Swarm Sequence Research Articles

The Evolution Process between the Earthquake Swarm Beneath the Noto Peninsula, Central Japan and the 2024 M 7.6 Noto Hanto Earthquake Sequence

Several physical mechanisms of earthquake nucleation, such as pre-slip, cascade triggering, aseismic slip, and fluid-driven models, have been proposed. However, it is still not clear which model plays the most important role in driving foreshocks and mainshock nucleation for given cases. In this study, we focus on the relationship between an intensive earthquake swarm that started beneath the Noto Peninsula in Central Japan since November 2020 and the nucleation of the 2024 M7.6 Noto Hanto earthquake. We relocate earthquakes listed in the standard Japan Meteorological Agency (JMA) catalog since 2018 with the double-different relocation method. Relocated seismicity revealed that the 2024 M7.6 mainshock likely ruptured a thrust fault above a parallel fault where the M6.5 Suzu earthquake occurred in May 2023. We find possible along-strike and along-dip expansion of seismicity in the first few months at the beginning of the swarm sequence, while no obvious migration pattern in the last few days before the M7.6 mainshock was observed. Several smaller events occurred in between the M5.5 and M4.6 foreshocks that occurred about 4 and 2 minutes before the M7.6 mainshock. The Coulomb stress changes from the M5.5 foreshock were negative at the hypocenter of the M7.6 mainshock, which is inconsistent with a simple cascade triggering model. Moreover, an M5.9 foreshock was identified in the JMA catalog 14 s before the mainshock. Results from back-projection of high-frequency teleseismic P waves show a prolonged initial rupture process near the mainshock hypocenter lasting for ∼25 seconds, before propagating bi-laterally outward. Our results suggest a complex evolution process linking the earthquake swarm to nucleation of the M7.6 mainshock at a region of complex structures associated with the bend of a mapped large-scale reverse fault. A combination of fluid migration, aseismic slip and elastic stress triggering likely work in concert to drive both the prolonged earthquake swarm and the nucleation of the M7.6 mainshock.

Fluids-Triggered Swarm Sequence Supported by a Nonstationary Epidemic-Like Description of Seismicity

Abstract The variation in Coulomb failure stress (CFS) plays a crucial role in either increasing or decreasing seismic activity. In cases in which the standard epidemic-type aftershock sequence (ETAS) model does not adequately fit seismicity data, the potential deviations from empirical laws are explored. These deviations may arise from stress changes imparted by aseismic transients that lead swarm-like earthquake sequences to occur. The time-dependent background rate of seismicity serves as an indicator for detecting changes in CFS or the presence of transient aseismic forcing. We investigate seismic anomalies in the slow deforming Mt. Pollino, Italy seismogenic area, where a 4-yr-long swarm-like sequence partially filled a previously hypothesized seismic gap. The primary process of this seismic swarm is still under debate. Employing a nonstationary ETAS model on a new template-matching high-resolution catalog, we suggest a slow-slip event and fluid interplay as the main aseismic forces in triggering and developing this swarm-like sequence.

Unraveling Earthquake Clusters Composing the 2014 Alto Tiberina Earthquake Swarm via Unsupervised Learning

AbstractEarthquake swarms represent a particular mode of seismicity, not directly related to the occurrence of large earthquakes (e.g., aftershocks) but rather driven by external forcing such as aseismic deformation or fluid migration in fault systems. Sometimes their occurrence overlaps with observable geodetic signals in space and time, indicating a direct link. However, the low resolution of geodetic observations tends to obscure the small scale spatial and temporal dynamics of swarms. In this work, we automatically extract clusters of seismicity related to the 2014 Alto Tiberina swarm sequence (Italy) using an unsupervised clustering approach that exploits space and time information of the seismicity. The quantitative characterization of each cluster indicates that the overall swarm is composed of spatially and temporally confined (sub) swarms each of which could potentially be driven by small‐scale aseismic deformation process. This observation aligns with similar findings during slow slip events in subduction zones.

