 for the aftershock sequences in the whole Gulf of Aqaba, and the Eilat and Aragonese basins, respectively, correlative with regions of low heat flow.For 1630 events in the local magnitude range of 0.6≤ML≤6.2 we found seismic moment estimates, M0, of 5×1018≤M0≤7.7×1026 dyn cm, and Brune stress drop estimates, Δσ, between 1 and 200 bars, with a characteristic value of 90 bars in the Eilat and Aragonese basins for M0>5×1021 dyn cm. These characteristics are comparable to those for events occurring on the main Dead Sea–Jordan fault. Most of the moment release in the basins occurred shortly after the occurrence of the main shock. The total moment release in the Eilat and Aragonese basins is only a fraction of the moment release of the main shock suggesting an almost complete stress release.
 
Article
A dense grid of petroleum industry seismic reflection profiles, coupled with field mapping, exploratory trenching and geomorphic and structural analysis are used to characterize the Quaternary growth history of the Capriano del Colle Fault System, one of several inferred active buried thrusts that extend across the Po Plain in northern Italy. Shortening is characterized here by a deeply buried south-vergent forethrust and an associated north-vergent backthrust whose upward propagation is expressed by fault-propagation folding near the surface. Structural interpretation based on seismic data suggests that strain is accommodated at very shallow levels by secondary flexural-slip thrusts and reverse faults developed on synclinal flanks that emanate from active axial surfaces. Analysis of syntectonic growth strata document maximum rates of dip-slip of 3.45 ± 0.66 mm/yr (1.6 Myr–1.2 Myr) and 0.47 ± 0.22 mm/yr during a more recent time period (0.89 Myr–present). A quarry excavation at Capriano del Colle allows a preliminary paleoseismologic analysis of coseismic surface faulting and liquefaction exposed near the core of an active mid-Pleistocene to Holocene anticline. These features are interpreted to be generated during strong local earthquakes, consistent with the environmental effects and ground motions of an event similar to the December 25, 1222, Brescia earthquake (Io = IX MCS). This indicates, for the first time, that compressive folds and blind thrusts in the Po Plain are currently accommodating slow rates of modern contraction in an active zone of the Southern Alps that extends from Lake Garda to Varese. We thus argue that earthquakes similar to the December 25, 1222 Brescia event are likely to occur in this region and pose a direct threat to such a densely populated and developed area.
 
Article
The age of separation of Australia and Antarctica (that is, the onset of seafloor spreading) has usually been determined by the identification of seafloor-spreading magnetic anomalies adjacent to the margin, or by extrapolation of the spreading rate/time span between the oldest identified anomaly and the continent-ocean boundary (COB) as interpreted from magnetic and single-channel seismic data. The most recent estimate of the age of breakup using these methods is 95 ± 5 Ma, in the Cenomanian. However, identification of the oldest magnetic anomalies, formed during the early phase of slow drift between Australia and Antarctica, is tenuous, particularly between southwest Australia and the central Great Australian Bight (GAB).
 
Article
The spatial and temporal evolution of the aftershock sequence of the Kalamata (South Greece), 1986 earthquake is examined by the method of principal parameters (Ebbling and Michelini, 1986).The application of the method to 515 aftershocks of known location reveals a variety of orientations and dips for the subfaults which were activated during the aftershock sequence. Events which occurred in the northern area indicate the existence of two types of orientation which dip at four different angles, while the southern area is characterized by almost uniform behavior, activated later in the sequence.Displacement spectra obtained from hand digitized seismograms have been interpreted as evidence for a possible regional variation in the source parameters. Visual techniques were replaced by a numerical generalized inversion method which was applied to the spectra to evaluate spectral source parameters. Initial pulse widths, measured on the seismograms, were also used for the evaluation of source dimensions and average path attenuation. Theoretical seismograms were computed, according to the model of Sato and Hirasawa (1973) and were then compared to the obtained initial pulses.The results revealed that both regions are characterized by similar spectral properties typified by ω-square or higher high-frequency decay. Statistical methods showed no significant difference in the source parameters of the earthquakes between the two regions.
 
Article
In 1983 the SeaMARC I and the Sea Beam systems imaged the East Pacific Rise (EPR) and the Clipperton Transform Fault from 13°10′N to 9°50′N revealing both magmatic and tectonic segmentation along the rise axis. Tectonic segments are defined by an undulating axial zone of extension which widens and narrows at an average wavelength of 45 km. In addition, axial fissure-fault-density and -length curves reveal wavelengths of 30 and 56 km. The narrowest axial zones, < 0.5 km in width, correspond to regions of hydrothermal activity and are located atop regional morphologic highs quasi-harmonically spaced at 155 km. These highs define the major magma centers on the rise axis about which seamounts cluster. Superimposed on the intermediate-wavelength highs are smaller highs (from 10 to 50 km in length) which are truncated by overlapping spreading centers (OSC's) at their distal edges. The rise axis widens near OSC's while it narrows and disappears near the Clipperton Transform Fault suggesting that transforms and OSC's have different origins. Imagery on and adjacent to a few OSC's reveals that in some cases OSC's migrate along the axis forming relict off-axial structural traces at oblique angles (285°–330°) to the axis.
 
Article
We hypothesize that crustal deformation occurs on a scale-invariant matrix of faults. For simplicity we consider a two-dimensional pattern of hexagons on which strike-slip faulting occurs. The behavior of the system is controlled by a single parameter, the fractal dimension. Deformation occurs on all scales of faults. The fractal dimension determines the fraction of the total displacement that occurs on the first-order or primary faults. The value of the fractal dimension can be obtained from the frequency-magnitude relation for earthquakes. Our results are applied to the San Andreas fault system in central California. Earthquake studies give D = 1.90. We associate the main strand of the San Andreas fault with the primary faults of our fractal system. We predict that the relative velocity across the main strand is 2.93 cm/yr. The remainder of the relative velocity of 5.5 cm/yr between the Pacific and North American plates occurs on higher-order faults. The predicted value is in reasonably good agreement with the value 3.39 ± 0.29 cm/yr obtained from geological studies.
 
