Seismogenic Fault Research Articles

Lowering of the kinetic barrier of clay-mineral reactions along a seismogenic fault: Example from the Nankai subduction zone, Japan

An analysis of clay minerals in core samples around the megasplay fault of the Nankai Trough, Japan, recovered during the Integrated Ocean Drilling Program Nankai Trough Seismogenic Zone Experiment reveals a local progression of the smectite-to-illite conversion (S–I) reaction in the slip zone, possibly associated with local thermogenesis caused by seismic slip on the fault. Our semiquantitative X-ray diffraction analysis demonstrates that the illite content in mixed-layer illite/smectite is ∼15% higher in the gouge than in the host sediments. The thermal history of the fault, constrained in a previous study, requires a reduction of the apparent activation energy by 20–30% compared with the literature value to explain the observed illitization. The Shirako Fault in the Miura–Boso accretionary complex, which is thought to have been activated in a tectonic setting similar to that of the megasplay fault of the Nankai Trough, also shows a 20–30% reduction in the apparent activation energy. These results suggest that the kinetic barrier of the smectite–illite reaction is lowered as a result of mechanochemical processes in seismogenic fault zones.

A catastrophic, buried fault-generating earthquake: The 1937 M7.0 Heze earthquake in the south-central North China Plain

The North China Plain (NCP) is a region with a high level of seismic hazards. Since 1830, eight historical M7.0–7.9 earthquakes have been recorded in the NPC, seven of which have occurred on the boundary of the subblocks, and only the 1937 Heze earthquake has occurred in the interior of the Ji-Lu Subblock. Why did this intraplate earthquake occur there? What are its seismogenic conditions and seismogenic mechanisms? The Liaocheng-Lankao Fault (LLF) is an important buried active fault in the south-central NCP, and the 1937 Heze earthquake occurred 10 km east of it. What role did the LLF play in this earthquake? Using shallow seismic reflection detection and drilling combined with Quaternary dating methods reveals that the southern segment of the LLF is a Holocene active fault, and the northern segment is a late Pleistocene active fault. The BCS drilling profile reveals four paleoseismic events in the southern LLF since the late Pleistocene, and the average vertical slip rate of the southern LLF since 21 ka is approximately 0.35 ± 0.04 mm/a. The analysis of the intensity lines, surface rupture distribution, and focal mechanism solution of the 1937 Heze M7.0 earthquake implies that the NNE-striking Xiaoliu-Xieyuanji Fault and the WNW-striking Dongming-Chengwu Fault are the seismogenic faults of the 1937 Heze M7.0 earthquake. The LLF, as the deep major fault in the region, controlled the accumulation of stress, stimulated the earthquake with its deep movement, and reduced the effect of the seismic energy westward, acting as the regional seismic controlling fault of the 1937 Heze M7.0 earthquake. Another branch fault near Yangji, approximately 50 km north of Heze, is a new stress concentration area and may be at risk of a large earthquake. In addition, the northern segment of the LLF may also be reactivated in the future and produce an M ≥ 7.0 earthquake.

New Zealand Fault-Rupture Depth Model v.1.0: A Provisional Estimate of the Maximum Depth of Seismic Rupture on New Zealand’s Active Faults

ABSTRACT We summarize estimates of the maximum rupture depth on New Zealand’s active faults (“New Zealand Fault-Rupture Depth Model v.1.0”), as used in the New Zealand Community Fault Model v1.0 and as a constraint for the latest revision of the New Zealand National Seismic Hazard Model (NZ NSHM 2022). Rupture depth estimates are based on a combination of two separate model approaches (using different methods and datasets). The first approach uses regional seismicity distribution from a relocated earthquake catalog to calculate the 90% seismicity cutoff depth (D90), representing the seismogenic depth limit. This is multiplied by an overshoot factor representing the dynamic propagation of rupture into the conditional stability zone, and accounting for the difference between regional seismicity depths and the frictional properties of a mature fault zone to arrive at a seismic estimate of the maximum rupture depth. The second approach uses surface heat flow and rock type to compute depths that correspond to the thermal limits of frictional instabilities on seismogenic faults. To arrive at a thermally-based maximum rupture depth, these thermal limits are also multiplied by an overshoot factor. Both the models have depth cutoffs at the Moho and/or subducting slabs. Results indicate the maximum rupture depths between 8 (Taupō volcanic zone) and >30 km (e.g., southwest North Island), strongly correlated with regional thermal gradients. The depths derived from the two methods show broad agreement for most of the North Island and some differences in the South Island. A combined model using weighting based on relative uncertainties is derived and validated using constraints from hypocenter and slip model depths from recent well-instrumented earthquakes. We discuss modifications to the maximum rupture depths estimated here that were undertaken for application within the NZ NSHM 2022. Our research demonstrates the utility of combining seismicity cutoff and thermal stability estimates to assess the down-dip dimensions of future earthquake ruptures.

