Burst Of Seismicity Research Articles

Lifecycle of an Isolated Seismic Swarm in Continental Southern Norway from Nucleation, Acceleration, and Yielding to the Demise of Fault Slip

Abstract An intraplate earthquake swarm started near the Bitdal valley in the seismically inactive mountains of Southern Norway in summer 2020 and continued for more than two years. We detected 1600 earthquakes through template matching and obtained high-resolution relative magnitudes and locations for the events to resolve the swarm’s spatiotemporal evolution and tectonic implications. The detection results reveal that no earthquakes occurred at the swarm site for at least 22 yr prior to the current activity. In July 2020, six months before the swarm’s initially recognized onset, weak precursory events appeared. Hypocenter relocation shows that the swarm develops along a single 1.5 km patch on a northwest–southeast-striking fault with oblique-normal sense containing a left-lateral component. The seismicity follows a dual-velocity migration for which the slow, general migration reverses sense twice, and 30 short bursts show internal migration both with and against the general migration direction. Changes in the distribution of events let us divide the swarm into four phases: precursory activity, accelerating growth, main slip, and slow decline. The observed seismicity bursts, pauses, and dual migration velocities are consistent with a rupture-driven mechanism for which slip initiates spontaneously and keeps going in slow and quick failure cascades due to stress transfer and slip weakening.

Identifying earthquake swarms at Mt. Ruapehu, New Zealand: a machine learning approach

Mt. Ruapehu is an active andesitic stratovolcano, consisting of several peaks with the summit plateau at 2,797 m, making it the tallest active volcano in New Zealand. The extent of the volcano spreads 40 km across with a series of complex faults encompassing almost the entire base of the volcano. A series of earthquakes occurring 20 km west of the summit of Mt. Ruapehu, near the small town of Erua, which preceded the 1995/1996 major volcanic eruption sequence has been proposed as a medium-term precursor for eruptions at Mt. Ruapehu. We use unsupervised machine learning clustering algorithms HDBSCAN and DBSCAN to define anomalous earthquake swarms in the region and determine whether the Erua swarm was unique by identifying key characteristics in space, time and magnitude distribution. HDBSCAN found six spatial cluster zones to the west of Mt. Ruapehu, which have temporal seismic bursts of activity between 1994 and 2023. DBSCAN identified the seismic swarm that preceded the 1995/1996 major eruption, along with one other similar cluster in the same region, which did not coincide with any documented magmatic unrest, suggesting distal seismic swarms at Mt. Ruapehu may not serve as a reliable eruption precursor when observed in isolation. We instead found that earthquake swarms are relatively common at Mt. Ruapehu and the temporal evolution of the earthquake clusters west of Mt. Ruapehu share similar characteristics to seismic swarms identified in other settings related to fluid migration, typical of fault-valve models.

Observation of a Synchronicity between Shallow and Deep Seismic Activities during the Foreshock Crisis Preceding the Iquique Megathrust Earthquake

We analyze at a broad spatial scale the slab seismicity during one of the longest and best recorded foreshock sequence of a subduction earthquake to date: the M8.1 2014 Iquique earthquake in Chile. We observe the synchronisation of this sequence with seismic events occurring in the deep slab (depth ~100km). This synchronisation supports the existence of long-range seismic bursts already observed in the Japan Trench subduction. It suggests that, like for the 2011 Tohoku earthquake, the deep slab was involved in the nucleation process of the Iquique earthquake. We interpret these observations by the presence of pressure pulses propagating in transient fluid channels linking the deep slab where dehydration occurs to the shallow seismogenic zone before the earthquake. These observations may seem surprising but they are in line with the short-lived pulse-like channelized water escape from the dehydration zone predicted by recent studies in slab mineralogy and geochemistry.