Analysis of Swarm Earthquakes on the Territory of the Elbrus Volcanic Center

The paper presents the results of the analysis of swarm seismic events recorded in 2018 in the area of the Elbrus volcanic center by the North Caucasian seismological network of the FRC GS RAS. The standard method of station processing shows the hypocenters of recorded events in the area of the Elbrus volcanic center. The hypocenters of seismic events in swarm sequences were refined using the LOS software package, a complex combined location algorithm, methods of minimizing the time discrepancy in the source and grid search with depth enumeration. The described approach improves the reliability of location due to accounting for inaccuracies associated with incorrectly taken times of phase arrivals and minor deviations in travel times, depending on the velocity model of the medium used. According to the results of the study of the focal mechanisms of the strongest earthquakes, the type of movement was established, which is consistent with the kinematics of the Elbrus-Mineralnye Vody fault zone. Correlation analysis of the waveforms of the vertical component filtered in the frequency range of 1‒10 Hz, including P and S waves, for the nearest station shows a high coincidence of the analyzed data (correlation coefficient R ≥ 0.85), which indicates a high probability that events have similar sources and paths propagation of seismic waves. Comparison of the power spectral density of individual earthquakes in swarm sequences also demonstrates high similarity.

Современная группирующаяся низкомагнитудная сейсмичность срединно-океанического хребта Гаккеля

Earthquakes at mid-ocean ridges (MOR) reflect the active magmatic and tectonic processes that form new oceanic crust. Active spreading processes of ultraslow ridges, with spreading rates of less than 20 mm/yr, are still poorly understood compared to the MOR in the Atlantic and Pacific oceans, with spreading rates of more than 25 mm/yr. Thanks to the installation of stationary seismic stations in the 21st century on the Arctic archipelagos of Franz Josef Land and Severnaya Zemlya, it became possible to record and study low-magnitude grouping earthquakes within the Gakkel Ridge, including swarm sequences of earthquakes. This article presents the first results of registration, location and study of swarm sequences of low-magnitude earthquakes within the Gakkel Ridge for the period from 2012 to 2022. It is shown that swarm sequences are recorded to a greater extent in the western volcanic and eastern volcanic segments of the ridge, while no such sequences were found within the central amagmatic segment. The structure formation of this part of the ridge is dominated by tectonic rather than magmatic and metamorphic processes. Two large swarm of low-magnitude earthquakes that occurred in the eastern volcanic segment of the ridge are considered in detail.

Seismic swarms as intermittent quasi-static ruptures driven by pore pressure variations due to the water reservoir impoundment

Seismic swarms are characterized as time-space clustered sequences of earthquakes without a dominating strong event. The occurrence of earthquake swarms has been observed in various environments and found out to be closely related to underground migration of fluids. Mechanical interactions between fluid-conduit properties of fault and rupture determine the spatio-temporal complexity of rupturing process and earthquake occurrence. Often, seismic swarms are triggered by Water Reservoir Impoundment (WRI). Although many studies have investigated the relationship between the pore pressure changes due to WRI and observed seismicity, hydromechanical models that explain the observed processes are limited. Here, we investigate the role of hydromechanical interactions in triggering earthquake swarms in WRI conditions, using data from the Song Tranh 2 Reservoir in Vietnam as an example. We found that seismic swarms evolve due to stress redistribution and subsequent induced fluid flow. Swarm sequences form interconnected rupture increments and their aggregated behaviour is similar to a single large earthquake that ruptures the fault segment at once. The evolution of friction with the mean slip retrieved from the swarm earthquakes exhibits both, slip weakening and strengthening stages, resembling linear slip-weakening friction. The effective slip over which the weakening is completed, is modulated by the pore pressure diffusion. These results provide insights into the physical mechanisms of seismic swarms and have implications for seismic hazard.

