Aftershock Catalog Research Articles

Implications of Fault‐Valve Behavior From Immediate Aftershocks Following the 2023 Mj6.5 Earthquake Beneath the Noto Peninsula, Central Japan

AbstractThe Mj6.5 (Mw6.2) event that occurred on 5 May 2023 near the northern shoreline of the northeastern tip of the Noto Peninsula, central Japan, is the largest event to date in a long‐lasting, intense earthquake swarm. Here we have created a more precise aftershock catalog associated with the 2023 Mj6.5 and the second‐largest 2022 Mj5.4 sequence to understand the rupture process of this largest earthquake. Most of the aftershocks are aligned along a ∼45° SE‐dipping plane. The mainshock initially ruptured the same deep section of the fault zone that had been ruptured by the 2022 Mj5.4 event, before propagating rapidly to shallow depths and to offshore along the ruptured fault plane. The aftershock front migrated at a speed of ∼20 km/hr. This rapid upward migration of the immediate aftershocks might be driven by upwelling of crustal fluids along the intensely fractured and permeable fault zone via mainshock dynamic rupture.

High-resolution seismicity imaging and early aftershock migration of the 2023 Kahramanmaraş (SE Türkiye) MW7.9 & 7.8 earthquake doublet

We build a high-resolution early aftershock catalog for the 2023 SE Türkiye seismic sequence with PALM, a seamless workflow that sequentially performs phase picking, association, location, and matched filter for continuous data. The catalog contains 29,519 well-located events in the two mainshocks rupture region during 2023-02-01–2023-02-28, which significantly improves the detection completeness and relocation precision compared to the public routine catalog. Employing the new PALM catalog, we analyze the structure of the seismogenic fault system. We find that the Eastern Anatolian Fault (EAF) that generated the first MW7.9 mainshock is overall near-vertical, whereas complexities are revealed in a small-scale, such as subparallel subfaults, unmapped branches, and stepovers. The seismicity on EAF is shallow (<15 km) and concentrated in depth distribution, indicating a clear lock-creep transition. In contrast, the Sürgü Fault (SF) that is responsible for the second MW7.8 mainshock is shovel-shaped for the nucleation segment and has overall low dip angles (∼40°–80°). Aftershocks on the SF distribute in a broad range of depth, extending down to ∼35 km. We also analyze the temporal behavior of seismicity, discovering no immediate foreshocks within ∼5 days preceding the first mainshock, and no seismic activity on the SF before the second mainshock.

Precursory Tidal Triggering of the 2011 Mw 9.0 Tohoku Aftershocks

Precursory earth tidal triggering is believed to influence earthquake timing preferentially when a region is critically stressed. However, whether and how the recurrence of aftershocks after a giant earthquake is affected by tidal triggering remains perplexing. To provide insight into this study, we utilized the Schuster test to explore the tidally induced stress variation correlated with the 2011 Mw 9.0 Tohoku earthquake aftershock sequence by determining the tidal phase angle at the occurrence time of events and the periodic characteristics of the aftershocks. Our results show that the aftershocks were triggered by short-period tides, including semidiurnal and diurnal tides. The rupture associated with the mainshock likely resulted in a critical stress state in the focal region, which is conducive to tidal triggering. We subdivided the aftershock catalog into several subsets, using a depth of 30 km and a magnitude of 5 as discriminators. The analysis of these subsets reveals that weaker and deeper earthquakes are best correlated with Earth tides, which will be helpful to investigate the mechanisms of tidal correlation.

Detecting and Locating Aftershocks for the 2020 Mw 6.5 Stanley, Idaho, Earthquake Using Convolutional Neural Networks

Abstract Our study is to build an aftershock catalog with a low magnitude of completeness for the 2020 Mw 6.5 Stanley, Idaho, earthquake. This is challenging because of the low signal-to-noise ratios for recorded seismograms. Therefore, we apply convolutional neural networks (CNNs) and use 2D time–frequency feature maps as inputs for aftershock detection. Another trained CNN is used to automatically pick P-wave arrival times, which are then used in both nonlinear and double-difference earthquake location algorithms. Our new one-month-long catalog has 4644 events and a completeness magnitude (Mc) 1.9, which has over seven times more events and 0.9 lower Mc than the current U.S. Geological Survey National Earthquake Information Center catalog. The distribution and expansion of these aftershocks improve the resolution of two north-northwest-trending faults with different dip angles, providing further support for a central stepover region that changed the earthquake rupture trajectory and induced sustained seismicity.

