![Extent of the west Andean thrust (WAT; Armijo et al., 2010), South America (SAm). A: The WAT is the main active continental-scale fault system at the western foot of the Andes, breaking the west margin of the continent ~200 km eastward from the trench. The Santiago segment of the WAT is in red, and other parts are in black (dashed where less certain). B: Three-dimensional view of Santiago conurbation (more than six million inhabitants) and fault scarps at the piedmont of the abrupt Andean mountain front [SPOT satellite image draped over 30 m digital elevation model, courtesy of Centre National d'Etudes Spatiales]. Orange star shows location of paleoseismological trench.](https://www.researchgate.net/profile/Gabriel-Easton/publication/267269254/figure/fig1/AS:614148203360263@1523435750291/Extent-of-the-west-Andean-thrust-WAT-Armijo-et-al-2010-South-America-SAm-A-The_Q320.jpg)
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Probing large intraplate earthquakes at the west flank of the Andes
Abstract and Figures
Estimating the potential for the occurrence of large earthquakes on slow-slip-rate faults in
continental interiors, away from plate boundaries, is possible only if the long-term geological
record of past events is available. However, our knowledge of strong earthquakes appears to
be incomplete for thrust faults flanking large actively growing mountain ranges, such as those
surrounding Tibet and the Andes Mountains. We present a paleoseismic study of a prominent
fault scarp at the west flank of the Andes in Santiago, Chile. The evidence demonstrates recurrent
faulting with displacement of ~5 m in each event. With two large earthquake ruptures
within the past 17–19 k.y., and the last event occurring ~8 k.y. ago, the fault appears to be
ripe for another large earthquake (moment magnitude, Mw 7.5). These results emphasize the
potential danger of intraplate continental faults, particularly those associated with youthful
mountain fronts.
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... Four major crustal-scale structures accommodate shortening in this region: (i) San Ramon-Pocuro; (ii) El Diablo-Las Leňas-El Fierro; (iii) Malargüe-Aconcagua fold and thrust belt; and (iv) Frontal Cordillera (Figure 1a) (Farías et al., 2010). The partitioning of deformation across these four crustal-scale structures (Figure 1a), however, is highly contested (Armijo et al., 2010;Farías et al., 2010;Giambiagi et al., 2003;Vargas et al., 2014;Riesner et al., 2018), though linked to local variability in subduction geometry and convergence rate (Arriagada et al., 2013). ...
... The celerity of knickpoint propagation they estimated to be 10-40 mm a -1 . The uplifting fault, the San Ramon-Pocuro fault, is now believed to only be seismically active in the north of the study area (Vargas et al., 2014;Riesner et al., 2017). These four major structures were incorporated into the tectonic model. ...
... Malargüe fold-and-thrust belt (Figure 3d). There is ample structural and geomorphic evidence to suggest both of these structures have been major active fronts accommodating shortening throughout the Quaternary (Armijo et al., 2010;Branellec et al., 2016;Farías et al., 2010bFarías et al., , 2008Messager et al., 2010;Riesner et al., 2018;Vargas et al., 2014). In the north of the study area, the preferential accommodation of rock uplift along the west-verging, San Ramon-Pocuro fault, and the passive nature of the El Fierro-El Diablo-Las Leňas fault systems, are in agreement with existing evidence of a bi-verging orogeny (Armijo et al., 2010;Riesner et al., 2018). ...
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... The western border of this range at the latitudes (Fig. 2) is bounded by mega-faults (e. g., Pocuro Fault System at 32 • -33 • S, San Ramón Fault at 33 • -34 • S), which have maintained their current position since the opening of the Abanico extensional basin during the early Neogene (Charrier et al., 2002;Farías et al., 2010). These high-angle thrusts have accommodated minor shortening compared to the uplift (Farías et al., 2008(Farías et al., , 2010Armijo et al., 2010;Vargas et al., 2014). At the latitude of this study, the eastern limit of the basin deposits is located in the Principal Cordillera in the vicinity of the international boundary (Chile-Argentina), and are affected by the westernmost structures of the FTBs, and in some cases by backthrusts associated with out-of-sequence deformation in the evolution of the Andean FTBs (LRFTB and AFTB in Fig. 2). ...
