Large Continental Earthquakes Research Articles

The dynamic threat from landslides following large continental earthquakes.

Earthquake-triggered landslides show three important characteristics: they are often responsible for a considerable proportion of the damage sustained during mountain region earthquakes, they are non-randomly distributed across space, and they continue to evolve in the years after the earthquake. Despite this, planning for future earthquakes rarely takes into consideration either landslides or their evolution with time. Here we couple a unique timeseries of mapped landslides between 2014-2020 across the area of Nepal impacted by the 2015 Mw 7.8 Gorkha earthquake and a numerical landslide runout model overlain with building locations to examine how the distributions of both evolving landslide hazard and exposure intersect to generate a dynamic threat to buildings. The threat from landslide runout is shown to change in predictable ways after the earthquake, becoming more pronounced at mid- and lower-hillslope positions and remaining in the landscape for multiple years. Using the positions of our mapped landslides as a starting point, we can identify a priori the locations of 78% of buildings that were subsequently impacted by landslide debris. We show that landslide exposure and hazard vary from negligible to high, in relative terms, over lateral distances of as little as 10s of m. Our findings hold important implications for guiding reconstruction and for taking steps to reduce the risks from future earthquakes.

Improving Early Estimates of Large Earthquake's Final Fault Lengths and Magnitudes Leveraging Source Fault Structural Maturity Information

AbstractEarthquake early warning would be improved if the final size of an ongoing earthquake could be predicted early in the rupture process. Previous research relies largely on parameters derived from seismic waveforms, resulting in widely varied estimates of how much of the rupture must be complete before final sizes can be predicted. We demonstrate here that incorporating prior information on along‐strike variation in source fault structural maturity helps improve constraints on the earthquake's final size. Using surface slip profiles from 26 large continental earthquakes worldwide, we derive a generic empirical relation between earthquake slip, rupture length, and along‐strike variability in source fault maturity. Using this equation, we fit successive slip profiles developing during each earthquake and find that, on average, we predict the final length and magnitude of an earthquake once it has reached ~20% of its total length. Our findings thus demonstrate a clear determinism in the rupture process.

Magnitude-Dependent Transient Increase of Seismogenic Depth

AbstractThe thickness of the seismogenic zone in the Earth’s crust plays an important role in seismotectonics, affecting fault-system architecture and relative fault activity, earthquake size and distribution within a fault system, as well as long-term accumulation of tectonic deformation. Within the last two decades, several studies have revealed that aftershocks of large continental earthquakes may occur below the background depth of the seismogenic zone, that is, below the seismic–aseismic transition zone. This observation may be explained with a strain- and strain-rate-induced shift in rheological behavior that follows large mainshocks, transiently changing the deformation style below the seismogenic zone from incipient ductile to seismically brittle failure. As large earthquakes transiently deepen the seismic–aseismic transition zone, it is plausible to assume that larger mainshocks may cause stronger deepening than smaller mainshocks. Corresponding observations, however, have not yet been reported. Here, we use well-located seismic catalogs from Alaska, California, Japan, and Turkey to analyze if mainshock size positively correlates with the amount of transient deepening of the seismic–aseismic transition zone. We compare the depths of background seismicity with aftershock depths of 16 continental strike-slip earthquakes (6≤M≤7.8) and find that large mainshocks do cause stronger transient deepening than moderate-size mainshocks. We further suggest that this deepening effect also applies to the mainshocks themselves, with larger mainshock coseismic ruptures being capable of extending deeper into the normally aseismic zone. This understanding may help address fundamental questions of earthquake-source physics such as the assumed scale invariance of earthquake stress drop and whether fault-slip scales with rupture length or rupture width.

