Seismic Gap Research Articles

Holocene Earthquake Cycles of an Active Tectonic Block Boundary Fault Zone: A Case Study in the Qilian–Haiyuan Fault Zone, Northeastern Tibet Plateau

Abstract Fault zones along active tectonic block boundaries are a significant source of devastating continental earthquakes. Strong earthquakes produce disruptions of sediment and induce characteristic sediments near the fault, which serve as valuable sedimentary evidence for identifying and dating of paleoearthquakes. In this study, we aimed to reconstruct the earthquake history of the Qilian–Haiyuan fault zone in the northeastern Tibetan Plateau during the Holocene. We reanalyzed forty-four trenches and used the sedimentary sequences, event indicators, and age constraints to determine the earthquake history. Our analysis revealed the paleoearthquakes of 6 subsidiary faults of the Qilian–Haiyuan fault zone with accurate event ages and rupture extents. Based on the spatial and temporal distributions of strong earthquakes since 10 ka, we identified five earthquake clusters around the central-eastern Qilian–Haiyuan fault zone including seven rupture cascades where the earthquakes migrated gradually from east to west. The existing seismic gap reveals that the latest migration may not yet be complete and suggests a high probability of M ≥ 7 earthquakes occurring on the Jinqianghe fault, Maomaoshan fault, and the central part of the Lenglongling faults. We concluded that, in order to better understand earthquake cycles and seismic hazards, it is important to consider a fault zone as a whole, including multiple faults and their interaction on the earthquake triggering between nearby faults.

Coseismic Coulomb stress changes induced by a 2020–2021 MW > 7.0 Alaska earthquake sequence in and around the Shumagin gap and its influence on the Alaska-Aleutian subduction interface

Three MW > 7.0 earthquakes in 2020–2021 occurred in the Shumagin seismic gap and its adjacent area of the Alaska-Aleutian subduction zone, including the MW7.8 Simeonof thrust earthquake on July 22, 2020, the MW7.6 Sand Point strike-slip earthquake on October 19, 2020, and the MW8.2 Chignik thrust earthquake on July 29, 2021. The spatial and temporal proximity of these three earthquakes prompts us to probe stress-triggering effects among them. Here we examine the coseismic Coulomb stress change imparted by the three earthquakes and their influence on the subduction interface. Our results show that: (1) The Simeonof earthquake has strong loading effects on the subsequent Sand Point and Chignik earthquakes, with the Coulomb stress changes of 3.95 bars and 2.89 bars, respectively. The Coulomb stress change caused by the Sand Point earthquake at the hypocenter of the Chignik earthquake is merely around 0.01 bars, suggesting the negligible triggering effect on the latter earthquake; (2) The triggering effects of the Simeonof, Sand Point, and Chignik earthquakes on aftershocks within three months are not well pronounced because of the triggering rates of 38%, 14%, and 43% respectively. Other factors may have played an important role in promoting the occurrence of these aftershocks, such as the roughness of the subduction interface, the complicated velocity structure of the lithosphere, and the heterogeneous prestress therein; (3) The three earthquakes caused remarkable coseismic Coulomb stress changes at the subduction interface nearby these mainshocks, with an average Coulomb stress change of 3.2 bars in the shallow region directly inwards the trench.

Dynamic Rupture Process of the 2023 Mw 7.8 Kahramanmaraş Earthquake (SE Türkiye): Variable Rupture Speed and Implications for Seismic Hazard

AbstractWe considered various non‐uniformities such as branch faults, rotation of stress field directions, and changes in tectonic environments to simulate the dynamic rupture process of the 6 February 2023 Mw 7.8 Kahramanmaraş earthquake in SE Türkiye. We utilized near‐fault waveform data, GNSS static displacements, and surface rupture to constrain the dynamic model. The results indicate that the high initial stress accumulated in the Kahramanmaraş‐Çelikhan seismic gap leads to the successful triggering of the East Anatolian Fault (EAF) and the supershear rupture in the northeast segment. Due to the complexity of fault geometry, the rupture speed along the southeastern segment of the EAF varied repeatedly between supershear and subshear, which contributed to the unexpectedly strong ground motion. Furthermore, the triggering of the EAF reminds us to be aware of the risk of seismic gaps on major faults being triggered by secondary faults, which is crucial to prevent significant disasters.