Article
An updated version of the catalogue of earthquakes in northern Europe is presented. It comprises events inside a region window of about 55–80°N and 10°W–45°E, covering a 615-year period from 1375 to 1989. The data have been compiled using all the available historical publications, catalogues, studies and reports of the region, and for events from 1984 to 1989, a regional seismic parameter database maintained by the Institute of Seismology of the University of Helsinki. The number of events exceeds 5200, of which more than 40% have been registered since 1980. The huge increase of event reports is the result of the installation of new high-frequency networks for microearthquake studies and the upgrade of the seismic instrumentation.Because of the different degree of accuracy and homogeneity of the parameter data, the events were divided into historical (1375–1964) and instrumental (1965–1989) datasets. The source parameters are mainly macroseismic for historical events, and obtained by iterative location procedure for instrumental data. By assuming internal homogeneity between the different regional magnitude scales, we estimated the catalogue to be complete for events with magnitude ≥ 4.5 since the 1880's, magnitude 4.0–4.4 since the 1940's and magnitude 3.5–3.9 since the 1970's.Although the distribution of seismicity exhibits similar patterns in both datasets, the clustering of epicenters and offshore seismicity is more distinct in the instrumental period. The occurrence of the greatest events can be seen more clearly in the historical data. The earthquake epicenters are concentrated in the geologically younger tectonic provinces in the western part of the region and follow, with some exceptions, the tectonic fracture and fault zones.In the Fennoscandian area, 80% of the earthquakes occurred in the upper 20 km of the earth's crust. The deepest epicenters are in the southwestern corner of the region where the crust is thinnest.
 
Article
Reliable dating is an essential element of palaeoseismological studies, yet whilst a suite of geochronological methods can now provide late Quaternary age control it remains very difficult to date modern events (i.e., those occurring within the last 150 years). This is significant because the starting point for many palaeoseismological investigations is a modern surface-rupturing event, whose geological effects need to be disentangled in trench stratigraphies from palaeoseismic ruptures. Two dating methods which, in combination, can provide robust dating control in recently deposited sediments are the 210Pb and 137Cs dating methods. Here, we test the applicability of using 210Pb and 137Cs to date colluvial sediments exposed in three trenches excavated across an earthquake fault—the Eliki fault, Gulf of Corinth, Greece—which ruptured in an earthquake in 1861. The 210Pb and 137Cs profiles observed in these colluvial sequences are relatively erratic due to the mixed nature of the sediments, i.e., their deposition in an environment where the supply of slope sediments is driven by seasonal rainfall, causing non-uniform sediment accretion and sediment reworking. In one trench, however, 210Pb dating, corroborated by 137Cs dating, indicates that a proposed post-1861 surface colluvial unit has been deposited over the period 1950 AD–present (at a rate of ca. 9 mm/year), and overlies a significantly older unit (>120 years old). The dating control provided here by 210Pb and 137Cs dating corroborates the published interpretation of the trench stratigraphy, and refines the 14C-based estimated dates for the upper unit. At two other trenches 210Pb and 137Cs dating only provided minimum ages (based on the presence or absence of 210Pbexcess and 137Cs). Such approximate ages, however, may still useful in corroborating interpretations made using the trench stratigraphy, or, at sites which have long earthquake recurrence intervals, determining which earthquake event was responsible for a particular bed offset.
 
Article
We investigate the rupture process of the strongest events of the 14 February 2008 sequence which ruptured ~ 30 km of the western Hellenic trench. Three were the strongest events of the sequence: the first one (M6.7) occurred on 14 February 2008 at a depth of ~ 30 km, on a low-angle (10°) thrust fault, followed within 2 h by an M6.1 aftershock at a depth of ~ 33 km, also on a low-angle thrust fault and the third one on 20 Feb 2008 of M6.0 which is characterized as a strike-slip event at shallower depth (~ 10 km). We used the M6.1 aftershock as empirical Green's function (EGF) to invert for the slip distribution of the strongest M6.7 event of the sequence. We were able to identify the low-angle plane (strike 288°, dip 10° and rake 73°) as the fault plane. The slip model is dominated by two distinct slip patches, which extend towards SSE from the hypocenter, thus implying rupture directivity toward the island of Crete, a result which is further supported by our teleseismic waveform inversion results. Maximum slip is estimated at ~ 270 cm, while average slip on the ruptured area (area of subfaults of non-zero slip) was ~ 70 cm. Most of the slip (> 95% of total) is confined in an area of 34 × 22 km2. We attribute the occurrence of the shallow strike-slip event, just above the interplate thrusting at depth, to slip partitioning, due to the deflection of slip vectors from the expected plate convergence vector.
 
Article
On 14 November 2007, a subduction thrust earthquake, magnitude Mw = 7.8, occurred in the coastal region of northern Chile, causing substantial damage to the city of Tocopilla. We investigate the source fault of the earthquake, slip distribution and fault interaction by integrating aftershock locations, satellite interferometry data and stress model simulations. Aftershock measurements allow us to locate the area and geometry of the rupture plane in the coastal region between the cities of Tocopilla and Antofagasta. Combining two satellite viewing geometries, acquired in Envisat's Wide Swath and Image modes, we observe decimetre-scale coseismic deformation. The maximum line-of-sight displacement is found to be about 40 cm, located at the Mejillones Peninsula. Slip inversions using elastic half-space models with geometry constrained by aftershocks suggest rupturing of an area of ∼ 160 km by ∼50 km along the Nazca–South America convergent margin between latitudes 22°S and 23.5°S. The main slip is concentrated on two asperities, the largest being located in the southern part of the rupture area at a depth of approximately 30–50 km with a magnitude of about 2.5 m. Because aftershock distribution may also suggest a region of shallow crustal deformation activity located offshore, we investigate whether the 2007 Tocopilla earthquake also involved shallow crustal fault slip offshore. Although we find that the latter assumption is supported by Coulomb stress modelling and geologic inferences, our geodetic and seismic data provide insufficient constraints to resolve the exact geometry and kinematics of dislocation on this structure.
 
Article
Radon alpha tracks monitored at six sites increased gradually three months before (detected by microscopic counting), and increased remarkably 2 weeks before (detected by both mechanical and microscopic counting) the western Nagano Prefecture earthquake (M 6.8) which occurred on September 14, 1984. The monitoring sites were located about 65 km northwest of the epicenter, on the shear zone of the Atotsugawa fault. The changes in radon emanation are considered to be related to the earthquake rather than to changes in meteorological conditions.
 