... TL was later replaced by Optically Stimulated Luminescence applied to quartz (OSL) and Infra-Red Stimulated Luminescence (IRSL), and post Infra-Red (pIRIR225) on K-feldspar rich grains (e.g. Rockwell et al., 2009Rockwell et al., , 2014Blisniuk et al., 2012;Vargas et al., 2014;Ferrater et al., 2016). Choice of where to sample, sample collection protocol, and laboratory method are more complex in active tectonics sites, where all types of sediments are possibly deformed or have a complex history. ...
... Large earthquakes along thrust systems transfer towards the surface large amounts of seismic slip accumulated at mid-crustal depth during centuries [1][2][3][4][5][6][7] . The thrusting of the hanging wall by several meters during those large events is often accompanied by dramatic surface breaks 8-10 that can be distributed along several parallel strands (e.g. ...
Large earthquakes breaking the frontal faults of the Himalayan thrust system produce surface ruptures, quickly altered due to the monsoon conditions. Therefore, the location and existence of the Mw8.3 1934 Bihar–Nepal surface ruptures remain vividly disputed. Even though, previous studies revealed remnants of this surface rupture at the western end of the devastated zone, ruptures extent remains undocumented in its central part. Evidence for recent earthquakes is revealed along the frontal thrust in this region. The Khutti Khola river cuts an 8 m-high fault scarp exposing Siwalik siltstone thrusted over recent alluvial deposits, with faults sealed by a colluvial wedge and undeformed alluvial sediments. Detrital charcoals radiocarbon dating reveals that the last event occurred between the seventeenth century and the post-bomb era, advocating for the 1934 earthquake as the most recent event. In the hanging wall, fluvial terraces associated with fault scarps were abandoned after a penultimate event that happened after the tenth century, a rupture we associate with the historic earthquake of 1255CE. Slips of 11–17 m and 14–22 m for the 1934 and 1255 earthquakes, respectively, compare well with the ~ 10–15 m slip deficit accumulated between the two earthquakes, suggesting that most of the deformation along the front is accommodated by surface-rupturing earthquakes.
The thickness of the seismogenic crust (Ts) controls crustal earthquakes. Its upper limit is the seismicity onset depth (SOD) while its base corresponds to the seismicity cutoff depth (SCD) that correlates with the brittle-ductile transition. Thus, it influences the magnitude and location of crustal earthquakes and knowledge of its geometry may aid in seismic hazard assessment. Here we present the first Ts map of the Andean margin. We follow the standard methodology using the statistical depth distribution of events on a grid of equally size square cells. However, we find it has flaws and develop a new approach, based on circular cells of variable radius that changes according to earthquake density. Our results indicate that Ts is heterogenous, showing three controls: thermal structure, subduction geometry and crustal thickness.
In the South Andes western edge, a very active seismic contact, with earthquakes up to magnitude $9.5$ and ca. $4000\thinspace\textnormal{km}$ extension threatens cities and very large populations. The existence of modern seismological networks along the contact allowed the observation of unprecedented earthquake cycle characteristics, which can improve our ability to estimate earthquake hazard, a main objective of seismology. Using dimensional and similarity analysis techniques, we show precise mechanical conditions under which the earthquake generation process unfolds, and derive a set of scaling equations linking renormalized variables. Later on, we test our theoretical results using a curated earthquake point-catalog by using gridding, box-counting, statistical bootstrap and fixed-point iteration collapse techniques. We found non-trivial scaling laws valid across multiple orders of magnitude capable of describing a complex interplay between renormalized earthquake occurrence and renormalized moment release rate. We discuss finite-strain and seismic-moment release-rate conditions; declustering, foreshock, mainshock, aftershock notions; cutoff magnitudes, earthquake hazard implications and a possible large-scale tectonic energy transfer mechanism.