Two Mw ≥ 6.5 Earthquakes in Central Pamir Constrained by Satellite SAR Observations

The Pamir, situated in central Asia, is a result of the ongoing northward advance of the Indian continent, leading to compression of the Asian landmass. While geodetic and seismic data typically indicate that the most significant deformation in Pamir is along its northern boundary, an Mw 7.2 earthquake on 7 December 2015 and an Mw 6.8 earthquake on 23 February 2023 have occurred in the remote interior of Pamir. These two Mw ≥ 6.5 earthquakes, with good observations of satellite synthetic aperture radar data, provide a rare opportunity to gain insights into rupture mechanics and deformation patterns in this challenging-to-reach region. Here, we utilize spaceborne synthetic aperture radar data to determine the seismogenic faults and finite slip models for these two earthquakes. Our results reveal that the 2015 earthquake ruptured a ~88 km long, left-lateral strike-slip fault that dips to northwest. The rupture of the 2015 earthquake extended to the ground surface over a length of ~50 km with a maximum slip of ~3.5 m. In contrast, the 2023 earthquake did not rupture the ground surface, with a maximum slip of ~2.2 m estimated at a depth of ~9 km. Notably, the seismogenic fault of the 2015 earthquake does not align with the primary strand of the Sarez–Karakul fault system (SKFS), and the 2023 earthquake occurred on a previously unmapped fault. The well-determined seismogenic faults for the 2015 and 2023 earthquakes, along with the SKFS and other distributed faults in the region, suggest the existence of a wide shear zone extending from south to north within the central Pamir.

Earthquake relocation using a 3D velocity model and implications on seismogenic faults in the Beijing-Tianjin-Hebei region

To enhance the understanding of the geometry and characteristics of seismogenic faults in the Beijing-Tianjin-Hebei region, we relocated 14 805 out of 16 063 earthquakes (113°E−120°E, 36°N–43°N) that occurred between January 2008 and December 2020 using the double-difference tomography method. Based on the spatial variation in seismicity after relocation, the Beijing–Tianjin–Hebei region can be divided into three seismic zones: Xingtai–Wen'an, Zhangbei–Ninghexi, and Tangshan. (1) The Xingtai–Wen'an Seismic Zone has a northeast-southwest strike. The depth profile of earthquakes perpendicular to the strike reveals three northeast-striking, southeast-dipping, high-angle deep faults (>10 ​km depth), including one below the shallow (<10 ​km depth) listric, northwest-dipping Xinghe fault in the Xingtai region. Two additional deep faults in the Wen'an region are suggested to be associated with the 2006 M 5.1 Wen'an Earthquake and the 1967 M 6.3 Dacheng earthquake; (2) The Zhangbei-Ninghexi Seismic Zone is oriented north-northwest. Multiple northeast-striking faults (10–20 ​km depth), inferred from the earthquake-intensive zones, exist beneath the shallow (<10 ​km depth) Xiandian Fault, Xiaotangshan Fault, Huailai-Zhuolu Basin North Fault, Yangyuan Basin Fault and Yanggao Basin North Fault; (3) In the Tangshan Seismic Zone, earthquakes are mainly concentrated near the northeast-striking Tangshan-Guye Fault, Lulong Fault, and northwest-striking Luanxian-Laoting Fault. An inferred north-south-oriented blind fault is present to the north of the Tangshan-Guye Fault. The 1976 M 7.8 Tangshan earthquake occurred at the junction of a shallow northwest-dipping fault and a deep southeast-dipping fault. This study emphasizes that earthquakes in the region are primarily associated with deep blind faults. Some deep blind faults have different geometries compared to shallow faults, suggesting a complex fault system in the region. Overall, this research provides valuable insights into the seismogenic faults in the Beijing–Tianjin–Hebei region. Further studies and monitoring of these faults are essential for earthquake mitigation efforts in this region.