Nearby fault interaction within the double-vergence suture in eastern Taiwan during the 2022 Chihshang earthquake sequence

Nearby faults interact through stress changes induced by fault slip and viscoelastic flow. The process is, however, often elusive and can be geometry-dependent and time-variant. Here, we combine geodetic and field observations to characterize the interaction of two head-to-head, conjugate faults in eastern Taiwan during the 2022 Chihshang earthquake sequence. We map the coseismic slip on the Central Range fault and dynamically-triggered shallow slip on the Longitudinal Valley fault, which has been creeping interseismically. Overlapping of seismic and aseismic slip suggests that the Longitudinal Valley fault is capable of hosting a variety of distinct slip behaviors. Moreover, substantial slip on the Central Range fault suppresses Coulomb stress on the Longitudinal Valley fault, and vice versa, resulting in seismic bursts in an out-of-phase pattern on the two faults as seen in the hundred-year historical records. Such fault interaction implies the need for time-dependent seismic hazard reassessment for the complex fault system.

Hydroacoustic Study of a Seismic Swarm in 2016–2017 near the Melville Transform Fault on the Southwest Indian Ridge

ABSTRACT Hydroacoustic monitoring has become particularly efficient for studying the low-magnitude seismicity occurring at mid-ocean ridges. In 2016–2017, a seismic swarm occurred near the Melville transform fault of the ultraslow-spreading Southwest Indian Ridge in the Indian Ocean. It comprised 258 events in the land-based International Seismological Centre catalog, extending from June 2016 to March 2017. We examined this seismicity using hydroacoustic records from three to nine hydrophones moored in the southern Indian Ocean from the Hydroacoustic Observatory of Seismicity and Biodiversity in the Indian Ocean (OHASISBIO) temporary network and the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization. We detected 27,624 hydroacoustic events spanning 298 days (1 June 2016–25 March 2017) with lower localization and origin time errors. These include several energetic, short-duration impulsive events, which we interpret as due to lava–water interactions on the seafloor. The spatio-temporal distribution of all the detected events does not show clear tectonic mainshock–aftershock sequences and suggests a magmatic origin of the swarm with bursts of seismicity likely caused by dike emplacements.

Cluster Analysis of Seismicity in the Eastern Gulf of Corinth Based on a Waveform Template Matching Catalog.

The Corinth Rift, in Central Greece, is one of the most seismically active areas in Europe. In the eastern part of the Gulf of Corinth, which has been the site of numerous large and destructive earthquakes in both historic and modern times, a pronounced earthquake swarm occurred in 2020-2021 at the Perachora peninsula. Herein, we present an in-depth analysis of this sequence, employing a high-resolution relocated earthquake catalog, further enhanced by the application of a multi-channel template matching technique, producing additional detections of over 7600 events between January 2020 and June 2021. Single-station template matching enriches the original catalog thirty-fold, providing origin times and magnitudes for over 24,000 events. We explore the variable levels of spatial and temporal resolution in the catalogs of different completeness magnitudes and also of variable location uncertainties. We characterize the frequency-magnitude distributions using the Gutenberg-Richter scaling relation and discuss possible b-value temporal variations that appear during the swarm and their implications for the stress levels in the area. The evolution of the swarm is further analyzed through spatiotemporal clustering methods, while the temporal properties of multiplet families indicate that short-lived seismic bursts, associated with the swarm, dominate the catalogs. Multiplet families present clustering effects at all time scales, suggesting triggering by aseismic factors, such as fluid diffusion, rather than constant stress loading, in accordance with the spatiotemporal migration patterns of seismicity.

Machine Learning Reveals Additional Hydraulic Fracture‐Induced Seismicity in the Eagle Ford Shale

