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Genuine non‐double‐couple (non‐DC) components of a seismic source, defined here as the non‐DC components that are not due to summation of pure double‐couple (DC) components, provide important insight into special physical processes in non‐earthquake sources such as explosion, volcano eruption and collapse etc. Yet they remain challenging to be resolved. To address the issue and explore the physical mechanism of those special events, we develop a waveform‐polarity‐based moment tensor (WPMT) inversion method and employ it to study physical process in the 2014–2015 Bárðarbunga volcano event sequence. The WPMT method incorporates P‐wave polarity data and seismic waveforms in the source inversion, designs a source simplicity test to check possible complex rupture in the seismic source, and employs a simulated annealing algorithm to search the best source solution. The simplicity test checks consistency of the source processes in the initiation stage of the event and the major energy release process of the event, thus ensuring that the inferred non‐DC components are genuine to the seismic source. Real event and synthetic tests indicate that the WPMT method can identify and resolve genuine non‐DC components in a seismic source. The WPMT inversions of the Bárðarbunga sequence yield many genuine non‐DC source components and reveal that the eruptions are accompanied by seismic activities in depths of 1–5 km with magma migrations out of chambers, collapses of conduits, failures of normal faults, and a magma recharge at a depth of 9 km accompanied by a failure on a nearby normal fault. Plain Language Summary: Special seismic events such as explosion, collapse and volcanic eruption possess source components that are different from fault solutions used to represent a typical tectonic earthquake of simple shear. The identification and quantification of the source components of those special seismic sources thus play an important role in deciphering the physical process during those events. Yet, it remains challenging to resolve the source components of special seismic events due to the ambiguity of distinguishing them from an apparent one due to complex earthquake rupture. We develop a waveform‐polarity‐based moment‐tensor inversion to resolve the source components of a special seismic source and employ the method to study the physical process in the 2014–2015 Bárðarbunga volcano event sequence in Iceland. Real event and synthetic tests confirm method's capability of identifying and constraining the source components of special non‐earthquake events. The study of the Bárðarbunga sequence reveals that the eruptions are accompanied by seismic activities in depths of 1–5 km with magma migrations out of chambers, collapses of conduits, failures of normal faults, and a magma recharge at a depth of 9 km accompanied by a failure on a nearby normal fault. Key Points: We propose a waveform‐polarity‐based moment tensor inversion method to extract non‐double‐couple components of moderate seismic eventsReal event and synthetic tests show the method is effective in distinguishing and resolving genuine non‐double‐couple componentsMethod application reveals magma activities, conduit collapses and induced earthquakes during the 2014–2015 Bárðarbunga eruptions [ABSTRACT FROM AUTHOR]

All instrumented basaltic caldera collapses have generated Mw > 5 very long period earthquakes. However, previous studies of source dynamics have been limited to lumped models treating the caldera block as rigid, leaving open questions related to how ruptures initiate and propagate around the ring fault, and the seismic expressions of those dynamics. We present the first 3D numerical model capturing the nucleation and propagation of ring fault rupture, the mechanical coupling to the underlying viscoelastic magma, and the associated seismic wavefield. We demonstrate that seismic radiation, neglected in previous models, acts as a damping mechanism reducing coseismic slip by up to half, with effects most pronounced for large magma chamber volume/ring fault radius or highly compliant crust/compressible magma. Viscosity of basaltic magma has negligible effect on collapse dynamics. In contrast, viscosity of silicic magma significantly reduces ring fault slip. We use the model to simulate the 2018 Kı̄lauea caldera collapse. Three stages of collapse, characterized by ring fault rupture initiation and propagation, deceleration of the downward‐moving caldera block and magma column, and post‐collapse resonant oscillations, in addition to chamber pressurization, are identified in simulated and observed (unfiltered) near‐field seismograms. A detailed comparison of simulated and observed displacement waveforms corresponding to collapse earthquakes with hypocenters at various azimuths of the ring fault reveals a complex nucleation phase for earthquakes initiated on the northwest. Our numerical simulation framework will enhance future efforts to reconcile seismic and geodetic observations of caldera collapse with conceptual models of ring fault and magma chamber dynamics. Plain Language Summary: Caldera collapse manifests as the rapid subsidence of a kilometer‐scale block of crust circumscribed by a near‐circular fault on top of a volcano. The subsidence of the caldera block is caused by the eruption‐induced withdrawal of magma and reduction in pressure in the underlying magma chamber. All scientifically instrumented caldera collapses at volcanoes with low‐viscosity magma are accompanied by earthquakes of magnitude 5 and above. How do magma viscosity and the seismic wave radiation influence the amount of slip per earthquake on the fault? What can we learn about the dynamics of these earthquakes from seismic records? We address these questions by performing computer simulations of caldera collapse earthquakes and compare the results to the seismic records from the Kı̄lauea caldera collapse of 2018. Key Points: Seismic wave radiation through ring fault and magma, as well as high magma viscosity, reduce fault slip by up to half during collapseRupture propagation, downward momentum transfer via magma pressure waves, and chamber pressurization are identified in unfiltered seismogramsComparison between simulated and observed near‐field seismograms from Kı̄lauea 2018 reveals complex nucleation phase on the ring fault [ABSTRACT FROM AUTHOR]

Submarine volcano monitoring is vital for assessing volcanic hazards but challenging in remote and inaccessible environments. In the vicinity of Kita‐Ioto Island, south of Japan, unusual M ∼ 5 non‐double‐couple volcanic earthquakes exhibited quasi‐regular recurrence near a submarine caldera. Following the earthquakes in 2008 and 2015, a distant ocean bottom pressure sensor recorded distinct tsunami signals. In this study, we aim to find a source model of the tsunami‐generating earthquake and quantify the pre‐seismic magma overpressure within the caldera's magma reservoir. Based on the earthquake's characteristic focal mechanism and efficient tsunami generation, we hypothesize that submarine trapdoor faulting occurred due to highly pressurized magma. To investigate this hypothesis, we establish mechanical earthquake models that link pre‐seismic magma overpressure to the size of the resulting trapdoor faulting, by considering stress interaction between a ring‐fault system and a reservoir of the caldera. The trapdoor faulting with large fault slip due to magma‐induced shear stress in the submarine caldera reproduces well the observed tsunami waveform. Due to limited data, uncertainties in the fault geometry persist, leading to variations of magma overpressure estimation: the pre‐seismic magma overpressure ranging approximately from 5 to 20 MPa, and the co‐seismic pressure drop ratio from 10% to 40%. Although better constraints on the fault geometry are required for robust magma pressure quantification, this study shows that magmatic systems beneath calderas are influenced significantly by intra‐caldera fault systems and that tsunamigenic trapdoor faulting provides rare opportunities to obtain quantitative insights into remote submarine volcanism hidden under the ocean. Plain Language Summary: Monitoring submarine volcanoes is essential to understand and prepare for potential volcanic hazards in/around oceans, but it's challenging because these volcanoes are located in inaccessible environments. In a submarine volcano with a caldera structure in the south of Japan, unusual volcanic earthquakes took place every several years. After one of these earthquakes in 2008, a pressure sensor deployed on the sea bottom recorded a clear signal of tsunami waves. By utilizing the tsunami signal from the earthquake, we attempt to measure how much magma pressure was building up beneath the volcano before the earthquake. Assuming that the earthquake happened with sudden rupture on an intra‐caldera fault system due to highly pressurized magma beneath the volcano, we develop a method to assess the built‐up magma pressure through quantification of the earthquake and tsunami sizes. By applying the method, we estimate that the volcanic edifice was under a highly stressed condition before the earthquake, suggesting the active magma accumulation process that has continued beneath the volcano. Signals emitted from volcanic earthquakes under oceans shed light on the activity of poorly monitored submarine volcanoes. Key Points: Non‐double‐couple earthquakes with seismic magnitudes of 5.2–5.3 recurred in the vicinity of a submarine caldera near Kita‐Ioto IslandA mechanical model of trapdoor faulting based on tsunami data of the 2008 earthquake infers pre‐seismic overpressure in a magma reservoirUncertainty in fault geometry varies our estimate of pre‐seismic overpressure (5–20 MPa) and co‐seismic pressure drop ratio (10%–40%) [ABSTRACT FROM AUTHOR]

This study describes an ongoing intense earthquake swarm in the crust of the northeastern Noto Peninsula, Japan, that began around the end of 2019. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust. However, no volcanic activity has occurred in this region since the Middle Miocene (15.6 Ma). This study investigates the cause of this earthquake swarm based on the spatiotemporal evolution of earthquake hypocenters and seismic reflectors. The hypocenter relocation of 10,940 earthquakes (M > 1) reveals that they are all crustal and migrated upward, activating a complex network of faults at depths shallower than 20 km. The initiation of this earthquake swarm occurred at a locally deep depth (z = 17 km), and the local hypocenter distribution shows a characteristic circular pattern. We find a distinctive S‐wave reflector in the immediate vicinity. A low‐velocity anomaly exists below the reflector, and a high helium isotope ratio and a low‐gravity anomaly were observed at the surface. These observations suggest that the current seismicity is associated with fluids released by ancient or possibly unrecognized modern magmatic activity, although no volcanic activity has been documented in this area for 15 million years. Significant crustal deformation was observed during this swarm and is probably related to the fluid movement and aseismic deformation that contributed to this earthquake swarm. The strongest M5.4 earthquake (as of October 2022) occurred near the migration front on the largest planar structure, leaving the shallow extension unruptured. Plain Language Summary: An intense earthquake swarm is currently occurring in the crust of the northeastern Noto Peninsula, Japan. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust, but no volcanic activity has occurred in this region since the middle Miocene (15.6 Ma). We investigate the cause of this earthquake swarm using precisely determined earthquake locations and seismic reflectors. We find that the earthquakes moved from deep to shallow depths via many planes, similar to earthquake swarms near volcanoes. The strongest M5.4 earthquake occurred near the migration front on the largest planar structure. Further earthquakes may be possible in the shallow part of this fault. We find a distinctive S‐wave reflector, suggesting a fluid source, in the immediate vicinity of the initiation point of this swarm. The local hypocenters show a circular pattern similar to the ring fault that forms above the magma reservoir. These observations may indicate that the current seismic activity is being impacted by fluids related to ancient or new hidden magmatic activity. The present results suggest that hidden magma‐induced structures and fluids can generate earthquakes even in areas where no volcanic activity has been observed for over 10 million years. Key Points: An intense seismic swarm continues in a non‐volcanic region, with upward earthquake migration through a complex network of faultsA deep S‐wave reflector located near the seismicity initiation point and above the low‐velocity anomaly may represent a fluid sourceThe significant surface deformation observed during this swarm was caused by aseismic deformation activated by fluid migration [ABSTRACT FROM AUTHOR]

A high‐precision earthquake catalog is made from continuous waveform data recorded at 360 portable seismographs deployed for one month in the Changbaishan‐Tianchi volcanic area based on an automatic earthquake detection and location workflow. Then we investigate detailed 3‐D structures of P and S wave velocities (Vp and Vs) and Vp/Vs ratio of the crust beneath the study region. Our results reveal a high‐Vp/Vs zone accompanied with active volcanic earthquakes beneath the Tianchi caldera at ∼5 km depth, which may reflect a magma chamber. The intense seismic activities within 2 km depth beneath the Tianchi caldera from 2002 to 2005 occurred directly above the magma chamber revealed by this study, which could be related to upward migration of high‐pressure volatiles originating from the magma chamber. A potential middle‐crustal magma chamber might provide continuous magmatic materials feeding the upper‐crust magma chamber and lead to ruptures of surrounding rocks, resulting in two swarms of volcanic seismicity during the observation period. We deem that the diverse eruption styles of the Changbaishan volcano are closely associated with the multilevel structure of magma chambers in the upper and middle crust. Plain Language Summary: It is difficult for the earthquake monitoring system to identify and locate microearthquakes by using traditional seismic detection methods due to their weak signals, leading to a relatively low quantity and quality of seismic data in a study region, which cannot meet the demands of high‐resolution seismic tomography. In this study, we detect and locate microseismic events beneath the Changbaishan‐Tianchi volcanic area using 1‐month waveform data recorded at our 360 portable seismographs deployed in the volcanic area. Then we investigate high‐resolution 3‐D crustal structures of P and S wave velocities (Vp, Vs) and Vp/Vs ratio beneath the study area. Our results reveal a possible crustal magma reservoir at ∼5 km depth beneath the Tianchi caldera, which is accompanied with volcanic earthquakes during our observation period. In addition, small earthquakes occurred actively at a depth of ∼2 km during 2002–2005, which are located right above the magma reservoir. We deem that the joint effect of magma reservoirs in the upper and middle crust could explain the active volcanic seismicity and distinctive eruption behaviors of the Changbaishan volcano. Key Points: A magma chamber at ∼5 km depth beneath the Tianchi caldera of the Changbaishan volcano (CBV) is revealedThe CBV unrest during 2002–2005 was related to reactivation of the crustal magma chamberDistinctive eruption behaviors of the volcano are related to multilevel magma chambers in the crust [ABSTRACT FROM AUTHOR]

