Aftershock Distribution Research Articles

Seismotectonic assessment of the High Atlas orogen: Implications for the 8 September Mw 6.8 El-Haouz earthquake

The Moroccan High Atlas is a slowly deforming intracontinental orogenic belt, characterized by moderate and diffuse seismic activity. The 8 September 2023 Mw 6.8 El-Haouz earthquake, one of the most powerful quakes recorded in North Africa, has intensified investigations into the seismotectonics of the High Atlas. This study examines seismotectonic patterns in the High Atlas using seismological and geodetic data to better understand the mechanisms driving such seismic events. Seismological data indicate active shortening in the region, contributing to ongoing mountain building. Present-day deformation is partitioned between thrust and strike-slip faulting, with NW-SE compression, consistent with the broader stress field in North Africa. Frequency-magnitude distribution analysis indicates that the Western High Atlas exhibited low b-values, slightly lower than 1, in the eight years preceding the El-Haouz earthquake, with an low b-value (∼ 0.8 ± 0.1) near the epicenter, suggesting high stress accumulation in the region. GNSS observations reveal that the High Atlas experiences low geodetic velocities compared with the Rif collision belt, with displacements below 1 mm/year. Notably, the axial zone of the Western High Atlas exhibits an uplift of 1.1 mm/yr. The combination of moderate shortening and relatively higher uplift prior to the El-Haouz earthquake suggests that present-day deformation in the Western High Atlas is predominantly accommodated by the reactivation of inherited faults in the axial zone. This is further corroborated by the distribution of aftershocks, which supports a steeply dipping seismogenic fault manifested by the Tizi n'Test Fault.

Kinematic Source Properties of the 2023 Mw 7.7 Türkiye Earthquake Inferred from Near-Fault Strong Ground Motions

Abstract The 2023 Mw 7.7 Türkiye earthquake provided densely observed near-fault ground-motion data, which enabled us to investigate their characteristics and generations. This study aimed to obtain an intuitive understanding of the characteristics of near-fault strong ground motions and their relationship with kinematic source properties using a simple source model. Synthetic ground motions were calculated using the discrete wave number method and compared with observed ground motions. The observed strong motions were used after a time correction based on the first P-wave arrival. Results showed that near-fault pulse-like velocities (0.02–0.6 Hz) were generally explained by the simple kinematic source model that assumed a continuous rupture propagation along the faults. Comparisons at individual sites indicate the various rupture properties of the fault segments. Near-fault ground motions along the Narli segment were explained by the fault plane along the surface trace and not by aftershock distribution. A supershear rupture propagation speed is necessary for a part of the Amanos segment. Rupture stops and restarts were implied near the stepover along the Amanos segment. Empirical site factors estimated by generalized inversion techniques did not show peaks around the frequency of observed large velocity pulses, indicating that the velocity pulses were mainly generated by the source. The model bias and uncertainties were also examined to supplement the simplified subsurface structure used in the simulation. Our results will help to associate the qualitative and quantitative aspects of the kinematic source process with near-fault strong ground motions.

Focal mechanics and disaster characteristics of the 2024 M 7.6 Noto Peninsula Earthquake, Japan

On January 1, 2024, a devastating M 7.6 earthquake struck the Noto Peninsula, Ishikawa Prefecture, Japan, resulting in significant casualties and property damage. Utilizing information from the first six days after the earthquake, this article analyzes the seismic source characteristics, disaster situation, and emergency response of this earthquake. The results show: 1) The earthquake rupture was of the thrust type, with aftershock distribution showing a north-east-oriented belt-like feature of 150 km. 2) Global Navigation Satellite System (GNSS) and Interferometric synthetic aperture radar (InSAR), observations detected significant westward to north-westward co-seismic displacement near the epicenter, with the maximum horizontal displacement reaching 1.2 m and the vertical uplift displacement reaching 4 m. A two-segment fault inversion model fits the observational data well. 3) Near the epicenter, large Peak Ground Velocity (PGV) and Peak Ground Acceleration (PGA) were observed, with the maxima reaching 145 cm/s and 2681 gal, respectively, and the intensity reached the highest level 7 on the Japanese (Japan Meteorological Agency, JMA) intensity standard, which is higher than level 10 of the United States Geological Survey (USGS) Modified Mercalli Intensity (MMI) standard. 4) The observation of the very rare multiple strong pulse-like ground motion (PLGM) waveform poses a topic worthy of research in the field of earthquake engineering. 5) As of January 7, the earthquake had left 128 deaths and 560 injuries in Ishikawa Prefecture, with 1305 buildings completely or partially destroyed, and had triggered a chain of disasters including tsunamis, fires, slope failures, and road damage. Finally, this paper summarizes the emergency rescue, information dissemination, and other disaster response and management measures taken in response to this earthquake. This work provides a reference case for carrying out effective responses, and offers lessons for handling similar events in the future.