AbstractA recent study found ∼90% of earthquakes in the Eagle Ford shale in Texas from 2014 to 2018 were spatiotemporally related to hydraulic fracturing (HF) wells. Since then, earthquakes have remained prevalent, including a new region of seismicity in Live Oak county. We sought to perform a deeper exploration of how HF has contributed to recent seismicity using template matching and repeating signal detection (RSD), which employs machine learning to search for repeating earthquakes with the potential to find something different than the template catalog. RSD identified new bursts of seismicity with a shorter S‐P time (∼2 s) than the previous catalog (>4 s), which temporally correlated with HF near station N4 735B. The short S‐P events have smaller magnitudes (ML < 2.0), consistent with the idea that HF‐induced seismicity in the Eagle Ford is likely more pervasive than previously reported, but detection is limited by the density of stations. RSD and template matching identified 1,600 earthquakes correlated with HF from 2019 to February 2020. We confirmed newly detected HF‐induced seismicity in Live Oak county did not occur until January 2019. Despite similar cumulative volume prior to and after 2019, the onset of detectable seismicity did not occur until HF injection exceeded 2 million barrels per month over this area, supporting the notion that injection flux is a stronger influence on the seismicity occurrence than cumulative volume. When considering the full catalog, the likelihood of seismicity also correlated with proximity to mapped faults, with 13% of wells <2 km from faults having seismicity compared to only 3% of wells >2 km.

Modeling frictional precursory phenomena using a wear-based rate- and state-dependent friction model in the laboratory

We examine numerical models that employ the rate-and-state frictional (RSF) framework to investigate earthquake sequences using laboratory driven descriptions of heterogeneous frictional properties. Using previously obtained experimental measurement of roughness, we observed that wear produced a bimodal Gaussian distribution of surface heights, which we hypothesized produced spatial heterogeneity of the critical slip distance Dc. In this numerical study, the fault surface was binarized into discrete smooth or rough sections, producing a barcode style version of frictional heterogeneity. The fault was predominantly rough except for two dominant asperities (A1 and A2) representative of larger polished sections. We simulated the resistive effect of increasing the fracture energy (toughness) of the rough barriers while maintaining constant properties of the embedded brittle/smooth asperities. Our numerical simulations generated burst-like seismic events and aseismic transients throughout the interseismic phase. At the late interseismic phase, bursts of seismicity (foreshocks) interacted with the accelerating preslip region at the transition to the preseismic (nucleation) phase. At lower levels of toughness heterogeneity, the slip rate increase was roughly inversely proportional to the time-to-failure tf for larger events. As fault toughness was increased, the dominant asperities initiated nucleation and thus force deviations of the fault from the smooth 1/tf acceleration observed for the homogeneous case, producing a rate-dependent cascade response. The calculations were validated by comparing two independently measured metrics from the experiments: (1) The expansion rate of slow ruptures during the interseismic and preslip phase and (2) the scalar seismic moment and source dimensions. While our study does not address the scaling problem, these results help to understand laboratory experiments that investigate transition to the preseismic (nucleation) phase during complex earthquake sequences.

Spatiotemporal Evolution of the Seismicity in the Alto Tiberina Fault System Revealed by a High‐Resolution Template Matching Catalog

AbstractThe Alto Tiberina Fault system, located in the Northern Apennines (Italy), consists of a low angle normal fault (LANF) which radiates micro‐seismicity that can be explained by continuous creep. On top of the LANF, a network of syn‐ and antithetic high angle faults frequently hosts seismic swarms, one of which has been associated with a transient aseismic deformation signal. To study in detail the seismicity and its relationships with aseismic deformation processes occurring in this fault system, we apply template matching on seismic data recorded at an array of borehole stations, to derive a high‐resolution earthquake catalog. Thanks to the additionally detected events, we are able to reveal time periods of increased spatial‐ and temporal clustering during an aseismic deformation event. This reflects the complex evolution of aseismic slip together with the complexity of the shallow fault system. Along the LANF, we observe a bimodal type of seismicity, with diffuse seismicity active continuously, and short‐lived bursts of seismicity that could indicate rapid fluid releases. We additionally identify repeating earthquakes. These events not necessarily match a simple creep model and therefore open the possibility for new models to explain the seismicity along the LANF.