Article
Tertiary volcanism around the Japan Sea is closely related to the opening of the Japan Sea. Paleogene to early Middle Miocene formations in the Pohang-Yangnam area of the Korean Peninsula mainly consist of volcanic rocks that are stratigraphically and petrographically similar to similar age rocks on the Japan Sea side of the Japanese Islands, especially those of Sado Island. We propose that all these early Cenozoic rocks formed part of a single tectonic-volcanic belt called Sado-Pohang Belt. It is inferred that this belt was located on the south side of the Yamato Ridge in Paleogene time. After the Japan Basin opened, the Yamato Basin opened during Early Miocene to the early Middle Miocene time and part of the Sado-Pohang Belt migrated southward.The tectonics of the Sado-Pohang Belt is considered to support the hypothesis of the two-stage opening of the Japan Sea.
 
Macroseismic intensity database derived from Stucchi and Monachesi (1997), Boschi et al. (2000) and Cecic´(Cecic´Cecic´(2001). In the sites, identified with an empty triangle, the values of the intensity given by different sources are different.
Macroseismic field computed from the reported intensities using the Polynomial Filtering Method (Molchan et al., 2002). Two highintensity areas, elongated towards the southeast and the southwest, are located at the border between Italy and Slovenia.
Maximum horizontal velocity field computed in the scaled point-source approximation for an upper frequency cutoff equal to 0.1 Hz. Source parameters are consistent with the seismotectonic setting of the Alps-Dinarides junction: top, a gentle north-dipping thrust; bottom, a sub-vertical right-lateral strike-slip.
Distribution of d i corresponding to the least total misfit scenario of the single-event case I-F(SN), see Table 3, on the Idrija Fault, schematically represented by a line. The position of the nucleation points (stars) implies bilateral rupture propagation (arrows).
Input fault models parameters used in the two-and single-source event scenarios
Article
The 1511 Western Slovenia earthquake (M = 6.9) is the largest event occurred so far in the region of the Alps–Dinarides junction. Though it strongly influences the regional seismic hazard assessment, the epicenter and mechanism are still under debate. The complexity of the active tectonics of the Alps–Dinarides junction is reflected by the presence of both compressional and transpressional deformations. This complexity is witnessed by the recent occurrence of three main earthquake sequences, the 1976 Friuli thrust faulting events, the 1998 Bovec–Krn Mountain and the 2004 Kobarid strike-slip events. The epicenters of the 1998 and 2004 strike-slip earthquakes (Ms = 5.7 and Ms = 4.9, respectively) lie only 50 km far from the 1976 thrust earthquake (Ms = 6.5).We use the available macroseismic data and recent active tectonics studies, to assess a possible epicenter and mechanism for the 1511 earthquake and causative fault. According with previous works reported in the literature, we analyze both a two-and a single-event case, defining several input fault models. We compute synthetic seismograms up to 1 Hz in an extended-source approximation, testing different rupture propagations and applying a uniform seismic moment distribution on the fault segments. We extract the maximum horizontal velocities from the synthetics and we convert them into intensities by means of an empirical relation. A rounded-to-integer misfit between observed and computed intensities is performed, considering both a minimized and a maximized databases, built to avoid the use of half-degree macroseismic intensity data points. Our results are consistent with a 6.9 magnitude single event rupturing 50 km of the Idrija right-lateral strike-slip fault with bilateral rupture propagation.
 
Article
The subcrustal mantle to about a depth of 160 km below the Baltic Shield can be divided seismically into two main intervals. The upper interval between ca. 45 and 100 km is characterized by refracted waves with velocities between 8.0 and 8.6 km/s, interpreted as a sandwich-layered model of intermixed high- and low-velocity layers. The lower interval is a ca. 40-km-thick zone of relatively low average velocity at a depth increasing from 95 km in the southern part to 120 km in the northern part of the shield. Seismic sections show that isolated bodies in this interval generate scattered reflections with substantial amplitude variation over short distance. The maximum velocity difference at the reflectors is 1.0 km/s which is larger than in the upper main interval. Our model is based on a general gap in correlation of seismic arrivals at offsets between 800 and 1100 km. At larger offsets we observe continuous linear first arrivals, indicating a relatively homogenous layer of high (> 8.6 km/s) velocity below a depth of 160 km. Petrologically, the lower reflective interval is explained by partially melted small bodies with patches of carbonatitic or kimberlitic magma, of horizontal extent less than 20 km. This new model of the upper mantle is based on seismic data from the Baltic Shield and there is clear indication that it is equally valid for other shield and platform areas.
 
Regional tectonic features of the Rivera plate and Jalisco block area. Black and shaded triangles are talc-alkaline and alkaline volcanic centers, respectively. CG = the Colima graben; TZG = the Tepic Zacoalco graben; RFZ = the Rivera Fracture zone; EPR = the East Pacific Rise; PRR = the Pacific-Rivera ridge; M4T = the Middle American Trench; TFZ = the Tamayo fracture zone. The diamond labeled RI-NA indicates the location of the Rivera North America pole of relative motion. Focal mechanism solutions are in a lower hemispheric projection where dark quadrants indicate compressional arrivals. Lines on the continent represent mapped faults and the bathymetry is in meters (after Pardo and Suarez, 1993). 
Article
On December 27th, 1568, a large earthquake occurred to the southwest of Guadalajara, Mexico, near the northeastern corner of the Jalisco block, in an area where no great earthquakes have been reported before. It caused heavy damage in the region where the Colima and Tepic-Zacoalco grabens intersect. Many churches, houses and convents in the neighboring towns collapsed and severe deformation of the ground was observed in the area. Landslides apparently dammed the Ameca River for several days and the opening of large cracks was reported in the lowlands. The flow of natural springs and the level of Lake Zacoalco changed dramatically after the earthquake. All of these reports strongly suggest that a local fault was the source of this large and destructive earthquake. Based on the intensity data inferred from the historical reports, the 1568 event is perhaps the largest earthquake to date in the Trans-Mexican Volcanic Belt. Compared to other well documented, large earthquakes that occurred in the volcanic belt in 1875, 1912 and 1920, the magnitude appears to be greater than 7.0 (MW). The Jalisco block is presumed to be rifting away from the North American plate. The Colima and Tepic-Zacoalco grabens, which bound the Jalisco block to the east and north, respectively, are apparently the boundaries where rifting is taking place in a complex and highly faulted environment. Based on the data available, it is impossible to identify the specific fault ruptured during the earthquake unequivocally. However, the occurrence of this large event in 1568 confirms that active deformation is still taking place in the Jalisco block. The apparently long recurrence times of these large events suggest that tectonic deformation is slow.
 