Characteristics of earthquake swarm activity observed in the Palghar region of Indian Peninsula from January 2019 to October 2020

The present study emphasizes the seismic characteristics of earthquake swarm activity of the Palghar region that occurred in the hard strata of Deccan traps of the Indian peninsula. This activity is well monitored by National Centre for Seismology (NCS) from December 2018 to October 2020. The related data inferred that initially before the swarm activation there was a barricade to stop the entry of fluids into the permeable zone. Later the fluids are injected into the fault zone at a feasible place may be near the surface at the time of swarm initiation. Further the associated fluids percolated through several channels in many directions forming a multi-dimensional system of cracks and fissures at the level of swarm progression through two distinctive clusters during the time of present study along normal fault orientations. During this process, the heterogeneous material within the subsurface layers might have experienced a decrease in the effective stress due to the increase in the pore pressure which is the resultant of the percolation of fluids within the permeable zone. The outcome of the results shows a comparable strategic increase in the seismicity after the soil saturation during and after the rainfall in the affected zone. Also, the study of the seismicity accompanying fluid injections indicates that the fluid triggering seismicity is demonstrated by the increased slope of the magnitude frequency distribution in the form of spatial b-value which has been found reliable for a catalogue for Palghar swarm activity. The coulomb stress change is also examined to find out the consistency of stress patterns with the earthquake occurrences in the Palghar region which gives the characteristics of swarm sequence. The study emphasized the fundamental and geo-mechanical behavior of fluid migration through multi-directional cracks and fissures as the cause for the evolution and further growth of the Palghar swarm activity.

Swarms and mainshock–aftershocks sequences are both triggered by fluids in the Ubaye Region (Western Alps)

SUMMARYThe Ubaye Region (French Western Alps) is one of the most seismically active regions in France. It is regularly struck by mainshock–aftershocks sequences like in 1959 (ML 5.3), seismic swarms (2003–2004), and complex sequences (2012–2015) characterized by successive mainshocks clustered in time and space. This diversity of seismic behaviour highlights the complex processes at play in this area. To improve our understanding of these processes, in this study, we compile a regional catalogue of existing focal mechanisms, completed by 100 new calculated focal mechanisms of aftershocks following the 07/04/2014 mainshock (ML 5.1). The oriented stress-state we reconstruct for different periods and subareas are similar to each other and to previous published values focusing on swarm periods. We then calculate fluid pressure required to trigger the earthquakes. Most of the events (65 per cent) need fluid overpressure between 15 and 40 MPa (17 to 40 per cent of the hydrostatic pressure) with a median value of 24 per cent. Moreover, even the largest events, like the mainshocks in the 2012–2015 sequence, appear to be triggered by fluid pressure, similarly as events within swarm sequences. However, while fluid overpressure decreases with time in an aftershock sequence, it varies randomly at high levels during a swarm sequence. Therefore, based on a fault-valve model, we propose that: (1) the fluids trapped in the fault plane tend toward lithostatic pressure and trigger the mainshock rupture and (2) part of the aftershocks are induced by the diffusing fluid pressure. On the contrary, swarms need external, likely deep, fluid pressure feedings. Fluid pressure is likely to be a common triggering mechanism of the seismicity in the Ubaye Region, even if the involved processes should differ to explain the different types of seismic sequences.

Seismicity modulation due to hydrological loading in a stable continental region: a case study from the Jektvik swarm sequence in Northern Norway

SUMMARYSeismic swarms have been observed for more than 40 yr along the coast of Nordland, Northern Norway. However, the detailed spatio-temporal evolution and mechanisms of these swarms have not yet been resolved due to the historically sparse seismic station coverage. An increased number of seismic stations now allows us to study a nearly decade-long sequence of swarms in the Jektvik area during the 2013–2021 time window. Our analysis resolves four major groups of seismic events, each consisting of several spatial clusters, that have distinct spatial and temporal behaviours. Computed focal mechanism solutions are predominantly normal with NNE–SSW strike direction reflecting a near-vertical maximum principal stress and a NW–SE near-horizontal minimum principal stress, which are controlled by local NW–SE extension. We attribute the swarms to fluid-saturated fracture zones that are reactivated due to this local extension. Over the time period, the activity tends to increase between February and May, which coincides with the late winter and beginning of spring time in Norway. We hypothesize that the seismicity is modulated seasonally by hydrological loading from snow accumulation. This transient hydrological load results in elastic deformation that is observed at local Global Navigation Satellite System stations. The loading is shown to promote failure in a critically stressed normal faulting system. Once a segment is activated, it can then also trigger neighboring segments via stress transfer. Our new results point to a close link between lithosphere and hydrosphere contributing to the occurrence of seismic swarm activity in northern Norway.

