Earthquake Swarm Research Articles

Long Swarms and Short Swarms at Volcanoes: Evidence for Different Processes

Many earthquake swarms at volcanoes last several months, then have a sharp uptick in rate in the hours before eruption. Examples include 2006 Augustine, 8.5 months then 10 hours; 1992 Spurr, 10 months then 4 hours; 1994 Rabaul, ~2 years then 27 hours; 2008 Kasatochi, 6 weeks then 31 hours; and 2011 Puyehue Cordon Caulle, 7 weeks then 3.5 days. For the well-studied Augustine case, broadband data showed that very long period (VLP) energy accompanied 221 of 722 located earthquakes in the 10 hours before the first explosive eruption on 11 January 2006. This was revealed by low-pass filtering and the period of the VLP signal was up to 50 sec. The Augustine broadband stations were campaign instruments at distances of 2-3 km from the vent. No similar VLP energy has been found in events during the 8.5-month long swarm. Okmok volcano had a short swarm only, lasting 5 hours prior to its 12 July 2008 eruption. Low-pass filtering of data from broadband station OKSO, 10 km from the vent, showed that 23 of 42 located events had VLP energy with a period of 30-40 sec. Events from Kasatochi volcano were scanned on station ATKA. Here the broadband station is much farther away at 88 km but the earthquakes in the short swarm 6-7 August 2008 were much larger with many M > 3 events. The station suffered data gaps so only a few hours of data were scanned but numerous events were observed with VLP energy starting just after the P phase. Low-pass filtering showed VLP energy with a period of 10-12 sec. No VLP energy has been found in events of the preceding 6-week long swarm. These observations at different volcanoes suggest that the short swarms represent a different process than the long swarms. The long swarms likely reflect pressure increases in the surrounding country rock caused by increasing magma pressure. The short swarms in contrast, appear to represent discrete pulses of magma injection at shallow depths. For all the examined volcanoes most of the earthquakes looked like typical volcano-tectonic (VT) earthquakes on short-period stations. The short swarms are very short – 4 hours to 3.5 days – and have important implications for hazards assessments. It is not known how commonly the long swarm-short swarm pairs occur and thus the false alarm rate is also not known.

Generation of the 2022 earthquake swarm in intersection of four geological units in the seismic experimental Site, southwest China

Generation of the 2022 earthquake swarm in intersection of four geological units in the seismic experimental Site, southwest China

Seabed stability inferred from the 2019–2020 earthquake swarm under a volcanic cone field and slopes of Condor Seamount, Azores

Seabed stability inferred from the 2019–2020 earthquake swarm under a volcanic cone field and slopes of Condor Seamount, Azores

Thrust-dominated unilateral rupture of a blind listric fault associated with the 2024 Hualien earthquake

The 2024 Hualien Mw 7.4 earthquake struck the Longitudinal Valley, which accommodates the partial collision between the Eurasian and Philippine Sea plates. As the most significant event in Taiwan since the 1999 Chi-Chi Mw 7.6 earthquake, it presents a distinct opportunity for investigating the current rupture behavior related to the northern Longitudinal Valley. The spatiotemporal rupture process of the Hualien earthquake is reconstructed through the analysis of geodetic and seismic observations. We demonstrate that the Hualien earthquake occurs on a blind listric fault, manifesting as a unilateral rupture primarily extending toward the NNE. The slip distribution exhibits a compact pattern dominated by the thrust faulting. As indicated by the increased Coulomb failure stress, the 2022 Chihshang Mw 6.5 and Mw 6.9 earthquakes cause a triggering effect on the Hualien earthquake. The Hualien earthquake also promotes the occurrence of a seismic swarm at the southernmost tip of its rupture area.

