Earthquake Swarm Research Articles

The propagation velocity of seismic activity migrating along the directions of the geodynamic forces prevailing in the northeastern Baikal rift system, Russia

The recent tectonic stress field in the northeastern Baikal rift system (BRS) corresponds to the crustal deformation field. The stress-strain state of the Earth’s crust determines the fault network geometry and spatiotemporal structure of the epicentral field characterized by many earthquake swarms and earthquake migrations in the study area. In order to study the seismic process dynamics in different directions of the crustal deformation, the spatiotemporal analysis of earthquake time series has been made over the 1964–2015 instrumental period. To determine the relationship between crustal stress and spatiotemporal features of the epicentral field the seismic data were projected along horizontal stress tensor axes σ3 and σ2, consistent with major directions of the crustal deformation, a strike of major rifting structures, and a general azimuth of active fault groups. The NE-SW direction along the intermediate horizontal stress axes and main faulted arears exhibits slow earthquake migrations up to 60 km long, propagating with a modal velocity of about 30 kilometers per year. The NW-SE direction along the principal horizontal stress axes, orthogonal to the main faulted areas, is characterized by shorter migration sequences of less duration, propagating with a higher velocity than sequences registered in the NE-SW. The difference between the migration dynamics in mutually orthogonal directions can be attributed to the fault network configuration and the differences in the deformation process.

Features of the Kizimen Volcano area seismicity prior to and during the 2010–2013 eruption

After 82 years of dormancy, Kizimen Volcano in Kamchatka (Russia) produced its first eruption that was supported by seismological observations. The 2010–2013 eruption lasted about three years and was preceded by high rates of seismicity lasting more than one and a half years. In this study we conduct a time-space seismicity analysis and consider the dynamics of the b-value (the slope of the cumulative distribution function of magnitudes). It is shown that there was a constant increase in stress. Which ended with an earthquake with MLmax = 5.2 followed by an eruption. The eruption was characterized by explosive phases and the squeezing out of a very viscous lava flow in 2011–2012. A change from effusive activity to explosive following swarms of volcano-tectonic earthquakes was observed throughout the entire period of the eruption. This confirms the poor mixing of magma in the chamber under the Kizimen Volcano.

The TsuJal Amphibious Seismic Network: A Passive-Source Seismic Experiment in Western Mexico

The geodynamic complexity in the western Mexican margin is controlled by the multiple interactions between the Rivera, Pacific, Cocos, and North American plates, as evidenced by a high seismicity rate, most of whose hypocenters are poorly located. To mitigate this uncertainty with the aim of improving these hypocentral locations, we undertook the TsuJal Project, a passive seafloor seismic project conducted from April to November 2016. In addition to the Jalisco Seismic Network, 10 LCHEAPO 2000 Ocean Bottom Seismometers (OBSs) were deployed by the BO El Puma in a seafloor array from the Islas Marías Archipelago (Nayarit) to the offshore contact between the states of Colima and Michoacan. We located 445 earthquakes in four or more OBSs within the deployed array. Most of these earthquakes occurred in the contact region of the Rivera, Pacific, and Cocos plates, and a first analysis suggests the existence of three seismogenic zones (West, Center, and East) along the Rivera Transform fault that can be correlated with its morphological expression throughout the three seismogenic zones. The seismicity estimates that the Moho discontinuity is located at 10 km depth and supports earlier works regarding the West zone earthquake distribution. Subcrustal seismicity in the Central zone suggests that the Intra-Transform Spreading Basin domain is an ultra-low spreading ridge. A seismic swarm occurred during May and June 2016 between the eastern tip of the Paleo-Rivera Transform fault and the northern tip of the East Pacific Rise-Pacific Cocos Segment, illuminating some unidentified tectonic feature.