The Tengchong volcano (TCV) is a large active volcano system in the southeastern Tibetan Plateau. It is characterized by large‐volume magmatic gas emission, active hydrothermal circulation, and intense volcanic and earthquake activities, posing a threat of near future eruptions. However, there is still no available model of the magmatic plumbing system beneath the volcano system, limiting the quantitative assessments of the eruption hazards. Here, we present a high‐resolution 3D model of the TCV constructed using ambient noise adjoint tomography. Our 3D model reveals a large basaltic magma reservoir with a volume of ∼7,000 km3 at depths of 20–35 km, which has a melt fraction of ∼2%–4.5%. Our results suggest that the deep crustal magma reservoir is fed by partial melting in the uppermost mantle and is recharging the shallow magma chambers beneath the TCV. Our results are key to understanding the volcanic activities and assessing future eruption hazards. Plain Language Summary: The Tengchong volcano (TCV) is one of the largest active intraplate volcanoes in southwest China. The unrests of the TCV in recent years have attracted a lot of attention and aroused concerns for the future explosive eruptions. In this study, we present a high‐resolution 3D model of the TCV using an innovative method of ambient noise adjoint tomography. Our results reveal a multi‐layered magmatic plumbing system in the lower crust and uppermost mantle beneath the TCV. The large basaltic magma reservoir in the lower crust has a volume of ∼7,000 km3 at depths of 20–35 km and melt fraction of ∼2%–4.5%. Thus, the total of melt volume reaches 140–315 km3. The deep crustal magma reservoir is fed by partial melting in the uppermost mantle and is recharging the shallow magma chambers. If such a large volume of melt gets erupted in the future, the lava could cover a huge area, threatening the livelihood of the half million people living nearby. It, therefore, becomes urgent to quantitatively assess and model the potential of future eruptions. Key Points: We present a high‐resolution 3‐D velocity model of the Tengchong volcano (TCV) using ambient noise adjoint tomographyWe reveal a multi‐level magma system in TCV, that is, a low velocity zone in the uppermost mantle feeds the crustal magma reservoirThe lower crustal basaltic magma reservoir has a volume of ∼7,000 km3 at depths of 20–35 km with melt fraction of ∼2%–4.5% [ABSTRACT FROM AUTHOR]

The "wet" silicate solidus of mantle peridotite defines the initial melting temperature of Earth's mantle under water‐saturated conditions and the second critical endpoint (SCEP) marks the high P‐T end of the wet solidus. However, the location of the wet solidus has remained an outstanding issue for over 50 years and the position of the SCEP is hotly debated. Published wet solidi show a difference of 200–600°C at a given pressure while reported SCEPs range from <4 to >6 GPa. Using a large‐volume multianvil apparatus, we investigated the water‐saturated melting behavior of a fertile peridotite at 3–6 GPa, 950–1200°C, and obtained well‐preserved quenched materials. On the basis of textures and compositions of the quenched materials, we bracket the wet solidus to between 950°C and 1000°C at 3 GPa and the SCEP between 3 and 4 GPa. Combining our experimental results with seismologic and petrologic observations, we propose that the lithosphere‐asthenosphere boundary in subduction zones should be constrained by the wet solidus and emphasize the role of a deep hydrous partial‐melting zone (DHPMZ) on magma genesis within the mantle wedge. We suggest that the DHPMZ is a source of hydrous melts to the primary melting zone in the mantle wedge and that the position of the volcanic front and its magma production rate may largely be controlled by melting and melt segregation processes within the DHPMZ. Our experimental results also suggest that high‐magnesian magmas (e.g., boninite, picrite, and komatiite) could be formed at conditions representative of subduction zones. Key Points: We investigated H2O‐saturated melting behavior of peridotite with a large‐volume multianvil apparatus and obtained well‐preserved quenched materialsWe determined the water‐saturated solidus and SCEP of peridotite from the textures and compositions of the quenched materialsThe experimental results provide new constraints on the melting processes and magma genesis within the mantle wedge [ABSTRACT FROM AUTHOR]

Okmok volcano located on the northeastern part of the Umnak Island is one of the most active volcanoes in the Aleutian Arc. It was initially built as a large shield volcano, but 12,000 and 2,050 years ago, it experienced two caldera‐forming eruptions that destroyed the central part of the volcano. The post‐caldera eruptions have occurred mostly along the inner perimeter of the caldera from cinder and tuff cones. Here, we use the local earthquake data of the Alaska Volcano Observatory (AVO) in the time period from 2003 to 2017 to build a model with the 3D distributions of the P and S wave velocities and Vp/Vs ratio. At depths of more than 10 km, we observe a vertically aligned anomaly of high Vp/Vs ratio interpreted as a long‐lived conduit likely responsible for the volcano evolution since its origin. Above this conduit, we reveal a large anomaly of high Vp/Vs ratio representing the main magma reservoir that is connected with several shallow anomalies located below the centers of recent eruptions in the caldera. One of such anomalies represents a large shallow reservoir below the Cone A, which was the source of most of Okmok's historical eruptions. The most recent eruption occurred in 2008 and was fed by a magma diapir that was initially formed in the central magma reservoir and then slowly ascended through a ductile silica rich layer at depths from 6 to 2 km. This interpretation is consistent with the petrology studies and modeling of ground deformations. Plain Language Summary: Okmok is one of the most active volcanoes of the Aleutian Arc. Initially it was formed as an isometrical shield volcano, which was later destroyed by two caldera‐forming eruptions. The post‐caldera activity mostly occurred at several cones distributed along the inner perimeter of the caldera. The latest eruption with the explosivity index of VEI 4 took place in 2008. This eruption, which produced a new large cone Ahmanilix in the northeastern part of the caldera, was significantly different in composition and eruption style compared to other intra‐caldera eruptions. We present a new seismic tomography model, which was constructed based on the arrival times of the P and S waves from local seismicity. Below 10 km depth, we observe an anomaly of high Vp/Vs ratio, which may represent a steady magma conduit that is responsible for the long‐term formation of the entire Okmok volcanic complex. In the upper crust, the model reveals a series of separate local magma sources beneath volcanic centers where historical eruptions took place. The 2008 eruption was fed by a magma diapir that was initially formed in the deep conduit and then slowly ascended through a ductile layer at depths from 6 to 2 km. Key Points: The new crustal model beneath the Okmok Caldera with the 3D distributions of Vp, Vs and Vp/Vs ratio reveals the geometry of magma sourcesBelow 10 km depth, an anomaly of high Vp/Vs ratio indicates the location of the main magma conduit that fed the long‐term formation of OkmokAt shallow depths, seismic anomalies reveal several magma sources that fed eruptions along the inner perimeter of the caldera [ABSTRACT FROM AUTHOR]

Adakite‐like potassic rocks are widespread in postcollisional settings where they provide potential insights into deep crustal processes that include partial melting, lower crustal flow and thickening, plateau uplift, and the creation of porphyry metal deposits. Although substantial progress has been made in characterizing the geochemical and geophysical features of postcollisional adakite‐like potassic rocks, their petrogenesis remains controversial. Here we report direct experimental evidence for the origins of these rocks with partial melting experiments on (1) garnet amphibolite and (2) the same garnet amphibolite mixed with 20 wt. % of a primitive Tibetan shoshonite. The experiments were conducted at 1.5–2.0 GPa and 800–1,000 °C. The partial melts of garnet amphibolite have typical adakitic signatures and are calc‐alkalic but lack the enrichment in potassium and other strongly incompatible elements (Rb, Ba, Th, U) that are characteristic of Tibetan adakite‐like rocks. In contrast, all characteristic features of the natural adakite‐like rocks are convincingly reproduced by the hybrid experiments. This includes negative inflections for Nb, Ta, and Ti on mantle‐normalized plots which are inherited from source materials rather than the effects of rutile in melting residues. The input of mantle‐derived shoshonitic mafic melts to a crustal source can be argued to provide not only the high concentrations of incompatible elements characteristic of adakite‐like potassic magmas but also the heat necessary for crustal melting. Our experimental results demonstrate that, in the case of the Tibetan Plateau at least, the production of adakite‐like potassic rocks in postcollisional settings can be best explained by such a model. Key Points: Adakite‐like potassic rocks are partial melts of hybrid crustal and mantle sourcesMantle‐derived magma provides a heat source for melting in thickened lower crustMantle‐derived magma provides high concentrations of K and incompatible elements [ABSTRACT FROM AUTHOR]

Determining whether seismicity near volcanoes is due primarily to tectonic or magmatic processes is a challenging but critical endeavor for volcanic eruption forecasting and detection, especially at poorly monitored volcanoes. Global statistics on the occurrence and timing of earthquakes near volcanoes both within and outside of eruptive periods reveal patterns in eruptive seismicity that may improve our ability to discern magmatically driven seismicity from purely tectonic seismicity. In this paper, we catalog magnitude four and greater (M4+) earthquakes near volcanoes globally and compute statistics on their occurrence with respect to various eruptive and volcanic attributes, evaluating their utility as diagnostic indicators of eruptions. Using a 2‐week time window and a 30 km radius around the volcanoes, we find that 11% of eruptions are preceded by at least one M4+ earthquake, but only 1% of such earthquakes is followed by eruption. However, earthquakes located 5–15 km from the volcano, those with normal faulting mechanisms and/or large nondouble‐couple components, and those occurring as groups are more commonly associated with eruptions, providing significant forecasting utility in some cases. Similarly, certain volcanoes are more likely to exhibit such precursors, such as those with long repose periods. We illustrate the use of these data in eruption forecasting scenarios, including rapid identification of analogous earthquake sequences at other volcanoes. When integrated within the context of multiparametric, multidisciplinary probabilistic assessments of volcanic activity, global earthquake statistics can improve eruption forecasts, and our work provides a model for use on other rapidly expanding global volcanological databases. Plain Language Summary: Magnitude four and larger (M4+) earthquakes in volcanic regions are only rarely followed rapidly by volcanic eruptions. In fact, most such earthquakes are caused by normal tectonic activity in the regions hosting the volcanoes. However, quickly determining the cause of any such earthquake is difficult. In this paper, we compute statistics of M4+ earthquakes near volcanoes globally in order to reveal patterns in the earthquake data that might be useful for improving volcano eruption forecasting and detection. Using a 2‐week time window and a 30 km radius around the volcanoes, we find that 11% of eruptions are preceded by at least one M4+ earthquake, but only 1% of such earthquakes is followed by eruption. However, certain kinds of M4+ earthquakes are more likely to be directly caused by magma movement and certain kinds of volcanoes are more likely to experience these earthquakes prior to erupting than others. Our results provide significant forecasting utility in some cases and provide a model for future statistical analyses of other global volcano data sets, which promise to further improve volcano eruption forecasting and detection efforts. Key Points: Eruption forecasting and detection benefits from analyses of global earthquake data, including seismicity unassociated with eruptionsAnalysis of global magnitude four and greater (M4+) earthquakes near volcanoes reveals patterns in seismic activity precursory to eruptionsGlobally, 1% of M4+ earthquakes within 30 km of volcanoes are followed by eruption within 2 weeks [ABSTRACT FROM AUTHOR]