Spatial and temporal variations of the 3-year earthquake swarm activities leading up to the M7.6 Noto Peninsula earthquake and interpretations of their activities

The seismic swarm activity that began and expanded in four separate regions in the northern Noto Peninsula at the end of 2020 was followed by the M6.5 earthquake in May 2023, which cascaded into the M7.6 earthquake on New Year’s Day 2024. To this series of earthquake events, we estimate temporal changes in the background intensity of the nonstationary ETAS model by inversion for each region. We then interpret how this series of earthquakes was driven by subsurface fluid motion and slow slip at each stage, based on the correspondence between the background seismicity changes and the temporal changes in oblique distance and elevation differences between nearby GNSS stations, as well as the spatiotemporal earthquake patterns.Graphical

KOLKHOZABAD EARTHQUAKE of APRIL 24, 2020 with KR=11.4, MwGCMT=4.6, I0=6 (TAJIKISTAN)

The results of a field survey of the effects and consequences of Kolkhozabad earthquake with KR=11.4, MwGCMT=4.6, I0=6, which occurred in the southwestern part of Tajikistan on April 24, 2020, are pre sented. The earthquake is a part of a swarm of 25 earthquakes and is the maximum event of the swarm. An iso seismal map was compiled, macroseismic parameters and the association of the earthquake with the geological structures of the region were determined. A rupture plane operating in the source coinciding with the strike and dip of the Karatau fault has been established. It is shown that the 6-point macroseismic effect could be the result of the total impact of the swarm events of April 24, several of which are close in energy to the maximum shock. The depth values of the maximum shock on April 24 and the accompanying swarm of earthquakes during the day are considered, their connection with the deep structure of the region is established.

An unobserved lava fountain deciphered in real-time by high-precision borehole strainmeter and contribution to hazard evaluation: the Etna 21 May 2023 eruption

A powerful lava fountain took place on 21 May 2023 at Mt. Etna after a year of repose, preceded a few days earlier by a shallow seismic swarm. The main critical issue with the eruption was the difficulty to track this eruptive event with conventional remote sensing devices, such as webcams and satellite instruments, due to the bad weather conditions and especially the dense cloud cover. Despite the sophisticated monitoring available at the volcano, the eruption effectively remained “hidden”. It was here that the borehole dilatometers excelled: as with all recent lava fountains at Etna, these high-precision instruments detected significant strain changes and proved a valuable contribution to real-time monitoring of the event. Through recently implemented approaches, the analysis of the strain data allowed us to decipher the eruption: namely identify the timing of the events, evaluate the “size” of the fountain (i.e. its eruptive intensity), and also estimate the erupted volumes in near real-time. This provided a useful support during the Civil Protection emergency meeting at the Prefecture of Catania, where these results were presented and jointly discussed. Overall, the 21 May 2023 lava fountain was an important showcase, demonstrating the strategic contribution that real-time high-precision strain signals may have in defining and assessing the hazard associated with an eruptive event even in adverse weather conditions, when remote sensing systems may be unable to furnish direct information on the ongoing phenomenon.

Seismicity and Present‐Day Crustal Deformation in the Southern Puna Plateau

AbstractThe Southern Puna plateau in the central Andes has a complicated tectonic history that includes episodes of distributed shortening and extension, lithospheric delamination, uplift and Quaternary backarc volcanism. In this study, the upper crustal structure and present‐day deformation in this area is investigated using a new regional earthquake catalog derived with a deep‐learning‐based phase picker. Results show abundant strike‐slip seismicity at shallow depths in the eastern Southern Puna plateau that reveals active fault systems in the area and indicates N‐S extension/E‐W compression that changes orientation and relative magnitude from north to south. A broad zone of seismic quiescence in the western plateau may indicate a zone of upper crustal decoupling from large‐scale deformation. The region separating the western and eastern plateau exhibits a complex stress field that can be related to the boundary of east/west oriented middle‐to‐lower crustal flow in the main volcanic arc. Southeast of the plateau in the Sierras Pampeanas, crustal seismicity deepens and is dominated by conjugate reverse faulting structures associated with the direction of plate convergence. Vp and Vs seismic velocity models of the upper crust obtained through local earthquake tomography with the improved seismic catalog show low‐velocity anomalies near intermontane basins, except in the Antofagasta basin where a high‐velocity anomaly possibly represents shallow intrusive component of Quaternary basaltic volcanism. Below the Cerro Galan caldera, an upper crustal 10‐day long earthquake swarm is observed which may indicate local stress perturbations from fluids at the top of the crustal magmatic system that feeds this volcano.