Volcanic earthquake catalog enhancement using integrated detection, matched-filtering, and relocation tools

Volcanic earthquake catalogs are an essential data product used to interpret subsurface volcanic activity and forecast eruptions. Advances in detection techniques (e.g., matched-filtering, machine learning) and relative relocation tools have improved catalog completeness and refined event locations. However, most volcano observatories have yet to incorporate these techniques into their catalog-building workflows. This is due in part to complexities in operationalizing, automating, and calibrating these techniques in a satisfactory way for disparate volcano networks and their varied seismicity. In an effort to streamline the integration of catalog-enhancing tools at the Alaska Volcano Observatory (AVO), we have integrated four popular open-source tools: REDPy, EQcorrscan, HypoDD, and GrowClust. The combination of these tools offers the capability of adding seismic event detections and relocating events in a single workflow. The workflow relies on a combination of standard triggering and cross-correlation clustering (REDPy) to consolidate representative templates used in matched-filtering (EQcorrscan). The templates and their detections are then relocated using the differential time methods provided by HypoDD and/or GrowClust. Our workflow also provides codes to incorporate campaign data at appropriate junctures, and calculate magnitude and frequency index for valid events. We apply this workflow to three datasets: the 2012–2013 seismic swarm sequence at Mammoth Mountain (California), the 2009 eruption of Redoubt Volcano (Alaska), and the 2006 eruption of Augustine Volcano (Alaska); and compare our results with previous studies at each volcano. In general, our workflow provides a significant increase in the number of events and improved locations, and we relate the event clusters and temporal progressions to relevant volcanic activity. We also discuss workflow implementation best practices, particularly in applying these tools to sparse volcano seismic networks. We envision that our workflow and the datasets presented here will be useful for detailed volcano analyses in monitoring and research efforts.

Detection and Characterization of Earthquake Swarms in Nankai and Its Association With Slow Slip Events

AbstractVarious types of slow earthquakes occur in the Nankai subduction zone, among which the slow slip events (SSEs) seem to connect the shallow and deep part of the subduction plate and further trigger seismic or aseismic transients in surrounding areas. This study uses a version of space‐time epidemic‐type aftershock sequence (ETAS) model with non‐stationary background rate to detect earthquake swarms that may be related to SSEs in the Nankai region. We detect 999 swarm sequences including 13,387 earthquakes, accounting for about 18 percent of all M ≥ 1.0 earthquakes in the selected region along the Nankai trough. The detected swarms are mostly distributed in the Hyuga‐nada, Bungo Channel, Kii Channel, and Tokai regions, forming a complementary pattern to SSEs. Nearly 70 percent of swarm events are located in western Nankai, with the southern part characterized by a higher occurrence rate, which is consistent with the smaller recurrence interval of SSEs in this region. By comparing occurrence times of swarm events to those of SSEs, we find that some SSEs are accompanied by enhanced swarm activities during their occurrence periods. In addition, the swarm rates increase to higher levels in the southern part of western Nankai during the 2003 and 2010 Bungo SSEs, indicating the high sensitivity of earthquake swarms to the migrations of slow earthquakes in this region.