The San Ramón Fault is an active west-vergent thrust fault system located along the eastern border of the city of Santiago, at the foot of the main Andes Cordillera. This is a kilometric crustal-scale structure recently recognized that represents a potential source for geological hazards. In this work, we provide new seismological evidences and strong ground-motion modeling from hypothetic kinematic rupture scenarios, to improve seismic hazard assessment in the Metropolitan area of Central Chile. Firstly, we focused on the study of crustal seismicity that we relate to brittle deformation associated with different seismogenic fringes in the main Andes in front of Santiago. We used a classical hypocentral location technique with an improved 1D crustal velocity model, to relocate crustal seismicity recorded between 2000 and 2011 by the National Seismological Service, University of Chile. This analysis includes waveform modeling of seismic events from local broadband stations deployed in the main Andean range, such as San José de Maipo, El Yeso, Las Melosas and Farellones. We selected events located near the stations, whose hypocenters were localized under the recording sites, with angles of incidence at the receiver <5° and S–P travel times <2 s. Our results evidence that seismic activity clustered around 10 km depth under San José de Maipo and Farellones stations. Because of their identical waveforms, such events are interpreted like repeating earthquakes or multiplets and therefore providing first evidence for seismic tectonic activity consistent with the crustal-scale structural model proposed for the San Ramón Fault system in the area (Armijo et al. in Tectonics 29(2):TC2007, 2010). We also analyzed the ground-motion variability generated by an M
w
6.9 earthquake rupture scenario by using a kinematic fractal k
−2 composite source model. The main goal was to model broadband strong ground motion in the near-fault region and to analyze the variability of ground-motion parameters computed at various receivers. Several kinematic rupture scenarios were computed by changing physical source parameters. The study focused on statistical analysis of horizontal peak ground acceleration (PGAH) and ground velocity (PGVH). We compared the numerically predicted ground-motion parameters with empirical ground-motion predictive relationships from Kanno et al. (Bull Seismol Soc Am 96:879–897, 2006). In general, the synthetic PGAH and PGVH are in good agreement with the ones empirically predicted at various source distances. However, the mean PGAH at intermediate and large distances attenuates faster than the empirical mean curve. The largest mean values for both, PGAH and PGVH, were observed near the SW corner within the area of the fault plane projected to the surface, which coincides rather well with published hanging-wall effects suggesting that ground motions are amplified there.
It is unclear where plate boundary thrusts generate giant rather than great earthquakes. Along the Himalayas, the source sizes and recurrence times of large seismic events are particularly uncertain, since no surface signatures were found for those that shook the range in the twentieth century. Here we challenge the consensus that these events remained blind and did not rupture the surface. We use geomorphological mapping of fluvial deposits, palaeo-seismological logging of river-cut cliffs and trench walls, and modelling of calibrated 14 C ages, to show that the M w 8.2 Bihar–Nepal earthquake on 15 January 1934 did break the surface: traces of the rupture are clear along at least 150 km of the Main Frontal Thrust fault in Nepal, between 85 • 50 and 87 • 20 E. Furthermore, we date collapse wedges in the Sir Valley and find that the 7 June AD 1255 earthquake, an event that devastated Kathmandu and mortally wounded the Nepalese King Abhaya Malla, also ruptured the surface along this stretch of the mega-thrust. Thus, in the past 1,000 years, two great earthquakes, 679 years apart, rather than one giant eleventh-century AD event, contributed to the frontal uplift of young river terraces in eastern Nepal. The rare surface expression of these earthquakes implies that surface ruptures of other reputedly blind great Himalayan events might exist.
It is unclear where plate boundary thrusts generate giant rather than
great earthquakes. Along the Himalayas, the source sizes and recurrence
times of large seismic events are particularly uncertain, since no
surface signatures were found for those that shook the range in the
twentieth century. Here we challenge the consensus that these events
remained blind and did not rupture the surface. We use geomorphological
mapping of fluvial deposits, palaeo-seismological logging of river-cut
cliffs and trench walls, and modelling of calibrated 14C
ages, to show that the Mw 8.2 Bihar-Nepal earthquake on 15
January 1934 did break the surface: traces of the rupture are clear
along at least 150km of the Main Frontal Thrust fault in Nepal, between
85°50' and 87°20'E. Furthermore, we date collapse wedges in the
Sir Valley and find that the 7 June AD 1255 earthquake, an event that
devastated Kathmandu and mortally wounded the Nepalese King Abhaya
Malla, also ruptured the surface along this stretch of the mega-thrust.
Thus, in the past 1,000 years, two great earthquakes, 679 years apart,
rather than one giant eleventh-century AD event, contributed to the
frontal uplift of young river terraces in eastern Nepal. The rare
surface expression of these earthquakes implies that surface ruptures of
other reputedly blind great Himalayan events might exist.