Seismically active structures of the Main Himalayan Thrust revealed before, during and after the 2015 Mw 7.9 Gorkha earthquake in Nepal

SUMMARY The Mw 7.9 2015 April 25 Gorkha earthquake is the latest of a millenary-long series of large devastating Himalayan earthquakes. It is also the first time a large Himalayan earthquake and its aftershocks were recorded by a local network of seismic stations. In the 5 yr following the main shock, more than 31 000 aftershocks were located by this permanent network within the ruptured area, including 14 362 events with ML greater than 2.5, 7 events with ML > 6, including one large aftershock with Mw 7.2 on 2015 May 12. In 2020, 5 yr after the main shock, the seismicity rate along the ruptured fault segments was still about 5 times higher than the background seismicity before the Gorkha earthquake. Several bursts of earthquakes, sometimes organized in clusters, have been observed from a few days to several years after the main shock. Some of these clusters were located at the same place as the clusters that happened during the decades of interseismic stress build-up that preceded the large earthquake. They also happened in the vicinity of the high frequency seismic bursts that occurred during the main shock. These heterogeneities contribute to a persistent segmentation of the seismicity along strike, possibly controlled by geological structural complexities of the Main Himalayan Thrust fault. We suggest that these pre-2015 clusters revealed the seismo-geological segmentation that influences both the coseismic rupture and the post-seismic relaxation.

Observation of rapid long-range seismic bursts in the Japan Trench subduction leading to the nucleation of the Tohoku earthquake

Earthquake precursory phenomena like foreshocks are common but the physical processes which produce them and lead to rupture are poorly understood. Looking for clues about these processes, we explore here the long-term evolution of seismicity in a broad section of the Japan Trench slab around the 2011 Tohoku earthquake location and report the existence of long-range seismic bursts moving rapidly (∼1-2 days or less) between the deep and the shallow slab. Their presence is evidenced by synchronizations of deep and shallow seismic activities in the slab. Probabilities that these synchronizations could be due to chance are infinitesimal (<10−5). We show that a significant part of the activity in this subduction occurs during these bursts and that they are responsible for the foreshock sequence which preceded the earthquake. These observations support the existence of fluid channels connecting the deep slab, where water is released during dehydration, to the shallow seismogenic zone.

Multisegment ruptures andVp/Vsvariations during the 2020–2021 seismic crisis in western Corinth Gulf, Greece

SUMMARYOn 2020 December 23, a seismic crisis initiated in the western Corinth Gulf offshore Marathias, lasted several months, and generated thousands of small magnitude earthquakes. The Gulf of Corinth is well known for earthquake swarm occurrence and short-lived burst-like earthquake sequences, mostly triggered by crustal fluids. Here, we perform a detailed seismic analysis aiming to identifying earthquake clusters within the seismic crisis and define their spatial and temporal characteristics. Thanks to the dense seismic station coverage in the area, operated by the Hellenic Unified Seismological Network and Corinth Rift Laboratory, we relocate shallow seismicity and compile a high-resolution earthquake catalogue containing ∼1400 earthquakes spanning the first two months of the seismic crisis. We identify 19 earthquake clusters by applying spatio-temporal criteria and define the geometry (strike and dip) using principal component analysis for 11 of them. Our results are consistent with moment tensor solutions computed for the largest earthquake in each cluster. A striking feature of the seismic activity is the west-towards-east migration with a notable increase in Vp/Vs values for each cluster and a slight increase of the dip angle for the identified fault segments. Furthermore, we find that each cluster contains several burst-like, short interevent time, repeating earthquakes, which could be related to aseismic slip or fluid migration. Overall, we show that the 2020–2021 seismic crisis consists of earthquake clusters that bifurcate between swarm-like and main shock–aftershock-like sequences and ruptured both north- and south-dipping high-angle fault segments. The 2020–2021 seismic activity is located between 5 to 8 km, shallower than the low angle north-dipping (∼10°) seismic zone which hosts long-lived repeating sequences at ∼9–10 km depth. This study supports a hypothesis that the low-angle north-dipping seismicity defines the brittle-ductile transition in the western Corinth Gulf, with seismic bursts occurring at shallower depths in the crust.