Contribution of aseismic slips to earthquake swarms at the Hakone volcano

Recent studies have proposed the contribution of aseismic slip (AS) to earthquake swarms. We investigated the role of AS in earthquake swarms that occurred in 2009, 2015, and 2019 at the Hakone volcano, central Japan, through highly resolved hypocenter distribution analysis, geodetic observation analysis, and identification of similar earthquakes. We observed diffusion-like migration of hypocenters during these swarms. The hydraulic diffusivity varied among the swarms, indicating differing dynamics. The 2015 swarm exhibited rapid hypocenter migration and significant crustal deformation, as revealed by the temporal sequences of tiltmeters near the swarm region. Right-lateral shear dislocation on fault planes could explain the crustal deformation observed in 2015, indicating that AS released approximately 90% of the moment. However, the 2009 swarm lacked evidence of significant AS contribution, indicating that the primary mechanism was fluid pressure diffusion. The substantial contribution of AS to the 2015 swarm might be attributed to increased fluid pressure due to the intrusion of hydrothermal fluid into the shallow part beneath the volcano during volcanic unrest. Our findings imply that the temporal and spatial patterns of seismicity can provide valuable insights into the underlying mechanics of earthquake swarms.Graphical abstract

The role of fluids in earthquake swarms in northeastern Noto Peninsula, central Japan: insights from source mechanisms

AbstractA prolonged earthquake swarm has persisted since June 2018 in northeastern Noto Peninsula (central Japan), with activity focused into distinct southern, western, northern, and eastern clusters. To explore the role of fluids in the occurrence of this swarm, we analyzed the focal mechanisms of the earthquakes occurring from 1 January 2018 to 30 November 2022 and performed stress tensor inversions. The western, northern, and eastern clusters were dominated by a reverse fault-type mechanism with a horizontal P-axis oriented NW–SE. One of the nodal planes of those mechanisms aligns closely with the precisely relocated hypocenter distribution. The stress fields in these three clusters, as determined by stress tensor inversion, have maximum principal stresses oriented horizontally in the NW–SE direction and minimum principal stresses oriented vertically, aligning with the regional stress field. From these focal mechanisms and this stress field, we derived small misfit angles and large slip tendencies. These findings suggest that, in these three clusters, fluids diffused into faults largely aligned with the regional stress field, resulting in earthquakes with compatible focal mechanisms. Conversely, normal and strike-slip fault-type focal mechanisms, which are unfavorable to the regional stress field, dominated in the southern cluster. The estimated stress fields, deviating from the regional stress field, have maximum principal stresses closer to vertical and minimum principal stress oriented horizontally in the ENE–WSW direction. For earthquakes deeper than 15 km, this local stress field results in relatively larger misfit angles and lower slip tendency than the other clusters. These results suggest that those earthquakes in the southern cluster occurred on misoriented fault planes due to elevated pore fluid pressures. These findings provide strong evidence of the ascent of high-pore-pressure fluids from depth in the southern cluster and their subsequent diffusion into a southeast-dipping fault zone. Graphical Abstract

Real-time modeling of transient crustal deformation through the quantification of uncertainty deduced from GNSS data