Spatiotemporal Aftershock Evolution of the 2014 M 6.4 and 5.9 Kangding Double Shocks in Sichuan, Southwestern China

Abstract We apply the graphics processing unit-based match and locate method to detect missing small aftershocks of the 2014 M 6.4 and 5.9 Kangding double shocks (hereafter KD1 and KD2, respectively) in Sichuan, southwestern China. KD1 and KD2 occurred on two adjacent branch faults of the Xianshuihe fault zone, respectively. We use the relocated routine catalog events as templates to scan through the continuous waveforms from KD1 to approximately 5 days after KD2. Approximately 7.8 times more aftershocks than the routine catalog are obtained. Our results show that the aftershocks occurring between the two mainshocks first expanded northwestward along the KD1 seismogenic fault and then migrated southeastward to the KD2 seismogenic fault. This feature may reflect the stress transfer process after KD1, which may represent the triggering process of KD1 to KD2. These aftershocks also show minor migration with logarithmic time in the along-strike and up-dip directions, which suggests that the afterslip may occur between the two mainshocks and improve the stress level around KD2 to some extent. Based on the improved aftershock catalog, we also calculate the b-values within different space–time windows in the aftershock zone. A relatively low b-value around the KD2 nucleation point after KD1 and before KD2 is observed, which may indicate that this area contained a high stress level and might be in the critical state of rupture, in which KD2 occurred at this particular moment. Our study suggests that after a moderate-to-large earthquake a more complete aftershock catalog can be obtained by detecting microearthquakes with which the triggering processes of subsequent mainshocks could be revealed; based on the detailed spatiotemporal distribution of aftershocks and b-values, the subsequent moderate-to-large earthquake hazard could be assessed to some extent.

An Earthquake Detection and Location Architecture for Continuous Seismograms: Phase Picking, Association, Location, and Matched Filter (PALM)

Abstract We developed an earthquake detection and location architecture for continuous seismograms that incorporates phase picking, phase association, location, and matched-filter techniques (PALM). The PALM architecture incorporates two modules: (1) PAL, the initial detection following picking, association, and location processes, and (2) match, expand, shift, and stack (MESS), a matched-filter detector that augments the template catalog. The effectiveness of PALM is demonstrated in building an early aftershock catalog for the 2019 Ridgecrest, California, earthquake. By comparing with Southern California Seismic Network arrival times, we show that the PAL picker combines the strengths of short-term average/long-term average and the kurtosis picker, realizing robust phase detection and precise picking. Our final MESS catalog is compared with two other matched-filter catalogs by Ross, Idini, et al. (2019) and Shelly (2020). We find that PALM directly recovers unbiased and detailed features in seismicity from continuous seismograms, which can be efficiently implemented to scan continuous waveforms without the need for visual inspection.

Repeating Earthquakes at the Edge of the Afterslip of the 2016 Ecuadorian MW7.8 Pedernales Earthquake

AbstractRepeating earthquakes repeatedly rupture the same seismic asperity and are strongly linked to aseismic slip. Here, we study the repeating aftershocks of the April 16, 2016 MW 7.8 Pedernales earthquake in Ecuador, which generated a large amount of afterslip. Using temporary and permanent stations, we correlate waveforms from a one‐year catalog of aftershocks. We sort events with a minimum correlation coefficient of 0.95 into preliminary families, which are then expanded using template‐matching to include events from April 2015 to June 2017. In total, 376 repeaters are classified into 62 families of 4–15 events. They are relocated, first using manual picks, and then using a double difference method. We find repeating earthquakes during the whole period, occurring primarily within large aftershock clusters on the edges of the areas of largest afterslip release. Their recurrence times, shortened by the mainshock, subsequently increase following an Omori‐type law, providing a timeframe for the afterslip's deceleration. Although they are linked temporally to the afterslip, repeater‐derived estimates of slip differ significantly from GPS‐based models. Combined with the fact that repeaters appear more spatially correlated with the afterslip gradient than with the afterslip maxima, we suggest that stress accumulation at the edge of the afterslip may guide repeater behavior.