Relocation of the 2021 MW 7.4 Maduo, Qinghai, China earthquake sequence and implications for seismogenic structure

An MW 7.4 earthquake struck Maduo County, Qinghai, China on 22 May 2021. To better understand the seismogenic structure of this region, we collect local earthquake arrival time data at the MAD station in the China Earthquake Networks Center observational bulletins during 1 June to 20 September 2021, and manually pick P and S wave arrival times from high-quality seismograms recorded at 34 recently deployed MaduoArray portable seismic stations. Using these arrival times, we relocate the Maduo earthquake sequence using earthquake association, absolute location and relative location methods. Our results show that the 2021 Maduo earthquake sequence occurred along the Kunlun Mountain Pass-Jiangcuo fault zone in the NWW-SEE direction and the aftershocks are located on both sides of the mainshock, showing characteristics of bilateral rupture. Vertical cross-sections of the aftershock distribution illustrate a nearly vertical shape of the seismogenic fault that tilts toward the northeast and southwest in different sections, reflecting a complex geometry of the fault plane. There is a horsetail bifurcation phenomenon at the eastern end of the fault zone. A sparse area of aftershocks appears at about 30 km east of the mainshock epicenter, which may be associated with a uniform fault friction and sufficient release of rupture energy caused by super-shear rupture of the mainshock. Taking into account many geophysical results including seismic tomography and magnetotelluric soundings, we speculate that the occurrence of the Maduo earthquake could be affected by crustal fluids in the fault zone. The fluids may ascend from the lower crustal flow beneath northeastern Tibet.

Localization of Deformation on Faults Driven by Fluids During the L’Aquila 2009 Earthquake

AbstractCoseismic rupture and aftershock development on a fault plane are complex and heterogeneous processes. The Mw 6.1 L’Aquila 2009 normal faulting earthquake is a perfect case to explore how fault geometry and rheology influence the rupture process and aftershocks distribution. In this study, we use for the first time a dense set of earthquake data to obtain enhanced images of the causative normal fault structure to the kilometer scale. The hypocenter of the emergent onset of the mainshock took place within a low Vp/Vs volume, while the large coseismic slip occurred a few kilometers above, as the rupture propagated through a high Vp and high Vp/Vs fluid‐filled rock volume. The increase of Vp/Vs in the fault hanging wall during the sequence suggests a strong dehydration in the earthquake asperity, with an upward fluid pressure migration along the fault toward the host rock volume. We propose that the localization of deformation on the fault plane is favored by high fluid pressure, while the spreading of aftershocks on a wide volume around the fault is driven by the depletion of fluids from the slipped portion of the fault plane and migration to small segments within the fault host rocks.

The 17 January 1994 Northridge, California, Earthquake: A Retrospective Analysis

Abstract The 17 January 1994 Northridge, California, earthquake was a watershed event with far-reaching societal and scientific impacts. The earthquake, which occurred in the early days of both broadband seismic networks and the Internet, spurred advances in seismic monitoring, real-time systems, and development of data products. Motivated by the 30th anniversary of the earthquake, we present a brief retrospective of the earthquake and its impact, and reconsider both ground motions and the aftershock distribution using modern tools and the best-available data. With improvements in instrumentation and analysis methodology, recent earthquakes continue to reveal the increasing complexity of ground motions, fault systems, and earthquake ruptures. Even in the absence of data from state-of-the-art instrumentation, a retrospective consideration of ground-motion data from the Northridge earthquake reveals complexities beyond what could be characterized (and modeled) 30 yr ago. Aftershock relocations for both the 1971 Sylmar and 1994 Northridge earthquakes also reveal an updated view of fault complexity. Our study does provide a cautionary tale regarding legacy data sets and research results that are not easily accessible, which can result in discrepancies between catalog data and products from the best available science. We also briefly describe outreach products produced as a part of the anniversary commemoration.