Akutan Island hosts a volcano considered as one of most active in the Aleutian Arc. We build a new tomography model including the 3D distributions of the Vp, Vs, and Vp/Vs ratio based on arrival time data from more than 4,000 local earthquakes recorded by 13 seismic stations. In this model, we reveal a columnar anomaly of high Vp, low Vs, and high Vp/Vs ratio with a top boundary at a depth of ∼6 km below sea level, which represents a steady conduit feeding the Akutan volcano. In the upper part of the tomographic model, the highly heterogeneous structures are associated with interactions of shallow magmatic sources, meteoric and magmatic fluids, as well as degassing. Beneath the summit area of Akutan, we observe a prominent anomaly of high Vp/Vs, which may represent a shallow magma reservoir directly responsible for recent eruption activity and ongoing gas emission through the summit fumarole. The other fumarole field at the eastern flank of the volcano appears to be associated with a seismically active zone of low Vp/Vs ratio at depths of 2–4 km below surface. These structures indicate different depths of magma degassing in these two areas, which may explain distinct geochemical features of emitted gases. Besides the prominent anomaly representing the shallow magma reservoir beneath the caldera and the active cone, we observe several areas with high Vp/Vs ratio, some of which are interpreted as shallow magma storage regions, and some as zones of meteoric water penetration. Key Points: The new tomography model of upper crust beneath Akutan reveals complex interaction of magma sources and geothermal systemsBelow 5 km depth, we reveal an anomaly of high Vp/Vs representing a steady magma conduit beneath AkutanTomography reveals two distinct fluid pathways beneath the summit and flank fumaroles, which explains their different properties [ABSTRACT FROM AUTHOR]

Large earthquakes alter the crustal stress field across great distances (hundreds to thousands of kilometers) over geologically short timescales (seconds to years). These stress changes can affect magmatic systems, triggering (or suppressing) volcanic unrest and eruption, along with other deeper processes. We use simple kinematic source models in an isotropic elastic half‐space to assess earthquake‐induced static stress changes (>1 kPa) over the entire thickness of the lithosphere and consider the implications for magma ascent and storage. Modeling subduction zone earthquakes, we calculate static normal stress changes with depth on three mutually perpendicular end‐member magma pathways: vertical arc‐parallel, vertical arc‐perpendicular, and horizontal. From this, we define seven stress change regimes within the adjacent volcanic arc. Three of these regimes may strongly encourage magma ascent in dykes by inducing unclamping (decreased compressive normal stress) of vertical pathways which increases in magnitude toward the surface and clamping of horizontal pathways. Two of the regimes may encourage stalling and storage of magma in sills near the base of the crust by inducing unclamping of horizontal pathways at depth. The spatial distribution of the regimes is largely dependent on earthquake magnitude, but also varies with slip distribution and interface dip. We show how the responses of magmatic systems to earthquakes also depends on the stress change magnitude and the state of the magmatic system, with a greater impact expected for larger stress changes acting on weaker, more thermally mature systems. Key Points: We map seven normal stress change regimes (>1 kPa) on three perpendicular magma pathways after subduction thrust earthquakesSlip magnitude, distribution, and dip control regime distribution, with hotter, mature arcs more sensitive to stress changesWe show how rapid stress field variations caused by earthquakes could affect patterns of magma ascent and storage [ABSTRACT FROM AUTHOR]

We present a local earthquake tomography to illuminate the crustal and uppermost mantle structure beneath the southern Puna plateau and to test the delamination hypothesis. Vp and Vp/Vs ratios were obtained using travel time variations recorded by 75 temporary seismic stations between 2007 and 2009. In the upper crust, prominent low Vp anomalies are found beneath the main volcanic centers, indicating the presence of magma and melt beneath the southern Puna plateau. Beneath the Moho at around 90‐km depth, a strong high Vp anomaly is detected just west of the giant backarc Cerro Galan ignimbrite caldera. This high Vp anomaly is only resolved if earthquakes with an azimuthal gap up to 300° are included in the inversion. However, we show through data subset and synthetic tests that the anomaly is robust due to our specific station‐event geometry and interpret it as a delaminated block of lower crust and uppermost mantle lithosphere under the southern Puna plateau. The low velocities in the crust are interpreted as a product of the delamination event that triggered the rise of fluids and melts into the crust and induced the high topography in this part of the plateau. The tomography also reveals the existence of low‐velocity anomalies that link arc magmatism at the Ojos del Salado volcanic center with slab seismicity clusters at depths of about 100 and 150 km and support fluid transport in the mantle wedge due to dehydration reaction within the subducted slab. Key Points: A local earthquake tomography is performed for the southern Puna plateau and adjacent regionA high mantle Vp anomaly beneath Cerro Galan provides evidence for lithospheric delaminationA low Vp zone is observed beneath Ojos del Salado, reflecting fluid transport supplied by slab dehydration at two depths [ABSTRACT FROM AUTHOR]

Deep low‐frequency (DLF) earthquakes occur beneath the Kirishima volcanoes in southwest Japan at depths of 10–30 km. In this study, we aim to reveal the relationship between DLF earthquakes and volcanic activity including eruptions by relocating the hypocenters of the earthquakes using the network correlation coefficient method and detecting the earthquakes comprehensively using the matched filter technique. Hypocenters of DLF earthquakes are found to be concentrated in some separated small clusters within depths of 10–15 and 20–27 km. Activation of deeper DLF earthquakes had been observed for approximately 2 years from December 2009, during which various styles of eruptions occurred. Such a 2‐year increase in DLF seismicity was well correlated with crustal deformation because of the volume change of a magma reservoir. The waveforms and hypocenters of DLF earthquakes during the activation period were different from those during other time periods. The activated DLF earthquakes mostly had low dominant frequencies and were located in four deeper clusters. The activation of each cluster was switched 3 times at the transition of the eruption styles. These results suggest that DLF earthquakes might occur near magma sills and could be triggered by fluid flow in the changing paths by complex eruption processes. In addition, the waveforms and hypocenters of DLF earthquakes associated with the 2018 eruptions are different from those associated with the 2011 eruptions. The fluid paths of the 2018 eruptions might be different from those of the 2011 eruptions. Plain Language Summary: Deep low‐frequency (DLF) earthquakes occur at depths of 10–30 km beneath the Kirishima volcanoes, which is one of the most active volcanoes in Japan. In the last decade, two major series of eruptions were observed there. The relationship between DLF earthquakes and volcanic activity such as eruptions is still unknown; therefore, we conduct two analyses, namely, relocation and detection to obtain the precise spatial distribution and comprehensive temporal distribution of DLF earthquakes based on waveform correlations. The result is that DLF earthquakes occurred in some small clusters, having been activated for approximately 2 years from December 2009, during which various styles of eruptions including subplinian eruptions on 26–27 January 2011 occurred. The long‐term increase in DLF earthquakes was well correlated with crustal deformation because of the volume change of the magma reservoir. Most DLF earthquakes associated with the 2011 eruptions are distributed in four deeper clusters that are different from those of DLF earthquakes in the other time periods. Moreover, the clusters were switched 3 times, which approximately correlates with the transitions of eruption styles. These results suggest that DLF earthquakes were triggered by fluid flow in the deep crust related to the eruptions. Key Points: Activation of low‐frequency earthquakes at depths of 20–27 km started 1 year before the 2011 eruptions of Shinmoe‐dakeWaveforms of low‐frequency earthquakes associated with the 2011 eruptions indicate lower dominant frequencies than those of the othersActivated low‐frequency earthquake locations might have been switched by fluid path redistribution with eruption style transitions [ABSTRACT FROM AUTHOR]

The Laguna del Maule volcanic field is a large rhyolitic magmatic system in the Chilean Andes, which has exhibited frequent eruptions during the past 20 ka. Rapid surface uplift (>20 cm/year) has been observed since 2007 accompanied by localized earthquake swarms and microgravity changes, indicating the inflating magma reservoir may interact with a preexisting weak zone (i.e., Troncoso fault). In this investigation, we model the magma reservoir by data assimilation with Interferometric Synthetic Aperture Radar data. The reservoir geometry is comparable to the magma body inferred by seismic tomography, magnetotelluric, and gravity studies. The models also suggest that a weak zone, which has little effect on surface displacement, is important as a fluid transport channel to promote earthquakes and microgravity changes. In particular, concentrated dilatancy within the weak zone facilitates the microfracture formation during reservoir inflation. High‐pressure fluid can inject into the weak zone from the magma reservoir to trigger earthquakes and further migrate upward to create positive gravity changes by occupying unsaturated storages. The pore pressure will then decrease, halting the seismicity swarm until the next cycle. This "hydrofracturing" process may release some accumulated stress along the magma reservoir delaying an eventual eruption in turn. Besides, the resultant models are propagated forward in time to evaluate potential stress trajectories for future unrest. Key Points: The presence of a weak, Troncoso fault zone significantly impacts the stress field of the host rock surrounding the LdM magma systemFault zone permeability can increase due to deformation of an adjacent, inflating magma reservoirPore fluid injecting from a magma system into a dilatant fault zone can trigger earthquakes and microgravity changes [ABSTRACT FROM AUTHOR]

Understanding the mechanism of moderate earthquake swarm is important for the time‐dependent hazard assessment. Five Ms ≥ 5.0 earthquakes occurred between October and November 2013 in Qianguo, Jilin, a region with low seismicity in northeast China. After 4 years, two more Ms > 5.0 earthquakes struck Ningjiang, Jilin. To investigate the swarm mechanism and the relationship between these two swarm activities, we determine the full moment tensor solutions for 12 Ms > 3.5 earthquakes, including the seven Ms ≥ 5.0 events. The focal mechanisms show that the 2013 Ms ≥ 5.0 events mostly exhibit thrust compression with significant volume closure components, whereas the 2017 Ms ≥ 5.0 earthquakes illustrate a dominated strike‐slip type. In order to explain such differences, we conduct double‐difference tomography in the study region. Our tomographic results reveal that both swarms are located in the low‐velocity zones and the fault materials exhibit structural variations along the strike, possibly indicating that the structural contrast could be critical for the variations in the focal mechanism and hypocenter migration. The faults provide pathways for the upward intrusion of fluids that control the distribution and migration of the swarm sequences. A long‐term fluid‐rock interaction could lead to the fault materials intensely fractured in the 2013 swarm area but relatively less damaged in the 2017 swarm area. We can conclude that fluids play an important role in triggering the two seismic swarms. Key Points: The 2013 swarm earthquakes show a significant non‐DC component, but the 2017 swarm earthquakes are dominated by a strike‐slip typeLow‐velocity zones are revealed around these two swarms, and the fault materials exhibit structural variations along the strikeSeismic structures control these two swarms, and fluids could play an important role in triggering the two moderate earthquake swarms [ABSTRACT FROM AUTHOR]