Nature of the Himalayan Seismicity Belt in the Garhwal–Kumaun Segment and Its Tectonic Implications

Abstract High-quality data recorded by a dense network of 53 seismic stations in the Garhwal–Kumaun Himalaya between February 2017 and December 2021 is analyzed. A total of 813 local earthquakes are relocated using a newly developed regional 1D velocity model incorporating station corrections. In addition, focal mechanism solutions of M ≥ 3.8 events are estimated using waveform inversion. The relocated seismicity patterns along with the focal mechanism solutions are utilized to present a seismotectonic scenario of the region. Almost 95% of the relocated seismicity is found to be clustered along the Himalayan seismic belt (HSB), down to ∼24 km depth. Seismicity in this belt is interpreted to be caused due to interseismic stress loading associated with the ongoing India–Eurasia collision tectonics. A few scattered hypocenters in the deeper crust between 30 and 50 km depth attest the strength of the downgoing Indian plate. Focal mechanisms in the seismogenic upper crust reveal thrusting of the Indian plate beneath the Lesser Himalaya, with compression normal to the strike of the Main Central Thrust (MCT). The north-dipping thrust mechanisms can be associated with a near-horizontal Main Himalayan Thrust (MHT). In addition, more steeply dipping faults above it define the Lesser Himalayan duplex systems, similar to those in western and Nepal Himalaya. A prominent ∼50 km wide seismicity gap region observed within the HSB is probably due to (1) a locally varying locking width of the MHT; (2) an unruptured, ductile segment at the eastern end of the rupture zone of the great 1803 earthquake (Mw 7.8 ± 0.2); and (3) a slab tear in the MHT, similar to those in subduction zones.

Relation Between Oceanic Plate Structure, Patterns of Interplate Locking and Microseismicity in the 1922 Atacama Seismic Gap

AbstractWe deployed a dense geodetic and seismological network in the Atacama seismic gap in Chile. We derive a microseismicity catalog of >30,000 events, time series from 70 GNSS stations, and utilize a transdimensional Bayesian inversion to estimate interplate locking. We identify two highly locked regions of different sizes whose geometries appear to control seismicity patterns. Interface seismicity concentrates beneath the coastline, just downdip of the highest locking. A region with lower locking (27.5°S–27.7°S) coincides with higher seismicity levels, a high number of repeating earthquakes and events extending toward the trench. This area is situated where the Copiapó Ridge is subducted and has shown previous indications of both seismic and aseismic slip, including an earthquake sequence in 2020. While these findings suggest that the structure of the downgoing oceanic plate prescribes patterns of interplate locking and seismicity, we note that the Taltal Ridge further north lacks a similar signature.

Neotectonics of the eastern Matano fault, Sulawesi, Indonesia: Preliminary results

The Matano fault in central Sulawesi is one of the major strike-slip faults that accommodate the rapid left-lateral slip between the Pacific and Australian plates. The Matano fault has a high seismic potential of producing large (Mw ≥7) earthquakes. However, the Matano fault was less studied, unlike the Palu-Koro fault. This paper presents preliminary results of our active fault investigation in the eastern Matano fault which may help in understanding the neotectonics and earthquake hazard in central Sulawesi. The late Quaternary deformation of the eastern Matano fault is evidenced by deformed young geomorphic surfaces. The fault shows predominantly left-lateral motion and has steeply dipping fault plane. The eastern portion of the Matano fault may form a seismic gap that potentially host the next surface-rupturing earthquake.