Seismotectonic evaluation of off Nicobar earthquake swarms, Andaman Sea

The Andaman Sea has experienced a number of earthquake swarms as a result of complex tectonics arising from oblique subduction, active volcanic arc, the backarc spreading center, and sliver fault systems. One of the most energetic offshore earthquake swarms occurred in the off Nicobar region of the Andaman Sea in January 2005, following the 26th December 2004 (9.1 Mw) Sumatra megathrust event. After a brief quiescence, this region got reactivated again after the 21st March 2014 (6.5 Mw) event, followed by the occurrence of earthquake swarms in March 2014, October 2014, November 2015, and April 2019. In the present study, we analysed the temporal variation of the b-value of these swarms using the global network data and the Ocean Bottom Seismometer data to understand their genesis. Temporal variation of b-value suggests that b-values are larger than unity for earthquake swarms, indicating volcanic origin. Bimodal distribution of frequency magnitude relation suggests that the earthquake swarms have occurred due to complex seismic processes controlled by both tectonic and volcanic activities. We propose that a combination of magmatic pulsation in the arc volcanism in response to 2004 and 2005 megathrust events and the 6.5 Mw magnitude 21st March 2014 event in off Nicobar region, and reactivation of sliver fault systems, are the dominant mechanisms for the observed frequent earthquake swarms.

Migration of Fluid‐Induced Seismicity Reveals the Seismogenic State of Faults

AbstractSeismic swarms, in natural contexts or induced by anthropogenic fluid injections, commonly show migrations of earthquake hypocenters. Classically, such migrations are interpreted as the diffusion of the fluid pressure. However, recent studies show that the seismic front maps the stress concentration at the tips of propagating aseismic slip that is primarily induced by the fluid pressure increase. In this case, seismic migration might depend on the hydromechanical properties that control the dynamics of aseismic slip propagation rather than the hydraulic diffusivity. Here, we use synthetic seismic catalogs obtained from the hydromechanical modeling of a slip‐weakening, permeable fault response to a fluid injection. By varying the fault permeability and the stress state, we show that the shape of the seismic front in a distance‐time plot depends on the initial fault criticality to failure, and not on the hydraulic properties. We then extrapolate this numerical result to seven observed injection‐induced earthquake swarms by showing that the shape of the migration front directly depends on the seismogenic index. The seismic front has a diffusive behavior when the aseismic slip is directly driven by the pressure increase, either at the swarm beginning or later if the faults are not initially critically stressed. On the contrary, an accelerating seismic front indicates a high‐criticality stressed fault, on which the aseismic slip runs away from the fluid pressurized area. Therefore, this study highlights that the seismic front shape may prove a useful indicator to characterize the initial stress state of faults over which the swarms occur.

The correlation of earthquake swarms and local velocity heterogeneities in the Brawley seismic zone, southern California

Earthquake swarms with the possible largest magnitude greater than 5.0 frequently strike the Brawley seismic zone in southern California. The latest swarm of earthquakes occurred on 5 June 2021, including a magnitude 5.2 quake. The Salton Sea Geothermal Field lies at the northern end of the Brawley seismic zone. A good understanding of the detailed subsurface structure is necessary for seismic hazard assessment and the monitoring of geothermal resources exploration in this region. We apply the novel adjoint-state traveltime tomography method to image the crustal P- and S-wave velocity structures of the Brawley seismic zone and the adjacent area to the west. This new method requires no ray tracing but maintains a high level of accuracy in complex media. High-resolution P- and S-wave velocity models are constructed by inverting 85,321 first P-wave arrival times and 43,912 first S-wave arrival times, respectively. The velocity models reveal strong crustal heterogeneities. At 5–12 km depth the Brawley seismic zone is occupied by earthquake-prone high-velocity rocks; while the near surface is dominated by low-velocity sedimentary rocks. The source zone of the 1987 Superstition Hills earthquake sequence is filled with low-velocity rocks, possibly suggesting the presence of fluids. Fluids may have also played a critical role in the generation of earthquake swarms in the Brawley seismic zone.

Multitemporal and Multisensor InSAR Analysis for Ground Displacement Field Assessment at Ischia Volcanic Island (Italy)