Axial Seamount is a prominent volcano located at the intersection of the Juan de Fuca Ridge and the Cobb‐Eickelberg hot spot in the northeast Pacific Ocean that has erupted in 1998, 2011, and 2015. The 2015 eruption was recorded by a seven‐station seismic network in the southern part of the summit caldera that forms part of the Ocean Observatories Initiative Cabled Array. We utilize a data set of ~3,900 well‐recorded earthquakes from January 2015 to February 2017 and a three‐dimensional P wave velocity model obtained previously from active source data to conduct a joint inversion for three‐dimensional P and S wave velocities and hypocentral parameters. The resulting velocity models are used to relocate >76,000 earthquakes with ≥10 arrival times. The velocity models reveal a low‐velocity anomaly in the center of the southern caldera at depths less than ~2 km, which corresponds to the top of the magma chamber and is interpreted as a region that is intensely fractured by the cyclical deformation of the caldera. High velocities around the caldera rim are likely due to consolidated undeformed lava flows. Low VP/VS in the southern caldera is consistent with the presence of hydrothermal vapor. Low S wave velocities and high VP/VS in the northern caldera may indicate a region dominated by thin cracks caused by dike injection. The relocated earthquakes delineate outward‐dipping ring faults more clearly than previous studies and image a subvertical inward‐dipping fault within the network that connects to the east caldera wall and eruptive fissures. Plain Language Summary: Axial Seamount is a volcano in the northeast Pacific Ocean that has erupted in 1998, 2011, and 2015. The 2015 eruption was recorded by a small seismic network that is part of a cabled observatory that connects scientific instruments on the volcano to shore. We analyzed the recordings of 3,900 small earthquakes to determine how the speed of compressional and shear seismic waves varies spatially within the volcano. We use the results to locate 76,000 earthquakes that occurred under the volcano over 2 years. The speed of seismic waves is low beneath the southern part of the caldera. This region is heavily fractured because the volcano repeatedly inflates and deflates as magma enters the magma chamber and then erupts. The relative speed of seismic compressional and shear waves beneath the southern caldera is consistent with the presence of boiling water in the subsurface beneath several sites of hydrothermal venting on the seafloor. The earthquakes are mostly located on a buried fault that wraps all around the caldera and slopes outwards with some others located on a near vertical fault that forms the caldera wall. Key Points: An inversion for hypocenters and velocity on Axial Seamount images low VP and VS in the caldera indicative of altered and fractured rockLow VP/VS beneath the hydrothermal vent fields in the southern caldera is consistent with the presence of hydrothermal vaporThe 3‐D velocity models yield hypocenters that resolve an inward‐dipping ring fault inside the network connecting to the east caldera wall [ABSTRACT FROM AUTHOR]

The southern Aegean is characterized by an ongoing process of oceanic subduction. In this study, 1,718 intermediate depth (>35 km) events recorded in the southern Aegean are used to calculate the peak delay times (tp). The tp are measured as the time difference between S wave onset and the peak of the S wave envelope in 2‐to 4‐, 4‐ to 8‐, 8‐ to 16‐, and 16‐ to 32‐Hz bands. The tp are also inverted to estimate the scattering parameters (κ and εparam) and power spectral density function (P (m)) at large wave numbers (m = 15 km−1 or ml) of the medium modeled as von Kármán type. κ controls frequency dependence of scattering while P (ml) represents the scattering strength in high frequencies for ɑml >> 1 (ɑ = correlation length of inhomogeneities). High P (ml) with low κ across Peloponnese in 0‐ to 20‐km depth may result from intermixing of oceanic material with continental rocks at different levels with a possibility of fluid activity and salt diapirism in northwest Peloponnese. Moderate P (ml) in 0‐ to 60‐km depth across Crete is likely due to a combination of factors including past megathrust earthquakes, sediment underplating, and flow of metamorphosed material in a subduction channel. Cyclades shows low κ in 0‐ to 40‐km depth, indicative of inhomogeneities produced by metamorphic core complexes. Very low κ in 60‐ to 80‐km depth is consistent with the location of mantle magma reservoir in the back‐arc region. Our results suggest that a significant portion of total S wave attenuation (Qs−1) in eastern Cyclades for 20‐ to 80‐km depth may be due to scattering losses. Key Points: Peak delay times of S waves are calculated to study the scattering strength in southern AegeanSpectral density of inhomogeneous medium is modeled as von Kármán type and its parameters are evaluated by inverting peak delay timesHigh inhomogeneity zones are observed in Cyclades, Crete, and upper crust of Peloponnese [ABSTRACT FROM AUTHOR]

We estimate the velocity structures of P (Vp) and S waves (Vs) in southern Hokkaido, Japan, where a large number of crustal low‐frequency earthquakes (LFEs) have occurred. To investigate the relationship between structure and crustal seismicity, we compare the obtained velocity heterogeneity between the crustal LFE regions, which are characterized by active volcanoes, Quaternary volcanoes, and seismic swarms. The tomographic results show that low‐velocity zones (LVZs) with high Vp/Vs are located around the Moho discontinuity beneath active volcanoes, and the LVZs extend horizontally from beneath the active volcanoes to other LFE regions. In southern Hokkaido, crustal LFEs also appear inside and below the upper crust as well as around the Moho discontinuity. These intermediate‐depth and deep LFEs are located around the LVZs with a high Vp/Vs ratio, indicating the contribution of partial melt to the LFEs genesis, even at a distance from the active volcanoes. At intermediate depths of the crust, LFEs are distributed even in the high‐velocity zones, adjacent to the LVZs. This indicates that the partial melt migrations within the crust affect the activity of LFE at the intermediate depths. Below the seismic swarms in the upper crust, LVZs with crustal LFEs are distributed, probably owing to upward migrations of fluid originating from the solidification of the partial melt; this indicates a link between the shallow and deep crustal seismicity in southern Hokkaido. Key Points: Constructing detailed P and S wave velocity structures in southern Hokkaido, JapanPartial melt and their movement are related to the genesis of crustal low‐frequency earthquakesShallow and deep crustal seismicity are linked through fluid and melt migrations [ABSTRACT FROM AUTHOR]

Two‐phase systems, where one phase is solid and the other is fluid, are widespread in nature. Examples include reservoir rocks holding vital fluids like water or petroleum, slurries of partially crystallized magmas, fluids migrating along faults filled with fault gouge, and even the semibrittle crust. Previous studies of two‐phase systems have shown that the fluid phase plays an important role in deformation localization and dynamics (e.g., Higashi & Sumita, 2009, https://doi.org/10.1029/2008JB005999; Reber et al., 2014, https://doi.org/10.1002/2014GL059832). Here, we present results from experiments investigating the influence of a fluid phase on force distribution in a granular media during simple shear. We use photoelastic polyurethane discs as the granular or solid phase and a linear viscous silicone as the fluid phase. The photoelastic property of the discs allows for direct observation and measurement of force magnitude and distribution. We compare the two‐phase experiments to granular experiments without silicone. The addition and percentage of the fluid phase has a strong impact on the force distribution and the overall force chain orientations. In the two‐phase system, the force chains form parallel to the shear plane and only rotate to the principal stress direction with an increase in strain. Locally, the fluid phase can support forces and terminate force chains over an extended period of time. Our results are in line with findings from numerical studies investigating the formation of slow slip events, proposing that these events are the result of dynamic interactions between solid and viscous phases. Plain Language Summary: Systems containing a solid and a fluid phase are common in nature, including settings such as aquifers, petroleum reservoirs, water saturated fault sediments, and partially molten magmas. Previous work on two‐phase systems has shown that the fluid phase is able to change deformation dynamics and increase deformation localization when compared to single‐phase brittle systems. We conduct experiments using photoelastic discs as the solid phase and a silicone as the fluid phase. The photoelastic property allows for visualization and quantification of forces. We compare the two‐phase experiments to granular experiments without silicone. Without silicone, force distributes along chains of discs that span the experimental apparatus. These force chains form in characteristic orientations that hold true throughout an experiment. The addition of the fluid phase significantly changes the force distribution. In two‐phase experiments, force chains initially do not span the entirety of the experimental box and instead localize along a shear plane in the middle of the experiment. These force chain orientations change over time. We find that the fluid can support forces for extended periods of time. Our findings highlight the importance of the fluid phase on deformation in two‐phase systems, which may have both economic and societal consequences. Key Points: The presence of a fluid phase affects the force distribution in a granular system during shear deformationThe presence of a fluid phase changes the average force chain orientation and deformation localizationThe force can be supported for a limited time by the fluid phase [ABSTRACT FROM AUTHOR]

We develop a new 3‐D shear velocity model for the lithosphere and sublithospheric mantle under Saudi Arabia by jointly inverting P wave receiver functions, S wave receiver functions, and fundamental‐mode Rayleigh wave group velocities. P and S receiver functions are calculated from earthquakes recorded between 2012 and 2015 at 156 Saudi National Seismic Network stations operated by the Saudi Geological Survey. Rayleigh wave dispersion data are extracted from independent tomographic studies. Our model reveals significant lateral variations in crustal and upper‐mantle S velocity below Saudi Arabia. Particularly, a low‐velocity zone, with minimum S velocity of ~4.0 km/s in the depth range of 70‐190 km, is observed under the Arabian Shield coinciding with Cenozoic surface volcanism. The low‐velocity zone is found consistent with the presence of partial melts in the mantle and interpreted as a potential deep magma source for the volcanism in western Arabia. We propose that magmas responsible for the Arabian volcanism may be derived from multiple sources. Both lithospheric thinning and the Afar plume trigger magma production beneath southwestern Arabia, while decompression melting caused by the lithospheric thinning may be the main factor in the central and northern portions. Furthermore, we find evidence for localized crustal low shear velocity anomalies (2–4% reductions) that appear spatially correlated to the volcanism; these may be due to fractures caused by magma ascent and small amounts of partial melt in the crust. This spatial distribution of S velocity reductions may indicate the plumbing system for magma ascent underneath western Arabia. Key Points: We build a 3‐D S velocity model of the crust and upper mantle for Saudi Arabia by jointly inverting PRFs, SRFs, and Rayleigh wave dispersionOur model shows an upper‐mantle LVZ, which illustrates the presence of partial melts and is interpreted as a magma source for the volcanismWe find localized crustal S velocity reductions that may be due to fractures caused by magma ascent and small amounts of partial melt [ABSTRACT FROM AUTHOR]

Seismicity during explosive volcanic eruptions remains challenging to observe through the eruptive noise, leaving first‐order questions unanswered. How do earthquake rates change as eruptions progress, and what is their relationship to the opening and closing of the eruptive vent? To address these questions for the Okmok Volcano 2008 explosive eruption, Volcano Explosivity Index 4, we utilized modern detection methods to enhance the existing earthquake catalog. Our enhanced catalog detected significantly more earthquakes than traditional methods. We located, relocated, determined magnitudes and classified all events within this catalog. Our analysis reveals distinct behaviors for long‐period (LP) and volcano‐tectonic (VT) earthquakes, providing insights into the opening and closing cycle. LP earthquakes occur as bursts beneath the eruptive vent and do not coincide in time with the plumes, indicating their relationship to an eruptive process that occurs at a high pressurization state, that is, partially closed conduit. In contrast, VT earthquakes maintain a steadier rate over a broader region, do not track the caldera deflation and have a larger b‐value during the eruption than before or after. The closing sequence is marked by a burst of LPs followed by small VTs south of the volcano. The opening sequence differs as only VTs extend to depth and migrate within minutes of the eruption onset. Our high‐resolution catalog offers valuable insights, demonstrating that volcanic conduits can transition between partially closed (clogged) and open (cracked) states during an eruption. Utilizing modern earthquake processing techniques enables clearer understanding of eruptions and holds promise for studying other volcanic events. Plain Language Summary: Sparse instrumentation and high noise levels prevent us from seeing most of the quakes that occur. This problem is particularly severe during large volcanic eruptions when the explosive processes hide most of the seismic waves. Here, we overcome the problem by utilizing a suite of signal processing methods to dramatically increase our ability to see earthquakes during the noisiest, most difficult to monitor stage of a volcanic eruption. This high‐resolution data set unveils features of the volcanic structure as well as the dynamic interaction between the solid medium and the fluids. When the system is closed (clogged), the rocks are stressed by the trapped pressurized fluids and then break in earthquakes. When the system is open (cracked), ash and steam plumes grow higher, the rock is not being stressed as much and the number of observed earthquakes is lower around the eruptive vents. The volcano is displaying both open and closed vent behavior during a single eruption. Key Points: We analyzed the enhanced seismic catalog of a long‐lived explosive volcanic eruptive sequenceLong‐period earthquakes track eruptive processes when the system is partially closed and they are not associated with mass ejectionVolcano‐tectonic earthquakes are a secondary effect that does not track the deflation and occur at a low stress‐state [ABSTRACT FROM AUTHOR]