Asymmetric Bilateral Rupture of the 2022 Ms 6.8 Luding Earthquake on a Continental Transform Fault, Tibetan Border, China

Abstract The Xianshuihe (XSH) fault on the eastern boundary of the Tibetan plateau is one of the most active faults in the world. However, the Moxi fault, the southernmost segment of the XSH fault, remained seismically quiescent for over 230 yr, with a large slip rate and high locking degree. Thus, this region is considered to be a seismic gap capable of hosting earthquakes of Mw 7.0 or greater. In September 2022, the Ms 6.8 Luding earthquake occurred on the Moxi fault, resulting in destructive landslide damage, with 93 people dead and 25 missing. Using regional and global seismic recordings, we integrate the multiple-point-source analysis, finite-fault inversions, and backprojection imaging to investigate in detail the rupture process associated with the 2022 Luding, China, earthquake. Our results show that this event is characterized by an asymmetric bilateral rupture with three episodes: (1) an initial bilateral rupture propagation mainly toward north-northwest; (2) major rupture propagating toward south-southeast and up-dip direction; (3) south-southeast-rupture propagation speed accelerates to ∼2 km/s. Over 70% of the seismic moment is released at shallow depths <12 km thus explaining significant damaging effects. Furthermore, the seismic potential of the remaining locked portions of the XSH and Anninghe faults is still high and positive increases of the Coulomb stress on these faults due to the Luding earthquake might bring them closer to future failure.

Spatio-temporal variations of seismic coupling along a transform fault: the western North Anatolian Fault Zone

SUMMARY The Main Marmara Fault (MMF) forms a major segment of the North Anatolian Fault Zone (NAFZ) in northwestern Türkiye. The MMF represents a seismic gap with currently high seismic hazard and associated risk for the Istanbul metropolitan area. Here we estimate the seismic coupling defined as the ratio of the seismic strain rate to the tectonic strain rate, for the MMF and adjacent NAFZ segments. This ratio indicates the fraction of total strain accumulated with time that is released seismically. We compare the results of seismic strain rates and coupling estimated from earthquakes included in historical and instrumental catalogues, which allows us to identify fault segments that represent a considerable seismic threat during the current seismic cycle. We find that along the main fault traces hosting the large events, seismic strain rates from the historical catalogue are of the same order as the tectonic strain rates. In contrast, coupling estimates based on seismic data from the instrumental catalogue covering also off-fault areas, are up to 100 times smaller, highlighting that most of the seismic energy is released in large earthquakes with recurrence times longer than the time covered by the instrumental catalogue. Within the Sea of Marmara, a significant portion (48%) of shear strain from the instrumental catalogue is currently being accommodated by seismic deformation. Significant variations of the seismic coupling are observed before and after the 1999 M > 7 Izmit earthquake, highlighting the different contribution of aseismic slip over different portions of the seismic cycle. A comparison of the temporal evolution of the 1999 Izmit and Düzce post-seismic deformation with seismic strain rates shows that the largest seismic strain rates coincide with the largest post-seismic deformation.

Relocation of the 8 January 2022 Menyuan (Ms 6.9), China, earthquake sequence: A conjugated type of rupturing event with a seismic gap in the Qilian-Haiyuan fault in northeast Tibet

On 8 January 2022, an earthquake with Ms 6.9 struck Menyuan County, Qinghai Province, China, which provides an opportunity to investigate the seismotectonics of the Qilian-Haiyuan fault in northeast Tibet. We collect seismic observation data from China Seismic Network Center from 8 January to 6 February 2022 around the 2022 Menyuan mainshock. The double-difference relocation algorithm is applied to relocate 904 earthquakes, and 820 high-precision hypocenter locations are obtained. The mainshock epicenter was located at 10.2 km depth in the Tuolaishan fault. The aftershock sequence extended towards nearly E-W along the Tuolaishan fault and NW-SE along the Lenglongling fault, parts of the Qilian-Haiyuan fault, but there were fewer aftershocks at the beginning of the rupture near the intersection between the Lenglongling and Tuolaishan faults, suggesting a conjugated type of rupturing with a seismic gap. Integrating with focal mechanism solutions of mainshock and one large aftershock, we suggest that the rupture is a left-lateral strike-slip event with a high dip. The stress change around the faults intersection. The 1986 Ms 6.4 earthquake may trigger the 2016 Ms 6.4 earthquake, and then the extrusion stress could transfer to around the 2022 Ms 6.9 earthquake source area due to the Indo-Eurasian collision. Previous tomographic models revealed low-velocity anomalies under the Menyuan earthquakes, reflecting that fluids could decrease friction coefficient and normal stress on the fault planes and further promote the generation of these earthquakes. These fluids could be contained in the lower crustal flow and mantle upwelling flow under northeast Tibet.