Source Parameter Inversion and Century-Scale Stress Triggering Analysis of the 2021 Maduo MW7.4 Earthquake Using GNSS and InSAR Displacement Fields

To explore the degree of constraint by Global Navigation Satellite System (GNSS) and Interferometric Synthetic Aperture Radar (InSAR) data on the Maduo earthquake within a layered earth model structure and to gain an insight into the seismogenic mechanism and the seismic risk in the surrounding area, this study employs D-InSAR technology to acquire the InSAR co-seismic deformation field of the Maduo earthquake on 22 May 2021. Utilizing both GNSS and InSAR data, the inversions constrained by single and joint data are conducted and compared to determine the co-seismic slip model and fault plane stress distribution of the Maduo earthquake. Additionally, this paper calculates the Coulomb stress changes induced by 14 M ≥ 7 strong earthquakes, considering co-seismic effects, post-seismic viscoelastic relaxation, and inter-seismic tectonic stress loading, on 19 fault segments within the Bayan Har block research area (96°E~106°E, 29°N~36°N) since 1900. The findings are as follows: (1) The maximum line-of-sight (LOS) deformation was approximately 0.9 m. The joint inversion rupture was primarily located in the Dongcao Along Lake section (~98.6°E), aligning with previous research outcomes. (2) The cumulative Coulomb stress at the Maduo earthquake’s source location was −0.1333 MPa, while the inter-seismic stress loading amounted to 0.0745 MPa. The East Kunlun Fault, Maduo–Gande Fault, Ganzi–Yushu Fault, and Dari Fault C exhibited considerable stress loading, warranting attention due to heightened seismic risk. (3) Based on three different co-seismic slip models, the stress disturbance results caused by the Maduo earthquake to the surrounding area and fault did not differ significantly. After the earthquake, the seismogenic fault still has high seismic risk.

Study on deformation characteristics and dynamic cause of the Luding MS6.8 earthquake

A MS6.8 earthquake struck Luding Country in Ganzi Prefecture, Sichuan Province on 5 September 2022. The earthquake occurred on the Moxi segment of Xianshuihe fault zone (XFZ), one of the most seismically active faults in the Chinese mainland. In this study, multiple periods of the Global Positioning System (GPS) velocity field and continuous observational data are collected to analysis the tectonic deformation and evolution characteristics before the Luding earthquake, from the perspectives of the kinematic behavior of seismogenic fault, the multi-scale strain features around the study region, and the variation of GPS baselines across the epicenter area. Then the following conclusions are obtained: 1) The accelerated compression of baselines SCGZ-SCXJ (Ganzi to Xiaojin in Sichuan province) and SCLH-SCXJ (Luhuo to Xiaojin in Sichuan province) in Bayan Har block indicate that under the influence of the coseismic rupture of Maduo MS7.4 earthquake, the boundary faults decoupled and accelerated the push southward and eastward, leading to the acceleration of strain accumulation and the increase of seismic risk in the divergence area bounded by the southeastern XFZ and the southwestern Longmenshan fault zone (LFZ). 2) Luding earthquake located in the weakened region around the edge of the large strike-slip fault zone with high shear strain rate, and the tensile zone of the strain perpendicular to the fault direction, denoting that the reduction of the normal strain in the locked background is strongly related to fault rupture and earthquake nucleation.

Viscoelastic Slider Blocks as a Model for a Seismogenic Fault.

In this work, a model is proposed to examine the role of viscoelasticity in the generation of simulated earthquake-like events. This model serves to investigate how nonlinear processes in the Earth's crust affect the triggering and decay patterns of earthquake sequences. These synthetic earthquake events are numerically simulated using a slider-block model containing viscoelastic standard linear solid (SLS) elements to reproduce the dynamics of an earthquake fault. The simulated system exhibits elements of self-organized criticality, and results in the generation of avalanches that behave similarly to naturally occurring seismic events. The model behavior is analyzed using the Epidemic-Type Aftershock Sequence (ETAS) model, which suitably represents the observed triggering and decay patterns; however, parameter estimates deviate from those resulting from natural aftershock sequences. Simulated aftershock sequences from this model are characterized by slightly larger p-values, indicating a faster-than-normal decay of aftershock rates within the system. The ETAS fit, along with realistic simulated frequency-size distributions, supports the inclusion of viscoelastic rheology to model the seismogenic fault dynamics.

