Earthquake Swarm Research Articles

Two Sets of High-Conductivity Systems with Different Scales Reveal the Seismogenic Mechanism of Earthquakes in the Songyuan Area, Northeastern China

To reveal the deep seismogenic environment and mechanism of earthquakes in Songyuan City, Northeastern China, 59 broadband magnetotelluric sites in the Songyuan area were arranged in this study at a spacing of 5 km. In addition, two intersecting magnetotelluric profiles, with a total of 23 measuring sites and a spacing of 2 km, were established near the Ningjiang earthquake swarm. Using a nonlinear conjugate gradient (NLCG) algorithm, resistivity structures in the lithosphere were obtained at different scales using three-dimensional (3D) inversion. The research results show that: a deep high-conductivity system (<10 Ω·m) was identified at 25–85 km depth in the lithosphere under Songyuan, corresponding closely to a region of high heat flow. It is inferred to be the molten material of mantle upwelling. In addition, a shallow high-conductivity system (<10 Ω·m) was identified beneath the Ningjiang earthquake swarm, which is interpreted to correspond to the Fuyu North fault. It is the main seismo-controlling structure of the Ningjiang earthquake swarm. The deep seismogenic environment and seismogenic mechanism of the Ningjiang earthquake swarm can be described as a deep upwelling of molten mantle material, which provides the power source. The deep magma intruded into the lower crust and accumulated, then intruded along faults and fissures, resulting in the activation of the North Fuyu fault and triggering the Ningjiang earthquake. It is attributed to the activation of shallow faults caused by the upwelling of molten mantle material.

Enigmatic Tsunami Waves Amplified by Repetitive Source Events Near Sofugan Volcano, Japan

AbstractOn 8 October 2023, mysterious tsunamis with a maximum wave height of 60 cm were observed in Izu Islands and southwestern Japan, although only seismic events with body‐wave magnitudes mb 4–5 have been documented to the west of Sofugan volcano. To investigate the source process, we analyze tsunami waveforms recorded by an array network of ocean bottom pressure gauges. Stacked waveforms of pressure gauge records suggest recurrent arrivals of multiple wave trains. Deconvolution of the stacked waveforms by tsunami waveforms from an earlier event revealed over 10 source events that intermittently generated tsunamis for ∼1.5 hr. The temporal history of this sequence corresponds to the origin times of T‐phases estimated by an ocean bottom seismometer and of the seismic swarm, implying a common origin. Larger events later in the sequence occurred at intervals comparable to the tsunami wave period, causing amplification of later phases of the tsunami waves.

Slow slip event displacement on 2018 offshore Boso Peninsula detected by Sentinel-1 InSAR time-series analysis with numerical weather model assistance

SUMMARY The south-eastern offshore of the Boso Peninsula in Japan periodically experiences short-term slow slip events (SSEs) every few years. On 2018 June, an SSE occurred with the maximum surface horizontal displacement reaching up to 4.7 cm by according to the operational global navigation satellite system (GNSS) network. This study performed a time-series analysis of interferometric synthetic aperture radar (InSAR) with Sentinel-1 SAR images to investigate detailed spatial pattern of surface displacements caused by the SSE. With the assistance of an atmospheric delay correction with a regional numerical weather model output, the InSAR time-series analysis successfully captured displacement signals in three paths, whose maximum amplitudes in line-of-sight directions were 1.46, 1.86 and −0.80 cm. A checkerboard test revealed that the resolution of the slip inversion was higher when InSAR was used than that using GNSS, especially in and around the inland. The slip inversion with the actual displacement data derived from the InSAR time-series analysis was performed with the L-curve optimization, showing that the estimated slip area was concentrated offshore south-eastward from the Boso Peninsula with the maximum slip of 5 cm and the estimated moment magnitude of 6.4. As similar to previous SSEs in the Boso Peninsula, a seismic swarm simultaneously occurred in the downdip area adjacent to the estimated slip with the SSE occurrence, suggesting a different friction characteristics between them. This study demonstrates usefulnesses of the InSAR observation for capturing detailed spatial characteristics of small-displacement events like SSEs and of the hybrid use of the externally derived delay correction with the time-series analysis to improve the displacement detection accuracy.