Characteristics and Predictive Significance of Spatio-Temporal Space Images of M ≥ 4.0 Seismic Gaps on the Southeastern Margin of the Tibetan Plateau

In the present study, seismic gaps were identified as periods with no occurrence of M ≥ 4.0 earthquake over dT ≥ 400 days. After examining all records in the Sichuan–Yunnan–Tibet–Qinghai junction area on the southeastern margin of the Tibetan Plateau in 1970–2022, a total of six M ≥ 4.0 seismic gaps were identified. Spatio-temporal images of the seismic gaps had similar characteristics and demonstrated spatial overlapping and statistical significance. The quiet periods of the six seismic gaps included 419–777 days (approximately 580 days on average). The semi-major-axis and semi-minor-axis lengths were in the 880–1050 km (approximately 987 km on average) and 500–570 km (about 533 km on average) ranges, respectively. Case analysis results revealed that the images of M ≥ 4.0 seismic gaps were of high significance in predicting M ≥ 6.7 strong earthquakes in the region, and they could be used as a predictive index on a time scale of about 1–0.5 years or less.

Investigation of Pounding between Two Adjacent Building Models with Experimental Methods

This study investigates the pounding phenomenon by shaking table experiments on two scaled building models. Representing the situation where the seismic gap is insufficient, two building models are adjacently positioned on the shaking table, and pounding was investigated for harmonic and strong ground motion excitations. Displacement and acceleration responses were obtained to observe the pounding effect experimentally from video and accelerometer recordings, respectively. The Kelvin-Voigt Model consisting of spring and damper was used for numerical pounding analysis. The most critical parameters of the Kelvin–Voigt model, which are the spring stiffness (ks) and the damping, are calculated according to the coefficient of restitution (r) and were investigated and compared with harmonic experimental results. The obtained parameters, compatible with the harmonic experiments, were used to examine structural behavior under earthquake effect for the case where the building models are positioned for an insufficient seismic gap. For comparison, numerous numerical simulations were realized using different spring stiffnesses and coefficients of restitution. The study shows that when the coefficient of restitution is taken as 0.2 or 0.4, and the ratio of spring stiffness to shear stiffness (ks/k) is 1 or 5, reasonable results in numerical earthquake simulations can be obtained.

Abundance, characteristics, and removal of microplastics in the Cihu Lake-wetland microcosm system.

Sewage treatment plants (STPs) are significant routes through which microplastics (MPs) are released into the aquatic environment. Constructed wetland is an effective facility for deep treatment of tailwater. At present, research on the removal of MPs in the tailwater of STPs by multi-stage constructed wetlands is limited. This work investigated and analyzed the removal characteristics of MPs in the tailwater treatment system of Cihu wetland park in Huangshi, Hubei Province of China. The abundance/removal of MPs in the Cihu Lake-wetland microcosm system was investigated. The results showed that the multi-stage constructed wetlands achieved a total removal rate of 94.7% for MPs with 2.2 particles/L MPs in the effluent. The removal rates of MPs reached 89 and 37.5%, respectively, in the (horizontal/vertical) subsurface flow constructed wetland and surface flow constructed wetland. The abundance of MPs in receiving water of Cihu Lake substantially decreased due to the dilution of wetland effluents. This study partially bridged the knowledge gap hypothesis on the treatment of MPs in tailwater by multi-stage constructed wetlands.