Seismicity in the upper plate of the Northern Chilean offshore forearc: Evidence of splay fault south of the Mejillones Peninsula

The region around the Mejillones Peninsula is characterized by major onshore and offshore fault systems. The existence of major splay faults in these fault systems has been debated. To improve our undestanding of seismotectonic processes affecting the MP, we built a new catalog of aftershocks of the 1995 Antofagasta earthquake (Mw 8.1) recorded by the CINCA seismic network. In addition, we build a three-dimensional tomographic model to determine the physical properties of the upper plate using arrival times from our new catalog combined with other earthquakes catalogs derived from several networks deployments across the MP. We detected seismicity in the outer wedge of the upper plate. The seismicity is partially defined by a clear lineament between 22 and 5 km depth that we defined as a splay fault. The focal mechanisms indicate a rupture plane with NE orientation and SE dip. The seismicity in the upper plate and splay fault are located in the areas in which the Vp and Vs velocities are decreased and the Vp/Vs ratio is increased, suggesting a fractured fault zone within the outer wedge of the upper plate. We propose the presence of fluids in the fracture fault zone within the outer wedge because the decrease in Vs larger than that of Vp and repeating earthquakes clusters were detected. Our observations present clear evidence of the activation of the splay fault in the outer wedge of the North Chilean offshore forearc.

On the Relationship between the Omori and Gutenberg–Richter Parameters in Aftershock Sequences

Abstract—The issues concerning the relationship between two self-similarity parameters—the Gutenberg–Richter b- and Omori p-values—in the aftershock sequences are explored. In the laboratory experiments, under fracture initiation in the rock by sharp jumps in the axial stress, a correlation between the p- and b-values is revealed in the fracture relaxation regimes similar to aftershocks. The correlation observed in the experiments on water-saturated sandstone samples with the preliminarily formed faults is negative and clearly pronounced. The correlation in the case of dry samples of migmatite and concrete proved to be positive but its statistical significance is lower than for the wet samples. The analysis of the literature data on detecting the connection between parameters p and b in the natural aftershock sequences shows that the reported results are heterogeneous. Some authors conclude that these parameters are connected and that both positive and negative correlation is noted between them. Other authors present evidence suggesting the absence of any correlation. Our study of the natural aftershocks based on the data of regional earthquake catalogs has shown that the statistical estimates of the Gutenberg–Richter and Omori parameters are fairly sensitive to the quality and homogeneity of the input data. The key factors affecting the estimation quality of these parameters are established, and the procedure for selecting the aftershock catalogs for subject analysis is developed. The results of statistical estimating the Gutenberg–Richter and Omori parameters in the aftershock processes in the regions with different types of the tectonic regimes—subduction zones and regions of shear transform faults—have shown that that the correlation of these parameters in the subduction zones can be positive and negative either. In the zones of the transform faults, the connection between these parameters is not detected. Our study generalizes K. Scholtz’s idea that the Omori law can be explained by the superimposition of the relaxation processes having different relaxation times. According to the generalized model, the different sign of the correlation between the self-similarity parameters in the aftershock processes correspond to the different relaxation mechanisms with different types of the dependence of the relaxation time on the “size” of the relaxator. It is currently unclear which particular mechanisms are implemented in the aftershock processes. The relationship between the Omori and Gutenberg–Richter parameters revealed by our laboratory experiments and field studies (positive correlation, negative correlation, or lack of correlation) may indicate the implementation of different relaxation mechanisms in some or other particular conditions.

3D Fault Structure Inferred from a Refined Aftershock Catalog for the 2015 Gorkha Earthquake in Nepal

ABSTRACT In this article, we created a well-resolved aftershock catalog for the 2015 Gorkha earthquake in Nepal by processing 11 months of continuous data using an automatic onset and hypocenter determination procedure. Aftershocks were detected by the NAMASTE temporary seismic network that is densely distributed covering the rupture area and became fully operational about 50 days after the mainshock. The catalog was refined using a joint hypocenter determination technique and an optimal 1D velocity model with station correction factors determined simultaneously. We found around 15,000 aftershocks with the magnitude of completeness of ML 2. Our catalog shows that there are two large aftershock clusters along the north side of the Gorkha–Pokhara anticlinorium and smaller shallow aftershock clusters in the south. The patterns of aftershock distribution in the northern and southern clusters reflect the complex geometry of the Main Himalayan thrust. The aftershocks are located both on the slip surface and through the entire hanging wall. The 1D velocity structure obtained from this study is almost constant at a P-wave velocity (VP) of 6.0 km/s for a depth of 0–20 km, similar to VP of the shallow continental crust.