Hierarchical Exploration of Continuous Seismograms With Unsupervised Learning.

Continuous seismograms contain a wealth of information with a large variety of signals with different origin. Identifying these signals is a crucial step in understanding physical geological objects. We propose a strategy to identify classes of signals in continuous single‐station seismograms in an unsupervised fashion. Our strategy relies on extracting meaningful waveform features based on a deep scattering network combined with an independent component analysis. Based on the extracted features, agglomerative clustering then groups these waveforms in a hierarchical fashion and reveals the process of clustering in a dendrogram. We use the dendrogram to explore the seismic data and identify different classes of signals. To test our strategy, we investigate a two‐day‐long seismogram collected in the vicinity of the North Anatolian Fault, Turkey. We analyze the automatically inferred clusters' occurrence rate, spectral characteristics, cluster size, and waveform and envelope characteristics. At a low level in the cluster hierarchy, we obtain three clusters related to anthropogenic and ambient seismic noise and one cluster related to earthquake activity. At a high level in the cluster hierarchy, we identify a seismic burst that includes around 200 events with similar waveforms and high‐frequent signals with correlating envelopes and an anthropogenic origin. The application shows that the cluster hierarchy helps to identify particular families of signals and to extract subclusters for further analysis. This is valuable when certain types of signals, such as earthquakes, are under‐represented in the data. The proposed method may also successfully discover new types of signals since it is entirely data‐driven.

Interplay of large-scale tectonic deformation and local fluid injection investigated through seismicity patterns at the Reykjanes Geothermal Field, Iceland

SUMMARY Occurrence of seismicity sequences as consequence of fluid injection or extraction has long been studied and documented. Causal relations between injection parameters, such as injection pressure, injection rates, total injected volumes and injectivity, with seismicity derived parameters, such as seismicity rate, cumulative seismic moment, distance of seismicity (RT-plot), b-values, etc. have been derived. In addition, reservoir engineering parameters such as permeability/porosity relations and flow types play a role together with geology knowledge on fault and fracture properties, influenced by the stress field on different scales. In this paper, we study observed seismicity related to water injection at the Reykjanes Geothermal Field, Iceland. The region near the injection well did not experience seismicity before the start of injection. However, we observed continued seismic activity during the 3 months of injection in 2015, resulting in a cloud of about 700 events ranging in magnitude from Mw 0.7 to 3.3. We re-located these events using a modified double-difference algorithm and determined focal mechanism of event subsets. Characteristic for the site is that the events are bound to about 4 km distance to the injection point, and moreover known faults seem to act as barrier to fluids and seismicity. Several repeating sequences of seismicity, defined as bursts of seismicity have hypocenter migration velocities larger than 4 km d–1 and their dominant direction of propagation is away from the injection point towards larger depths. The seismic events within the bursts lack larger magnitude events, have elevated b-values (∼1.5) and consist of many multiplets. Except from the coinciding onset of seismicity with the start of fluid injection, no correlation between injection rates and volumes could be identified, neither could hydraulic diffusivity models explain observed seismicity patterns. Comparison of our results with investigations on background seismicity from 1995 to 2019 and from a seismic swarm in 1972 revealed similar focal mechanism patterns and burst-like seismicity patterns. We finally present a conceptual model where we propose that the observed seismicity patterns represent a stress release mechanism in the area close to the injection well, controlled by an interplay of local pore pressure and stress field changes with continued extensional stress build up at the Reykjanes Ridge.