Article
We examined the spatial variation in the aftershock activity from the 17 August 1999 Izmit, Turkey earthquake. We found that this aftershock sequence is non-uniform both in space and time, aspects that need to be taken into account in any further statistical analysis. Other aspects of this aftershock sequence are similar to other aftershock sequences, namely low b-values and a high degree of spatial variation. We have detected three zones of relatively high b-values, two of which coincide with asperities revealed by previous slip inversion studies. The third zone with an anomalous b-value is located beyond the fault rupture and indicates a weakened fractured zone in the Yalova-Tuzla area. This b-value analysis provided no evidence for any significant difference that may exist between the two sides of the mainshock fault plane.
 
Article
Geophysical evidence for continental origin and growth may be summarized as follows: 1.(1) The overall mass of the crust and mantle to a depth of about 500 km is the same on a broad scale beneath oceans and continents.2.(2) The heat flow in shield areas is less than in orogenic areas and the mantle under shield areas is cooler than under the oceans.3.(3) The average continental crust is about 40 km thick. Shield areas are 40–45 km thick and orogenic areas have thinner crust, typically about 35 km thick, but much thinner (20 km) in areas such as the Pacific coastal region of California. Geological evidence indicates that: 1.(1) mountainous areas are destroyed by erosion in a few million years;2.(2) shield areas remain stable for long periods;3.(3) the continents have grown throughout geological time and are now more extensive than previously;(4) calc-alkaline volcanic rocks, in particular andesites, are characteristic of orogenic areas. Geochemical evidence shows that: 1.(1) the continental crust has an overall composition (for major elements) equivalent to an intermediate rock with a silica content about 60%;2.(2) the sedimentary rare-earth pattern is typical for the crust exposed to weathering, and may be typical of the whole crust;3.(3) the Sr isotope data indicate that the amount of Rb available is sufficient only for about 20 km of granodiorite, so that the lower crust must contain much lower concentrations of Rb;(4) there has been a strong vertical concentration of the heat producing elements in the earth. A simplified model for continental origin and growth is proposed, with the following stages: 1.(1) An initial geochemical fractionation leads to the concentration of the lithophile elements in the upper 1,000 km.2.(2) Lateral variations in the distribution of the heat producing elements in the uppermost mantle localise the sites of continental nuclei.3.(3) Continental areas grow mainly by the addition of andesites and associated calc-alkaline rocks in orogenic areas. Granodiorités and granites are mainly formed by partial melting, leaving a lower anorthositic crust. The overall composition of the continental crust is close to that of calc-alkaline or orogenic andesites.4.(4) Depletion of the subcontinental mantle in heat-producing elements terminates the process.5.(5) Erosion of shield areas removes K, U and Th and reduces the heat flow to levels below those of present orogenic areas.
 
Article
To reveal a subsurface structure beneath the southern part of the Boso Peninsula, Japan, where the Philippine Sea plate is subducting and great interplate earthquakes associated with the subduction occur repeatedly, we conducted a new seismic reflection survey from March to April 2005 (Boso05). We also reanalyzed old multi-channel seismic (MCS) survey data that had been collected off the Boso Peninsula in 1978 (SK78). We found clear strong reflectors beneath the southern coast of the Boso Peninsula. Since common mid points (CMPs) were distributed widely beneath the study area owing to the design of the receiver and shot lines of Boso05, we selected appropriate directions of stacking lines to get the best image of the dipping reflectors by optimum azimuth search (OAS) processing. We carefully checked the seismic profiles at the intersections of the survey lines to confirm the NNE-dipping configuration of the strong reflectors. These strong reflectors were interpreted as the upper surface of the subducting PHS plate from their locations and the estimated velocities beneath the reflectors. Furthermore, these reflectors revealed a topographic high (bump) beneath the southern coast of the Boso Peninsula where the source fault of the Genroku earthquake of 1703 is thought to be located.
 
Article
Damaging earthquakes in the South Iceland Seismic Zone (SISZ) occur fairly regularly and often as a series of events with a few days only between individual events. Tolerably reliable information on epicentre locations and mechanisms are available for 13 M ≥ 6 events between 1706 and 2000. For these events, we computed the co- and post-seismic stress fields, hereby approximating the SISZ by a mixed elastic/visco-elastic layered half-space. The horizontal shear stress and the Coulomb stress changes were analysed to detect possible trigger mechanisms, which may aid future earthquake mitigation efforts. We tested several criteria but must conclude that the start of an earthquake series in the SISZ cannot be explained by triggering through previous events. Inside an individual series, however, one may infer triggering. Our results are in contradiction with the findings in other regions of the world. The reason might be related to the fact that the SISZ is not a mature fault zone, in which old faults are re-activated if a certain stress level threshold is passed. In addition, uncertainties in the model parameters as well as the neglect of horizontal variations in the model and of possible stress transfer due to volcanic activity further complicate the evaluation of our results and need to be taken into account in future studies.
 