Seismic activity in the Ubaye Region (French Alps): a specific behaviour highlighted by mainshocks and swarm sequences

Seismic activity in the Ubaye Region (French Alps): a specific behaviour highlighted by mainshocks and swarm sequences

Harmonic convergence coordinates swarm mating by enhancing mate detection in the malaria mosquito Anopheles gambiae

The mosquito Anopheles gambiae is a major African malaria vector, transmitting parasites responsible for significant mortality and disease burden. Although flight acoustics are essential to mosquito mating and present promising alternatives to insecticide-based vector control strategies, there is limited data on mosquito flight tones during swarming. Here, for the first time, we present detailed analyses of free-flying male and female An. gambiae flight tones and their harmonization (harmonic convergence) over a complete swarm sequence. Audio analysis of single-sex swarms showed synchronized elevation of male and female flight tones during swarming. Analysis of mixed-sex swarms revealed additional 50 Hz increases in male and female flight tones due to mating activity. Furthermore, harmonic differences between male and female swarm tones in mixed-sex swarms and in single-sex male swarms with artificial female swarm audio playback indicate that frequency differences of approximately 50 Hz or less at the male second and female third harmonics (M2:F3) are maintained both before and during mating interactions. This harmonization likely coordinates male scramble competition by maintaining ideal acoustic recognition within mating pairs while acoustically masking phonotactic responses of nearby swarming males to mating females. These findings advance our knowledge of mosquito swarm acoustics and provide vital information for reproductive control strategies.

Hydraulically conductive fault zone responsible for monsoon triggered earthquakes in Talala, India

The Deccan volcanic province (DVP) in the northwestern part of the Indian subcontinent is the largest continental volcanic province in the world. Talala, located in the middle of the DVP, experience seismicity with swarm-like behavior towards the end of, or shortly after, the annual monsoon period. Between the years 2007 and 2012, seismic activity was recorded with moment magnitudes up to Mw 5.1. Although there is a statistical correlation between seismic activity and monsoon rainfall, there is little information about the geometry or attributes of the potentially causative fault system to date. We conducted geophysical surveys in the Talala seismogenic region to investigate the structure of the causative fault, with two Time-Domain Electromagnetic (TDEM) profile measurements, and Magnetotelluric (MT) measurements across the fracture zone to characterize deeper structures. TDEM measurements in the E-W direction revealed an electrically conductive zone within highly resistive surroundings, and the MT measurements revealed high electrical resistance along a curved structure that correlates with the seismogenic area and is likely associated with fractured crystalline rock. These fracture systems or networks can act as conduits for channeling and focusing intense rainfall events and develop significant fluid over-pressures to drive the swarm sequences. Numerical modelling results of fluid pressure evolution indicate increased groundwater and reservoir levels are likely responsible for earthquake generation.

Earthquake swarms in Taiwan: A composite declustering method for detection and their spatial characteristics

Earthquake swarms can act as indicators of fluid activity and aseismic transients at depth. As a young and active collisional mountain belt with frequent earthquakes, Taiwan is a natural laboratory for studying the physical mechanisms controlling the characteristics of swarm activity in this complex tectonic environment. Where the earthquake swarms tend to occur and how they reflect the aseismic deformation in this young orogen? In this study, we propose a composite swarm detection method that relies on the correlation of mainshock density rates derived from three distinct declustering algorithms. Using earthquakes in Taiwan from 1990–2019 with magnitudes greater than or equal to three and at depths less than 50 km, 153 swarm sequences were identified as characterized by an event number greater than ten, a time span of less than fifteen days, and a maximum magnitude of 5.79. The swarm sequences were found to be active at locations with a high seismicity rate (>0.2yr−1km−2) but not an anomalous b-value, suggesting that the environment with a high rate of earthquake occurrence is linked with the mechanism for swarm generation. They were found to largely overlap with the repeating earthquakes, as approximately 86% of the 1857 repeating events in Taiwan are closely located to swarm events. Such overlap is found to be largest in the southern Longitudinal Valley where a creeping fault is located and where a low coupling ratio was inferred. Consequently, it is proposed that the active collision boundary experiencing a high creep rate and the areas experiencing episodic aseismic slip are capable of producing active swarm sequences in Taiwan.