Late Medieval seismicity on the Himalayan Frontal Thrust Fault at Lal Dhang, Uttarakhand, India

The Himalayan Frontal Thrust Fault (HFT) lies at the active tectonic boundary between Eurasia and the Indian subcontinent. Past earthquakes on the HFT have resulted in significant ground displacement that is evidenced by sizable scarps along the fault. Here we present findings from an ~28-m-long x 5-m-wide x 8-m-deep paleoseismic trench excavated across an ~10-m-high fault scarp at the site of Lal Dhang in the Central Seismic Gap. Excavation exposed three recumbent fault-propagation folds that support occurrence of at least two earthquakes with >25 m of cumulative coseismic slip. Findings suggest that two great (Mw ≥ 8.0) earthquakes may have occurred in rapid succession at the site during the Late Medieval period. Radiocarbon dating of stratigraphic units that are modeled using the OxCal software program constrain timing of the events to 1317–1391 CE and 1447–1572 CE, which is coincident with historical earthquakes in 1344 CE and 1505 CE. With over 500 years of quiescence since the last great earthquake in the Central Seismic Gap, it is possible that another devastating rupture could be imminent.

2022 Mw 6.6 Luding, China, Earthquake: A Strong Continental Event Illuminating the Moxi Seismic Gap

Abstract On 5 September 2022, an Mw 6.6 strike-slip earthquake occurred in Luding, Sichuan, China. It illuminates the seismic gap on the southern end of Xianshuihe fault zone, that is Moxi segment, and is the most damaging earthquake in Chinese Mainland since 2014. In this article, we use multiple seismological methods to analyze the basic characteristics of the Luding earthquake, including its source parameters, rupture process, aftershock distribution, and further discuss its implications on preseismic fault behavior and impact on short-term seismic hazard. Using near-field stations, we first revisit the source parameters of the mainshock and obtain a much shallower hypocentral depth (∼9.3 km) than previously reported. Next, we jointly inverse the teleseismic P-wave data and near-field strong-motion data to constrain the mainshock rupture process. The coseismic slip consists of two subevents that spans for ∼20 km and shows an overall unilateral rupture to the south-southeast direction. The earthquake has 90% of the seismic moment released in its first 10 s. Third, we relocate the first 34 days of aftershocks with both manual phase picks and cross correlation, and finally obtain 5269 well-located events. The aftershocks reveal a near-vertical Xianshuihe fault and two sets of unmapped faults off the major Xianshuihe fault: one set of strike-slip faults on the north-northwest of rupture and the other one on the west of mainshock that generates the most intense seismicity with prevalent normal faulting. By examining the aftershock evolution and Coulomb stress change, we find that postseismic processes evolve both on and off the major fault, and the western off-fault system has highly variable fault orientations. Finally, we calculate the Coulomb stress change by the Luding earthquake on surrounding faults. Results show that the north Anninghe fault is significantly loaded by the Luding earthquake (>0.1 MPa), which poses high seismic hazard in the short term.

Earthquake spatial probability and hazard estimation using various explainable AI (XAI) models at the Arabian peninsula

Earthquakes are the most destructive natural hazards because of their adversely severe impacts on urban areas. Earthquakes affect people's lives and properties, thus captivating the extensive attention of seismologists. Carrying out probability and hazard assessment for the prevention, and reduction of mega-events and recovery will be of great significance in affected areas. Given that limited studies have attempted to estimate earthquake Spatial Probability Assessment (SPA) in the Arabian Peninsula, this study aims to evaluate the SPA and Earthquake Hazard Assessment (EHA). This study implements and evaluates various machine learning and explainable-AI (XAI) techniques for the estimation of SPA and EHA in the Arabian Peninsula, explores the contribution and highlights the importance of different factors in the development of AI-based models. A total of twelve factors ranging from seismological to geophysical factors were evaluated. Two machine learning models namely Light Gradient Boosting Machine (LightGBM) and deep Recurrent Neural Networks (RNN) along with three XAI approaches (i.e, Smart predictor, Smart Explainer and Local Interpretable Model-Agnostic Explanation (LIME) model) were investigated. Results of the comparative earthquake SPA estimation demonstrated that the accuracy of 89% and 87% were achieved by LightGBM and RNN models. Moreover, the results of the XAI models show that the Smart Predictor provides better spatial outputs than the other evaluated XAI models. The stable factors identified by Smart Predictor were magnitude variation and earthquake frequency whereas the important factors were magnitude variation, earthquake frequency, depth variation, and seismic gap. Collectively, results of SPA show that, the Gulf of Aden, Red Sea, Iran, and Turkey are falling under a very-high SPA index (0.991–1). Correspondingly, Gulf areas, coastal areas of Saudi Arabia, and areas in the Zagros fault and Anatolian fault zone fall under a very-high hazard zone. This research could support planners, and decision-makers for emergency planning, infrastructure development, and reconstruction projects.