We propose a new method for real-time uncertainty monitoring of earthquake and volcano source models using data from the global navigation satellite system (GNSS) and explore its application concerning observation station placement. The Geospatial Information Authority of Japan operates two main types of GNSS earth observation network system (GEONET) coordinates for crustal deformation monitoring on different time scales: post-processing analysis values and real-time GEONET analysis system for rapid deformation monitoring (REGARD). REGARD uses the Markov Chain Monte Carlo (MCMC) method termed real-time automatic uncertainty estimation of a coseismic single rectangular model using GNSS data (RUNE) for single rectangular fault model estimation to handle uncertainty. Thus far, no GNSS monitoring system can automatically detect transient crustal deformation events, such as volcanic activity and earthquake swarms, on timescales of a day or less. We extended RUNE and developed a core program for a new monitoring system for earthquake and volcanic source models and their uncertainties. Our program achieved automatic and stable MCMC utilization for rectangular fault, dike, Mogi, and spheroid models by increasing the computational speed, improving search efficiency, and adjusting hyperparameters. The program automatically determines the standard deviation of the likelihood function assuming a normal distribution with weights for each observation station. The calculation time was within 15 s for 1 × 106 samples on a standard 1U server. We assessed the reliability of the developed method using synthetic and observed GNSS data from the 2015 Sakurajima volcanic event. The results were consistent with the assumed model and previous studies and indicated an advantage in automatically quantifying uncertainty in a short computation time. Based on MCMC samples, we developed a new visualization algorithm to indicate areas on a map in which the number of observation stations should be expanded. We assessed the reliability using data from the 2023 Noto Peninsula earthquake [Mj 6.5]. The results indicate that the algorithm is helpful in studying the placement of stations. The above model extensions and their application are essential to achieve a rapid quantitative understanding of disaster events near urban areas and for utilizing this information in emergency response activities.Graphical

Rapid Gas Bubble Growth in Basaltic Magma as a Source of Deep Long Period Volcanic Earthquakes

AbstractIn this paper, we present numerical modeling aimed to explain Deep Long Period (DLP) events occurring in middle‐to‐lower crust beneath volcanoes and often observed in association with volcanic eruptions or their precursors. We consider a DLP generating mechanism caused by the rapid growth of gas bubbles in response to the slow decompression of H2O–CO2 over‐saturated basaltic magma. The nucleation and rapid growth of gas bubbles lead to rapid pressure change in the magma and elastic rebound of the host rocks, radiating seismic waves recorded as DLP events. The magma and host rocks are modeled as Maxwell bodies with different relaxation times and elastic moduli. Simulations of a single sill‐shaped intrusion with different parameters demonstrate that realistic amplitudes and frequencies of P and S seismic waves can be obtained when considering intrusions with linear sizes of the order of 100 m. We then consider a case of two closely located sills and model their interaction. We speculate on conditions that can result in consecutive triggering of the bubble growth in multiple closely located batches of magma, leading to the generation of earthquake swarms or seismic tremors.

Crustal Deformation and Seismic Velocity Perturbations in the Alto Tiberina Fault Zone (Northern Apennines, Italy)

AbstractCrustal perturbations related to seismic activity can generally be observed with the occurrence of a large magnitude event. For less energetic seismic sequences though, the associated transient crustal variations are questionably measurable, and their observation gets easily obscured by relatively stronger perturbations such as the ones related to hydrological processes. In this study we reveal the significant role that terrestrial water‐storage variations play in governing temporal crustal changes in the tectonically active Northern Apennines of Italy, and discuss the potential of accounting for its correction in order to monitor the relatively weaker transient perturbations caused by local seismic swarms. This area is characterized by an extensive level of low‐energetic seismic activity, typically clustered in time and space, of which three main seismic swarms outstand during the 12 year period of study (2010–2021). Our analysis compares independent observations and processing methods of Global Navigation Satellite System measurements and ambient seismic noise recordings. We adopt a multivariate statistical approach to discriminate between independent sources of ground deformation, and seismic noise cross‐correlation analysis to monitor relative seismic‐velocity variations. The result shows how the perturbation effects produced by variations in total water content are dominant in both time series of ground deformations and seismic‐velocity variations. After correcting for the water‐related variation effects, our monitoring results reveal perturbations in the crustal properties whose activation time and depth range correlate with the occurrences of the seismic swarms.