Volcanic islands are often affected by ground displacement such as slope instability, due to their peculiar morphology. This is the case of Ischia Island (Naples, Italy) dominated by the Mt. Epomeo (787 m a.s.l.), a volcano-tectonic horst located in the central portion of the island. This study aims to follow a long temporal evolution of ground deformations on the island through the interferometric analysis of satellite SAR data. Different datasets, acquired during Envisat, COSMO-SkyMed and Sentinel-1 satellite missions, are for the first time processed in order to obtain the island ground deformations during a time interval spanning 17 years, from November 2002 to December 2019. In detail, the multitemporal differential interferometry technique, named small baseline subset, is applied to produce the ground displacement maps and the associated displacement time series. The results, validated through the analysis and the comparison with a set of GPS measurements, show that the northwestern side of Mt. Epomeo is the sector of the island characterized by the highest subsidence movements (maximum vertical displacement of 218 mm) with velocities ranging from 10 to 20 mm/yr. Finally, the displacement time series allow us to correlate the measured ground deformations with the seismic swarm started with the Mw 3.9 earthquake that occurred on 21 August 2017. Such correlations highlight an acceleration of the ground, following the mainshock, characterized by a subsidence displacement rate of 0.12 mm/day that returned to pre-earthquake levels (0.03 mm/day) after 6 months from the event.

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.

Analysis of the 2016\u20132018 fluid-injection induced seismicity in the High Agri Valley (Southern Italy) from improved detections using template matching

Improving the capability of seismic network to detect weak seismic events is one of the timeless challenges in seismology: the greater is the number of detected and locatable seismic events, the greater insights on the mechanisms responsible for seismic activation may be gained. Here we implement and apply a single-station template matching algorithm to detect events belonging to the fluid-injection induced seismicity cluster located in the High Agri Valley, Southern Italy, using the continuous seismic data stream of the closest station of the INSIEME network. To take into account the diversity of waveforms, albeit belonging to the same seismic cluster, eight different master templates were adopted. Afterwards, using all the stations of the network, we provide a seismic catalogue consisting of 196 located earthquakes, in the magnitude range − 1.2 ≤ Ml ≤ 1.2, with a completeness magnitude Mc = − 0.5 ± 0.1. This rich seismic catalogue allows us to describe the damage zone of a SW dipping fault, characterized by a variety of fractures critically stressed in the dip range between ~ 45° and ~ 75°. The time-evolution of seismicity clearly shows seismic swarm distribution characteristics with many events of similar magnitude, and the seismicity well correlates with injection operational parameters (i.e. injected volumes and injection pressures).

Interpretation of Recent Unrest Events (Bradyseism) at Campi Flegrei, Napoli (Italy): Comparison of Models Based on Cyclical Hydrothermal Events versus Shallow Magmatic Intrusive Events

Several recent models that have been put forth to explain bradyseism at Campi Flegrei (CF), Italy, are discussed. Data obtained during long-term monitoring of the CF volcanic district has led to the development of a model based on lithological-structural and stratigraphic features that produce anisotropic and heterogeneous permeability features showing large variations both horizontally and vertically; these data are inconsistent with a model in which bradyseism is driven exclusively by shallow magmatic intrusions. CF bradyseism events are driven by cyclical magmatic-hydrothermal activity. Bradyseism events are characterized by cyclical, constant invariant signals repeating over time, such as area deformation along with a spatially well-defined seismogenic volume. These similarities have been defined as “bradyseism signatures” that allow us to relate the bradyseism with impending eruption precursors. Bradyseism is governed by an impermeable shallow layer (B-layer), which is the cap of an anticlinal geological structure culminating at Pozzuoli, where maximum uplift is recorded. This B-layer acts as a throttling valve between the upper aquifer and the deeper hydrothermal system that experiences short (1-102 yr) timescale fluctuations between lithostatic/hydrostatic pressure. The hydrothermal system also communicates episodically with a cooling and quasi-steady-state long timescale (103-104 yr) magmatic system enclosed by an impermeable carapace (A layer). Connectivity between hydrostatic and lithostatic reservoirs is episodically turned on and off causing alternatively subsidence (when the systems are connected) or uplift (when the systems are disconnected), depending on whether permeability by fractures is established or not. Earthquake swarms are the manifestation of hydrofracturing which allows fluid expansion; this same process promotes silica precipitation that seals cracks and serves to isolate the two reservoirs. Faults and fractures promote outgassing and reduce the vertical uplift rate depending on fluid pressure gradients and spatial and temporal variations in the permeability field. The miniuplift episodes also show “bradyseism signatures” and are well explained in the context of the short timescale process.