Aftershock Analysis of the 2018 Mw 7.1 Anchorage, Alaska, Earthquake: Relocations and Regional Moment Tensors

AbstractThe 30 November 2018 magnitude 7.1 Anchorage earthquake occurred as a result of normal faulting within the lithosphere of subducted Yakutat slab. It was followed by a vigorous aftershock sequence with over 10,000 aftershocks reported through the end of July 2019. The Alaska Earthquake Center produced a reviewed aftershock catalog with a magnitude of completeness of 1.3. This well‐recorded dataset provides a rare opportunity to study the relationship between the aftershocks and fault rupture of a major intraslab event. We use tomoDD algorithm to relocate 2038 M≥2 aftershocks with a regional 3D velocity model. The relocated aftershocks extend over a 20 km long zone between 47 and 57 km depth and are primarily confined to a high VP/VS region. Aftershocks form two clusters, a diffuse southern cluster and a steeply west‐dipping northern cluster with a gap in between where maximum slip has been inferred. We compute moment tensors for the Mw&amp;gt;4 aftershocks using a cut‐and‐paste method and careful selection of regional broadband stations. The moment tensor solutions do not exhibit significant variability or systematic differences between the northern and southern clusters and, on average, agree well with the mainshock fault‐plane parameters. We propose that the mainshock rupture initiated in the Yakutat lower crust or uppermost mantle and propagated both upward into the crust to near its top and downward into the mantle. The majority of the aftershocks are confined to the seismically active Yakutat crust and located both on and in the hanging wall of the mainshock fault rupture.

Multi-segment rupture of the 2016 Amatrice-Visso-Norcia seismic sequence (central Italy) constrained by the first high-quality catalog of Early Aftershocks

We present the first high-quality catalog of early aftershocks of the three mainshocks of the 2016 central Italy Amatrice-Visso-Norcia normal faulting sequence. We located 10,574 manually picked aftershocks with a robust probabilistic, non-linear method achieving a significant improvement in the solution accuracy and magnitude completeness with respect to previous studies. Aftershock distribution and relocated mainshocks give insight into the complex architecture of major causative and subsidiary faults, thus providing crucial constraints on multi-segment rupture models. We document reactivation and kinematic inversion of a WNW-dipping listric structure, referable to the inherited Mts Sibillini Thrust (MST) that controlled segmentation of the causative normal faults. Spatial partitioning of aftershocks evidences that the MST lateral ramp had a dual control on rupture propagation, behaving as a barrier for the Amatrice and Visso mainshocks, and later as an asperity for the Norcia mainshock. We hypothesize that the Visso mainshock re-activated also the deep part of an optimally oriented preexisting thrust. Aftershock patterns reveal that the Amatrice Mw5.4 aftershock and the Norcia mainshock ruptured two distinct antithetic faults 3–4 km apart. Therefore, our results suggest to consider both the MST cross structure and the subsidiary antithetic fault in the finite-fault source modelling of the Norcia earthquake.

Evolution and Distribution of the Early Aftershocks Following the 2008 Mw 7.9 Wenchuan Earthquake in Sichuan, China