The 1912 Seismic Swarm in Guadalajara, Mexico: An Example of Intense and Unusual Seismicity in the Trans-Mexican Volcanic Belt

Abstract The residents of the Guadalajara, Mexico metropolitan area awakened on 8 May 1912 to a strong earthquake. In the next two months, ∼1500 shocks followed this first event. Two additional clusters of microearthquakes occurred in July and September. Although no significant structural damage was reported, several churches, public buildings, and private dwellings in the cities of Guadalajara and Zapopan suffered damage. Based on the macroseismic data, the intensity of the larger earthquakes was modified Mercalli intensity (MMI) VI–VII. The area that strongly felt the earthquakes and where the damage was concentrated is contained within a radius of ∼5 km. We assume that the swarm took place within this circle. Based on the relations of MMI versus peak ground acceleration (PGA), it is estimated that the PGA during the swarm was ∼190 gals. The magnitude of the largest earthquakes in the swarm was initially estimated as Mw 5.7, using a published PGA-to-Mw relation. However, this relation was based on earthquakes located 20 km away from downtown Guadalajara. Considering that the distance of the swarm from the city was ∼5 km, the magnitude of the largest events in the swarm was recalibrated as Mw 4.7. A second estimate of the magnitude was based on ground-motion models to calculate the PGA resulting from an earthquake located 5 km away, at a depth of 1 km. The resulting magnitude is Mw 5.3 ± 0.5. Thus, we suggest that the largest earthquakes of the swarm had an approximate magnitude of Mw 5.0. The location of the swarm suggests the presence of active faults underlying the metropolitan area of Guadalajara, where no seismicity has been recorded. This should be carefully considered to assess the seismic hazard of this important city of over 5 million people.

First Installation of an Optical Ocean-Bottom Seismometer, Cabled Offshore Les Saintes, Lesser Antilles

Abstract The detection and analysis of offshore seismic processes worldwide often require the use of ocean-bottom seismometers (OBSs). However, most OBS deployments are done with stand-alone stations, with data recovery delayed by months. On the other hand, electrically cabled OBS, which allows for real-time monitoring, remains exceptional due to the high cost of manufacturing, installation, and maintenance. Here, we present a new perspective for cabled array of OBSs, using purely optical seismometers, plugged at the end of long fiber-optic cables, aimed at reducing their cost for observatories requesting real-time data. The optical seismometer was developed in the last decade by the École Supérieure d’Électronique de l’Ouest, based on the Fabry–Perot interferometer, tracking at high resolution the displacement of the mobile mass of a mechanical geophone (no electronics nor feedback). A prototype was successfully installed at the top of La Soufrière volcano of Guadeloupe in 2019. We replicated this sensor and installed it 5 km offshore Les Saintes islands, at 43 m depth (Guadeloupe, Lesser Antilles) to characterize the swarm-type activity persistent after the 2004 M 6.3 earthquake (Interreg Caraïbe PREST project). The installation cruise, FIBROSAINTES, was supported by the Flotte Océanographique Française. A plow designed by GEOAZUR carried the cable and was pulled on the seafloor by the vessel ANTEA. The landing cable was connected to the interrogator, with a real-time telemetry to the Institut du Physique du Globe de Paris/Observatoire Volcanologique et Sismologique de Guadeloupe. The OBS has been qualified with local land-based velocity broad band stations. The analysis of local earthquake swarms suggests transient creep on the major normal faults. This successful installation opens promising perspectives for real-time monitoring in on-land or offshore sites, presenting harsh environmental conditions, in which commercial, electrical seismic sensors are difficult and/or costly to install and maintain.

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.