The 2022 Har Lake earthquake sequence highlights a complex fault system in the western Qilian Shan, northeastern Tibetan Plateau

SUMMARY The 2022 Har Lake earthquake sequence, which began in 2022 January and lasted for ∼70 d, jolted the Har Lake area, which is located in the western Qilian Shan, northeastern Tibetan Plateau. Two Mw > 5.5 earthquakes occurred during the earthquake sequence, among which the March 25 Mw 5.8 event is considered the largest event recorded in the area. However, determining the seismogenic faults of the earthquake sequence, as well as the detailed rupture features, is difficult due to the lack of geological data and near-field seismological observations. In this study, we use Sentinel-1 synthetic aperture radar (SAR) data to obtain the coseismic deformation field, identify possible ruptured faults and associated fault geometries, and further estimate detailed coseismic slip models of the two Mw > 5.5 earthquakes. The results show that the January 23 Mw 5.6 earthquake (Earthquake A) occurred on a N15° W-trending dextral-slip fault with a dip angle of ∼61°. For the March 25 Mw 5.8 earthquake (Earthquake B), the interferometric synthetic aperture radar (InSAR) data can be described by either an ∼N–S-trending dextral-slip fault or an ∼E–W-trending sinistral-slip fault. The ∼N–S-trending fault better describes the aftershock distribution, while the ∼E–W-trending model is more consistent with the regional geological setting. We suggest that the complex coseismic ruptures in the multiple-fault system are driven by widespread NE–SW-trending compression in the western Qilian Shan. This study demonstrates the importance of integrating geodetic and seismological observations to capture the full complexity of moderate earthquakes and further suggests potential seismic hazards in the Har Lake area.

Source Models of the 2016 and 2022 Menyuan Earthquakes and Their Tectonic Implications Revealed by InSAR.

The Haiyuan fault system plays a crucial role in accommodating the eastward expansion of the Tibetan Plateau (TP) and is currently slipping at a rate of several centimeters per year. However, limited seismic activities have been observed using geodetic techniques in this area, impeding the comprehensive investigation into regional tectonics. In this study, the geometric structure and source models of the 2022 Mw 6.7 and the 2016 Mw 5.9 Menyuan earthquakes were investigated using Sentinel-1A SAR images. By implementing an atmospheric error correction method, the signal-to-noise ratio of the 2016 interferometric synthetic aperture radar (InSAR) coseismic deformation field was significantly improved, enabling InSAR observations with higher accuracy. The results showed that the reliability of the source models for those events was improved following the reduction in observation errors. The Coulomb stress resulting from the 2016 event may have promoted the strike-slip movement of the western segment of the Lenglongling fault zone, potentially expediting the occurrence of the 2022 earthquake. The coseismic slip distribution and the spatial distribution of aftershocks of the 2022 event suggested that the seismogenic fault may connect the western segment of the Lenglongling fault (LLLF) and the eastern segment of the Tuolaishan fault (TLSF). Additionally, the western segment of the surface rupture zone of the northern branch may terminate in the secondary branch close to the Sunan-Qilian fault (SN-QL) strike direction, and the earthquake may have triggered deep aftershocks and accelerated stress release within the deep seismogenic fault.

Fault Kinematics of the 2023 Mw 6.0 Jishishan Earthquake, China, Characterized by Interferometric Synthetic Aperture Radar Observations

Characterizing the coseismic slip behaviors of earthquakes could offer a better understanding of regional crustal deformation and future seismic potential assessments. On 18 December 2023, an Mw 6.0 earthquake occurred on the Lajishan–Jishishan fault system (LJFS) in the northeastern Tibetan Plateau, causing serious damage and casualties. The seismogenic fault hosting this earthquake is not well constrained, as no surface rupture was identified in the field. To address this issue, in this study, we use Interferometric Synthetic Aperture Radar (InSAR) data to investigate the coseismic surface deformation of this earthquake and invert both ascending and descending line-of-sight observations to probe the seismogenic fault and its slip characteristics. The InSAR observations show up to ~6 cm surface uplift caused by the Jishishan earthquake, which is consistent with the thrust-dominated focal mechanism. A Bayesian-based dislocation modeling indicates that two fault models, with eastern and western dip orientations, could reasonably fit the InSAR observations. By calculating the coseismic Coulomb failure stress changes (∆CFS) induced by both fault models, we find that the east-dipping fault scenario could reasonably explain the aftershock distributions under the framework of stress triggering, while the west-dipping fault scenario produced a negative ∆CFS in the region of dense aftershocks. Integrating regional geological structures, we suggest that the seismogenic fault of the Jishishan earthquake, which strikes NNE with a dip of 56° to the east, may be either the Jishishan western margin fault or a secondary buried branch. The optimal finite-fault slip modeling shows that the coseismic slip was dominated by reverse slip and confined to a depth range between ~5 and 15 km. The released seismic moment is 1.61 × 1018 N·m, which is equivalent to an Mw 6.07 earthquake. While the Jishishan earthquake ruptured a fault segment of approximately 20 km, it only released a small part of the seismic moment that was accumulated along the 220 km long Lajishan–Jishishan fault system. The remaining segments of the Lajishan–Jishishan fault system still have the capability to generate moderate-to-large earthquakes in the future.