Using control theory for preventing induced seismicity due to fluid injections in a reservoir

Deep Geothermal Energy, Hydrogen Underground Storage and Carbon Capture Utilization and Storage are promising techniques to satisfy the large-scale needs of the energy sector. However, they all depend on the injection of fluids into the earth’s crust, which, in turn, can cause earthquakes [1, 2]. Earthquakes nucleate when large amounts of elastic energy, stored in the earth's crust, are suddenly released due to abrupt sliding over faults. Fluid injections can create new, or reactivate existing, seismogenic faults and, therefore, cause earthquakes [2, 3].&#x0D; More recently, new results for controlling the earthquake instability of a single, natural, mature seismic fault were obtained [4, 5, 6, 7, 8]. These works were based on the mathematical control theory to stabilize and control the underlying physical system, which is underactuated, strongly non-linear and uncertain. The above works could inspire new approaches for preventing induced seismicity too [4, 9].&#x0D; In this work, a 3D diffusion equation is considered to model induced seismicity due to fluid injections in a geological reservoir. A robust tracking control is then designed to force the seismicity rate to follow a desired reference, minimize induced seismicity and assure fluid circulation. The designed regulator ensures the control task despite system uncertainties and external perturbations. Simulations of the process are presented to show the reliability and performance of the control approach.

Earthquakes Induced by Rapid Loading of Faults During Pulichintala Reservoir Impoundment in the Stable Continental Region of India

AbstractThe first stage impoundment of the Pulichintala reservoir in the Proterozoic Cuddapah basin in the southeastern Indian shield began in August 2014. The second stage of impoundment started in 2019 when the height of the dam was increased and the filling of the reservoir was rapid. Soon after earthquakes started occurring which continued for a few months with a maximum magnitude of ML 4.6 on 25 January 2020. Two distinct clusters within the shallow crust were identified, which follow the faults/shear zones in the region. The estimated focal mechanisms with predominant strike slip motion on steep planes and the derived stress state are consistent with the regional stress and plate motion of the Indian plate. We simulated the influence of reservoir impoundment on the inferred seismogenic faults and found that the stress and pore pressure due to reservoir load indeed promoted failure, thus inducing earthquakes. We ascribe the induced earthquakes to rapid filling, longer and higher retention of high‐water levels during 2019–2020, which caused a stress change of 4–8 kPa. This stress change was at least two orders of magnitude higher than the typical tectonic loading rate in the stable continental region of the Indian Shield.

Low-magnitude seismic swarms in the Calabrian Arc (Italy)

AbstractSeismic swarms of low magnitude earthquakes occur frequently in the Calabrian Arc. During the last few years, several earthquakes of magnitude up to ML4.4 occurred both on land and offshore near the coast of Calabria. Some of them were followed by a sequence of tens to hundreds of smaller, well-clustered earthquakes that occurred during the following weeks or months. In other cases, swarms of low-magnitude earthquakes occur without a classical mainshock-aftershock evolution. In this work, we selected swarms that were well recorded by a high number of seismic stations to perform a detailed analysis consisting of the determination of the relative location and focal mechanism for as many earthquakes as possible. In some cases, the relative location allows to recognize the seismogenic fault and to distinguish the fault plane from the auxiliary plane of the focal mechanism solution. In other cases, the relative location unravels a small cloud of events that is not compatible with a unique fault plane, suggesting the occurrence of the swarm in highly fractured seismogenic volume. The relative hypocenter positions allow to estimate the size of the seismogenic volume, which is very small in most of the cases, often less than 1 km3. However, its extension is greater than the size computed for the mainshock rupture in many cases. The most common source mechanism is of normal type, but strike-slip and reverse kinematics are also found, in particular for swarms located offshore and near the coast. The temporal distribution of events does not show any evident migration of the sources, thus suggesting that the driving mechanism is not related with aseismic phenomena like fluid diffusion and stress waves.