Volcanic inflation and deflation often precede eruptions and can lead to seismic velocity changes (dv/v $dv/v$) in the subsurface. Recently, interferometry on the coda of ambient noise‐cross‐correlation functions yielded encouraging results in detecting these changes at active volcanoes. Here, we analyze seismic data recorded at the Klyuchevskoy Volcanic Group in Kamchatka, Russia, between summer of 2015 and summer of 2016 to study signals related to volcanic activity. However, ubiquitous volcanic tremors introduce distortions in the noise wavefield that cause artifacts in the dv/v $dv/v$ estimates masking the impact of physical mechanisms. To avoid such instabilities, we propose a new technique called time‐segmented passive image interferometry. In this technique, we employ a hierarchical clustering algorithm to find periods in which the wavefield can be considered stationary. For these periods, we perform separate noise interferometry studies. To further increase the temporal resolution of our results, we use an AI‐driven approach to find stations with similar dv/v $dv/v$ responses and apply a spatial stack. The impacts of snow load and precipitation dominate the resulting dv/v $dv/v$ time series, as we demonstrate with the help of a simple model. In February 2016, we observe an abrupt velocity drop due to the M7.2 Zhupanov earthquake. Shortly after, we register a gradual velocity increase of about 0.3% at Bezymianny Volcano coinciding with surface deformation observed using remote sensing techniques. We suggest that the inflation of a shallow reservoir related to the beginning of Bezymianny's 2016/2017 eruptive cycle could have caused this local velocity increase and a decorrelation of the correlation function coda. Plain Language Summary: Before eruptions, volcanoes inflate due to the rising magma from below. Previous studies have found that these deformations can lead to small changes in the properties of the surrounding rock. We use passive image interferometry, a method that relies on the omnipresent background vibration of the Earth—mostly induced by the oceans, to measure these changes at the Klyuchevskoy Volcanic Group in Kamchatka, Russia. However, in Kamchatka, this background noise is masked and distorted by small earthquakes and tremors originating from the volcanoes themselves. We combine machine learning techniques with established monitoring methods to find times when these tremors remain similar. Afterward, we use data from these time periods in the conventional way to observe changes in the soil and the rock. Our results show that rain‐ and snowfall and the thickness of the snow cover exert the strongest influence on the properties of the rocks. Additionally, we found that a large magnitude 7.2 earthquake, which struck Kamchatka during our study, caused a slight weakening of the rocks due to microstructural damage. We register changes shortly before an eruption and suggest a connection to the beginning of an eruptive cycle in 2016. Key Points: Fluctuating noise conditions lead to distortions in noise interferometry studies, which we avoid with the help of machine learningThe seismic velocity on Kamchatka is affected by numerous mechanisms, amongst them environmental, tectonic, and volcanic eventsWe observe a velocity increase at Bezymianny during February 2016 and link it to the beginning of the eruptive cycle [ABSTRACT FROM AUTHOR]

Volcanic activity on Tenerife Island is extremely diverse. Three radial rift zones are characterized by cinder cones from basaltic fissure eruptions. A triple junction in central Tenerife exhibits a complex of merged, predominantly phonolitic, stratovolcanoes. The Las Cañadas caldera and widespread ignimbrite deposits reveal high explosive potential. We investigated the crustal and upper mantle structure beneath Tenerife using local earthquake data recorded by two dense seismic networks on the island. For our tomographic inversion, we selected >130,000 P‐ and S‐wave arrivals from ∼6,300 events that occurred during seismic unrests in 2004–2005 and 2017–2021. Synthetic tests confirmed that we could robustly resolve seismic velocity structures to ∼20 km depth. In the upper crust (down to ∼7 km) beneath central Tenerife, a prominent high‐velocity anomaly represents the rigid core of the volcanic complex; at greater depths, a strong low‐velocity anomaly reveals abrupt crustal thickening. Vp and Vs contour lines of 5.2 and 2.85 km/s, respectively, reveal Moho depth variation; crustal thickness beneath Las Cañadas reaches ∼17 km, whereas that beneath other parts of Tenerife is ∼10 km. An anomaly at ∼5 km beneath the caldera with low Vp, low Vs, and high Vp/Vs might be associated with a major phonolitic magma reservoir. Similar anomalies at ∼ sea level may represent shallow magma sources responsible for recent eruptions. Seismicity occurs in a columnar area of high Vp, high Vs, and low Vp/Vs, and may represent hydrothermal fluid migration through brittle media. Based on our results, we constructed a conceptual model of volcanic activity on Tenerife. Plain Language Summary: Tenerife is the largest and most populated island of the Canary Islands archipelago. Volcanic activity in Tenerife has the potential to seriously threaten the population and infrastructure. Two main types of volcanism are observed: explosive phonolitic eruptions from the central part of the island and basaltic fissure eruptions along three radial ridges. We present a new seismic tomography model based on the inversion of P‐ and S‐wave data from local seismicity. Absolute P‐ and S‐wave velocities reveal abrupt crustal thickening from ∼10 to 17 km within an elliptical area coinciding with Las Cañadas caldera in the central part of Tenerife. We suggest a "push‐button" mechanism that facilitates vertical displacement of central Tenerife part, which has high topography and is of higher density than peripheral parts of the island. Beneath Teide, at a depth of ∼5 km, a low Vp, low Vs, high Vp/Vs anomaly may represent an intermediate phonolitic reservoir. Two similar anomalies at shallower depths may represent the uppermost magma reservoirs directly responsible for phonolitic eruptions. Basaltic eruptions along the three radial ridges appear to be directly fed from mantle sources that appear as high Vp/Vs anomalies. Key Points: We constructed a high‐resolution seismic velocity model extending to ∼20 km depth beneath TenerifeThe velocity model shows a crustal thickness of up to 17 km below the Las Cañadas caldera and ∼10 km below other parts of the islandThe reservoir of phonolitic magma is revealed as an anomaly of low Vp, low Vs, and high Vp/Vs at ∼5 km depth [ABSTRACT FROM AUTHOR]

We use ocean bottom seismometer data from the Endeavour segment of the Juan de Fuca ridge to construct a long‐term earthquake catalog for an intermediate spreading rate mid‐ocean ridge. We present >50,000 new earthquake locations for 2016–2021 from the Ocean Networks Canada NEPTUNE cabled observatory and relocate earthquakes from two autonomous networks in 1995 and 2003–2006. The catalog comprises >85,000 earthquakes located using three‐dimensional segment‐scale P and S wave velocity models from a prior tomography experiment. Despite the small footprints of networks near the segment center, locations show good agreement with geologic features at segment ends. The improved locations show that the northern Endeavour segment ruptured southwards from 48.3°N to 48.05°N during two diking events in early 2005, possibly accompanied by diking on the West Valley (WV) propagator. Persistent off‐axis seismicity near the segment center appears to be related to the WV and Cobb propagating rifts which we infer extend ∼10 km closer to the Endeavour segment center than is apparent in bathymetry. We suggest that the proximity of the propagators to the Endeavour vent fields (VFs) contributes to the localization, vigor, and longevity of the fields by focusing permeability through ongoing fracturing and by limiting extrusive magmatism through degassing of the axial magma lens. Increasing rates of seismicity beneath the VFs beginning in late 2018 and a deepening of earthquakes in 2020 indicate that the central portion of the segment may be entering the later stages of the eruptive cycle. Plain Language Summary: Seafloor spreading centers are boundaries where tectonic plates move apart and ocean crust is created by volcanism. Small earthquakes in these settings result from extensional faulting, magma injection, and cooling of the crust by circulating fluids. Seafloor spreading is cyclical: strain steadily builds up as the plates move apart and is relieved episodically when magma is injected into the crust. Earthquake activity must be observed on the order of decades to fully capture this cycle. Until recently, however, batteries have limited earthquake observations from autonomous seafloor seismometers to about a year. We combine observations from a new long‐term undersea cabled observatory and older experiments to present an earthquake catalog that spans 20 years for the Endeavour segment of the Juan de Fuca ridge. We improve earthquake locations relative to past studies and relate evolving earthquake activity to seafloor features. Our results suggest that the complicated geometry of nearby plate boundaries, through focused fracture creation, affects how fluids circulate through the crust. Earthquake patterns preceding the last observed magmatic event in 2005 indicate that the increases in earthquake rates and depths we observe in 2018–2020 may be signs of the center of the segment nearing the next spreading event. Key Points: Three‐dimensional velocity models facilitate reliable earthquake localization in the marine setting despite limited seismometer coveragePropagating rift tectonics enable long‐lived high‐temperature hydrothermal venting through amplified and spatially focused deformationEarthquakes across 20 years reflect strain that evolves cyclically as extension is accumulated and released on overlapping spreading axes [ABSTRACT FROM AUTHOR]

The strength and sliding behavior of faults in the crust is largely controlled by friction and effective stress, which is itself modulated by fluid pressure. Most earthquake models assume a fixed pore fluid pressure despite widespread evidence that is varies strongly in time due to changes in permeability. Here we explore how dynamic changes in pore pressure influence the properties of earthquakes in the upper crust. To study this problem we develop a two dimensional model that incorporates slow tectonic loading and fluid pressure generation during the interseismic period with frictional sliding on a thrust fault whose permeability evolves with slip. We find that the presence of relatively modest fluid overpressures tends to reduce coseismic slip, stress drop, maximum sliding velocity, rupture velocity and the earthquake recurrence time compared to models without fluids. Our model produces a wide range of sliding velocities from rapid to slow earthquakes, which occur due to the presence of high pore pressures prior to rupture. The models also show evidence for aftershocks that are driven by fluid transfer along the fault plane after the mainshock. Overall, this study shows that fluids can exert an important influence on earthquakes in the crust, which is mostly due to modulation of the effective stress and variations in permeability, and to a lesser extent to poro‐elastic coupling. Plain Language Summary: In this study we use a numerical model to investigate how fluid pressures vary over the seismic cycle and how they interact with and influence the properties of earthquakes that occur in the upper crust. In the model, fluid overpressures are generated slowly during the interseismic period by phenomena such as dehydration reactions while they are episodically released during earthquakes due to fracturing and a dramatic increase in permeability. The models show that the presence of high fluid pressures has an important influence on earthquakes. High fluid pressures favor smaller, more frequent earthquakes. Also high fluid pressures may sometimes be responsible for aftershocks and for anomalously slow earthquakes that involve slip over several months rather than several seconds. Overall, we show that the presence of fluids in the crust plays an integral part in the earthquake process. Key Points: Properties of earthquakes are markedly influenced by the presence of fluid overpressuresRuptures in wet models have relatively low stress drops, sliding speed and rupture velocitySlow slip events and aftershocks might be a fingerprint of fluid overpressures [ABSTRACT FROM AUTHOR]

Strain partitioning in oblique convergent margins results in margin‐parallel shear in the overriding plate. Margin‐parallel shear is often accommodated by margin‐parallel strike‐slip faults proximal to active volcanic arcs. Along the Nicaraguan segment of the Central American Forearc (CAFA) in the Cocos‐Caribbean plate convergent margin, there are no well‐expressed right‐lateral faults that accommodate CA‐CAFA relative motion. Instead, historical earthquakes and mapped fault orientations indicate that the ∼12 mm/yr of dextral motion is accommodated on arc‐normal, left‐lateral faults (i.e., bookshelf faults). We investigate three upper‐plate earthquakes; the 10 April 2014 (Mw 6.1), 15 September 2016 (Mw 5.7), and 28 September 2016 (Mw 5.5), using Global Position System co‐seismic displacements and relocated earthquake aftershocks. Our analyses of the three earthquakes indicate that the 10 April 2014 earthquake ruptured an unmapped margin‐parallel right‐lateral fault in Lago Xolotlán (Managua) and the September 2016 earthquakes ruptured arc‐normal, left‐lateral and oblique‐slip faults. These earthquakes represent a triggered sequence whereby the 10 April 2014 earthquake promoted failure of the faults that ruptured in September 2016 by imparting a static Coulomb stress change (ΔCFS) of 0.02–0.07 MPa. Likewise, the 15 September 2016, earthquake additionally promoted failure (ΔCFS of 0.08–0.1 MPa) on sub‐parallel faults that ruptured in two subsequent earthquakes. We also present an instance of magma‐tectonic interaction whereby the 10 April 2014 earthquake dilated (10s of μStrain) the shallow magmatic system of Momotombo Volcano, which led to magma injection, ascent, and eruption on 1 December 2015, after ∼100 years of quiescence. Plain Language Summary: The tectonic boundary between the northwest migrating Central American Forearc (CAFA), located between the active Central American Volcanic Arc and the Middle America Trench, and the CA plate poses significant seismic and volcanic hazards. Shallow, damaging earthquakes occur along the boundary and within close proximity to volcanic centers. We present models of GPS‐derived co‐seismic displacements and relocated seismicity that indicate these earthquakes primarily occur on faults perpendicular to the forearc, unlike most global analogs where motion is accommodated by faults parallel to the margin. Our analysis of these earthquakes also shows that they are a triggered sequence, where each earthquake primed nearby faults for failure, resulting in subsequent earthquakes. The triggered earthquake sequence has implications for seismicity migrating through the forearc and short‐ (years) to medium‐term (decadal) seismic forecasting. We also found that one of the earthquakes perturbed the Momotombo Volcano magmatic system thereby allowing the movement of magma, which lead to an eruption. Key Points: Three upper‐plate earthquakes show Central American Forearc‐CA plate motion is primarily accommodated by left‐lateral arc‐normal faultsThe three upper‐plate earthquakes were a triggered sequence, indicating stress migration through the arcThe 10 April 2014 earthquake dilated the Momotombo magma system, allowing magma injection and ascent, leading to the 2015 eruption [ABSTRACT FROM AUTHOR]