Co-seismic deformation and related hazards associated with the 2022 Mw 5.6 Cianjur earthquake in West Java, Indonesia: insights from combined seismological analysis, DInSAR, and geomorphological investigations

IntroductionOn November 21, 2022, a magnitude Mw 5.6 earthquake struck Cianjur Regency in the West Java Province of Indonesia. It was followed by at least 512 aftershocks that persisted from November to June 2023. This seismic event occurred in an area previously unrecognized as an active fault zone. The consequences of this earthquake in Cianjur were severe, leading to both loss of life and extensive structural damage. The substantial damage to buildings was likely a result of abrupt alterations in the local topography due to surface deformation effects.ObjectivesThis research endeavor aims to spatially determine the patterns of ground surface deformation and its relationship with local geomorphological setting due to earthquakes in Cianjur in 2022.MethodsIn this study we conduct seismological analysis of 45 seismic stations, statistical analysis of mainshock and aftershocks data, RADAR Sentinel-1 imagery and employed the DInSAR methodology. Field survey was also conducted to determine the geomorphological characteristics in the study area.ResultsThe outcomes disclosed that the deformation encompassed both subsidence and uplift. The results signify that there was subsidence deformation in the vicinity of Cianjur and its environs during the primary earthquake on November 21, 2022, with an average deformation value of approximately -5 cm. In contrast, the measured deformation during the aftershocks exhibited uplift deformation, with an average value of 10 cm. The examination of deformation patterns amid the 2022 Cianjur earthquake sequence detects elevated deformation values in the vicinity of Cugenang district, with an orientation running from northwest to southeast. The geomorphological investigation conducted indicates that the region of Cianjur encompasses a variety of landforms, such as volcanic, structural, fluvial, and denudational. These landforms exhibit distinct responses to seismic activities. Co-seismic hazards, such as landslides frequently occur as a consequence of seismic events in mountainous terrain.Main ConclusionsSpatio-temporal variation of ground deformation could arise from various causes, such as the number and distribution of aftershocks, stress redistribution, fault interactions, secondary effects, and local geological settings. The mainshocks release accumulated stress along a fault, resulting in particular types of deformation, whereas aftershocks may redistribute stress exhibiting on adjacent faults. Secondary effects triggered by aftershocks, coupled with local geological and geomorphological conditions, further contribute to the diverse patterns of ground deformation observed during seismic events. The results of the study revealed that ground deformation had the greatest impact on fluvial, volcanic, and denudational processes, resulting in notable subsidence and uplift in specific regions. The occurrence and magnitude of co-seismic landslides were triggered by both mainshock and aftershock events, which occurred on weathered geological materials. These effects were further amplified by the simultaneous presence of the rainy season.ImplicationsThe knowledge gained from this research can be applied to evaluate the impacts of earthquakes and to proactively reduce future risks.

Next Generation Seismic Source Detection by Computer Vision: Untangling the Complexity of the 2016 Kaikōura Earthquake Sequence

AbstractSeismic source locations are fundamental to many fields of Earth and planetary sciences, such as seismology, volcanology and tectonics. However, seismic source detection and location are challenging when events cluster closely in space and time with signals tangling together at observing stations, such as they often do in major aftershock sequences. Though emerging algorithms and artificial intelligence (AI) models have made processing high volumes of seismic data easier, their performance is still limited, especially for complex aftershock sequences. In this study, we propose a novel approach that utilizes three‐dimensional image segmentation—a computer vision technique—to detect and locate seismic sources, and develop this into a complete workflow, Source Untangler Guided by Artificial intelligence image Recognition (SUGAR). In our synthetic and real data tests, SUGAR can handle complex, energetic earthquake sequences in near real time better than skillful analysts and other AI and non‐AI based algorithms. We apply SUGAR to the 2016 Kaikōura, New Zealand sequence and obtain five times more events than the analyst‐based GeoNet catalog. The improved aftershock distribution illuminates a continuous fault system with extensive fracture zones beneath the segmented, discontinuous surface ruptures. Our method has broader applicability to non‐earthquake sources and other time series image data sets.