Characterization of the Structural Typologies of Buildings in the Lima Metropolitan Area

The Lima metropolitan area is in a prone earthquake zone, which is exposed to a seismic gap; this can result in Mw 8.9 megathrust earthquake in the central Peru. After the Pisco earthquake in 2007, risk management policies changed in Peru and are improving annually. Thus, more resources are destinated to assessing earthquake hazard, vulnerability, and risk in districts of the Lima metropolitan area mainly; nevertheless, they are limited to cover in detailed the whole extension of probable affected area after this potential megathrust earthquake. Data of buildings in the Lima metropolitan area were analyzed in this paper, to provide useful information in the development of rapid methodologies of identification of buildings. The Japan Peru Center for Earthquake Engineering and Disaster Mitigation (CISMID) gathered these data from 2010 to 2021 in field surveys to conduct vulnerability and risk studies; the survey of building characteristics consisted of obtain mainly the number of stories, predominant material, structural systems, type of occupancy, and state of conservation. Vulnerability and risk studies for districts of the Lima metropolitan area were conducted by considering one representative building in one block. Results showed that the predominant material in the Lima metropolitan area is masonry, used in different structural systems, such as confined masonry and other which are nonengineered structures. Also, two-story buildings are predominant in the target area, followed by one and three-stories buildings. Risk assessment for the Lima metropolitan area and the study area were conducted using the data from the characterization of structural typologies obtained in this study. Approximately, 47.72% and 58.67% of buildings presented a very high-risk level in the Lima metropolitan area and the study area, respectively, under the expected earthquake scenario due to the seismic gap in the central Peru.

Next generation ground-motion prediction equations for Indo-Gangetic Plains, India

Seismic hazard analysis (SHA) of Indo-Gangetic Plains (IGP), India, and its proximity to the Himalayas requires reliable ground motion prediction equations (GMPEs). This study attempts to derive the GMPEs for IGP using strong-motion accelerometer data recorded from 2005 to 2015 on IGP. For regression, peak ground acceleration (PGA) and pseudo-spectral acceleration (PSA) of 5% damped linear pseudo-absolute acceleration response spectra at 27 periods ranging from 0.01–10 s were used. Two-stage nonlinear regression trains the functional form of nonlinear magnitude scaling, distance scaling, and site conditions. The model includes a regionally independent geometric attenuation finite fault distance metric, style of faulting, shallow site response, basin response, hanging wall effect, hypocentre depth, regionally dependent anelastic attenuation, site conditions, and magnitude-dependent aleatory variability. Developed GMPEs are validated for active crustal continental earthquakes for epicentral distances REPI ranging from 10–1500 km, magnitude ranging from 3.3–7.9, and focal depths 1–70 km. The GMPEs developed are compared with the Campbell and Bozorgnia 2008, 2013 and 2014, and North Indian GMPEs, which are agreed upon consistently. The model can predict the horizontal ground motion for SHA on IGP, considering possible maximum magnitude earthquakes from the seismic gaps of the Himalayas.