AbstractUsing seismic data recorded by a dense temporary seismic array, we apply a waveform matched‐filter technique and obtain an early aftershock catalog around the mainshock rupture zone of the 2008 Wenchuan, Sichuan, China, earthquake. Specifically, we use 1,273 events in a relocated aftershock catalog as templates and scan through the waveforms within 7 days following the mainshock. We obtain up to ∼5.3 and ∼10 times more aftershocks than the standard and relocated catalogs, respectively. We find that early aftershocks mostly occurred below 10 km depth, downdip to large coseismic slip areas. Early aftershocks also show minor along‐strike migration with time since the mainshock, indicating triggering of afterslip following the mainshock. From the epicentral region to Beichuan where the mainshock slip was primarily reverse faulting, early aftershocks illustrated the Yingxiu‐Beichuan and Guanxian‐Jiangyou faults as high‐angle listric feature and rooted to a gentle dipping plane at the depth of ∼20 km. The aftershocks in the northeast segment of the Longmen Shan faults mostly occurred along a near‐vertical dipping fault, consistent with the primary strike‐slip component of mainshock slip in this region. In addition, differences in early and long‐term aftershock distribution indicate significant postseismic deformation along the conjugate Xiaoyudong fault. Our results help to illuminate the deep geometry of the Longmen Shan fault zone and the early evolution after the Wenchuan mainshock, which provides additional constraints to the mechanism of plateau uplifting.

New Data on Aftershocks of the December 7, 1988, Spitak Earthquake

The paper addresses the collection and analysis of new data on aftershocks that occurred within 20 days of the main shock of the December 7, 1988, Spitak earthquake, Mw = 6.8. The data were used to improve the location of aftershock hypocenters and magnitudes. Available data concerning this 20-day period were the least reliable in terms of completeness, representativeness, and the accuracy of hypocenter location and, in particular, estimation of energy classes and magnitudes. New data were retrieved from the records and bulletins of the seismic stations of the regional and global networks. Hypocenter parameters were determined by means of the minimization of wave travel-time residuals and subsequent double-difference hypocenter relocation. Digital records of the Obninsk and Arti seismic stations (Δ = 15°–18°) and five more distant stations (Δ = 34°–53°) were used to more accurately estimate the surface-wave magnitude of the main shock and strongest aftershock. The aftershock catalog of the Spitak earthquake was substantially revised. First, the previous hypocenter locations (Aref’ev et al., 1991) were improved using the double-difference method; second, new data were retrieved from the bulletins of Caucasian seismic stations. The minimum magnitude of completeness (M c = 1.9) of the new catalog for the first 20 days after the main shock (when there were no epicentral observations) is the same as that for the period from December 7, 1988, to December 31, 1989. The new catalog contains information on 2090 aftershocks with magnitude M = 1.9 and more for the period from December 7, 1988, to December 31, 1989. The double-difference method allowed the location of the epicenters of clustered earthquakes to be reliably estimated with a longitude error of no more than 4.6 km, a latitude error of 4 km, and a depth error of 5 km. The new spatial distribution of the aftershock hypocenters is better correlated with the tectonic setting than the old data. The new catalog can be used to assess seismic hazard after strong earthquakes in the region.

Foreshocks and delayed triggering of the 2016 MW7.1 Te Araroa earthquake and dynamic reinvigoration of its aftershock sequence by the MW7.8 Kaikōura earthquake, New Zealand

We analyze the preparatory period of the September 2016 MW7.1 Te Araroa foreshock–mainshock sequence in the Northern Hikurangi margin, New Zealand, and subsequent reinvigoration of Te Araroa aftershocks driven by a large distant earthquake (the November 2016 MW7.8 Kaikōura earthquake). By adopting a matched-filter detection workflow using 582 well-defined template events, we generate an improved foreshock and aftershock catalog for the Te Araroa sequence (>8,000 earthquakes over 66 d). Templates characteristic of the MW7.1 sequence (including the mainshock template) detect several highly correlating events (ML2.5–3.5) starting 12 min after a MW5.7 foreshock. These pre-cursory events occurred within ∼1 km of the mainshock and migrate bilaterally, suggesting precursory slip was triggered by the foreshock on the MW7.1 fault patch prior to mainshock failure. We extend our matched-filter routine to examine the interactions between high dynamic stresses resulting from passing surface waves of the November 2016 MW7.8 Kaikōura earthquake, and the evolution of the Te Araroa aftershock sequence. We observe a sudden spike in moment release of the aftershock sequence immediately following peak dynamic Coulomb stresses of 50–150 kPa on the MW7.1 fault plane. The triggered increase in moment release culminated in a MW5.1 event, immediately followed by a ∼3 h temporal stress shadow. Our observations document the preparatory period of a major subduction margin earthquake following a significant foreshock, and quantify dynamic reinvigoration of a distant on-going major aftershock sequence amid a period of temporal clustering of seismic activity in New Zealand.