Fluid Fluxing and Accumulation Drive Decadal and Short-Lived Explosive Basaltic Andesite Eruptions Preceded by Limited Volcanic Unrest

Abstract Some volcanoes are known for repeatedly producing explosive but short-lived eruptions (< half a day) every decade or so. These eruptions are often preceded by limited unrest signals and short run-up times to eruption (a few hours to months), and thus they are difficult to anticipate. Some well-documented examples are the 1990 and 2014 eruptions of Kelud volcano in Indonesia, or the 2015 Calbuco eruption in Chile. Here we interrogate the rock record and obtain insights into the processes and pre-eruptive conditions that led to the 1990 Kelud eruption, which we integrate with monitoring data (seismicity, lake temperature and hydro-acoustics, sulfur emissions) towards a conceptual model for this type of events. Mineral-melt geothermobarometers indicate that the basaltic andesite magma carried a crystal cargo from as deep as 15–19 km, and reached volatile saturation at 4–9 km with 2–4 wt.% water in the melt. The textures and compositional zoning of orthopyroxene and plagioclase do not support intrusion of more primitive magma as the driver for the eruption, and we instead propose that pre-eruptive fluid accumulation and high-temperature fluid fluxing from depth (likely dominated by CO2) played a major role in priming the eruption to occur. Such pre-eruptive gas accumulation is also supported by mass balance calculation of the emitted excess SO2 gas. Mg-Fe diffusion profiles in reversely zoned pyroxenes constrain timescales of weeks to months before eruption for fluid addition to the reservoir, and such events may be recorded in the monitoring signals, especially in the change of hydroacoustics and water lake temperature, and probably in the seismic swarms. We propose that fluid exsolution and accumulation in the shallow reservoir plays a crucial role in modulating and triggering short-lived explosive eruptions with brief unrest at Kelud and probably other volcanoes worldwide.

Characteristic of 27th September – 7th October 2017 earthquake swarms in Jailolo Volcano, West Halmahera, Indonesia, based on hypocenter and b-value

Halmahera is an area with active tectonics, so it has a high level of seismicity. Swarm earthquakes occurred in Jailolo from November to December 2015, and then in 2017, another earthquake swarm occurred from September to October. This earthquake is characterized by an increase in the number of earthquakes in a certain period with a relatively small magnitude, without mainshocks, and occurs in volcanic areas. This research used arrival time from P and S waves recorded at Taide Digital Seismograph (TDS) which was positioned at Ternate Geophysical Station (TNTI). We used cross-section on hypocenter to see the depth distribution using GMT and determination of b-value using ZMAP code. From the results of this study, the variation in the magnitude of the earthquake swarm obtained ranged from 0.7 to 5.0 with a depth of 7.7-12 km. Our results show a b-value of aproximately 1.0 in the area near Jailolo Volcano, 1.0-1.5 in the northwest of Jailolo and 1.0-2.0 in the southeastern part of Jailolo. Based on b-value we obtained, the characteristics of the Jailolo swarm earthquake tend to be influenced by magmatic activity.

Earthquake Swarm Analysis around Mt. Salak, West Java, Indonesia, Using BMKG Data from August 10 to November 24, 2019

Earthquake swarms commonly come approximately active tectonic and volcanic area. Interestingly, the swarm events occurred ~23 km southwest from Mt. Salak-Bogor, West Java, Indonesia, from August 10 to November 24, 2019, and were recorded by local/regional network of the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG). Our previous study showed that in this area a destructive ML 4.6 earthquake with thrust faulting occurred on September 8, 2012. The double-difference method was applied to update the hypocenter locations from the BMKG data. In the time period of ~3.5 months, we relocated 79 swarm events with ~9.4 km depth average for local magnitude (ML) 2.2 to 4.2. The source mechanism result for selected events shows a strike-slip faulting. Our interpretation is that these swarm events are probably related to stress change due to volcano-tectonic activity.