Coseismic Displacements and Surface Fractures from Sentinel-1 InSAR: 2019 Ridgecrest Earthquakes

Abstract Interferometric Synthetic Aperture Radar is an important tool for imaging surface deformation from large continental earthquakes. Here, we present maps of coseismic displacement and strain from the 2019 Ridgecrest earthquakes using multiple Sentinel-1 images. We provide three types of interferometric products. (1) Standard interferograms from two look directions provide an overview of the deformation and can be used for modeling coseismic slip. (2) Phase gradient maps from stacks of coseismic interferograms provide high-resolution (∼30 m) images of strain concentration and surface fracturing that can be used to guide field surveys. (3) High-pass filtered, stacked, unwrapped phase is decomposed into east–west and up–down, south–north components and is used to determine the sense of fault slip. The resulting phase gradient maps reveal over 300 surface fractures, including triggered slip on the Garlock fault. The east–west component of high-pass filtered phase reveals the polarity of the strike-slip offset (right lateral or left lateral) for many of the fractures. We find a small number of fractures that have slip polarity that is retrograde to the background tectonic stress. This is similar to observations of retrograde slip observed near the 1999 Mw 7.1 Hector Mine rupture, but the Ridgecrest observations are more completely imaged by the frequent and high-quality acquisitions from the twin Sentinel-1 spacecrafts. Determining whether the retrograde features are triggered slip on existing faults, or compliant fault deformation in response to stress changes from the Ridgecrest earthquakes, or new Coulomb-style failures, will require additional field work, modeling, and analysis.

A composite rupture model for the great 1950 Assam earthquake across the cusp of the East Himalayan Syntaxis

Although the M=w8.7, 1950 Assam earthquake endures as the largest continental earthquake ever recorded, its exact source and mechanism remain contentious. In this paper, we jointly analyze the spatial distributions of reappraised aftershocks and landslides, and provide new field evidence for its hitherto unknown surface rupture extent along the Mishmi and Abor Hills. Within both mountain fronts, relocated aftershocks and fresh landslide scars spread over an area of ≈330 km by 100 km. The former are more abundant in the Abor Hills while the later mostly affect the front of the Mishmi Hills. We found steep seismic scarps cutting across fluvial deposits and bounding recently uplifted terraces, some of which less than two thousand years or even a couple centuries old, at several sites along both mountain fronts. They likely attest to a minimum 200 km-long 1950 surface rupture on both the Mishmi and Main Himalayan Frontal Thrusts (MT and MFT, respectively), crossing the East Himalayan Syntaxis. At two key sites (Wakro and Pasighat), co-seismic surface throw appears to have been over twice as large on the MT as on the MFT (7.6 ± 0.2 m vs. >2.6 ± 0.1 m), in keeping with the relative, average mountain heights (3200 m vs. 1400 m), mapped landslide scar numbers (182 vs. 96), and average thrust dips (25–28° vs. 13–15°) consistent with relocated aftershocks depths. Corresponding average slip amounts at depth would have been ≈17 and ≈11 m on the MT and MFT, respectively, while surface slip at Wakro might have reached ≈34 m. Note that this amount of superficial slip would be out of reach using classic paleo-seismological trenching to reconstruct paleo-earthquake history. Most of the 1950 first arrivals fit with a composite focal mechanism co-involving the two shallow-dipping thrust planes. Their intersection lies roughly beneath the Dibang Valley, implying forced slip parallel to GPS vectors across the East Himalayan Syntaxis. Successive, near-identical, terrace uplifts at Wakro suggest near-characteristic slip during the last two surface rupturing earthquakes, while terrace boulder ages may be taken to imply bi-millennial return time for 1950-size events. As in Nepal, East-Himalayan mega-quakes are not blind and release most of the elastic, interseismic shortening that accumulates across the range.