In the South‐Central Andes, the crustal structures driving the tectonic evolution of the Andean Cordillera remain unresolved. So far, most seismological studies focused on the subduction interface, leaving crustal seismicity and its relationship with crustal deformation and Andean volcanism mostly unconstrained. However, because of their large number compared to higher magnitude events, the characterization of small‐magnitude crustal earthquakes is key to identify active structures and better constrain the tectonic models. In this work, we exploit 53 months of continuously recorded, three‐component waveforms from the permanent seismic network in central Chile using a deep‐learning approach to improve the detection of small‐magnitude earthquakes. To increase station coverage, we also use the seismic phases obtained from a previous temporary seismic deployment. We use the obtained seismicity catalog to refine tomographic models of that region, revealing a more detailed architecture of the Chilean forearc. Travel times calculated in the new 3‐D velocity model allowed us to locate ∼14,000 earthquakes. Refined double‐difference relocations of ∼4,900 events located beneath the West Andean Thrust suggest a large‐scale, west‐dipping structure which, together with the west‐verging tectonic front, likely contributed to the uplift and crustal deformation during the past ∼20 Myr. Plain Language Summary: The occurrence of earthquakes is closely related to plate tectonics and in particular, the creation of mountain ranges and their evolution over very long periods of time. In order to better understand how these processes work in the Andes of Central Chile, we analyze more than 4 years of earthquake records using a machine learning algorithm. This technique allows us to automatically and reliably detect very small earthquakes, that are, in general, missing in the seismicity catalogs. Since they are also more numerous, we can use them to improve the geophysical images and reveal more details of crustal structures. We find that most of the superficial earthquakes beneath the Andean Cordillera are aligned with a major structure, dipping toward the west. This structure accommodates processes of shortening and uplifting directly related to the construction of the Andes. Key Points: A large number of earthquakes detected using a deep‐learning approach are used for local earthquake tomographyThe obtained 3‐D seismic wave velocity structure is used to relocate the seismicity catalogEarthquake locations suggest a west‐dipping major structure that we interpret to have contributed to the uplift of the South‐Central Andes [ABSTRACT FROM AUTHOR]

The Makran Subduction Zone is the primary seismic/tsunami hazard of the northwestern Indian Ocean, but little is known of its on‐shore seismic structure. We derived a shear wave velocity model extending to > $ > $100 km depth beneath a ∼400 km‐long seismic profile oriented parallel to the convergence vector of the Arabian Sea Plate. Receiver function/surface wave analysis shows that the average structure in the coastal region comprises a ∼22–28 km‐thick low wavespeed sedimentary cover and a 6–8 km‐thick gradient zone overlying > $ > $100 km‐thick high wavespeed upper mantle. The ocean‐basement interface dips gently northward, remaining a positive impedance contrast to ∼50 km depth at ∼250 km north of the coast where it disappears as the basaltic/gabbroic oceanic crust has probably transformed to eclogite. Further north, a weak arrival at ∼5 s in the receiver functions appears, grading northward into the Moho arrival of the continental Iranian Plateau. This disruption in the seismic signature of the Moho occurs in the forearc region where the dip of the subducting oceanic plate steepens. The southern Iranian Plateau's continental crust has an average Vs of 3.55 ± 0.05 km s−1, an almost flat Moho 40–45 km deep, and a sub‐Moho mantle Vs of 3.75 ± 0.05 km s−1 in the 50–80 km depth range. Weak Moho conversions probably result from ∼20% serpentinization of peridotite in the mantle wedge. Receiver functions indicate a flat continental Moho – no crustal root beneath the high topography region of the volcanic belt, which therefore must be compensated by low upper mantle densities. The high Vp/Vs ratio observed for the mantle wedge suggests ∼1%–2% partial melt. Plain Language Summary: The Makran Subduction Zone is the primary seismic/tsunami hazard of the northwestern Indian Ocean, but little is known of its on‐shore structure. Although earthquake rates in the Makran are generally low, there have been historic destructive earthquakes accompanied by tsunamis. The poor knowledge of the Makran structure makes it difficult to evaluate the seismic and tsunami potential. From mid‐2016 to late 2020 we operated seismic stations along a ∼400 km‐long profile crossing the Makran Subduction Zone. Our analysis shows a shallow‐dipping oceanic crust beneath thick sediments near the coast. To the north, there is a normal continental crust, but the underlying mantle has a very low velocity. In the transition region, the seismic signature is disrupted in the forearc region where the dip of the subducting oceanic plate steepens. Weak Moho conversions probably result from ∼20% serpentinization of peridotite in the mantle wedge. Receiver functions indicate a flat continental Moho – no crustal root beneath the high topography region of the volcanic belt, which therefore must be supported by low upper mantle densities. The low Vs compared to the Vp observed for the mantle wedge suggests ∼1%–2% partial melt. Key Points: Due to past logistical constraints and low seismicity, little is known of the on‐shore structure of the Makran Subduction ZoneAnalysis shows a normal oceanic crust near the coast which transforms to eclogite at deeper depths, and a flat continental Moho to the northThe peridotite of the mantle wedge is ∼20% serpentinized and the high Vp/Vs ratio observed may result from ∼1%–2% partial melt [ABSTRACT FROM AUTHOR]

Deep low‐frequency earthquakes (LFEs) in Northeast Japan occur beneath active volcanoes at depths of 10–40 km. These volcanic LFEs radiate low‐frequency seismic waves, with most energy at 2–8 Hz, despite their low magnitudes (M < 2). Although many previous studies have obtained various focal mechanisms with non‐double‐couple components and suggested physical processes related to magma, the universal physical process is poorly understood. Therefore, we comprehensively determined the focal mechanisms of 264 LFEs for 26 volcanic regions in Northeast Japan using S/P amplitude ratios. Many of the obtained solutions have large double‐couple components with small compensated linear vector dipole components. Such source mechanisms can be explained by shear slip on the bending fault. We observed that the plunge of the null axis is as small as 20–30º at depths of 15–25 km, whereas it becomes larger and more varied at depths of 25–35 km. We interpreted that the regional stress field is relatively homogeneous in the middle of the crust, whereas it may be altered near the Moho discontinuity due to thermal stress or other effects related to potential magmatic processes. In addition, after quantitatively investigating the possible triggering of LFEs due to static stress changes in both the 2008 Iwate‐Miyagi earthquake and the 2011 Tohoku earthquake, we concluded that LFEs might have been triggered by substantial stress changes caused by large earthquakes in some regions. Plain Language Summary: Deep slow earthquakes occurring near active volcanoes are thought to be related to deep magmatism. However, their physical processes are not well understood because of the lack of observations. In this study, to reveal their universal processes, we determined the focal mechanisms of 264 events in 26 volcanic regions in Northeast Japan, where deep slow earthquakes occur actively, and high‐quality waveform data are available. Many of the obtained 264 focal mechanisms are dominated by shear slip components, similar to ordinary earthquakes. We found that the focal mechanisms of shallower events (<25 km) are consistent with the regional stress field in Northeast Japan, while those of deeper events (>25 km) vary. Such disturbances in the stress field at greater depths may be generated by stagnated magma near the Moho discontinuity. We also found that the activity of deep slow earthquakes is triggered by stress changes induced by large earthquakes. Key Points: Many of the 264 focal mechanisms of deep low‐frequency earthquakes in 26 regions have large double‐couple componentsThe plunge of the null axis is as small as 20–30º at 15–25 km, but it becomes larger and more diverse at 25–35 kmThe triggering potential of large earthquakes correlates with the activity changes of deep low‐frequency earthquakes [ABSTRACT FROM AUTHOR]

A three‐dimensional shear velocity model for the crust and upper mantle beneath the central United States is presented by inverting Rayleigh wave phase velocities from 20 s to 100 s periods. These phase velocities were determined using regional and teleseismic earthquakes recorded by the Northern Embayment Lithospheric Experiment stations, the CERI New Madrid Seismic Network, the Earthscope Transportable Array, and the Ozark Illinois INdiana Kentucky Flexible Array. A low Vs anomaly is imaged in the mantle below the Reelfoot Rift, which is the uppermost portion of connected low‐velocity zones dipping toward the southwest below the rift and to the northwest below the Illinois Basin. According to the analysis in previous tomographic studies using both Vp and Vs anomalies, the elevation of temperature and the enrichment of iron, water, and orthopyroxene contents are required factors to explain the reduced seismic velocities. These low‐velocity zones are produced by silica‐rich fluids rising from the stalled Farallon slab. Two areas with low shear strength characterized by low Vs are imaged below the Ste. Genevieve and the Wabash Valley seismic zones. The low‐velocity, weak areas may be responsible for stress concentration and thus the generation of intraplate seismicity. Plain Language Summary: We present a three‐dimensional seismic velocity model for the northern Mississippi Embayment and southern Illinois Basin derived from seismic waves that travel along the surface of the earth. These surface waves consist of many wavelengths. Short wavelengths travel through shallow parts of the crust while long wavelengths penetrate deeper into the earth, allowing construction of a velocity model from the near surface to a depth of 150 km. Our study utilizes seismic tomography to map velocity anomalies, much like tomography is used to produce images of the human body. We find anomalously high velocities in the crust below the three active seismic zones in the area, the New Madrid, Wabash Valley, and Ste. Genevieve seismic zones. Anomalously low velocities underlie the seismic zones at greater depths below the crust. The slow velocities imply that the rocks are weak, and their presence may be concentrating stress in the seismic zones. We find anomalously low velocities below the crust in the northern Mississippi Embayment that extend to the bottom of the model. This low‐velocity region has been detected in previous studies and is attributed to the effects of fluids coming from a trapped slab fragment at depths below 400 km. Key Points: We present a 3‐D Vs model for the upper mantle in the central United States by inverting Rayleigh wave phase velocitiesA low Vs anomaly is imaged below the Reelfoot Rift, which is the uppermost portion of a prominent LVZ in previous tomographic studiesLow Vs anomalies are found in the mantle below the Ste. Genevieve and Wabash Valley seismic zones [ABSTRACT FROM AUTHOR]