Spatiotemporal Characteristics of Relocated Deep Low‐Frequency Earthquakes Beneath 52 Volcanic Regions in Japan Over an Analysis Period of 14 Years and 9 Months

AbstractWe constructed a catalog of deep low‐frequency (DLF) earthquakes beneath 52 volcanic regions in Japan to investigate their seismicity based on three analyses: relocation, classification, and detection. Relocation and classification analyses were based on waveform correlation, and detection analysis was conducted using the matched filter technique. We detected a total of 105,327 DLF earthquakes and found that DLF earthquakes in many regions are spatially clustered with a spatial spread of only 1–2 km with approximately 5 km intervals between the lower limit of crustal earthquakes and Moho discontinuity. Based on temporal seismicity patterns, DLF earthquake groups in each region can be classified into episodic and non‐episodic types. Episodic groups consist of seismic swarms and quiescence. In some episodic groups, DLF earthquakes have constant recurrence interevent times or increasing interevent times as a function of time. Swarms of DLF earthquakes sometimes are associated with volcanic activities at the surface, which are crustal deformations of Meakan volcano, 2007 and 2014 eruptions of Ontake volcano, and the 2015 magma intrusion of Sakurajima volcano. The spatiotemporal characteristics of DLF earthquake groups may be linked to the movement of magmatic fluids. The discrete vertical separation of DLF earthquake groups may reflect small‐scale heterogeneities, such as injected magma. Periodic activity patterns may be caused by volcanic mechanisms, such as intermittent magma flow. The variety of DLF earthquake patterns may suggest that multiple mechanisms, including magma cooling and intermittent magma flow in complex magma supply systems, may trigger DLF earthquakes rather than a single mechanism.

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.

Kīlauea–Leilani 2018 lava flow delineation using Sentinel2 and Landsat8 images

Abstract Kīlauea is a broad shield volcano built against the southeastern slope of Mauna Loa. The summit presently has a caldera that is roughly 4 km by 3.2 km wide, and walls between 0 and 120 m high. In late April 2018, an eruption involving both the summit crater and the lower East Rift Zone occurred. In this work a quasi-real-time estimation of the evolution of radiant lava flow extension starting from May 2018 for Kīlauea–Leilani eruption using satellite image data is presented. The active lava flow evolution was obtained using Copernicus Sentinel2 (S2) and USGS-Landsat8 (L8) polar satellites acquiring medium/high-spatial-resolution images (20 × 20 m and 30 × 30 m, respectively) in the vis–SWIR–TIR (Short wave infrared-thermal infra red) spectral range. Because of the Kīlauea eruption extension and duration, a multisensor approach was used in order to improve the timing of the information derived by high-spatial-resolution remote-sensed data merging two missions with different revisit times. The 2018 eruptions at Hawaii's Kīlauea Volcano developed rapidly after the initial activity centred on the Pu‘u ‘Ō‘o crater floor on 1 May followed by draining of the lava lake at Halemáumáu Overlook Crater in the following days. During the magma extrusion from the summit, earthquake swarms and ground cracking hit the Leilani Estates neighbourhood on 2 May. With the S2 and L8 sensors we followed the lava flow from 5 May up to mid August, considering also that the activity started to decline from the beginning of August. At the end of the activity, Kīlauea Volcano had experienced its largest lower East Rift Zone eruption and caldera collapse in at least 200 years.

Seismicity patterns along the northern Manila Trench reflect crustal properties of the subducting plate

We examine the seismicity recorded by two temporary ocean bottom seismometer arrays deployed along the northeastern Manila Trench, covering the outer rise and part of the accretionary wedge. A total of 1437 events were determined during the 44-day recording period, showing a relatively more dynamic seismic activity than previously thought. Most events occurred in the subducting plate, oceanward of the trench, at depths between 25 and 50 km and revealed normal-fault type mechanisms. The relatively deeper focal depth and the lack of well-developed outer rise bulge near the seismic cluster suggest an untypical pattern of outer rise earthquakes. Seismicity with a high occurrence frequency and small magnitude is typical of earthquakes swarm, which is generally thought to be driven by fluid pressure variations. We suggest the infiltration of seawater through fractures into the rifted South China Sea margin is the main driver of the earthquakes detected during our deployments. Because earthquakes are also more abundant in the area where a significant change of crustal thickness is documented, we suggest the inherited crustal properties and tectonic structures strongly control the seismic behavior along the Manila subduction system.

On the role of thermal stress and fluid pressure in triggering seismic and aseismic faulting at the Brawley Geothermal Field, California.