Dual control of fault intersections on stop-start rupture in the 2016 Central Italy seismic sequence

Large continental earthquakes necessarily involve failure of multiple faults or segments. But these same critically-stressed systems sometimes fail in drawn-out sequences of smaller earthquakes over days or years instead. These two modes of failure have vastly different implications for seismic hazard and it is not known why fault systems sometimes fail in one mode or the other, or what controls the termination and reinitiation of slip in protracted seismic sequences. A paucity of modern observations of seismic sequences has hampered our understanding to-date, but a series of three Mw>6 earthquakes from August to November 2016 in Central Italy represents a uniquely well-observed example. Here we exploit a wealth of geodetic, seismological and field data to understand the spatio-temporal evolution of the sequence. Our results suggest that intersections between major and subsidiary faults controlled the extent and termination of rupture in each event in the sequence, and that fluid diffusion, channelled along these same fault intersections, may have also determined the timing of rupture reinitiation. This dual control of subsurface structure on the stop-start rupture in seismic sequences may be common; future efforts should focus on investigating its prevalence.

Omori‐like decay of postseismic velocities following continental earthquakes

AbstractVarious mechanisms have been proposed to explain the transient, enhanced surface deformation rates following earthquakes. Unfortunately, these different mechanisms can produce very similar surface deformation patterns leading to difficulty in distinguishing between them. Here we return to the observations themselves and compile near‐field postseismic velocity measurements following moderate to large continental earthquakes. We find that these velocities have a remarkably consistent pattern, with velocity inversely proportional to time since the earthquake. This suggests that postseismic velocities show an Omori‐like decay and that postseismic displacements increase logarithmically over time. These observations are inconsistent with simple, linear Maxwell or Burgers body viscoelastic relaxation mechanisms but are consistent with rate‐and‐state frictional afterslip models and power law shear zone models. The results imply that near‐field postseismic surface deformation measurements are primarily the result of fault zone processes and, therefore, that the inference of lower crustal viscosities from near‐field postseismic deformation requires care.

Location of largest earthquake slip and fast rupture controlled by along‐strike change in fault structural maturity due to fault growth

AbstractEarthquake slip distributions are asymmetric along strike, but the reasons for the asymmetry are unknown. We address this question by establishing empirical relations between earthquake slip profiles and fault properties. We analyze the slip distributions of 27 large continental earthquakes in the context of available information on their causative faults, in particular on the directions of their long‐term lengthening. We find that the largest slips during each earthquake systematically occurred on that half of the ruptured fault sections most distant from the long‐term fault propagating tips, i.e., on the most mature half of the broken fault sections. Meanwhile, slip decreased linearly over most of the rupture length in the direction of long‐term fault propagation, i.e., of decreasing structural maturity along strike. We suggest that this earthquake slip asymmetry is governed by along‐strike changes in fault properties, including fault zone compliance and fault strength, induced by the evolution of off‐fault damage, fault segmentation, and fault planarity with increasing structural maturity. We also find higher rupture speeds in more mature rupture sections, consistent with predicted effects of low‐velocity damage zones on rupture dynamics. Since the direction(s) of long‐term fault propagation can be determined from geological evidence, it might be possible to anticipate in which direction earthquake slip, once nucleated, may increase, accelerate, and possibly lead to a large earthquake. Our results could thus contribute to earthquake hazard assessment and Earthquake Early Warning.

Stress rise precursor to earthquakes in the Tibetan Plateau

Earthquake prediction thus far has proven to be a very difficult task, but changes in situ stress appear to offer a viable approach for forecasting large earthquakes in Tibet and perhaps other continental regions. High stress anomalies formed along active faults before large earthquakes and disappeared soon after the earthquakes occurred in the Tibetan Plateau. Principle stress increased up to ~2 - 5 times higher than background stress to form high stress anomalies along causative faults before the Ms 8.1 West Kunlun Pass earthquake in November 2001, Ms 8.0 Wenchuan earthquake in May 2008, Ms 6.6 Nimu earthquake in October 2009, Ms 7.1 Yushu earthquake in April 2010 and the Ms 7.0 Lushan earthquake in April 2013. Stress near the epicenters rapidly increased 0.10 - 0.12 MPa over 45 days, ~8 months before the Ms 6.6 Nimu earthquake occurred. The high principle stress anomalies decreased quickly to the normal stress state in ~8 - 12 months after the Ms 8.1 West Kunlun Pass and the Ms 8.0 Wenchuan earthquakes. These high stress anomalies and their demise appear directly related to the immediate stress rise along a fault prior to the earthquakes and the release during the event. Thus, the stress rise appears to be a viable precursor in prediction of large continental earthquakes as in the Tibetan Plateau.