Article
High-resolution shallow seismic reflection profiles across the northwesternmost part of the New Madrid seismic zone (NMSZ) and northwestern margin of the Reelfoot rift, near the confluence of the Ohio and Mississippi Rivers in the northern Mississippi embayment, reveal intense structural deformation that apparently took place during the late Paleozoic and/or Mesozoic up to near the end of the Cretaceous Period. The seismic profiles were sited on both sides of the northeast-trending Olmsted fault, defined by varying elevations of the top of Mississippian (locally base of Cretaceous) bedrock. The trend of this fault is close to and parallel with an unusually straight segment of the Ohio River and is approximately on trend with the westernmost of two groups of northeast-aligned epicenters (“prongs”) in the NMSZ. Initially suspected on the basis of pre-existing borehole data, the deformation along the fault has been confirmed by four seismic reflection profiles, combined with some new information from drilling. The new data reveal (1) many high-angle normal and reverse faults expressed as narrow grabens and anticlines (suggesting both extensional and compressional regimes) that involved the largest displacements during the late Cretaceous (McNairy); (2) a different style of deformation involving probably more horizontal displacements (i.e., thrusting) that occurred at the end of this phase near the end of McNairy deposition, with some fault offsets of Paleocene and younger units; (3) zones of steeply dipping faults that bound chaotic blocks similar to that observed previously from the nearby Commerce geophysical lineament (CGL); and (4) complex internal deformation stratigraphically restricted to the McNairy, suggestive of major sediment liquefaction or landsliding. Our results thus confirm the prevalence of complex Cretaceous deformations continuing up into Tertiary strata near the northern terminus of the NMSZ.
 
Article
The Royal Academy of History (RAH) of Spain collected very detailed information about the effects on the Spanish territory of the Lisbon, 1755 earthquake, reporting such information within a year after the shock. The data provided by the RAH has been thoroughly analyzed, deriving an M S K intensity value for each one of the localities (more than 1000) included in the report, and an isoseismal map has been obtained from such values. Observations reported, such as tsunami, surface effects, duration of shaking and effects on the population are also discussed. Finally, a comparison is made of the data with the results of recent investigations on the tectonics of the region.
 
Article
The Great Lisbon earthquake of 1755 with an estimated magnitude of 8.5–9.0 is the most destructive earthquake in European history, yet the source region remains enigmatic. Recent geophysical data provide compelling evidence for an active east dipping subduction zone beneath the nearby Gibraltar Arc. Marine seismic data in the Gulf of Cadiz image active thrust faults in an accretionary wedge, above an east dipping decollement and an eastward dipping basement. Tomographic and other data support subduction and rollback of a narrow slab of oceanic lithosphere beneath the westward advancing Gibraltar block.Although, no instrumentally recorded seismicity has been documented for the subduction interface, we propose the hypothesis that this shallow east dipping fault plane is locked and capable of generating great earthquakes (like the Nankai or Cascadia seismogenic zones). We further propose this east dipping fault plane to be a candidate source for the Great Lisbon earthquake of 1755. In this paper we use all available geophysical data on the deep structure of the Gulf of Cadiz–Gibraltar region for the purpose of constraining the 3-D geometry of this potentially seismogenic fault plane. To this end, we use new depth processed seismic data, have interpreted all available published and unpublished time sections, examine the distribution of hypocenters and perform 2-D gravity modeling. Finally, a finite-element model of the forearc thermal structure is constructed to determine the temperature distribution along the fault interface and thus the thermally predicted updip and downdip limits of the seismogenic zone.
 
Article
The “Lisbon” earthquake of November 1, 1755, the effects of which have been described at length in the Iberian Peninsula, was likewise strongly felt in Morocco, especially on the Atlantic coast, which was laid waste not only through the direct agency of seismic waves, but also through that of a formidable tsunami. The study of these effects in Morocco, based on a critical analysis of European and Arab sources entailing, whenever possible, a consultation of the original sources, has lead to the hypothesis that two distinct destructive earthquakes would have followed the main shock of November 1; one, on November 18, 1755, was probably an aftershock of the November 1 event and originated in the Gorringe Bank; the other, on November 27, 1755, was located in the Meknes area and is supposed to have destroyed that town. The values of intensities interpreted from the observations of the November 1 event have been compared with the values derived from the attenuation laws obtained from the recent earthquake of February 28, 1969 originating at the same source, the Moroccan isoseismals of which are well established. Extrapolated isoseismals for the effects of the November 1, 1755 event in Morocco as derived from this study are then assigned
 
Article
The Great Lisbon earthquake has the largest documented felt area of any shallow earthquake and an estimated magnitude of 8.5–9.0. The associated tsunami ravaged the coast of SW Portugal and the Gulf of Cadiz, with run-up heights reported to have reached 5–15 m. While several source regions offshore SW Portugal have been proposed (e.g.— Gorringe Bank, Marquis de Pombal fault), no single source appears to be able to account for the great seismic moment as well as all the historical tsunami amplitude and travel time observations. A shallow east dipping fault plane beneath the Gulf of Cadiz associated with active subduction beneath Gibraltar, represents a candidate source for the Lisbon earthquake of 1755.
 
Article
On September 18, 2004, a 4.6 mbLg earthquake was widely felt in the region around Pamplona, at the western Pyrenees. Preliminary locations reported an epicenter less than 20 km ESE of Pamplona and close to the Itoiz reservoir, which started impounding in January 2004. The area apparently lacks of significant seismic activity in recent times. After the main shock, which was preceded by series of foreshocks reaching magnitudes of 3.3 mbLg, a dense temporal network of 13 seismic stations was deployed there to monitor the aftershocks series and to constrain the hypocentral pattern. Aftershock determinations obtained with a double-difference algorithm define a narrow epicentral zone of less than 10 km2, ESE–WNW oriented. The events are mainly concentrated between 3 and 9 km depth. Focal solutions were computed for the main event and 12 aftershocks including the highest secondary one of 3.8 mbLg. They show mainly normal faulting with some strike-slip component and one of the nodal planes oriented NW–SE and dipping to the NE. Cross-correlation techniques applied to detect and associate events with similar waveforms, provided up to 33 families relating the 67% of the 326 relocated aftershocks. Families show event clusters grouped by periods and migrating from NW to SE. Interestingly, the narrow epicentral zone inferred here is located less than 4 km away from the 111-m high Itoiz dam. These hypocentral results, and the correlation observed between fluctuations of the reservoir water level and the seismic activity, favour the explanation of this foreshock–aftershock series as a rapid response case of reservoir-triggered seismicity, burst by the first impoundment of the Itoiz reservoir. The region is folded and affected by shallow dipping thrusts, and the Itoiz reservoir is located on the hangingwall of a low angle southward verging thrust, which might be a case sensible to water level fluctuations. However, continued seismic monitoring in the coming years is mandatory in this area to infer more reliable seismotectonic and hazard assessments.
 