Spatiotemporal evolution of the 2011 Prague, Oklahoma, aftershock sequence revealed using subspace detection and relocation

AbstractThe 6 November 2011 Mw 5.7 earthquake near Prague, Oklahoma, is the second largest earthquake ever recorded in the state. A Mw 4.8 foreshock and the Mw 5.7 mainshock triggered a prolific aftershock sequence. Utilizing a subspace detection method, we increase by fivefold the number of precisely located events between 4 November and 5 December 2011. We find that while most aftershock energy is released in the crystalline basement, a significant number of the events occur in the overlying Arbuckle Group, indicating that active Meeker‐Prague faulting extends into the sedimentary zone of wastewater disposal. Although the number of aftershocks in the Arbuckle Group is large, comprising ~40% of the aftershock catalog, the moment contribution of Arbuckle Group earthquakes is much less than 1% of the total aftershock moment budget. Aftershock locations are sparse in patches that experienced large slip during the mainshock.

Mapping the rheology of the Central Chile subduction zone with aftershocks

AbstractThe postseismic deformation following a large (Mw &gt;7) earthquake is expressed both seismically and aseismically. Recent studies have appealed to a model that suggests that the aseismic slip on the plate interface following the mainshock can be the driving factor in aftershock sequences, reproducing both the geodetic (afterslip) and seismic (aftershocks) observables of postseismic deformation. Exploiting this model, we demonstrate how a dense catalog of aftershocks following the 2015 Mw 8.3 Illapel earthquake in Central Chile can constrain the frictional and rheological properties of the creeping regions of the subduction interface. We first expand the aftershock catalog via a 19 month continuous matched‐filter search and highlight the log‐time expansion of seismicity following the mainshock, suggestive of afterslip as the main driver of aftershock activity. We then show how the time history of aftershocks can constrain the temporal evolution of afterslip. Finally, we use our dense aftershock catalog to estimate the rate and state rheological parameter (a − b)σ as a function of depth and demonstrate that this low value is compatible either with a nearly velocity‐neutral friction (a≈b) in the regions of the megathrust that host afterslip, or an elevated pore fluid pressure (low effective normal stress σ) along the plate interface. Our results present the first snapshot of rheology in depth together with the evolution of the tectonic stressing rate along a plate boundary. The framework described here can be generalized to any tectonic context and provides a novel way to constrain the frictional properties and loading conditions of active faults.

Preliminary Report on the 22 November 2014Mw 6.1/Ms 6.3 Kangding Earthquake, Western Sichuan, China

Online Material: Relocated aftershock catalog figures of moment rate function, magnitude–time distribution, observed strong-motion accelerograms, spectral accelerations versus period, and photos of damaged buildings. The M s 6.3 ( M w 6.1) Kangding earthquake struck western Sichuan Province on 22 November at 08:55:27 UTC (16:55:27 p.m. local time). Its epicenter was located approximately 37 km northwest of Kangding country and 236 km southwest of Chengdu city. The China Earthquake Networks Center (CENC) reported that the epicenter was located at 30.3° N, 101.7° E with a hypocentral depth of 18 km (Fig. 1). Three days after the first shock, another moderate earthquake ( M s 5.8/ M w 5.7) hit the area to the southeast. The distance between the two events is about 10 km. Detailed information of this earthquake is available on the website of China Earthquake Administration (CEA) (http://www.cea.gov.cn/publish/dizhenj/468/553/101420/index.html; last accessed December 2014, in Chinese) and on the website of U.S. Geological Survey (http://earthquake.usgs.gov/earthquakes/eventpage/usb000syy0; last accessed December 2014). The Kangding earthquake was located near the left-lateral, strike-slip Xianshuihe fault system (Allen et al. , 1991). The mainshock was characterized by a strike-slip mechanism, with two nearly vertical nodal planes striking approximately north-northwest and northeast-east, respectively. The maximum intensity was VIII on the Chinese intensity scale (http://en.wikipedia.org/wiki/China\_Seismic\_Intensity_Scale; last accessed December 2014), with the areas above intensity VI being 11,060 km2. The earthquake caused 5 deaths and 80 injuries. The direct economic loss has been estimated as more than 4.2 billion Renminbi (RMB; or about $0.7 billion U.S.). Figure 1. Tectonic blocks and strong earthquakes since 1997 in the Tibetan plateau. Dashed rectangle indicates the region in Figure 2. Large red arrow indicates the crustal motion direction. Black-and-white circles are the focal mechanism of recent strong earthquakes with magnitude larger than M s 7.0. Red-and-white circles are the focal mechanism of the M s 6.3 and 5.8 Kangding …