Deep infrasound radiated by the Sumatra earthquake and tsunami

Infrasound arrays in the Pacific and Indian oceans that are part of the International Monitoring System (IMS) of the Comprehensive Nuclear Test Ban Treaty (CTBT) recorded distinct signatures associated with the 26 December 2004 Sumatra earthquake (M/9, http://earthquake.usgs.gov/) and tsunami. Although the radiation of infrasound from large continental earthquakes is established [e.g., Le Pichon et al., 2003], the results presented in the present article indicate that islands undergoing significant surface displacements from submarine earthquakes can produce infrasound.Far more intriguing is the possibility that the initiation and propagation of a tsunami may produce low‐frequency sound near the source as well as along coastlines and basins. Since distant sound effectively propagates at ˜300 m/s and tsunamis propagate at ˜200 m/s, precursory sound could potentially be used as a discriminant for tsunami genesis.

The 2002 Denali Fault and 2001 Kunlun Fault Earthquakes: Complex Rupture Processes of Two Large Strike-Slip Events

We studied the source processes of two large continental earthquakes, the 3 November 2002 Denali fault earthquake and the 14 November 2001 Kunlun fault earthquake, associated with strike-slip faulting along ancient sutures. We inverted teleseismic P waveforms using a pulse-stripping method for multiple time windows with different focal mechanisms and derived composite source models. According to our results, the 2002 Denali fault earthquake began with initial thrusting ( M w 7.3) along a 40-km-long segment of the north-northwest-dipping Susitna Glacier fault and later ruptured a 300-km-long segment along the Denali and Totschunda faults with a right-lateral strike-slip mechanism ( M w 7.7). In contrast, the 2001 Kunlun fault earthquake nucleated near an extensional step-over with a subevent pair consisting of 30-km-long strike-slip ( M w 6.9) event and 40-km-long normal ( M w 6.8) faulting event and later ruptured a 350-km-long segment along the Kunlun fault with a left-lateral strike-slip mechanism ( M w 7.7). Both earthquakes propagated primarily unilaterally to the east and released most of their energy along slip patches (asperities) far from the hypocenter locations. We find that both the Denali fault and Kunlun fault earthquakes had high-average rupture velocities of 3.2 km/sec and 3.4 km/sec, respectively. We also compared the source properties of these two earthquakes with other strike-slip earthquakes. For scaling purposes, large strike-slip earthquakes were classified as interplate, oceanic intraplate, or continental intraplate events. By using this classification the Denali fault and Kunlun fault earthquakes have an interplate signature that suggests overall weak faulting.

Recent crustal deformation in the Antofagasta region(northern Chile) and the subduction process