Article
The different effects of cooling and alteration on magnetic properties, in single thick flows from subaerial and submarine eruptions, cored and logged during Ocean Drilling Program (ODP) Leg 183 at Sites 1137 and 1140 (Kerguelen Plateau) are examined. Downhole logging data from both sites is supplemented by petrology and geochemistry of 32 samples from three subaerial lava flows at Site 1137 and two flows units at Site 1140, covering transects from fresh to highly altered basalts.Changes in magnetic properties have previously been observed in several ODP drill holes, which penetrate basaltic basement. In subaerial basalts, a typical trend is that of high magnetic susceptibility and natural remanent magnetization (NRM) values in the altered flow top, and lower values in the less-altered massive flow interior. In contrast, submarine lava flows display the opposite behavior in their magnetic properties. Altered pillow rims have lower susceptibility and NRM values than the fresh pillow interiors. It is concluded that rate of cooling and degree of alteration are the main factors influencing the magnetization and, hence, the distribution of iron oxides. The effects of low-temperature alteration are most noticeable in the distribution of more mobile elements, such as K. Consequently, the spectral gamma ray (SGR) log, which in basaltic basement is largely controlled by K concentration, is an excellent proxy to the downhole identification of alteration. The strong positive correlation observed for the subaerial basalts between the downhole total magnetic field (Ftot) and SGR, suggest a potential link with alteration in the drilled sections. The alteration of the submarine basalts is not as pronounced and therefore no correlation is evident.
 
Article
A set of seven reliability criteria has been applied to a previously published Phanerozoic paleopole database for Europe and North America and a Late Precambrian data set for Africa. A quality factor (0 ⩽ Q ⩽ 7) is assigned to a result, based on the number of criteria satisfied. Three criteria, dealing with age reliability, structural control and laboratory demagnetization analysis are deemed the most important; for the Phanerozoic results these are satisfied by a large majority of the results, whereas for the majority (up to 80%) of the African Late Precambrian results such criteria are not met. Criteria based on tests that constrain the age of the magnetization, such as those dealing with folds, conglomerates, contacts or reversals, enhance the reliability of a result; for the Phanerozoic, they are generally satisfied by about one third of the data, but for the Precambrian only a few results incorporate such tests.The assertion is made in this study that these criteria indeed qualitatively describe the reliability of results in broad terms, so that a data set satisfying on average most of the criteria (Q ⩾ 4) can be described as more robust than a data set with average Q = 2. Statistical evaluations illustrate the difference in robustness of paleopole data sets between the well-studied Phanerozoic Era and the much more uncertain Late Precambrian.
 
Article
A catalogue of 1873–1972 earthquakes with M > 6.9 for the New Guinea—Solomon Islands region (130–165° E) is compiled. There are 152 events listed. Duda's (1965) results for 1900–1968 are improved for the Papua New Guinea area (141–156° E) because of the availability of historical data for that area.Although there is evidence of rapid Holocene uplift in the main seismic zones, there is little historical evidence for visible uplift or subsidence resulting directly from modern major earthquakes. Coastal subsidences commonly reported as a result of earthquakes are of smaller extent and appear to be due to settlement. However, the occurrence of tsunamigenic earthquakes does suggest that surface deformations do take place off-shore.Using Davies and Brune's (1971) method, regional fault slip rates over 5° -segments of the shallow seismic zone are determined from the seismicity catalogue. The slip rate for the island of New Guinea (Gutenberg and Richter's Region 16) is found to be at least 4.4 cm/y which is almost double the very anomalously low rate of 2.3 cm/y found by Davies and Brune (1971). If allowance is made for shear movement without seismicity and for the approximately ratio of dip-slip versus strike-slip faulting indicated by fault plane solutions, the agreement with Le Pichon's (1970) approach value of 10.7 cm/y for the Pacific—India (Australia) plates is reasonable. The fault slip rate in the area between east New Britain and Bougainville at the Pacific—Bismarck—Solomon triple junction is extremely high (20.6 cm/y at least). The smallest slip rate (1.5 cm/y) is found for westernmost New Guinea (130–135° E).Temporal cumulative summation of moments curves show a periodicity of approximately 25 years in the seismic activity at the triple junction (150–155° E). Elsewhere the rate of seismic activity is aperiodic.
 
Article
Two M6+ events occurred 15–20 km apart in central Greece on April 20 and April 27, 1894. We identify the April 27, 1894 rupture (2nd in the sequence) with the Atalanti segment of the Atalanti Fault Zone because of unequivocal surface rupturing evidence reported by Skouphos [Skouphos, T., 1894. Die swei grossen Erdbeben in Lokris am 8/20 und 15/27 April 1894. Zeitschrift Ges. Erdkunde zu Berlin, vol. 24, pp. 409–474]. Coulomb stress transfer analysis and macroseismic evidence suggest that the April 20, 1894 event (1st in the sequence) may be associated with the Martinon segment of the same fault zone. Our stress modelling suggests that this segment may have ruptured in an M = 6.4 event producing a 15-km long rupture which transferred 1.14 bar in the epicentral area of the April 27th, 1894 event, thus triggering the second M = 6.6 earthquake along the Atalanti segment and producing a 19-km long rupture. We also examined three alternative fault sources for the first event; however, all these produce smaller stress stresses for triggering the second event. The proposed slip model for the second earthquake is capable of producing coastal subsidence of the order of centimetres to decimetres, which fits the geological data. The 1894 earthquake sequence was followed by a difference in the timing of subsequent M > 5 events in each of the “relaxed” areas (stress shadows; a negative change in Coulomb failure stress > − 0.6 bar), which terminated between 22–37 years (north) and 80 years (south).
 