Surface deformation and earthquake swarms are correlated in space and time with operations at the Brawley geothermal field in southern California. The seismicity culminated in 2012, about 2 years after the onset of geothermal activity, with a M5.4 earthquake. These earthquakes occurred at a >5km depth, much larger than the ∼1km reach of the geothermal wells, raising questions about the triggering mechanism. Surface deformation shows that aseismic slip on a normal fault intersecting the geothermal reservoir preceded the swarm and possibly triggered it. In this study, we resort to geomechanical modeling to investigate how the sequence of aseismic and seismic slip unfolded. The modeling accounts for thermo- and poro-elastic stress changes induced by the geothermal operations and allows for inelastic deformation and faulting of the reservoir and surrounding medium. The simulation successfully reproduces the flow rates and well-head pressures reported by the operator as well as the measured surface subsidence. By varying the model parameters, we show that the surface subsidence is due to thermal contraction and normal faulting. The fault reactivation is driven by pressure changes and thermal unclamping. The pressure-driven reactivation is rapid and influences a larger area, while the temperature-driven reactivation is more gradual and more localized near the injection wells. In our simulation, aseismic normal faulting driven by the geothermal operation leads to elastic stress release via yielding and faulting within the reservoir volume and, conversely, to stress build-up beneath the reservoir, where the 2012 swarm developed. Such a stress transfer provides a plausible explanation for the 2012 Brawley swarm. Our study shows how a geothermal operation can, in principle, contribute to seismic hazard mitigation through the aseismic release of tectonic stresses within a geothermal field but points to the difficulty of mitigating the hazard posed by stress transfers in the surrounding area.

Characteristics of Foreshocks Revealed by an Earthquake Forecasting Method Based on Precursory Swarm Activity

AbstractWe have developed an empirical earthquake forecast method, Maeda's method, based on the statistical features of precursory seismic swarm activity, that is foreshocks, which sometimes appear before a mainshock, and issuing an alert of a mainshock occurrence within a certain period of time. In this study, we investigated the effectiveness of earthquake forecast of Maeda's method by applying it to seismicity under various tectonic environments of Japan such as regions characterized by interplate seismic activity, a tectonic fault line (concentrated deformation zone), and an island arc area of seismic and volcanic activity. As a result, we confirmed that Maeda's method yielded generally higher scores than a forecast model based on a stationary space‐time epidemic‐type aftershock sequence (ETAS) model. We also found that foreshocks detected along the Japan Trench were distributed along the edges of low‐velocity anomalies and among areas with background swarms related to slow slip events (SSEs). The foreshocks may have been caused by a heterogeneous stress distribution associated with the existence of a plate‐bending axis and a subducted seamount. Foreshocks off Iwate prefecture, in particular, were excited by periodic SSEs. In an inland tectonic zone and an island arc, swarm activity associated with magmatic or fluid activity related to low‐velocity anomalies tended to be followed by a mainshock. Maeda's method is a simple and efficient counting‐number‐based earthquake forecast model and may capture characteristics of foreshocks that reflect a physical phenomenon, such as a nucleation process involving precursory slip, which the stationary ETAS model is not able to represent.

A Bayesian reassessment of the relationship between seismic moment and magmatic intrusion volume during volcanic unrest

Volcano-tectonic (VT) earthquake swarms are usual precursors to volcanic eruptions and their dynamic phenomenologies are an invaluable eruption forecasting tool. A power-law relationship between cumulative seismic moment release and intrusion volume has been proposed for both well injection sites and VT swarms. Here we compile data from 17 geodetically-studied VT swarms and use a Bayesian methodology to assess the relationship between cumulative moment release and subsurface volume change. We find that the empirical relationship derived from injection sites systematically underpredicts volume changes observed during VT swarms near volcanoes. A new relationship derived specifically from VT swarms provides a better fit, but large uncertainties mean that estimates of intruded volume range by three orders of magnitude (95% confidence) for a given cumulative seismic moment release. We further subdivide the dataset, and although the sample size is too small to meet conventional statistic hypothesis significance tests, qualitative inspection suggests that the seismic moment release is proportionally larger for unrest swarms which culminate in eruption. We conclude that the currently available dataset is too sparse and the uncertainties too large for use for reliably forecasting eruption magnitude; however, as the dataset of cases grows, further analysis along these same lines may provide improved diagnostic insights into eruption-related processes that may be driving seismic moment release in VT swarms.