Rupture process of the January 8, 2022, Menyuan M 6.9 earthquake

After the occurrence of destructively strong earthquakes, rapid acquisition of the source rupture process can provide important reference information for post-earthquake disaster relief and aftershock trend determination. An M 6.9 earthquake occurred in Menyuan County, Qinghai Province on January 8, 2022. The epicenter is located in the seismic gap in the middle section of the Haiyuan fault belt. Such a typical strong earthquake was taken as an example to investigate the rupture process of strong earthquakes. Three days after the earthquake, the InSAR (Interferometric Synthetic Aperture Radar) coseismic deformation field was obtained by Sentinel radar, indicating that the surface ruptured obviously. The southern block of the earthquake faces towards the satellite about 95 ​cm along the LOS (line of sight) direction, and the northern block is away from the satellite by ​∼ ​74 ​cm, consistent with the characteristic of left-lateral strike-slip motion. In this study, InSAR coseismic deformation data and far-field waveform data were used to jointly invert the earthquake rupture process, and a four-segment finite fault model was constructed by referring to the surface deformation. The inversion results show that the focal depth of the Menyuan earthquake is about 7 ​km, and the strike of the seismogenic fault is 89.0°, 104.0°, 119.0° and 131.0° from west to east, respectively. It is a high-dip left-lateral strike-slip earthquake event lasting about 14 ​s. The rupture propagation mode is a bilateral extension. The maximum slip along the fault is about 380 ​cm, and the seismic moment magnitude is 6.7. The surface rupture length is about 24 ​km, which is consistent with that measured in the field survey. The detailed seismic source model can provide basic data for the aftershock trend determination and seismic risk analysis of the adjacent active faults.

Analysis of Peak Ground Acceleration and Seismogenic Fault Characteristics of the Mw7.8 Earthquake in Turkey

A Mw7.8 earthquake struck Turkey on 6 February 2023, causing severe casualties and economic losses. This paper investigates the characteristics of strong ground motion and seismogenic fault of the earthquake. We collected and processed the strong ground motion records of 379 stations using Matlab, SeismoSignal, and Surfer software: Matlab (Version R2016a), SeismoSignal (Version 5.1.0), and Surfer (Version 23.0.15), and obtained the peak ground acceleration (PGA) contour map. We analyzed the near-fault effect, the fault locking segment effect, and the trampoline effect of the earthquake based on the spatial distribution of PGA, the fault geometry, and slip distribution. We found that the earthquake generated a very strong ground motion concentration effect in the near-fault area, with the maximum PGA exceeding 2000 cm/s2. However, the presence of fault locking segments influenced the spatial distribution of ground motion, resulting in four significant PGA high-value concentration areas at a local dislocation, a turning point, and the end of the East Anatolian Fault. We also revealed for the first time the typical manifestation of the trampoline effect in this earthquake, which was characterized by a large vertical acceleration with a positive direction significantly larger than the negative direction. This paper provides an important reference for understanding the seismogenic mechanism, damage mode, characteristics, and strong earthquake law of the Turkey earthquake.

Kinematic deformation and intensity assessment of the 2021 Maduo MS7.4 earthquake in Qinghai revealed by high-frequency GNSS

Rapid acquisition of the kinematic deformation field and seismic intensity distribution of large earthquakes is crucial for postseismic emergency rescue, disaster assessment, and future seismic risk research. The advancement of GNSS observation and data processing makes it play an important role in this field, especially the high-frequency GNSS. We used the differential positioning method to calculate the 1 HZ GNSS data from 98 sites within 1000 km of the MS7.4 Maduo earthquake epicenter. The kinematic deformation field and the distribution of the seismic intensity by using the peak ground velocity derived from displacement waveforms were obtained. The results show that: 1) Horizontal coseismic response deformation levels ranging from 25 mm to 301 mm can be observed within a 1000 km radius from the epicenter. Coseismic response deformation on the east and west sides shows bilateral asymmetry, which markedly differs from the symmetry presented by surface rupture. 2) The seismic intensity obtained through high-frequency GNSS and field investigations exhibits good consistency of the scope and orientation in the high seismic intensity area, although the former is generally slightly smaller than the latter. 3) There may exist obstacles on the eastern side of the seismogenic fault. The Maduo earthquake induced a certain tectonic stress loading effect on the western Kunlun Pass-Jiangcuo fault (KPJF) and Maqin-Maqu segment, resulting in higher seismic risk in the future.