In 1861, one of the most destructive earthquakes in the history of Argentina destroyed the city of Mendoza (currently 1 million inhabitants). The magnitude MS∼7.0 earthquake is inferred to have occurred on the 31‐km‐long La Cal thrust fault, which extends from Mendoza to the north, where it offsets an alluvial fan and small inset terraces along a well‐preserved fault scarp. A trench excavated on a terrace that is vertically offset by ∼2.5 m exposes two main stratigraphic units separated by an erosional unconformity. The coarse‐grained upper unit is deformed by three east‐vergent folds (F1–F3). Retrodeformation of these folds yields total displacements of ∼2.0 m, ∼2.4 m, and ∼0.5 m on the underlying fault splays, respectively. The displacement of ∼2.0 m recorded by fold F1 is interpreted as the result of the fault rupture that caused the 1861 earthquake. F2 and F3 were presumably generated during the penultimate event with an inferred magnitude of Mw∼7.0, although formation during two distinct ruptures cannot be excluded. Finite‐element modeling shows that coseismic folding above the tip of a blind thrust fault is a physically plausible mechanism to generate these folds. A published luminescence age of 770±76 years, which is interpreted to date the formation of the deformed terrace, indicates that the two (or possibly three) scarp‐forming events occurred during the last ∼800 years. The fine‐grained sediments below the erosional unconformity—that contain evidence for at least one older earthquake—are dated at ∼12 kyr. Our results indicate that elastic strain energy, which is accumulating at the front of the Precordillera today as shown by Global Positioning System (GPS) data, was repeatedly released during earthquakes on the La Cal fault in the past. Hence, the La Cal thrust fault poses a serious threat to the city of Mendoza.
The North Tehran Fault (NTF) is located at the southernmost piedmont of
Central Alborz and crosses the northern suburbs of the Tehran metropolis
and adjacent cities, where ˜15 million people live. Extending over
a length of about 110 km, the NTF stands out as a major active fault and
represents an important seismic hazard for the Iranian capital after
historical seismicity. In order to characterize the activity of the NTF
in terms of kinematics, magnitude and recurrence intervals of
earthquakes, we carried out a first paleoseismological study of the
fault within its central part between Tehran and Karaj cities. We opened
a trench across a 3 m-high fault scarp affecting Quaternary deposits.
Our study shows that the scarp is the result of repeated events along a
main N115°E trending shallow dipping thrust fault, associated with
secondary ruptures. From the trench analysis and Infrared Stimulated
Luminescence (IRSL) dating of fault-related sediments, we interpreted
between 6 and 7 surface-rupturing events that occurred during the past
30 kyrs. Their magnitudes (estimated from the displacements along the
faults) are comprised between 6.1 and 7.2. The two last events - the
largest - occurred during the past 7.9 ± 1.2 ka, which yields a
Holocene slip rate of ˜0.3 mm/yr. The 7 earthquakes scenario
suggests a regular periodicity with a mean recurrence interval of
˜3.8 kyrs. However, the two most recent events could correspond to
the two largest historical earthquakes recorded in the area (in 312-280
B.C. and 1177 A.D.), and therefore suggest that the NTF activity is not
regular.
The Sierra Madre fault, along the southern flank of the San Gabriel Mountains in the Los Angeles region, has failed in magnitude
7.2 to 7.6 events at least twice in the past 15,000 years. Restoration of slip on the fault indicated a minimum of about 4.0
meters of slip from the most recent earthquake and suggests a total cumulative slip of about 10.5 meters for the past two
prehistoric earthquakes. Large surface displacements and strong ground motions resulting from greater than magnitude 7 earthquakes
within the Los Angeles region are not yet considered in most seismic hazard and risk assessments.
The Kobe earthquake of January 16, 1995, is one of the most damaging earthquakes in the recent history of Japan. This earthquake is also called “The Hyogo-ken Nanbu (Southern part of Hyogo prefecture) earthquake,” and the disaster caused by it is referred to as “Hanshin Daishinsai (A major earthquake disaster in the Osaka-Kobe area).” As of January 29, 1995, the casualty toll reached 5,094 dead, 13 missing and 26,798 injured.
This article presents some background on the earthquake and its setting and a summary of some preliminary seismological results obtained by various investigators.
The age of surface uplift in southeastern Tibet is currently unknown, but the initiation of major river incision can be used as a proxy for the timing of initial uplift. The topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated, low-relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern Tibetan Plateau. Thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca. 100 and ca. 10-20 Ma and a change to rapid cooling after ca. 13 Ma with initiation of rapid river incision at 0.25-0.5 mm/yr between 9 and 13 Ma. A rapid increase in mean elevation of eastern Tibet beginning at this time supports tectonic-climate models that correlate the lateral (eastern) expansion of high topography in Tibet with the late Miocene intensification of the Indian and east Asian monsoons.