A statistical analysis of volcano-tectonic earthquake swarms in the off Nicobar region of the Andaman Sea

A statistical analysis of volcano-tectonic earthquake swarms in the off Nicobar region of the Andaman Sea

The Alaska Makushin Volcano 2016–2018 inflation and its potential relation to the 2020 earthquake swarm, from GNSS observations

Makushin volcano on Unalaska Island along the Aleutian Chain in Alaska experienced a period of unrest from 2016 to 2020, including 2016–2018 surface inflation observed by 5 continuous Global Navigation Satellite System stations, and an earthquake swarm in the second half of 2020 consisting of hundreds of earthquakes, including two greater than M4. A Bayesian inversion of the geodetic data applied to a range of analytical source models suggests that the observed deformation pattern can be explained by an inflating pressure point source located 2 km east of the summit at a depth of approximately 5 km below sea level, with a chamber volume increase of approximately 0.004 km3. A stress change analysis suggests that the 2016–2018 magma intrusion may have played a role in advancing the 2020 seismicity anomaly by modulating the regional stress field. Our study highlights the importance of stress transfer between magmatic and tectonic processes and the potential for using this information to better understand and mitigate the risk of volcanic eruptions.

Magma migration below Tancítaro and Paricutin volcanoes revealed by seismology

SUMMARY In this study, we analyse the 2020 seismic swarm that lasted two months and occurred between the Tancítaro and the Paricutin volcanoes in the Michoacán Guanajuato Volcanic Field, Mexico. We developed a new method to automatically detect and locate about 100 000 earthquakes, enabling us to track the magma migration through narrow dykes. Additionally, we reveal the presence of two magma reservoirs from two seismic noise tomography results. The first reservoir is located from 8 to 20 km below sea level and beneath the Tancítaro volcano and probably corresponds to a complex network of dykes and sills. This crustal reservoir is fed by a mantle reservoir with a wide horizontal extension between 35 and 50 km below sea level. The seismic swarm initiated beneath the Tancítaro summit in the lower portion of the crustal magma reservoir. At this stage, the seismicity migration was mainly horizontal, which we interpret as its response to the higher normal stress caused by the gravitational load of Tancítaro. Once the magma was displaced laterally from beneath Tancítaro, magma migration became more vertical. The swarm reached the upper portion of the crustal magma reservoir but did not escape it. We also reveal the effect of a distant but strong tectonic earthquake on the seismic swarm. Before its occurrence, magma migration followed several paths; afterwards, it became more focused along a single path. Finally, after the swarm, we observed a second type of seismicity called post-swarm seismicity, with a lower earthquake rate but with higher magnitudes. The hypocentres were diffuse and horizontally centred on the previous swarm location. Furthermore, some earthquakes were aligned along shallow faults, generating a high seismic risk to the different Tancítaro nearby localities.

Control of Seismicity Migration in Earthquake Swarms by Injected Fluid Volume and Aseismic Crack Propagation

AbstractThe evolution of fluid injection‐induced seismicity, generally characterized through the number of events or their seismic moment, depends on, among other factors, the injected fluid volume. Migration of seismicity is observed during those sequences and might be caused by a range of mechanisms: fluid pressure diffusion, fluid‐induced aseismic slip propagating along a stimulated fault, interactions between earthquakes. Recent theoretical and observational developments underline the important effect on seismicity migration of structural parameters, like fault criticality, or injection parameters, like flow rate or pressurization rate. Here, we analyze two well‐studied injection‐induced seismic sequences at the Soultz‐sous‐Fôret and Basel geothermal sites, and find that the evolution of the seismicity front distance primarily depends on the injected fluid volume. Based on a fracture mechanics model, we develop new equations relating seismicity migration to injected fluid volume and frictional and structural properties of the fault. We find that the propagation of a fluid‐induced aseismic slip front along the stimulated fault, triggering seismicity, explains well the observations made on the two sequences. This model allows us to constrain parameters describing the seismicity front evolution and explains the diversity of migration patterns observed in injection‐induced and natural earthquake swarms.

On the diversity of fracture behavior in a brittle solid with sets of preexisting small-scale cracks

Understanding the multiscale fracture behavior of brittle materials is of crucial importance not only in engineering applications but also in seismology where an earthquake source, usually assumed as one single, relatively large fracture region but in reality composed of relatively smaller multiple fractures, behaves mechanically in diverse ways, emit waves with different characters and may cause a single seismic event or even a cluster of earthquakes and earthquake swarms. For understanding the complex fracture processes, by employing the experimental technique of dynamic photoelasticity with high-speed video cameras, we have been simultaneously observing global, large-scale material behavior and local, smaller-scale evolution of waves and fractures in two-dimensional linear elastic brittle polycarbonate specimens. Each specimen has sets of preexisting small-scale parallel cracks prepared by a digital laser cutter and modeling large-scale geological fault planes, and it is subjected to external (quasi-)static and impact loads. Here we show some recent examples of the diverse fracture behavior observed in brittle birefringent solid specimens under tensile/compressive external loading. The fracture behavior is considerably dependent on the loading conditions, and developing fractures do not always break the specimen in an “unzipping” way, i.e. the specimen is not always divided along a perforation line consisting of small-scale cracks. Rather, the diverse fractures can easily jump to remote places, propagate back-and-forth or reversely move in the opposite direction compared with the initial one. Our findings may play a role in comprehending the generation mechanism of a cluster of fractures in brittle solids in general.