Temporal Characteristics of the February 6, 2023 MW7.8, Turkey Earthquake Aftershock Sequence

The devastating earthquake with moment magnitude MW7.8, (local magnitude ML=7.5) which occurred in the East Anatolian Fault Zone, westwards of Gaziantep, Turkey on February 6, 2023 was followed by an intensive aftershock activity. Studying time distribution of aftershocks is important for understanding physics of the earthquake generation process.
 In the present study temporal pattern of aftershock sequence of the 2023 MW7.8 Turkey earthquake is analyzed. Because of the factors such as location and radiation pattern and the cumulative nature of building damage, aftershocks can cause more damage than the main shock. The temporal clustering of aftershocks is a dominant non-random element of the seismicity, so when the clusters are removed, the remaining activity can be modelled (as first approximation) as a Poisson process.
 On the assumption that aftershocks are distributed in time as a non-stationary Poisson process, we use the maximum likelihood method for estimating the parameters (K, c and p) of the modified Omori formula. For testing the goodness of fit between the aftershock occurrence and different statistical models, a transformation from the time scale t to a frequency-linearized time scale τ is applied. Akaike's information criteria is used to select the best model for the temporal distribution of aftershocks.
 It has been found that temporal distribution of the 2023 Turkey earthquake aftershocks is dominated by the classic power law decay in time and suggests the existence of secondary aftershock activity.

GNSS imaging coseismic and postseismic slip associated with the 2021 M 8.2 Chignik, Alaska earthquake

The coseismic and postseismic slip associated with the 2021 M 8.2 Chignik, Alaska earthquake is imaged using the geodetic modeling method. Further, the spatio-temporal evolution of afterslip and aftershocks (0–60 days) is also investigated based on the postseismic deformations captured by Global Navigation Satellite System (GNSS) sites in this study. The coseismic slip is mainly concentrated at depths of 20–50 km with a maximum of 3.92 m. After this earthquake, a significant afterslip can be observed in the shallow region, which is distributed around the coseismic rupture region. Afterslip is primarily accumulated at depths of 10–30 km, and the maximum reaches 1.29 m, which is located at a longitude of 157.19°W, latitude of 55.09°N, and depth of 21.80 km. The spatio-temporal properties of afterslip indicate that the spatial extent and magnitude of afterslip continuously increase during the 60 days after the earthquake, and the fault activity in the first seven days is significantly strong. Meanwhile, the fault slip rate during the time span of 0–7 days is the largest with a maximum of 0.054 m/day. The fault slip rate gradually decreases with time, implying that the fault activity is slowly weakening. The Coulomb Failure Stress (CFS) distribution suggests that afterslip and most aftershocks are located at the southeastern region of the hypocenter, which is covered by positive CFS. Moreover, in addition to coseismic rupture effects, afterslip may be a possible triggering mechanism of early aftershocks as evidenced by the fact that the spatial distributions of afterslip and aftershocks are highly consistent.

A High-Resolution Aftershock Catalog for the 2014 Ms 6.5 Ludian (China) Earthquake Using Deep Learning Methods

A high-resolution catalog for the 2014 Ms 6.5 Ludian aftershocks was constructed based on the deep learning phase-picking model (CERP) and seismic-phase association technology (PALM). A specific training strategy, which combines the advantages of the conventional short–long window average energy ratio algorithm (STA/LTA) and AI algorithms, is employed to retrain the CERP model. The P- and S-wave phases were accurately detected and picked on continuous seismic waveforms by the retained AI model. Hypoinverse and HypoDD were utilized for the precise location of 3286 events. Compared to the previous results, our new catalog exhibits superior performances in terms of location accuracy and the number of aftershock events, thereby enabling a more detailed depiction of the deep-seated tectonic features. According to the distribution of aftershocks, it can be inferred that (1) the seismogenic fault of the Ludian earthquake is the NW-trending Baogunao–Xiaohe Fault, (2) the Ludian aftershocks interconnected with the discontinuous NW-trending Baogunao–Xiaohe Fault, and they also intersected with the Zhaotong–Ludian Fault. (3) This suggests that the NE-trending Zhaotong–Ludian Fault may have been intersected by the NW-trending Baogunao–Xiaohe Fault, indicating that the Baogunao–Xiaohe Fault is likely a relatively young Neogene fault.