Article
We assume that the unusually deep, extensive and long-lasting floods of 1897 along the section of the Brahmaputra River north of the western Shillong plateau were due to local ground subsidence associated with the great earthquake which occurred on June, 12 of that year in the western part of northeast India. Numerical simulations of ground-level changes due to slip on a buried low-angle thrust fault, dipping due north, then show that the northern limit of the rupture zone of this earthquake should have been along the E-W-flowing Brahmaputra River, about 40 km north of the northern edge of the Shillong plateau and about 70 km south of the Himalayan mountain front. A similar interpretation of a ground tilt observation suggests that the western limit of the 1897 rupture zone was along the western margin of the Shillong plateau. The E-W and N-S dimensions of the rupture zone are estimated to be 170 km and 100 km respectively, so that it enclosed the western half of the Shillong plateau and areas north of it up to the Brahmaputra River. The rupture depth could not be estimated from the available data on ground-level changes, and was constrained at 15 km beneath the southern margin of the Shillong plateau, on other evidence. The above thrust fault should be of the nature of a detachment at midcrustal depth, which arose because the continental crust associated with the Indian Shield terrains of the Shillong plateau and Mikir Hills immediately to the east could not subduct under the continental crust of the Eurasian plate to the north and east. It is tentatively suggested that, although this detachment may extend under the Himalaya, it may not be the detachment on which the great earthquakes of 1905, 1934 and 1950 have occurred in the northwestern, central and eastern Himalaya, respectively. It is also suggested that a distinction should be made between the seismicity of the Himalaya and the seismicity of the Himalayan convergent plate margin (HCPM). An earthquake of the Himalayan seismic belt is also an earthquake of the HCPM, but the converse need not hold true. Since the inferred northern limit of the 1897 rupture zone is about 70 km south of the Himalayan mountain front, it is suggested that this earthquake belongs to the HCPM but not to the Himalayan seismic belt. Thus, conservatively, a seismic gap of about 700 km may exist along the Himalayan seismic belt between the eastern and western limits of the ruptures zones of the great 1934 and 1950 earthquakes respectively.
 
Article
The present-day crustal structure of the Andean chain results from deformation processes and magmatic activity since the onset of subduction. One line of investigation to resolve questions about the mechanism of crustal thickening is to get information about the composition of the crust and upper-mantle by imaging the structures in velocity and Poisson's ratio (or Vp/Vs). A high density seismic network was operated during six months in 1994 across the whole Andean chain. The P and S travel times of the best located local earthquakes, mainly situated in the subduction zone, have been inverted for Vp and Vp/Vs using Thurber's 3D iterative simultaneous inversion method. The resulting models, giving short-scale information about the crustal and upper-mantle composition, provide new limits on the respective involvement of crustal shortening and magmatism in the thickening of the Andean crust.Beneath the Western Cordillera, the seismic properties are well explained by typical active volcanic arc processes, and the thickening of the crust is explained by magmatic addition. Conversely, beneath the Central Altiplano Basin and the Eastern Cordillera, our results preclude significant volume of magmatic addition from the mantle and reinforce crustal shortening as the main thickening process. Moreover, cratonic lower crust and subcontinental lithospheric upper-mantle are observed under the Eastern Cordillera up to 67°W, evidence of the underthrusting of the Brazilian craton, which drives the crustal shortening. Beneath the Central Altiplano Basin, a thin lithospheric mantle is observed above a well marked asthenospheric wedge which extends in depth below the western part of the Eastern Cordillera. Thus partial melt is observed in the shallow mantle west of the Western Cordillera up to the Brazilian craton.
 
Article
Source parameters of the ML 5.7,19 July 1985 earthquake of northern Hawke's Bay, New Zealand are determined by modelling the teleseismic P and S body waves. A focal depth of 31 ± 1 km, scalar moment 5.6×1017 Nm and source duration of 3.0–3.5 s are estimated. The focal mechanism (strike 224°, dip 78°, rake 273°) shows normal faulting on a NW-dipping plane that is parallel to the strike of the subducted Pacific Plate. Thirty-five aftershocks were recorded by a temporary seismograph network, 21 of which were relocated using the JHD method. The hypocentres define a northwest dipping plane, 20×30 km, which extends from the hypocentre of the mainshock to a depth of 47 km. The position of the subducted plate in Hawke's Bay is well defined from previous micro-earthquake studies. In the region of the mainshock the plate interface is located at a depth of approximately 28 to 30 km, placing the hypocentre of the mainshock in the crust of the Pacific plate. The extent and location of the aftershock distribution suggest that the mainshock ruptured downward and extended through the crust of the plate. This event thus differs from other normal faulting events in the region that usually occur in the mantle of the subducted plate. The scalar moment at long periods (T > 45s, Dziewonski et al., 1986) exceeds that determined here by a factor of 2, suggesting a source rich in long period energy, an idea supported by the relatively large aftershock area.
 
Article
The Tauramena (Colombia) earthquake, Mw=6.5, occurred on January 19, 1995, in the Andean Eastern Cordillera foothill region, the so-called Piedemonte Llanero. The Harvard CMT focal mechanism indicates an almost pure reverse fault rupture. There was no surface faulting associated with this earthquake. This event was located at the northern tip of a zone, about 90 km in length, with relatively low microseismic activity along the central segment of the Piedemonte Llanero in Colombia. A field expedition to the epicentral area was organized and a temporary portable network was installed for 1 month to register aftershock activity. More than 800 events were recorded during this period. A subset of the best located aftershocks (319 events) shows epicenters extended over an area of 800 km², and suggests two antithetic planes on which most of the activity was concentrated. The main event and a subset of 41 aftershocks occurred before the installation of the portable network but were recorded by the Colombian National Seismic Network and were relocated. They show an epicentral distribution similar to that of the subsequent events.
 
Article
Determination of the fault-plane solution of the Kinnaur earthquake by first motion and amplitude data of P waves, along with other geological and geophysical data, indicates a N–S-trending and westward-dipping fault plane with purely dip-slip displacement. This plane coincides with the N–S extending zone of aftershocks, ground fissures and faults observed in the epicentral tract following the earthquake. The cumulative strain release pattern displays a characteristic release of logarithmic recovery followed by exponential recovery. The aftershocks display a notable migration in space. They also show two distinct phases of seismic activity. The main earthquake is estimated to have a fault length of 25 km, fault area of 575 km2, volume of strain of ~2900 km3, displacement of 59 cm, stress drop of 36 bars, and strain release of 5.9 · 10−5.
 