Origin of spring waters in the Matsushiro area: Implications for the 1965–1967 earthquake swarm and subsequent “water eruption”

Origin of spring waters in the Matsushiro area: Implications for the 1965–1967 earthquake swarm and subsequent “water eruption”

Intense seismic swarm punctuated by a magnitude 7.5 Japan shock

Intense seismic swarm punctuated by a magnitude 7.5 Japan shock

Implications of Fault‐Valve Behavior From Immediate Aftershocks Following the 2023 Mj6.5 Earthquake Beneath the Noto Peninsula, Central Japan

AbstractThe Mj6.5 (Mw6.2) event that occurred on 5 May 2023 near the northern shoreline of the northeastern tip of the Noto Peninsula, central Japan, is the largest event to date in a long‐lasting, intense earthquake swarm. Here we have created a more precise aftershock catalog associated with the 2023 Mj6.5 and the second‐largest 2022 Mj5.4 sequence to understand the rupture process of this largest earthquake. Most of the aftershocks are aligned along a ∼45° SE‐dipping plane. The mainshock initially ruptured the same deep section of the fault zone that had been ruptured by the 2022 Mj5.4 event, before propagating rapidly to shallow depths and to offshore along the ruptured fault plane. The aftershock front migrated at a speed of ∼20 km/hr. This rapid upward migration of the immediate aftershocks might be driven by upwelling of crustal fluids along the intensely fractured and permeable fault zone via mainshock dynamic rupture.

Earthquake Swarms in May 2015 at the Northern Shore of Lake Ashi, Hakone, Japan: What Caused the Activity?

Earthquake Swarms in May 2015 at the Northern Shore of Lake Ashi, Hakone, Japan: What Caused the Activity?

Earthquake swarms near the Mór Graben, Pannonian Basin (Hungary): implication for neotectonics

The central part of the Pannonian Basin is characterised by low to medium seismicity. North central Hungary is one of the most dangerous areas of the country in terms of earthquakes, which also includes the area of the Mór Graben where some of the largest earthquakes occurred in Hungary’s history. Recent activity has been observed in the Mór Graben. It has been established that earthquake swarms occur quite frequently in the graben. To further study these events, we deployed a temporary seismic network that operated for 20 months. Using the temporary network stations as well as permanent stations from the Kövesligethy Radó Seismological Observatory and the GeoRisk Ltd. networks we registered 102 events of small magnitudes. In this paper, we demonstrate and compare three different event detection methods based on the registered waveforms by the permanent and temporary stations to find the optimal one to collect a complete swarm list in the Mór Graben. After the hierarchical cluster analysis, we relocated the hypocentres using a multiple-event algorithm. Our results demonstrate that the most successful detector in this case is the “Subspace detector.” We managed to create a complete list of the events. Our results indicate that the Mór Graben is still seismically active.

Evolution of the magma system at Makushin volcano, Alaska, from 2004 to 2021

Located in the northeast of Unalaska Island in the central part of the Aleutian, Alaska, Makushin volcano is one of the more active volcanoes in the United States, with the most recent eruption occurred in 1995. Pre-eruptive inflation and post-eruptive deflation were identified from 1993 to 1995 and 1996 to 2000, respectively, and were interpreted as volume changes associated with magma movement within the plumbing system. In this study, we investigate the surface deformation of Makushin from 2004 to 2021 to evaluate its present-day activity, using Synthetic Aperture Radar (SAR) interferometry technique. SAR images acquired from C-band Envisat and Sentinel-1, X-band TerraSAR-X, and L-band ALOS-2 satellites are analyzed using an advanced multiple temporal Interferometric SAR (InSAR) algorithm. The produced deformation time series are then used to track the temporal evolution of the volcanic system with Ensemble Kalman Filter (EnKF) assuming Mogi sources. The results show that Makushin is characterized by time-varying deformation episodes including quiescence, and inflation/deflation cycles at varied rates since 2004. The derived deformation histories can be well reproduced by a spatially stationary pressure source located to the east-northeast of the summit, at a depth about 6 km beneath the sea level (BSL). The inflation/deflation cycles with distinct temporal patterns are interpreted as results from volatile-dominated volume accumulation/loss in the magmatic reservoir. A secondary reservoir connected to the main magma chamber is revealed from the deformation time series. No surface displacement associated with the large earthquake swarm in 2020 is identified from the InSAR-derived deformation. The cumulative volume change in the magma reservoir is about −6×106m3 from 2004 to 2021, indicating a volume/pressure deficit in the magma plumbing system.