Результаты сейсмологических наблюдений на территории Республики Северная Осетия - Алания и в прилегающих районах в 2020 году

North-Ossetian Division of the Geophysical Survey of the Russian Academy of Sciences (NOD GS RAS) carries out the continuous seismological observations of the central part of the North Caucasus region. The branch's seismological network includes 12 observation points located on the territory of the Republic of North Ossetia-Alania. The paper provides basic information about the places of installation of seismic stations and their equipment, and notes the registration possibilities of the network. The total number of registered seismic events and separately within the administrative territories in 2020 is given. The total released seismic energy is calculated. A comparison of the energy indicator with the previous year is given. The schedule of repeatability is calculated and constructed, which indicates the level of reliability of event registration. The analysis of the earthquakes foci distribution with ref-erence to geotectonic structures is presented on the map of the epicenters of recorded seismic events with an indication of the distribution against the background of the focal zones. Iden-tified and described the swarm sequences of seismic events, which occurred in the territories of the Republics of North Ossetia-Alania and Kabardino-Balkaria, as well as in the Stavropol Territory. The mechanisms of the foci for the three most powerful earthquakes are deter-mined and their stereograms are given. The obtained results will allow us to study the chang-es in the seismic regime of the North Caucasus region more effectively.

Stress transfer and poroelasticity associated to major earthquakes in Africa

This study focuses on the stress transfer using the Coulomb Failure Function (CFF) modeling and related poroelastic effect of four major earthquake sequences that occurred in different active zones of Africa: the May to July 1990 Sudan earthquake sequence, the May 2018 to June 2019 Mayotte-Comores earthquake swarm sequence, the 1980–2003 El Asnam-Zemmouri (Algeria) earthquake sequence, and the 1994–2016 Al Hoceima (Morocco) earthquake sequence. We observe the relationship between the stress transfer caused by mainshock fault ruptures and the post-seismic deformation controlled by the aftershock distribution. Based on other case studies, our hypothesis is that all seismic sequences are apparently controlled by the increase in pore fluid pressure caused by co-seismic phase and fluid-drained short-term post-seismic response. The poroelastic properties of any seismogenic zone appear to depend on the undrained and drained fluid conditions. The comparison between the 1990 Sudan sequence and the 1994–2016 Al Hoceima (Morocco) stress modeling shows that for most sequences the poroelastic response of the first mainshock play an important role in the occurrence of the second mainshock. Similar observations can be made for the 2018–2019 Mayotte-Comores and the 1980–2003 El Asnam-Zemmouri (Algeria) earthquake sequences. By comparing the case studies, we find that the value of fluid diffusivity controls the timing of earthquake sequences (e.g., more than three times larger between the Al Hoceima and Sudan earthquakes). The constraint of fault interactions with CFF modeling and fluid diffusivity allows a better estimate of the seismic hazard assessment.

Earthquake Swarm Detection Along the Hikurangi Trench, New Zealand: Insights Into the Relationship Between Seismicity and Slow Slip Events

AbstractEarthquake swarms, which are anomalous increases in the seismicity rate without a distinguishable mainshock, often accompany transient aseismic processes, such as fluid migration and episodic aseismic slip along faults. Investigations of earthquake swarm activity can provide insights into the causal relationship between aseismic processes and seismicity. Slow slip events (SSEs) along the plate interface in the Hikurangi Trench, New Zealand, are often accompanied by intensive earthquake swarms. However, the detailed spatiotemporal distribution of these earthquake swarms is still unclear. Here, we use the epidemic‐type aftershock‐sequence (ETAS) model to detect earthquake swarms (M ≥ 3) and create a new earthquake swarm catalog (1997–2015) along the Hikurangi Trench. We compare the earthquake swarm catalog with Global Navigation Satellite System (GNSS) time series data, and existing SSE and tectonic tremor catalogs. Most of the detected (119) earthquake swarm sequences were intraplate events, and their epicenters were mainly concentrated along the east coast of the North Island, whereas many tectonic tremors were located inland. Twenty‐five of the detected earthquake swarms occurred within 25 days before and after transient eastward GNSS displacements due to known or newly detected SSEs. We find that the earthquake swarms sometimes preceded the GNSS displacements by more than several days. SSE‐induced stress loading is therefore not a plausible triggering mechanism for these pre‐SSE earthquake swarms. We propose that high fluid pressure within the slab, which accumulated before the SSEs, may have caused intraplate fluid migration, which in turn triggered the pre‐SSE earthquake swarms.