Estimating the Quality of the Most Popular Machine Learning Algorithms for Landslide Susceptibility Mapping in 2018 Mw 7.5 Palu Earthquake

The Mw 7.5 Palu earthquake that occurred on 28 September 2018 (UTC 10:02) on Sulawesi Island, Indonesia, triggered approximately 15,600 landslides, causing about 4000 fatalities and widespread destruction. The primary objective of this study is to perform landslide susceptibility mapping (LSM) associated with this event and assess the performance of the most widely used machine learning algorithms of logistic regression (LR) and random forest (RF). Eight controlling factors were considered, including elevation, hillslope gradient, aspect, relief, distance to rivers, peak ground velocity (PGV), peak ground acceleration (PGA), and lithology. To evaluate model uncertainty, training samples were randomly selected and used to establish the models 20 times, resulting in 20 susceptibility maps for different models. The quality of the landslide susceptibility maps was evaluated using several metrics, including the mean landslide susceptibility index (LSI), modelling uncertainty, and predictive accuracy. The results demonstrate that both models effectively capture the actual distribution of landslides, with areas exhibiting high LSI predominantly concentrated on both sides of the seismogenic fault. The RF model exhibits less sensitivity to changes in training samples, whereas the LR model displays significant variation in LSI with sample changes. Overall, both models demonstrate satisfactory performance; however, the RF model exhibits superior predictive capability compared to the LR model.

Analysis of the Effect of Lateral Collision on the Seismic Response of Bridges under Fault Misalignment

Mutual dislocation of seismogenic faults during strong earthquakes will result in a large relative displacement on both sides of the fault. It is of great significance to explore the influence of the collision effect between the main beam and the transverse shear key on the seismic response of the bridge under fault dislocation. In this paper, a series of cross-fault ground motions with different ground permanent displacements are artificially synthesized using a hybrid simulation method. Based on the contact element theory, the Kelvin–Voigt model is used to simulate the lateral collision effect. The effect of lateral collision on the seismic response of the continuous girder bridge is compared from the two aspects of fault dislocation position and fault dislocation degree. On this basis, the analysis of lateral collision parameters is carried out with the aim of reasonably regulating the seismic response of the structure. The results show that, compared with the near-fault bridge, the influence of lateral collision on the cross-fault bridge is stronger. The amplification of the bending moment of the central pier and the limitation of the bearing displacement are five times and two times, respectively, for the near-fault bridge. When the fault has a large dislocation, the weak point of the structural damage is the bending failure of the pier bottom and the residual torsion after the earthquake. The collision parameters of conventional bridges will aggravate the bending moment demand of the pier bottom of cross-fault bridges and limit their bearing displacement too much. Therefore, by appropriately reducing the collision stiffness and increasing the initial gap, the internal force and displacement response distribution of the cross-fault bridge structure can be more reasonable. The study in this paper has reference significance for seismic analysis of cross-fault bridges with transverse shear keys.

Deep Postseismic Creep Following Large Earthquakes Revealed by Repeating Aftershocks in the Southeastern Tibetan Plateau

Abstract Large earthquake occurrence and the subsequent postseismic period are the most dramatic part of a seismic cycle that usually lasts months to years. However, the fault dynamics that account for the postseismic events are yet to be fully understood. Here, we use the repeating aftershock sequences (RASs) to investigate postseismic slips following the Mw 6.6 Lushan, Mw 6.5 Jiuzhaigou, Mw 6.1 Jinggu, and Mw 6.2 Ludian earthquakes in the southeastern Tibetan Plateau and find 135 RASs following the mainshocks. The RAS seismicity suggests that seismogenic faults began to creep in depth within a few hours after the Lushan, Jiuzhaigou, and Jinggu mainshocks. The deep creeps mainly follow a velocity-strengthening friction mode and decay with an Omori law p-value of ∼1. The results suggest that the combination of fault healing and geometry together controls deep fault behaviors. We develop two conceptual models to explain our observations. Our results provide new insights into spatiotemporal fault evolution after large earthquakes.