Crustal structure and fault geometries of the Garhwal Himalaya, India: Insight from new high-resolution gravity data modeling and PSO inversion

A high-resolution gravity survey from the Indo-Gangetic plain to the MCT zone is carried out across the Garhwal Himalaya, India, which lies in the central seismic gap. The gravity measurements along a ∼205 km transect with dense station interval (<650 m) are modeled using the power spectrum, gravity isolation technique, wavelet analysis, PSO inversion, 2D-forward modeling, and constraints from previous results. The spectral analysis suggests two dominant layer interfaces in the lithosphere at 44 km and 12 km, inferred as the Moho and the Himalayan decollement thrust, respectively. The gravity isolation technique contributes to assessing the previously known geophysical and geological information and separating individual gravity responses despite the complexity of the tectonics of the Himalaya and further deriving the gravity effect of the upper crustal region, including main fault/thrust zones. The computed residual gravity anomaly shows prominent signatures of the main tectonics structures. Wavelet analysis and the Particle Swarm Optimization (PSO) inversion constrain the geometries of the intracrustal fault/thrust zones. Our model shows an increase in the Moho depth towards the north (from ∼40.4 km to 43.5 km). The derived MHT geometry clearly shows the ramp structure and associated Lesser Himalayan Duplex structures towards the north. The proposed detailed density structure discerns the geometries of different crustal structures across the Garhwal Himalaya and gives insight to the upper crustal fault zones.

Imaging the 2007 Mw 7.7 Tocopilla earthquake from short-period back-projection

The 14 November 2007 Tocopilla (MW 7.7) earthquake was the first large rupture (M > 7.5) in the 1877 Northern Chile seismic gap. The event only ruptured the lower seismogenic part of the megathrust, raising questions about the mechanism that prevented the trenchward rupture propagation. Here, we present the short-period rupture process of the Tocopilla earthquake from local strong-motion (2.0–8.0 Hz) and teleseismic (0.5–2.0 Hz) back-projections. P and pP teleseismic back-projections were combined from two arrays formed on broadband networks in North America and Africa. The kinematics of the earthquake rupture was also characterized by back-propagating S-wave envelopes from local accelerometers (≤ 200 km from the mainshock). The results show a complex rupture process, including a sub-event, from the distribution of short-period rupture emissions and the main slip asperities. The sub-event location agrees with published estimates based on S-wave arrivals and kinematic inversion. We also observed rupture emissions around the slip patches and systemically classified them relative to the asperities, i.e., down-dip, up-dip, and inside. The short-period emissions are balanced around the asperities, highlighting up-dip rupture emissions. We interpreted the up-dip emissions as a proxy of the high-stress gradient caused by a kink in the slab interface, proposed in previous literature to be the mechanism that arrested the trenchward propagation of the 2007 rupture. The down-dip limit of the Tocopilla event coincided approximately where the slab interface intersects the continental Moho. Thus, it is a region expected to be extensively serpentinized, i.e., the most seaward part of the mantle wedge, defining a velocity-strengthening margin and a source of short-period radiation. The high-stress gradient around the asperities is proposed to define a third source of short-period radiation that may contribute to rupture encircling emissions.

Oblique Blind Faulting Underneath the Luzon Volcanic Arc During the 2022 Mw 7.0 Abra Earthquake, the Philippines

AbstractUnknown seismogenic structures lurking beneath convergent margins introduce substantial uncertainty in seismic hazard assessments. In northwestern Luzon, the Mw 7.0 Abra earthquake on 27 July 2022 highlights the seismic activity along an unmapped blind fault underneath the Cordillera Central. By integrating coseismic displacements constrained by radar satellite imagery and Global Navigation Satellite System, we image oblique coseismic slip at 11–22 km depth with peak slip of ∼1 m beneath the Philippine Fault Zone in northern Luzon. The southward propagation of coseismic slip and aftershocks terminated at a distance of 50 km from the northern end of the 1990 Luzon earthquake rupture, leaving a seismicity gap in between. Coulomb stress changes of reaching 2 bars are imparted at the shallow portions of the Vigan‐Aggao and Abra River faults, where the updated 100‐year seismic potential is increased to Mw 7.0–7.7, given the thick seismogenic layer of ∼30 km in northern Luzon.