Evolution in unrest processes at Campi Flegrei caldera as inferred from local seismicity

Changes in seismicity with time and location are diagnostic signals for understanding the dynamics of volcanic unrest. We used these signals at the Campi Flegrei caldera, in southern Italy, to investigate how structural changes have determined three styles of unrest since 1982, distinguished by a ground uplift (measured at Pozzuoli, near the centre of the caldera) of 178 cm in 1982–84; a subsidence of 93 cm in 1985–2005; and an uplift of 118 cm between 2005 and November 2023. Double-difference seismic locations and concentrations of seismic energy release have revealed impermeable horizons that correspond to the cap rock and self-sealed base of the geothermal system at depths of 1.5 and 3 km, respectively. Most earthquakes have been shallower than 3 km, consistent with the brittle upper crust being stretched over a zone of pressurization below the geothermal system. The 1982–84 uplift decayed after a major seismic swarm on April 1st, 1984, breached the lower impermeable horizon, which increased the flux of escaping gas and reduced the source pressure. Continued gas escape promoted subsidence until the lower horizon had resealed itself and initiated a new episode of uplift while gas from depth re-accumulated beneath. Compared with 1982–84, a greater proportion of recent seismicity has occurred at shallower depths below the actively degassing fumaroles of Solfatara-Pisciarelli, north-east of Pozzuoli. The associated high seismic b‐values, between 1 and 2, are consistent with high fluid pressures and, hence, with locations more favourable to fracturing. Continued uplift may thus persist until fracturing of the shallow crust allows faster rates of gas release and depressurization of the pressure source.

Preliminary Result of Swarm Activities in Toba Region Using Dense Temporary Network

For the first time, an earthquake swarm occurred in 2021 in Lake Toba (Indonesia), one of the largest calderas in the world. Although the earthquakes were located in the volcanic environment, the swarm activities could be related to the tectonic activities from the Sumatra fault system. The swarm activities occurred at a very shallow depth and were felt with the intensity of II–IV MMI by local people. The objective of this research was to investigate the characteristics of the earthquake swarm in the Toba Caldera from the waveforms recorded by temporary stations in March – June 2021. A total of 26 seismic stations were deployed for two months in May and June 2021. Automatic detection of seismic waves is important because it can be used for earthquake early warning systems and speeds up analysis of origin time, P-, and S arrival times, and earthquake locations. We found characteristics of spectrum waveform from low to high frequency of a non-swarm earthquake, while high frequency seismograms were generated by the swarm earthquake. Furthermore, we successfully captured the hypocentre that was distributed in the SW – NE direction and may figure out the unknown geological system that is responsible for the swarm activities.

An Analysis of Swarm Earthquakes in the Area of the Elbrus Volcanic Center

An Analysis of Swarm Earthquakes in the Area of the Elbrus Volcanic Center

The Cause of the Earthquake (4.7 mb) and Recent Stress Regime Occurring in Al-Refaei Area, Mesopotamian plain, August 18th, 2017

Al-Refaei area is considered one of the highest seismically activity in the Mesopotamian Plain according to its seismic history. Depending on the International seismological Center, four seismic swarms were diagnosed in this area; these are: August 2004, January 2013, May-Jun 2017, and August 2017. Based on three focal mechanism solutions of magnitude greater than 4 mb, the stresses in the area were analysed. The results of focal mechanisms suggest that the faults movements in the study area show reverse movements, which is generated by compression force. To determine the directions of principal stress axes that affecting the faults in the area, the Rotational Optimization method were used. The directions of the three stress axes are: 02°/32°, 10°/302°, and 80°/136° for σ1, σ2, and σ3, respectively. The collision between the Arabian and Iranian plates is the main cause of seismic activity in Al-Refaei area. Therefore, Al-Refaei active fault is responsible for most the earthquakes in the area, and responsible for the earthquake that occurred in August 18th, 2017.

Updip Fluid Flow in the Crust of the Northeastern Noto Peninsula, Japan, Triggered the 2023 Mw 6.2 Suzu Earthquake During Swarm Activity

AbstractAn Mw 6.2 earthquake occurred in Suzu, northeastern Noto Peninsula, Japan, on 5 May 2023, followed by many aftershocks. Before this mainshock‐aftershock sequence, an intense earthquake swarm lasted in the vicinity for 2.5 years. Here, we estimated the rupture process of the Mw 6.2 mainshock and relocated >20,000 surrounding small earthquakes. The results show that systematic upward migration occurred via a complex network of faults in the preceding swarm period and that the mainshock rupture was initiated near the shallow end of the swarm earthquakes. The mainshock rupture propagated farther updip, followed by many aftershocks in the shallow extension. Upward fluid movement likely caused systematic upward earthquake migration from a depth of 18–5 km. The present results indicate the importance of monitoring swarm events since large (M > 6) and dangerous earthquakes can occur during such swarms.

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.