Fine Seismogenic Fault Structures and Complex Rupture Characteristics of the 2022 M6.8 Luding, Sichuan Earthquake Sequence Revealed by Deep Learning and Waveform Modeling

AbstractAn approximate 236‐year interval in major seismic activity near the southern Xianshuihe fault terminated on 5 September 2022 until a destructive M6.8 Luding earthquake. The strike‐slip mainshock, accompanied by potent normal‐faulting aftershocks, fell short of the previously anticipated M7+ event. Utilizing deep‐learning and source inversion techniques, we analyzed dense near‐field seismograms to examine detailed fault structures and rupture characteristics. Our high‐quality relocated catalog with 7,388 events revealed that the earthquake ruptured an unmapped multiscale fault network. The analysis of the mainshock and 43 ML ≥ 2.8 aftershocks suggests normal‐faulting events relate to the vertical movement of the Gongga Mountain. The sequence length, seismic gaps, and asperities derived from seismicity and waveform modeling imply that the rupture of the M6.8 quake was incomplete, indicating a high risk of a future M6+ event at the northern Moxi segment. These findings hold crucial implications for assessing future seismic hazards in this region.

The 2021 Mw 7.2 Haiti earthquake: Blind thrust rupture revealed by space geodetic observations and Bayesian estimation

On 14 August 2021, a large earthquake struck the southern region of Haiti. The epicenter of this earthquake is located relatively close to the Enriquillo–Plantain Garden Fault (EPGF) zone, a major active fault with a strike-slip mechanism in the southern part of Hispaniola. Since the epicenter of this earthquake is located relatively close to the Enriquillo–Plantain Garden Fault zone, one might think that the EPGF is the causative fault. Using a Bayesian approach, the Sentinel-1 data is then utilized to investigate the seismogenic fault responsible for the 2021 Haiti earthquake. The Bayesian inversion indicated that the mainshock ruptured a north-dipping fault with a strike and a dip of 270.9° and 69.2°, respectively, and buried at a depth of 10.3 km from the earth’s surface. The preferred slip model showed that the rupture did not reach the surface and was confined at a depth of ∼6 km to ∼32 km. The preferred fault geometry is in good agreement with the relocated aftershock distribution and is inconsistent with the EPGF system configuration. It indicates that the EPGF is probably not the seismogenic fault responsible for the 2021 Haiti earthquake. Instead, results suggested that the 2021 Haiti earthquake ruptured an unmapped blind fault.

Co-Seismic Landslides Triggered by the 2014 Mw 6.2 Ludian Earthquake, Yunnan, China: Spatial Distribution, Directional Effect, and Controlling Factors

The 2014 Mw 6.2 Ludian earthquake exhibited a structurally complex source rupture process and an unusual spatial distribution pattern of co-seismic landslides. In this study, we constructed a spatial database consisting of 1470 co-seismic landslides, each exceeding 500 m2. These landslides covered a total area of 8.43 km2 and were identified through a comprehensive interpretation of high-resolution satellite images taken before and after the earthquake. It is noteworthy that the co-seismic landslides do not exhibit a linear concentration along the seismogenic fault; instead, they predominantly extend along major river systems with an NE–SW trend. Moreover, the southwest-facing slopes have the highest landslide area ratio of 1.41. To evaluate the susceptibility of the Ludian earthquake-triggered landslides, we performed a random forest model that considered topographic factors (elevation, slope, aspect, distance to rivers), geological factors (lithology), and seismic factors (ground motion parameters, epicentral distance, distance to the seismogenic fault). Our analysis revealed that the distance to rivers and elevation were the primary factors influencing the spatial distribution of the Ludian earthquake-triggered landslides. When we considered the directional variation in ground motion parameters, the AUC of the model slightly decreased. However, incorporating this variation led to a significant reduction in the proportion of areas classified as “high” and “very high” landslide susceptibility. Moreover, SEDd emerged as the most effective ground motion parameter for interpreting the distribution of the co-seismic landslides when compared to PGAd, PGVd, and Iad.