Tidal Triggering of Microseismicity at the Equatorial Mid‐Atlantic Ridge, Inferred From the PI‐LAB Experiment

AbstractThe gravitational pulls from the moon and the sun result in tidal forces which influence both Earth's solid and water mass. These stresses are periodically added to the tectonic ones and may become sufficient for initiating rupture in fault systems critically close to failure. Previous research indicates correlations between increased seismicity rates and low tides for fast‐ and intermediate‐spreading mid‐ocean ridges in the Pacific Ocean. Here, we present a microseismicity data set (4,719 events) recorded by an ocean bottom seismometer deployment at the equatorial Mid‐Atlantic Ridge. We show that low, as well as decreasing ocean water level, result in relatively elevated seismicity rates at higher magnitudes (lower b‐values), translated into increased probabilities of stronger event occurrence at or towards low tides. Moreover, seismic bursts (enhanced activity rate clusters), occurring at rates well above the reference seismicity, are exclusively present during values of either high tidally induced extensional stresses or high extensional stress rates. Although the b‐value differences are not significant enough to be conclusive, the seismicity rate variations exhibit statistical significance, supporting the previous findings for tidal triggering at low tides within normal‐faulting regimes and extending the range of observations to slow‐spreading ridges. Observed triggering of slip on low angle faults at low tides is predicted by Coulomb stress modeling. The triggering of slip on high angle faults observed here, is not easily explained without another factor. It may be related to the presence of a shallow magma body beneath the ridge, as supported by previous seismic imaging in the region.

Precursory Slow Slip and Foreshocks on Rough Faults

AbstractForeshocks are not uncommon prior to large earthquakes, but their physical mechanism remains controversial. Two interpretations have been advanced: (1) foreshocks are driven by aseismic nucleation and (2) foreshocks are cascades, with each event triggered by earlier ones. Here, we study seismic cycles on faults with fractal roughness at wavelengths exceeding the nucleation length. We perform 2‐D quasi‐dynamic, elastic simulations of frictionally uniform rate‐state faults. Roughness leads to a range of slip behavior between system‐size ruptures, including widespread creep, localized slow slip, and microseismicity. These processes are explained by spatial variations in normal stress (σ) caused by roughness: regions with low σ tend to creep, while high σ regions remain locked until they break seismically. Foreshocks and mainshocks both initiate from the rupture of locked asperities, but mainshocks preferentially start on stronger asperities. The preseismic phase is characterized by feedback between creep and foreshocks: episodic seismic bursts break groups of nearby asperities, causing creep to accelerate, which in turns loads other asperities leading to further foreshocks. A simple analytical treatment of this mutual stress transfer, confirmed by simulations, predicts slip velocities and seismicity rates increase as 1/t, where t is the time to the mainshock. The model reproduces the observed migration of foreshocks toward the mainshock hypocenter, foreshock locations consistent with static stress changes, and the 1/t acceleration in stacked catalogs. Instead of interpreting foreshocks as either driven by coseismic stress changes or by creep, we propose that earthquake nucleation on rough faults is driven by the feedback between the two.

Seismic rate variations prior to the 2010 Maule, Chile MW 8.8 giant megathrust earthquake

The MW 8.8 Maule earthquake is the largest well-recorded megathrust earthquake reported in South America. It is known to have had very few foreshocks due to its locking degree, and a strong aftershock activity. We analyze seismic activity in the area of the 27 February 2010, MW 8.8 Maule earthquake at different time scales from 2000 to 2019. We differentiate the seismicity located inside the coseismic rupture zone of the main shock from that located in the areas surrounding the rupture zone. Using an original spatial and temporal method of seismic comparison, we find that after a period of seismic activity, the rupture zone at the plate interface experienced a long-term seismic quiescence before the main shock. Furthermore, a few days before the main shock, a set of seismic bursts of foreshocks located within the highest coseismic displacement area is observed. We show that after the main shock, the seismic rate decelerates during a period of 3 years, until reaching its initial interseismic value. We conclude that this megathrust earthquake is the consequence of various preparation stages increasing the locking degree at the plate interface and following an irregular pattern of seismic activity at large and short time scales.