We 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. Plain Language Summary: In volcanic regions, deep low‐frequency (DLF) earthquakes are observed, which have lower dominant frequencies than regular earthquakes. We constructed a catalog of volcanic DLF earthquakes in 52 regions in Japan to investigate their distributions and temporal patterns. The catalog was constructed based on relocation, classification, and detection. Relocation and classification were conducted using waveform correlation based on the existing catalog and detection using the template‐matching method. The catalog shows that DLF earthquakes are divided into groups separately, with vertical intervals of approximately 5 km in many regions. The temporal patterns of DLF earthquakes in each group can be classified as episodic (i.e., associated with swarms) and non‐episodic. Some episodic DLF earthquakes have constant or increasing interevent times. In some volcanic regions, episodic activities of DLF earthquakes occurred with volcanic activity at the surface, such as eruptions or crustal deformations. The distribution of the groups of DLF earthquakes may reflect small‐scale heterogeneities beneath volcanoes, and multiple mechanisms may control the occurrence of the two types of DLF earthquakes. Key Points: Deep low‐frequency earthquakes beneath many volcanoes in Japan are clusteredThe temporal activity patterns of the earthquakes are classified into episodic type associated with seismic swarms and non‐episodic typeSpatiotemporal characteristics of deep low‐frequency earthquakes may be interpreted by magmatic fluid movement [ABSTRACT FROM AUTHOR]

In many subduction zones, the plate interface hosts intermittent, low‐frequency, low‐magnitude seismic tremor and low‐frequency earthquakes (LFEs). Seismic activity clusters in episodic bursts that migrate along the fault zone in complex ways. Geological structures in fossil tremor source regions testify to large and pervasive variations of fluid pressure and permeability. Here, we explore the potential of fluid pressure transients in a permeable subduction interface to trigger seismic sources and induce interactions between them. We show how variations of pore pressure and permeability can act in tandem to generate tremor‐like patterns. The core feature of the model is that low‐permeability plugs behave as elementary fault‐valves. In a mechanism akin to erosive bursts and deposition events documented in porous media, valve permeability opens and closes in response to the local fluid pressure distribution. The rapid pressure release and/or mechanical fracturing associated with valve opening acts as the source of an LFE‐like event. Valves interact constructively, leading to realistic, tremor‐like patterns: cascades, synchronized bursts, and migrations of activity along the channel, on both short and long time and space scales. Our model predicts that the input fluid flux is a key control on activity occurrence and behavior. Depending on its value, the subduction interface can be seismically quiescent or active, and seismicity can be strongly time‐clustered, quasi‐periodic or almost random in time. This model allows new interpretations of low frequency seismic activity in terms of effective fluid flux and fault‐zone permeability. Plain Language Summary: In between earthquakes, faults emit faint seismic signals, called tremor. Tremor is emitted as the fault channels fluid at very high pressures from deep sources towards the surface. As such processes can initiate earthquakes on the fault, they are the subject of intense scrutiny. In this work, we build a simple model to explore how fluids flow within a subduction fault and how it can be linked to tremor. In our model, the fault's conduits sometimes clog, because the fluid that circulates in it carries particles. As the fluid pushes through the plugs, it generates the seismic vibrations that we call tremor. If the fluid flux into the fault zone is just right, valves open and close in cycles, and the fault zone is permanently active. The patterns of model tremor it generates are very similar to the ones observed in real subduction zones: it migrates, and occurs in bursts of all sizes. However, if the flux is either too low or too high, the fault should remain silent. Our model provides a framework to interpret seismic patterns as symptoms of how closed or open a fault is to fluid circulation, and how strong the fluid flux is within it. Key Points: We model fluid pressure diffusion in a channel along the subduction interface in which the permeability varies through a valving mechanismTremor is generated as valves break open and interactions between valves through fluid pressure transients shape realistic event patternsThe input fluid flux in the fault controls seismic activity, opening possibilities to diagnose hydraulic conditions using low‐frequency earthquake patterns [ABSTRACT FROM AUTHOR]

The deformation model of antigorite and its relationship to intermediate‐depth earthquakes are still controversial. Here, we conducted deformation experiments on foliated iron‐poor antigorite at confining pressures of 1,100–1,500 MPa, temperatures of 400–600°C, and axial displacement rates of 3.75 × 10−6–7.24 × 10−4 mm/s using a solid‐medium Griggs‐type apparatus. As the temperature and/or pressure increases, the stress exponent decreases, and all samples show stable fault slip. Microstructural observations show ductile behavior indicated by kinking and bending of the originally straight foliation, and brittle characteristics, such as Riedel shear, comminution, and microcracking. Therefore, antigorite deformation is controlled by a semibrittle regime. Antigorite decomposed to forsterite and enstatite in samples sheared at temperatures ≥550°C but not in samples triaxially compressed at 600°C. This observation, combined with phase boundary information, implies that the deformation geometry (e.g., triaxial compression or shear) may change the phase stability of antigorite. Following dehydration, samples still show stable fault slip, indicating that dehydration does not directly trigger earthquakes. In cold subduction zones, fine‐grained antigorite in the core of the shear zone likely induces seismic activity by producing water and talc (or a talc‐like phase) in the process of shear friction because such phases within the fault zone may reduce the rate dependence, thereby inducing unstable slip. The comparison of the semibrittle strengths of antigorite and dunite and the weak strain rate dependence of stress suggest that a thin antigorite layer immediately above the subducting slab is not sufficient to cause significant decoupling between the slab and the mantle wedge. Plain Language Summary: As one of the most important hydrous minerals in subduction zone, antigorite has many geodynamic significances. However, low dehydration temperatures and complex crystal structure have long prevented good characterization of its peculiar rheological properties and its relationship to intermediate‐depth earthquakes. In this study, we deformed foliated iron‐poor antigorite. We found that all samples show stable fault slip dominated by a semibrittle regime and thus dehydration does not directly trigger earthquakes. Combined with the phase boundary information determined by phase equilibrium experiments under static compression, our results suggest that deformation geometry (such as compression, simple shear, and the direction of foliation plane with respect to the maximum compression axis) may change the phase stability of antigorite. As water or talc or a talc‐phase phase was evidenced previously to likely reduce the rate dependence, thus we proposed that in cold subduction zones, fine‐grained antigorite in the core of the shear zone likely induces earthquakes by producing such phases during shearing. In the semibrittle regime, small difference in strength between antigorite and dunite (σ(dunite)/σ(antigorite) < 1.6) suggest that a thin antigorite layer immediately above the subducting slab is not sufficient to cause significant decoupling between the slab and the mantle wedge. Key Points: The strength of antigorite has weak strain rate and temperature dependences, as well as moderate pressure dependenceDehydration cannot directly trigger earthquakes and the deformation geometry may change the phase stability of antigoriteA thin antigorite layer is not sufficient to cause significant decoupling between the slab and the mantle wedge [ABSTRACT FROM AUTHOR]

Low‐frequency earthquakes (LFEs) that predominantly occur at depths of 20–30 km are categorized as a particular class of earthquakes whose spectral power is concentrated at 1–4 Hz. While the tectonic LFEs along megathrust boundaries occur as shear failure, the genesis of LFEs in the continental plate is poorly understood due to the diversity of focal mechanism solutions. Here we conduct a systematic survey of LFEs using two metrics (frequency index and peak frequency) that characterize the frequency content of the waveforms, and show that LFEs are prevalent in the upper crust beneath the Japanese Islands, even in non‐volcanic regions. Shallow LFEs are most common near tectonic boundaries and are temporarily activated in the aftershock sequences of large (M ≥ 6.5) crustal earthquakes. The widespread distribution of shallow LFEs suggests that a lower crustal rheology is not necessary for their genesis. We infer that failure along frictionally weakened faults due to high pore‐fluid pressures is a primary control for the enrichment of low‐frequency energy. The observed differences in the frequency content are probably due to differences in the pore‐fluid pressure along each fault, which influences the rupture velocity and magnitude of the tensile component during shear failure. Our observations may lead to a more unified model of earthquake generation, thereby providing a better understanding of how earthquakes release the stress accumulated in the Earth. Key Points: Low‐frequency earthquakes are more common in the upper crust than previously thoughtEarthquakes with predominant low‐frequency energy are activated after large crustal earthquakesDifferences in the pore‐fluid pressure may be a major cause of the large variation in the frequency contents of earthquakes [ABSTRACT FROM AUTHOR]

The style of subduction that prevailed on the early Earth, or even whether subduction was prevalent at all, is an important question in the evolution of Earth's crust, mantle, and surface environment. Here, two‐dimensional numerical convection models, that include grain size evolution to generate weak plate boundaries, reveal a clear transition in subduction behavior with increasing internal heating rate. Sustained subduction with a coherent slab gives way to a style where slabs periodically detach and sink rapidly into the deep mantle, with increasing internal heating rate. In this latter, "drip‐like" subduction regime, repeating cycles of slab growth by subduction, followed by necking and detachment of the lower portion of the slab, are seen. These cycles are termed "slab detachment cycles," and similar behavior has been seen in regional scale subduction models of the early Earth. Fourier analysis is used to constrain the timescale of slab detachment cycles, and a simple scaling law for this timescale is developed. Applying the scaling law to the early Earth indicates that slab detachment cycles can occur on timescales of <10 Myr, even as low as <5 Myr if the lithosphere is thick and mantle temperature is >1900 K. These cycles may thus be capable of explaining repeating sequences of rocks with "arc" and "non‐arc" signatures seen in some Archean cratons. The drip‐like subduction regime could also have significant implications for the generation of the tonalite‐trondhjemite‐granodiorite (TTG) suite of rocks and exhumation of high pressure metamorphic rocks, two important features of the early Earth geologic record. Key Points: Global mantle convection models reveal a style of subduction with periodic slab detachment prevails for Archean mantle thermal conditionsScaling laws for the onset of this regime of subduction and timescale between successive slab detachment events are developedRepeated cycles of slab detachment can potentially explain observations from the Archean geologic record and the genesis of TTG rocks [ABSTRACT FROM AUTHOR]

Investigating heterogeneous structures beneath active volcanoes is important for better understanding of volcanic activity and improved mitigation of volcanic risk. In central Japan, Hakone volcano has recently shown shallow earthquake swarms and deep low‐frequency earthquakes (DLFEs), which are probably related to geothermal or deep magmatic activity. In order to image the feeding system beneath this volcano, we estimate 3‐D P wave attenuation structure using waveform data recorded at permanent and densely distributed temporary seismograph stations. We first determine corner frequencies of the earthquakes and then perform a joint inversion to obtain attenuation terms (t*) and site responses. Values of t* are finally inverted to estimate the attenuation structure to a depth of 50 km. High‐attenuation zones at depths ≤5 km suggest that fracture zones are permeated with hydrothermal fluids. A high–strain rate zone revealed by geodetic observations is spatially correlated with a high‐attenuation volume at depths of 5–10 km, suggesting that the anelastic deformation is dominant in the high‐attenuation zones. A subvertical volume of moderate attenuation that is imaged at depths of 10–20 km beneath Hakone volcano is connected to the top of a zone of partial melting beneath Mt. Fuji at depths ≥30 km through a subhorizontal channel at a depth of ~25 km. DLFEs in the lower crust and fluid‐related activity such as earthquake swarms in the upper crust occur in this volume. The zone is thus interpreted as a fluid‐bearing pathway, which allows the rise of magmatic fluids to Hakone volcano. Key Points: Large historical earthquakes occurred at the edge of high‐attenuation anomaliesA deep magmatic system to Mt. Fuji and Hakone volcano is common but is separated at a depth of ~25 kmMagma chamber with the dimension of as large as ~10 km is unlikely to exist at depths of 10–20 km beneath Hakone volcano [ABSTRACT FROM AUTHOR]

The south central Chilean margin regularly produces many of the world's largest earthquakes and tsunami, including the 2010 Mw 8.8 Maule and 1960 Mw 9.5 Valdivia events. In 2017, we acquired seismic reflection data along ~1,000 km of the margin using the R/V Langseth's 15 km long receiver array and 108.2 l (6,600 in3) seismic source to image structures associated with these ruptures. We focus on the Valdivia segment with the largest coseismic slip (~40 m). The outer 40 km of the forearc is an accretionary wedge constructed primarily of stacked sedimentary packages with irregular lengths and thicknesses and little along‐strike continuity. Forearc structures indicate that the accretionary wedge grows primarily through basal accretion of the downgoing trench fill. The décollement propagates along a weak boundary near the top of the trench fill but occasionally branches downward into the underthrust sediment along bedding horizons, peeling off slices that are underplated to the forearc. The shallow décollement level and the rarity of underplating events allow most of the trench sediment to subduct. As a result, only ~30% of the incoming sediment has been accreted since the Early Pliocene. This implies that, on average, ~1 km of sediment must subduct beyond the outer forearc, an inference that is supported by our seismic images. We propose that the thickness and great downdip and along‐strike extent of the underthrust layer, which separates the megathrust from the underlying roughness of the igneous ocean crust, ensures a smooth broad zone of strong coupling that generates the world's largest earthquakes and tsunami. Key Points: The south central Chilean margin grows primarily by basal accretion rather than frontal accretion processes common to many subduction zonesIntermittent basal accretion processes along the Chilean margin allow large fractions of the trench sediment to subduct into the seismogenic zoneThe large subducting sediment volume maintains a smooth, shallow décollement capable of building large strains and causing great earthquakes [ABSTRACT FROM AUTHOR]