Study on the Heterogeneity of the Stress Field in the Maduo Earthquake Fault Zone

Abstract The 2021 Maduo earthquake sequence occurred on the Jiangcuo fault zone in Qinghai, China. However, the earthquake sequence did not occur along a straight fault. Aftershocks in the southeast section deflected the aftershocks in the southeast section to the east, when the aftershocks in the northwest section bifurcated. To investigate the relationship between these eastward deflections, aftershock bifurcations, and fault activity, 150 focal mechanism solutions of the Maduo earthquake sequence are collected and processed, and then the stress fields in the subregion and whole region are subsequently determined by partitioning the sliding window from southeast to northwest. The results show that the overall tectonic stress field of the Maduo earthquake sequence exhibits northeast–southwest compression and northwest–southeast extension due to the northward compression of the Indian plate, causing rupture of the Kunlunshankou-Jiangcuo fault, which straightened the curved Maduo-Gander fault. The stress field at the deflection of the southeastern section of the source area differs significantly from the overall stress field. The plunge angle of the extensional stress axis in the southeastern deflection area is close to vertical, which is speculated to be due to the effect of the crack tip and the adjustment of local stress after the earthquake. The extensional stress axis at the bifurcated distribution of aftershocks in the northwestern section of the source area is slightly greater than of the overall stress field, indicating that the activation of the bifurcated hidden fault was triggered by the high rupture intensity and the adjustment of local stress. The reactivation of the hidden bifurcated fault results in local stress and causes decreasing seismicity west of the bifurcation area.

Rapid report of the December 18, 2023 MS 6.2 Jishishan earthquake, Gansu, China

On December 18, 2023, the Jishishan area in Gansu Province was jolted by a MS 6.2 earthquake, which is the most powerful seismic event that occurred throughout the year in China. The earthquake occurred along the NW-trending Lajishan fault (LJSF), a large tectonic transformation zone. After this event, China Earthquake Networks Center (CENC) has timely published several reports about seismic sources for emergency responses. The earthquake early warning system issued the first alert 4.9 ​s after the earthquake occurrence, providing prompt notification that effectively mitigated panics, injuries, and deaths of residents. The near real-time focal mechanism solution indicates that this earthquake is associated with a thrust fault. The distribution of aftershocks, the rupture process, and the recorded amplitudes from seismic monitoring and GNSS stations, all suggest that the mainshock rupture predominately propagates to the northwest direction. The duration of the rupture process is ∼12 ​s, and the largest slip is located at approximately 6.3 ​km to the NNW from the epicenter, with a peak slip of 0.12 ​m at ∼8 ​km depth. Seismic station N0028 recorded the highest instrumental intensity, which is 9.4 on the Mercalli scale. The estimated intensity map shows a seismic intensity reaching up to IX near the rupture area, consistent with field survey results. The aftershocks (up to December 22, 2023) are mostly distributed in the northwest direction within ∼20 ​km of the epicenter. This earthquake caused serious casualties and house collapses, which requires further investigations into the impact of this earthquake.

Double-Difference Method for Relocating the Hypocenter of Aftershock Earthquakes in Seririt Singaraja

A method that does not require a main earthquake (master event) that can be used simultaneously to relocate a very large number of earthquakes with wide hypocenter separation is called the double-difference method. A method used to relocate the aftershocks in Seririt Singaraja on November 14 2019 with coordinate positions 113.478 – 115.181 East Longitude and 8.357 – 7.894 South Latitude. The earthquake data used in this research was accumulated from 85 BMKG seismic stations. Data analysis uses cross-correlation time differences which can increase the accuracy of travel time between the receiving station and the earthquake, thereby reducing errors in calculations. The double difference method used to relocate the earthquake in the Seririt Singaraja area showed that there was a shift in the location of the earthquake hypocenter before and after it was relocated. Horizontally and vertically, the distribution of earthquake hypocenters before and after being relocated occurs when there is a collection of location shifts. The results of this research were able to relocate 152 aftershocks properly. The main earthquake after being relocated was at a depth of 17 km, while the distribution of aftershocks was at a depth of around 6-25 km, so that it can more accurately describe the position of the earthquake source and is able to show clearer and easier to interpret structural patterns.