New neotectonic observations, along with a detailed aerial photograph analysis, allow a new interpretation of the recent tectonic behaviour of the outer forearc in northern Chile between 22.5°S and 24.5°S (Antofagasta region). Both the Coastal Cordillera and the Mejillones Peninsula are under E–W extension. Normal faults dipping east with an almost N–S orientation are predominant. Large-scale Neogene to Recent deformation is characterized by vertical uplift and subsidence related to normal faulting. Recent kinematics along the Atacama Fault System, including the Atacama Fault itself, are controlled by the regional extensional stress regime, and the predominant component of recent displacement along the Atacama Fault is vertical (normal). However, strike-slip components are also observed, but with moderate offsets, no larger than one to two tens of metres. The sense of shear, and the ratio between vertical and horizontal offsets, vary coherently with the fault azimuth, compatible with E–W extension. Oblique-slip faulting, with normal and left-lateral offsets of similar amplitude, is observed on the most northeasterly oriented segment of the Atacama Fault. This small left-lateral component of displacement and the strike-slip motion predicted by models of strain partitioning of the convergence along the Atacama Fault are in opposite senses. This occurs because the stress regime in the coastal region is not compressional but extensional. Very fresh recent ruptures are numerous, especially between 23°S and 24°S, and provide clear indications of the occurrence of moderate to large continental earthquakes during the Late Quaternary. As pointed out by several authors, indirect lines of evidence suggest active subduction erosion and underplating along the margin of northern Chile. We propose that part of the long-term extension is due to the broad-scale flexure of the outer forearc. This flexure may be controlled by offshore subsidence caused by subduction erosion near the trench, and by onshore uplift related to the underplating of eroded low-density material beneath the Coastal Cordillera. Neotectonic observations show that the state of stress in the outer forearc remains extensional (in a deviatoric sense) in the long term. However, it is proposed that the state of stress has a strong time-dependent component, which is intimately linked to the subduction seismic cycle. The Mw=8.0 Antofagasta earthquake of 1995 July 30 showed that large subduction earthquakes produce E–W extension in the coastal region. The overall E–W deviatoric extension in the outer forearc should be reduced by interseismic contraction in the period separating two subduction earthquakes. This reduction means that continental faults will remain locked and aseismic in the interseismic period of the subduction cycle, in agreement with microseismic observations, which indicate an absence of shallow crustal seismicity. We assume that the amount of extension produced by subduction earthquakes in the coastal area is larger than the cumulative interseismic contraction. This occurs because the displacement field generated by subduction earthquakes is concentrated in the outer forearc. The E–W extension should grow over prepeated subduction seismic cycles and lead to large continental earthquakes occurring within the overriding block, simultaneous with large subduction events. We show new surface ruptures associated with normal faulting along the Atacama Fault, which were most probably triggered by the 1995 Antofagasta subduction earthquake.

Preliminary study of the uppermantle P wave velocity structure beneath central Asia

The travel-times of multi-bounce P waves have been used to investigate the lateral heterogeneity of the uppermantle P wave velocity structure beneath north-west part of China and middle Asia. The synthetic seismograms of P and PP waves for seven large Chinese continental earthquakes were calculated by using the WKBJ method. The PP-P residuals, which reflect the P wave structure of the midpoints of the seismic rays, were calculated by using the WKBJ method, The results show that: the uppermantle wave structure beneath Russian platform in the middle Asia area, the Seytho-Turanian and the west Siberian platform is similar to that of Canadian shield; the uppermantle P wave velocity in the fold system of west China, the center Asia fold system in Russia, and Kazak fold system is lower than that of Canadian sheild. It seems that the velocity structure is related to the tectonic structure.

ON RELATIVITY OF SEISMICITY BETWEEN NORTH CHINA AND JAPANESE ISLANDS AND THEIR TECTONIC ENVIRONMENT

From the earthquake records of about 2, 000 years, it can be seen that North China, Korea, and Japanese Islands are associated with seismicity. During this period there are several seismically active periods of about 200 years, and each period can be subdivided into several seismically active episodes. In 1966, the seventh episode of the present active period was initiated and four strong earthquakes (MS≥7.0) have occurred in North China. Before or after these events, noticeable anomalies emerged at several places, which are called regional stress windows. Eight (including six of seismicity, one of crustal deformation, and one of radon) are discovered. To our surprise, remarkable anomalies appeared at all the windows before the Japan Sea (or Akita-Oki) earthquake of 1983. It does not seem to be a mere coincidence. For a unified explanation of the phenomena of the episode and windows, the author presented the multiple stress concentration hypothesis in 1979. The locally stress-concentrated points would develop either into an earthquake focus or into a window. There seems to be a close tectonic relation between the eastern China and the island arc system. In terms of geological and geophysical data, this region can be divided into several laths (long and narrow pieces of plate), stretching along a co-latitudinal direction. One of them is the North China-Korea-Honshu, Japan lath, which is a seismically active lath with large continental earthquakes. The configuration of the lath appears to be closely related to the geodynamic process.