Seismicity in a low-strain area: Example of the Irauçuba fault in NE Brazil

The present study illustrates an example of methods and results of fault examination in a slow strain and elastic attenuation area in intraplate Brazil. We investigate the Irauçuba seismogenic fault, which occurs in the Precambrian crystalline basement of the Borborema Province, the Equatorial margin of Brazil. The focal mechanisms in the region show normal, reverse, and mainly strike-slip faulting indicating a regional strike-slip stress regime in the upper crust up to 12 km depth with maximum horizontal compression (SHmax) roughly following the coastline trend. Under such context, we present earthquake data from a swarm sequence, the Irauçuba fault, at which the seismicity does not correlate with surface features. However, events with a magnitude as small as 1.5 mb are felt by the local population, and, in some cases, events with magnitude 2.5 mb or greater have damaged poorly built constructions. Therefore, suitable aftershock monitoring demands detailed macroseismic surveillance and engagement of the affected population and the Civil Defense. This engagement is crucial for awareness of the affected cities and the sensor network design and sustainability of the many stages of the aftershock monitoring such as deployment and maintenance. The earthquake data reveals that the Irauçuba NW-SE-striking fault occurs at depths between 8 and 9 km and dips ~70° to NE. This fault has not been previously reported and is solely defined by the spatial distribution of hypocentres that coincides with the focal mechanism solution, and no obvious surface expression is observed. The fault plane reveals an initial rupture at shallower depths that progressively migrated to deeper portions of the crust. Therefore, we suggest that the initial seismic period transferred stresses to other near-critical areas of the fault, which allowed pore pressure to migrate within permeable portions of the fault zone within the upper crust.

2019 M6.7 Yamagata-Oki earthquake in the stress shadow of 2011 Tohoku-Oki earthquake: Was it caused by the reduction in fault strength?

Earthquake occurrence in the stress shadow provides a unique opportunity for extracting information about the physical processes behind earthquakes because it highlights processes other than the ambient stress change in earthquake generation. In this study, we examined the fault structure and the spatiotemporal distribution of the aftershocks of the 2019 M6.7 Yamagata-Oki earthquake, which occurred in the stress shadow of the 2011 M9.0 Tohoku-Oki earthquake, to better understand the earthquake generation mechanism. The detection and hypocenter relocation of the aftershocks led to the delineation of three planar earthquake structures consitent with their focal mechanisms. The results suggest that individual aftershocks were caused by a slip on these macroscopic planar structures. Aftershock hypocenters rapidly migrated upward from the deeper part of the major plane (fault), similar to the recent earthquake swarm sequences following the Tohoku-Oki earthquake in the upper plate. Moreover, we investigated the temporal evolution of the surface strain rate distribution in the source region using GNSS data. The east–west contraction strain rate in the source region of the Yamagata-Oki earthquake, with an E–W compressional reverse fault mechanism, changed to an E–W extension as a result of the Tohoku-Oki earthquake, and it continued until the occurrence of the Yamagata-Oki earthquake. The upward hypocenter migrations, together with the earthquake occurrence in the stress shadow and in the E–W extension strain rate field, suggest that a reduction in the fault strength due to uprising fluids contributed to the occurrence of this earthquake sequence. Localized aseismic deformations, such as aseismic creeps, beneath the fault zone may also have contributed to the earthquake occurrence. The results support the hypothesis that aseismic processes in the deeper part of the fault play crucial roles in the occurrence of shallow intraplate earthquakes.