Dynamic triggering of earthquakes in the North Island of New Zealand following the 2016 Mw 7.8 Kaikōura earthquake

Large earthquakes are capable of triggering microseismicity, deep tremor and slow-slip events from intermediate- to long-distance ranges. Unfortunately, earthquake catalogs are typically incomplete right after large mainshocks. Hence, mapping triggering patterns and understanding the underlying triggering mechanism are challenging. Here we present two different types of seismicity responses to dynamic stressing by passing seismic waves in the North Island of New Zealand following the 2016 Mw 7.8 Kaikōura earthquake. Based on a template matching technique, we identify up to 4-7 times more earthquakes than listed in New Zealand's GeoNet catalog. We also compute the dynamic stress perturbations in the North Island due to the Kaikōura mainshock and compare them to seismicity rate changes to identify regions with high susceptibility to dynamic stress triggering. Abundant triggered earthquakes occurred immediately following the mainshock in the shallow crust around the active Taupo Volcano Zone, likely related to activation of crustal faults/fluids associated with back-arc rifting and volcanism. Approximately 8 days after the initial dynamic stressing, an active burst of seismicity with the largest event of ML 5.55 occurred along the shallow megathrust near Porangahau on the east coast of the North Island. This burst of seismicity is likely driven by a ∼Mw 7.1 shallow slow slip event dynamically triggered by the mainshock. Our findings reveal the heterogeneous nature of dynamic triggering in a plate boundary region with recent large earthquake sequences and aseismic transient events and further highlight the difficulties in time-dependent earthquake forecasting following large mainshocks.

Monitoring a short-lived earthquake swarm during April–May 2020 in Haenam, Korea, and its preliminary results

On 3 May 2020, an ML 3.1 earthquake occurred in Haenam, southwestern Korea, in an area devoid of recorded seismicity since instrumental observations began in 1978. Careful examination of the temporal occurrence of seismicity, and the magnitude distribution of the sequence before and after the ML 3.1 earthquake, indicates typical swarm-like behavior. The earthquake swarm started with an ML 0.6 event on 26 April 2020, intensified up to 3 May 2020, and abruptly terminated with an ML 1.0 event on 9 May 2020. The Pusan National University Geophysics Laboratory (PNUGL) deployed a temporary seismic array with eight three-component short-period instruments to monitor the short-lived bursts of seismicity. During the monitoring campaign, we detected > 700 microearthquakes by applying a matched-filter technique to the combined dataset produced by PNUGL, the Korea Meteorological Administration, and the Korea Institute of Ocean Science and Technology. We determined earthquake parameters for 299 earthquakes that were detected at four or more seismic stations. We also determined the focal mechanism solutions of the 10 largest earthquakes in the swarm using first-motion polarities with S/P ratios. The focal mechanism, hypocentral depth, and stress orientation of the largest earthquake in the sequence were also determined using waveform inversions. The distribution of earthquake hypocenters, together with focal mechanism solutions, indicates that the earthquake swarm activated deeply-buried faults (~20 km) oriented either NNE-SSW or WNW-ESE. We also report details of the temporary seismic monitoring network, including the instrumentation, detection of microearthquakes, and variations in event-detection threshold influenced by anthropogenic and natural noise fluctuations. We also discuss the limitations associated with lowering the detection threshold of microearthquakes by increasing the number of seismic stations or by adopting advanced event-detection techniques.

Imbricated Aseismic Slip and Fluid Diffusion Drive a Seismic Swarm in the Corinth Gulf, Greece

AbstractThe primary processes driving seismic swarms are still under debate. Here, we study the temporal evolution of a seismic swarm that occurred over a 10‐day period in October 2015 in the extensional rift of the Corinth Gulf (Greece) using high‐resolution earthquakes relocations. The seismicity radially migrates on a normal fault at a fluid diffusion velocity (~125 m/day). However, this migration occurs intermittently, with periods of fast expansion (2 to 10 km/day) during short seismic bursts alternating with quiescent periods. Moreover, the growing phases of the swarm illuminate a high number of repeaters. The swarm migration is likely the results of a combination of multiple driving processes. Fluid upflow in the fault may induce aseismic slip episodes, separated by phases of fluid pressure build‐up. The stress perturbation due to aseismic slip may activate small asperities that produce bursts of seismicity during the most intense phases of the swarm.