Electrical Structure of Southwestern Longmenshan Fault Zone: Insights into Seismogenic Structure of 2013 and 2022 Lushan Earthquakes

On 1 June 2022, a magnitude 6.1 earthquake struck the southern segment of the Longmenshan fault zone on the eastern edge of the Tibetan Plateau, once again causing casualties and economic losses. Understanding the deep-seated dynamic mechanisms that lead to seismic events in the Lushan earthquake area and assessing the potential hazards in seismic gap areas are of significant importance. In this study, we utilized 118 magnetotelluric datasets collected from the Lushan earthquake area and employed three-dimensional electromagnetic inversion with topographic considerations to characterize the deep-seated three-dimensional resistivity structure of the Lushan earthquake area. The results reveal that the Shuangshi–Dachuan fault in the Lushan earthquake area can be divided into two relatively low-resistivity zones: a western zone dipping southeastward and an eastern zone with a steeper slightly northwestern dip. These two zones intersect at a depth of approximately 20 km, forming an extensional pattern resembling a “Y” shape. The epicenters of both the 2013 and 2022 Lushan earthquakes are primarily located in the upper constricted portion of the pocket-like low-resistivity body at depth. The distribution of seismic aftershocks is confined within the region enclosed by the high-resistivity body, following the pattern of the Y-shaped low-resistivity zone.

Spatial Relationships Between Coseismic Slip, Aseismic Afterslip, and On‐Fault Aftershock Density in Continental Earthquakes

AbstractDamaging aftershock sequences often exhibit considerable spatio‐temporal complexity. The stress changes associated with coseismic slip and aseismic afterslip are commonly proposed to drive aftershock sequences, but few systematic studies exist and do not always support strong, universal driving relationships. To investigate the roles that these two sources of stress changes may play in driving aftershocks, we assess the spatio‐temporal relationships between coseismic slip, afterslip, and on‐fault (within 5 km) aftershock density following seven Mw6.0–7.6 continental‐settings earthquakes, using available high‐quality slip models and regional seismic data. From previous empirical work and frictional considerations, near the mainshock we expect coseismic slip and afterslip to be anti‐correlated, and aftershocks to occur where coseismic slip is low/zero, near high slip gradients, and/or to migrate with afterslip. However, we find that spatial relationships between afterslip and coseismic slip, and between afterslip and aftershock density differ between earthquakes. Aftershock density correlates with coseismic slip following five of the earthquakes, and with total cumulative slip (coseismic slip + afterslip) following six: indicating that on‐fault aftershock distributions may be approximated by total slip (at current resolutions). Additionally, we find that the gradients of coseismic slip and afterslip (proxies for new stress concentrations) do not clearly correlate with aftershock distributions and that the choice of spatial domain over which relationships are tested can affect results significantly. A possible explanation of these results is that fault zones contain considerable fine‐scale structural and frictional heterogeneity. Nonetheless, the empirical evidence for frequently assumed relationships between coseismic slip, afterslip and on‐fault aftershocks is mixed.

Machine Learning Aids Rapid Assessment of Aftershocks: Application to the 2022–2023 Peace River Earthquake Sequence, Alberta, Canada

Abstract The adoption of machine learning (ML) models has ignited a paradigm shift in seismic analysis, fostering enhanced efficiency in capturing patterns of seismic activity with reduced need for time-consuming user interaction. Here, we investigate automated event detection and extraction of seismic phases using two widely used ML models: EQTransformer and PhaseNet. We applied both the models to four weeks of continuous recordings of aftershocks using a temporary array following the 30 November 2022, ML 5.6 earthquake near Peace River, Alberta, Canada. Both the tools identified >1000 events over the recording period. The aftershocks are located in close proximity to the ML 5.6 mainshock as well as to wastewater disposal operations that were ongoing at the time. Both the methods reveal an aftershock distribution that was not identified by the regional network; however, we find that events detected by PhaseNet have smaller event location errors and better depict subtle fault structures at depth, despite identifying ∼200 events less than EQTransformer. Our results highlight the advantages of using ML models for rapid detection and assessment of seismicity following felt events, which is important for rapidly assessing seismic hazard potential and risk.