Aftershock Observation and Analysis of the 2013Ms 7.0 Lushan Earthquake

Online Material: Figures of station deployment time and of spatial and temporal depiction of aftershocks, and tables of station information and aftershock catalogs. At 08:02 (local and Beijing time) on 20 April 2013, an earthquake of M s 7.0 ( M w 6.6) struck Lushan County in Sichuan Province, southwestern China (hereafter referred to as the Lushan earthquake). The Lushan earthquake resulted in casualties and severe damage to the buildings and to the economic activities of the region. The earthquake left 193 dead, up to 10,000 injured, and 25 missing. The direct economic loss hit over $1.6 billion U.S. The Lushan earthquake is the second destructive earthquake to have occurred in the southern segment of the Longmenshan fault zone since the 12 May 2008 M s 8.0 Wenchuan earthquake (Fig. 1). The distance between the epicenters of the Lushan earthquake and the Wenchuan earthquake is about 87 km (Fang, Wu, Wang, Lu, et al. , 2013). As happened following the Wenchuan earthquake, the occurrence of the Lushan earthquake also stimulated lots of discussions on the seismic risk potential and prediction in southwest China. In particular, whether it was a large aftershock of the Wenchuan earthquake has been heatedly debated (Chen et al. , 2013; Du et al. , 2013; J. Liu et al. , 2013; Wang et al. , 2013; Jia et al. , 2014). Figure 1. (a) Tectonic settings of the Longmenshan fault zone. Red and yellow stars indicate the epicenters of the Wenchuan and Lushan earthquakes, respectively. The focal mechanisms of the Lushan earthquake and Wenchuan earthquake are also shown. Red circles represent aftershocks of the Lushan and Wenchuan earthquakes. Black lines indicate the main faults. The inset map shows the location of the study region in the Chinese mainland. (b) The distribution of seismic stations in and around the epicentral region of the Lushan …

A High Resolution Aftershock Catalog of the Magnitude 7.5 Craig, Alaska, Earthquake on 5 January 2013

Abstract The M w 7.5 Craig, Alaska, earthquake on 5 January 2013 was the second largest earthquake on the Queen Charlotte fault (QCF) in 40 yrs, representing the transform plate boundary between the Pacific and North American plates. Earthquake hazard along the QCF was known from historical earthquakes, but the absence of major earthquakes in the digitally instrumented era has limited studies of earthquakes. Here, we present an analysis of aftershocks of the Craig earthquake. We first use waveform cross correlation and double‐difference relative relocation to relocate the aftershock catalog of the Alaska Earthquake Center (AEC). Our relocated catalog reveals a complex fault structure activated in response to the mainshock and provides insight into the faulting mechanics of this graduated transpressive plate boundary. We then use network‐matched filtering (NMF) to detect additional aftershocks. Using 351 aftershocks located by the AEC in 2013 as template events, we identify 1785 aftershocks through May 2013. This is a greater than fivefold increase in aftershock detection rate compared with the standard single‐event network processing. Utilizing regional seismic stations and waveform modeling, we are able to characterize the faulting parameters for 12 aftershocks. Six aftershocks show focal mechanisms consistent with the mainshock faulting parameters, and six aftershocks show either thrust or strike‐slip mechanisms with nodal planes rotated ∼45° from the mainshock mechanism. The six aftershocks that are inconsistent with the QCF have P axes that are perpendicular to the plate boundary, suggesting that transpression remains at this location. We show that five of these large aftershock families were activated within a day of the mainshock, whereas seven families were delayed by multiple days before activating. We show evidence for migrating patterns of seismic quiescence and elevated seismicity, relative to the predicted Omori–Utsu decay, which is related to the largest aftershock in the sequence. Online Material: Aftershock catalog of the 5 January 2013 Craig, Alaska, earthquake.