Characterization of Earthquake Clustering and Swarms in Central New Mexico from 2002–2009 Associated with the Socorro Magma Body

ABSTRACT Increased seismicity in central New Mexico is associated with a midcrustal magma body underlying an extensional rift zone, with earthquakes typically occurring in spatially compact clusters with occasional swarms occurring within limited time periods. Seismic swarms are observed in a range of environments and can be indicative of a variety of geophysical processes. To identify the primary geophysical processes governing central New Mexico earthquake clustering and swarming, we first relocated seven years (2002–2009) of earthquakes for the area above the Socorro magma body (SMB). The resulting catalog was used to define spatial–temporal and temporal–magnitude patterns, significant b-values, cluster and swarm variance and planarity, correlation coefficient variations for event pair P waves, and focal mechanisms. Spatial–temporal migration of swarms, typically indicative of aseismic or fluid-driven earthquake sequences, is not observed for the majority of SMB swarms. Other observed seismic characteristics of SMB clusters and swarms suggest complex rupture, as planarity, focal mechanisms, and available b-values are similar to those typically observed in mainshock–aftershock sequences. However, temporal–magnitude patterns, diffusivity rates of 1–10 m2/s, highly correlated waveforms, and swarm durations are indicative of fluid pressure-driven earthquake triggering. Based on these documented cluster and swarm patterns, we suggest complex rupture related to fluid-pressure triggering along pre-existing Rio Grande rift faults.

Parallel dynamics of slow slips and fluid-induced seismic swarms

Earthquake swarms may be driven by fluids, through hydraulic injections or natural fluid circulation, but also by slow and aseismic slip transients. Understanding the driving factors for these prolific sequences and how they can potentially develop into larger ruptures remains a challenge. A notable and almost ubiquitous feature of swarms is their hypocenters migration, which occurrence is closely related to the processes driving the observed seismicity, in a similar way as seismicity accompanies slow-slip events at subduction zones. Here, we analyze global data on migrating sequences, and identify scaling laws for migration velocity, moment and duration measured on natural and injection-induced swarms, foreshock sequences, and slow slip events. We highlight two different behaviors among these sequences: one linked to slow slips, with elevated migration velocities and moments, and the other related to fluid-induced processes, featuring lower velocities and moments. These results provide metrics for distinguishing between the drivers of earthquake swarms, fluid or slow-slip related, and prompt a reevaluation of scaling laws of fault slip transients, especially for swarms.

Nonoverlapped Sources of the Devastating 2023 Mw>6 Herat, Afghanistan, Earthquake Swarm Estimated by InSAR

Nonoverlapped Sources of the Devastating 2023 Mw>6 Herat, Afghanistan, Earthquake Swarm Estimated by InSAR

Monitoring seismic velocity changes at Campi Flegrei (Italy) using seismic noise interferometry

Campi Flegrei is a volcanic field located west of Naples (Italy) in a densely populated area. Since 2005, its ground has been rising steadily due to the accumulation of fluids at shallow depths. The inflation of volcanic edifices is a possible precursor of an impending eruption. The uplift is accompanied by increasing seismic activity. This raises concerns about the possibility that the volcano may be on the verge of an eruption. To track the fluid movement, it is possible to monitor subtle changes of velocities of seismic waves by exploring ambient seismic noise. By examining different frequency bands, we can observe velocity changes at different depths. We interpret these changes as a monitoring of depth-dependent deformation in addition to the standard monitoring of surface deformation. We observe a velocity decrease in the long-term trend, presumably due to the extension of the hydrothermal system at shallow depths. To explain the long-term changes, we model a spherical pressure source to simulate volumetric strain changes induced by recent fluid activity. The model explains both, surface and subsurface deformation which leads to the opening of microcracks and pores, resulting in the observed velocity decrease. The short-term velocity changes are mainly driven by temperature or groundwater level changes. Once velocity changes are corrected for seasonal effects, remaining short term velocity changes can be associated with volcanic activity and earthquake swarms.