Article
From the microfilms of Göttingen Observatory seismograms, magnitudes are verified for thirty-seven major and great earthquakes (magnitude ⩾ 7) in the Himalayan region for the period 1903–1985. Duda (1992) had earlier determined magnitudes of these earthquakes from Göttingen records. There were three other major earthquakes for which magnitudes could not be determined, as their records from Göttingen were not found. The magnitude values were determined using P-, S- and surface waves from long- and medium-period instruments. The magnitudes from short-period P- and S-waves were not redetermined to avoid uncertainties of such measurements from the microfilms. As compared to Duda (1992), the present values of the surface-wave magnitudes corresponding to 20 ± 3 s period from horizontal components are found to be higher for nine earthquakes by 0.2–0.5 units. The body-wave magnitudes are similarly changed.These changes bring the magnitudes closer to the values given in earlier catalogs. The relative quiescence after 1950 is striking. The seismogram portions of earthquakes (magnitude ⩾ 7.5) are reproduced.
 
Article
Sakurajima volcano, which is located on the southern rim of the Aira caldera, Kagoshima, is one of the most active volcanoes in Japan. The 1914 eruption of Sakurajima volcano was accompanied by an intensive fissure eruption and caused significant crustal deformations. The north side of the newly formed fissure craters, which are arranged in the WNW-ESE direction, moved toward the north by about 5 m, and the south side moved toward the south by 3 m. Furthermore, the ground surface around the Aira caldera was concentrically depressed by 80 cm.In order to explain these significant crustal deformations, a model consisting of a tensile fault and a deflation source is presented. By means of the least-squares fitting of the model to leveling data around the Aira caldera, we obtained the parameters of the deflation source as follows: depth 8.8 ± 1.2 km; subsidence just above the source 157 ± 27 cm. The source is located at the center of the Aira caldera. On the other hand, the tensile fault, which corresponds to the fissure craters, is estimated, on the basis of horizontal movements in Sakurajima, to be 7 km long, 1.5 km wide and 0.5 km deep at its upper margin, with an opening of 20 m. This model suggests that magma flowed out of the reservoir beneath the Aira caldera and broke the crust of Sakurajima, thus creating fissure.
 
Article
Extensive paleoseismological research was conducted in the Fucino Plain (central Italy) in order to better understand the seismogenic characteristics of the fault responsible for the 1915 Avezzano earthquake (Ms=7.0) and improve the general knowledge regarding active tectonics in the central Apennines. Evidence for Late Pleistocene–Holocene surface faulting events was obtained through the study of thirteen sites across four different fault branches. The paleoseismological analysis outlined the occurrence of ten surface faulting events in the past 33,000 years, seven of which occurred during the Holocene. Radiocarbon, thermoluminescence and archaeological dating permitted the definition of an event chronology and the estimation of a recurrence interval for surface faulting events ranging between 1400 and 2600 years. On the basis of the observed offsets it was possible to calculate vertical slip rates for the individual fault branches, ranging between 0.24 mm yr−1 and 0.5 mm yr−1 and an extension rate across the Fucino Plain ranging between 0.6 and 1 mm yr−1. The chronology of surface faulting events appears linearly distributed in time, and the observed recurrence times are similar to those inferred by other paleoseismological studies along different active Apennine faults. The general consequence of the large time span between strong earthquakes is that even a 2000 year long historical record, such as the Italian earthquake catalogue, does not cover the entire seismic cycle of all the active Apennine faults. A comparison of the extension rate across the Fucino Plain with that across the entire central Apennines (as inferred from the sum of seismic moments of earthquakes which occurred in the interval 1000–1992) shows a seismicity `deficit' in the period indicated. This confirms that a number of seismogenic faults were not active during the past 1000 years.
 
Article
The 1932 deep Fiji earthquake is studied from the point of view of comparing it to the recent large events in the transition zone. Spectral analysis and body-wave modeling suggest a moment of 3.4 × 1027 dyn cm (within a multiplicative or divisive factor of 1.5), which makes it comparable to, but not significantly larger than, the great 1994 earthquake, 770 km to the north. Its seismic moment remains well below that of the largest deep shocks (1994 in Bolivia or 1970 in Colombia). Relocation efforts show that the earthquake took place on the front edge of the Wadati-Benioff Zone, in an area where the latter features a complex geometry involving significant warping. The mechanism of the earthquake involves down-dip compression and can be readily explained in terms of the prevailing large-scale stresses in the slabs.
 
Article
The tectonic processes taking place along the southern part of the Japan trench are discussed on the basis of the focal mechanism of the 1938 Shioya-Oki event which consists of the five large earthquakes of . Detailed analyses of seismic waves and tsunamis are made for each of these earthquakes, and the dislocation parameters are obtained. The total seismic moment amounts to 2.3 · 1028 dyn.cm. The five earthquakes are grouped into either a low-angle thrust type or a nearly vertical normal-fault type. These mechanisms are common with other great earthquakes of the northwestern Pacific belt, and can be explained in terms of the interaction between the oceanic and continental plates. The vertical displacement inferred from the seismic results is in approximate agreement with the precise level data over the period from 1939 and 1897. This agreement suggests that the rate of the strain accumulation at the preseismic time is very small in the epicentral area. Repeated levelings at the postseismic time reveal a large-scale recovery of the coseismic subsidence. The postseismic deformation is one-third to one-half of the coseismic displacement. The time constant of the recovery is estimated to be 5 years or less. This type of deformation may be a manifestation of viscoelasticity of a weak zone underlying the continent. The amount of dislocation, together with the longterm seismicity, suggests a seismic slip rate of about 0.4 cm/year, which is one order of magnitude smaller than that for the adjacent regions. This suggests that a large part of the plate motion is taking place aseismically in this region. The tectonic process now taking place in the southern Japan trench can be considered to represent a stage just prior to a complete detachment of the sinking portion of the oceanic plate.
 
Top-cited authors
Xavier Le Pichon
  • Collège de France
Ali Mehmet Celal Sengor
  • Istanbul Technical University
H. Thybo
  • Istanbul Technical University
Bor-ming Jahn
  • National Taiwan University
Yücel Yilmaz
  • Istanbul Technical University