We present new seismic tomography of the Washington forearc using a suite of manually picked regional earthquake phase arrivals. The recovery of similarly sampled P and S velocity permits the robust calculation of Poisson's ratio throughout the region. The seismological signature of Siletzia, an accreted oceanic plateau that crops out in Washington as the Crescent Formation, is evident in our models as a continuous high Poisson's ratio body that coincides with subsurface structures estimated from potential field maps. Relocated earthquakes preferentially locate in low Poisson's ratio regions in the forearc crust and, in particular, in a diffuse layer located at 15–25 km depth in the crust beneath relatively aseismic Siletzia. Our imaging of the Puget Sound is consistent with previous interpretations of the architecture of major faults and blocks, as evident by distinct Poisson's ratio signatures that distinguish sedimentary basins from mafic rocks of the Crescent Formation. We speculate that seismicity below the Puget Sound is promoted by slab‐derived fluids that are localized beneath Siletzia as a result of intrinsic low vertical permeability. Key Points: We determine similarly sampled P and S velocity models for western Washington and southern Vancouver Island, enabling the determination of Poisson's ratio throughout the regionSiletzia, an accreted oceanic plateau, is imaged as a continuous high Poisson's ratio body in the forearc that thins to the northRelocated seismicity preferentially locates within low Poisson's ratio regions, whereas Siletzia is relatively aseismic [ABSTRACT FROM AUTHOR]

Klyuchevskoy and surrounding volcanoes in central Kamchatka form the Northern Group of Volcanoes (NGV), which is an area of particularly diverse and intensive Pleistocene‐Holocene volcanism. In this study, we present a new seismic tomographic model of the crust and uppermost mantle beneath NGV based on local earthquake data recorded by several permanent and temporary seismic networks including a large‐scale experiment that was conducted in 2015–2016 by an international scientific consortium. Having an unprecedented resolution for this part of Kamchatka, the new model reveals many features associated with the present and past volcanic activity within the NGV. In the upper crust, we found several prominent high‐velocity anomalies interpreted as traces of large basaltic shield volcanoes, which were hidden by more recent volcanic structures and sediments. We interpret the mantle structure to reflect asthenospheric flow up through a slab window below the Kamchatka‐Aleutian junction that feeds the entire NGV. The interaction of the hot asthenospheric material with fluids released from the slab determines the particular volcanic activity within the NGV. We argue that the eastern branch of the Central Kamchatka Depression, which is associated with a prominent low‐velocity anomaly in the uppermost mantle, was formed as a recent rift zone separating the NGV from the Kamchatka Eastern Ranges. Key Points: We present a new high‐resolution seismic model of the crust and upper mantle beneath the Northern Group of Volcanoes in KamchatkaThe volcanoes of the Northern group are fed by an asthenosphere flow ascending from a slab window below the Kamchatka‐Aleutian junctionEastern branch of the Central Kamchatka Depression is a rift separating the Northern Group of Volcanoes from the Eastern Ranges [ABSTRACT FROM AUTHOR]

Dehydration melting in subduction zones often produces cold plumes, initiated by Rayleigh‐Taylor instabilities in the buoyant partially molten zones lying above the dipping subducting slabs. We use scaled laboratory experiments to demonstrate how the slab dip (α) can control the evolution of such plumes. For α > 0°, Rayleigh‐Taylor instabilities evolve as two orthogonal waves, one trench perpendicular with wavelength λL and the other one trench parallel with wavelength λT (λT > λL). We show that two competing processes, (1) λL‐controlled updip advection of partially molten materials and (2) λT/λL interference, determine the modes of plume growth. The λT/λL interference gives rise to an areal distribution of plumes (Mode 1), whereas advection leads to a linear distribution of plumes (Mode 2) at the upper fringe of the partially molten layer. The λT wave instabilities do not grow when α exceeds a threshold value (α* = 30°). For α > α*, λL‐driven advection takes the control to produce exclusively Mode 2 plumes. We performed a series of 2‐D and 3‐D computational fluid dynamics simulations to test the criticality of slab dip in switching the Mode 1 to Mode 2 transition at α*. We discuss the effects of viscosity ratio (R) and the density contrast (Δρ) between the source layers and ambient mantle, source layer thickness (Ts), and slab velocity (Us) on the development of cold plumes. Finally, we discuss the areal versus linear distributions of volcanoes from natural subduction zones as possible examples of Mode 1 versus Mode 2 plume products. Key Points: Cold plumes are formed by two sets of Rayleigh‐Taylor instability waves: λL and λT along and across the slab dipThe λT interference and λL‐controlled updip partial melt advection are the key processes to decide distributed versus localized plume growthSteepening slab dip leads to a transition from distributed to localized plume development, as manifested in contrasting arc volcanisms [ABSTRACT FROM AUTHOR]

The bulk composition of overthickened Tibetan deep crust has been generally believed to be mafic granulite with eclogite at the lowermost crust. However, a granulitic/eclogitic deep crust is in contradiction to geological and geophysical observations in southern Tibet. Here we present petrofabrics and seismic properties of amphibolites from exhumed crustal part of the Indian plate in the eastern Himalayan syntaxis. Our results show strong fabrics of amphibole, nearly random fabrics of plagioclase and strong seismic anisotropies of amphibolites (AVp = 5.6–12.0% and Max. AVs = 5.1–7.7%). Comparing to a deep crust composed of nearly isotropic mafic granulite and weakly anisotropic eclogite, a thick amphibolitic layer in the middle‐lower crust would better account for the strong shear wave splitting (0.2–0.5 s in delay times or 4–15% in S wave anisotropy), the suture boundary parallel faster shear wave polarization, and the widespread postcollisional adakite‐like potassic rocks in southern Tibet. Plain Language Summary: We study the petrofabrics and seismic anisotropies of exposed lower crust of the subducting Indian plate. Our results show that a thick layer of amphibolites in the middle‐lower crust would agree better with the geological and geophysical observations than a deep crust entirely made of granulite and eclogite underneath southern Tibet. Key Points: Himalayan amphibolites are characterized by strong fabrics of amphibole and nearly random fabrics of plagioclaseHimalayan amphibolites show strong whole‐rock P wave and S wave seismic anisotropies mostly resulting from their amphibole componentsA thick amphibolite layer can account for the strong S wave splitting and the suture boundary parallel Vs1 polarization in southern Tibet [ABSTRACT FROM AUTHOR]

With substantial postglacial rhyolite eruptions and ongoing rapid uplift, the Laguna del Maule volcanic field in the southern Andes provides an exceptional opportunity to study the dynamics of an active silicic magmatic system. Using 4,093 P arrivals from 137 distant earthquakes recorded by 44 local stations over ∼2.25 years, we conduct teleseismic tomography to image the crustal structure down to 40 km below the volcanic field. A prominent low‐velocity body at depths between ∼0 and 12 km below sea level (b.s.l.), characterized by a volume of ∼500 km3 and a peak anomaly of −400 m/s (∼9%), overlaps the location of the upper‐crustal magma reservoir detected in recent gravity and surface wave tomography studies. Its estimated average P wave velocity of ∼4.6 km/s corresponds to an average melt fraction of about 14% and a melt volume of ∼70 km3. Petrologic observations are also consistent with generation and storage of rhyolitic melts at depths corresponding to the anomalous zone. Moreover, the tomographic results support a lower crust zone of MASH (melting, assimilation, storage, and homogenization) from a depth of ∼25 km to the base of the model, which likely reflects a deep crustal source of magma that contributes to and incubates the shallow silicic reservoir. Key Points: We present results of a teleseismic tomography study at Laguna del Maule volcanic field, ChileAn upper‐crustal magma reservoir of several hundred cubic kilometers is identified and characterizedFindings complement independent geophysical and petrologic observations and reveal a transcrustal magmatic system at LdMVF [ABSTRACT FROM AUTHOR]

Maximum depths of earthquakes in different settings are commonly thought to lie within a transition, many kilometers in width, from brittle deformation at shallow depths to plastic deformation by high‐temperature creep at greater depth. A review of temperatures and strain rates, both of which are low in intraplate settings, shows faster deformation in warmer tectonically active regions and highest strain rates in regions where the movement of magma affects strain rates and temperatures are especially high. Although intraplate earthquakes appear to occur in oceanic lithosphere only where colder than ~600°C, in tectonically active regions, where strain rates are several orders of magnitude higher than intraplate settings, earthquakes occur where temperatures exceed 600°C and perhaps 800°C. The temperature cutoff increases by ~100°C as strain rates increase by ~2 to 2.5 orders of magnitude. Nowhere, however, do any two of average strain rates, background stresses, and temperatures seem to be determined well enough that, when combined with laboratory‐based flow laws for high‐temperature creep, they can place a tight constraint on the third of these quantities. At the same time, the pattern of higher temperatures at higher strain rates follows the general forms of power law and Peierls creep that are appropriate for lithospheric conditions. Key Points: Earthquakes in tectonically active regions can occur in regions where temperatures reach 700–900°CStrain rates versus highest temperatures of seismic regions follow approximately laboratory‐based flow laws for lithospheric conditions [ABSTRACT FROM AUTHOR]

The water cycle plays an essential role in arc volcanism, earthquake generation, mantle rheology, and thermal structure of subduction zones. Previous seismic studies have revealed strong structural heterogeneities in the megathrust zone in Northeast Japan and Hokkaido. However, water transportation in the forearc region remains poorly understood due to the lack of long‐term seismic observatories at the seafloor. Using high‐quality data recently recorded by the permanent ocean‐bottom‐seismometer network (S‐net) in the Pacific Ocean off Northern Japan, we study the fine three‐dimensional P wave velocity (Vp) structure of the crust and upper mantle beneath the Kuril and Tohoku forearc region. Our results reveal high velocities in the mantle‐wedge corner, implying a low degree of serpentinization there. We suggest that the forearc mantle in the study region is cold and anhydrous. The slab interface under the forearc area may have a low permeability, which controls the fluid flux to the mantle wedge and the overriding plate. A low‐Vp layer atop the subducting Pacific plate is interpreted as the subducted oceanic crust. Dehydration of the subducted oceanic crust occurs at depths of 80–120 km, providing a large volume of water to the overriding mantle wedge to produce arc volcanoes, and part of the water may migrate upward to the shallow area. Large megathrust earthquakes (Mw ≥ 6.0) mainly occurred around low‐Vp patches in the megathrust zone. Destruction of the low‐permeability slab interface would result in fluid flow upward, which may trigger large megathrust earthquakes and seismicity in the mantle wedge under the forearc. Key Points: High‐resolution P wave tomography of the forearc off Northern Japan is derived from the S‐net OBS dataHigh‐velocity anomalies are revealed in the mantle wedge corner, implying a cold and dry forearc mantleA large amount of water in the subducted slab is transported into the deep upper mantle [ABSTRACT FROM AUTHOR]

We investigate 3‐D seismic structures (Vp, Vs, and Poisson's ratio) and Vp azimuthal anisotropy in the source area of the 2018 Eastern Iburi earthquake (M 6.7) in Hokkaido, Japan. Its mainshock occurred at the edge of a high‐Vp (2–4%) seismogenic zone. Significant low‐Vs (−1% to −3%) and high Poisson's ratio (2–7%) anomalies are imaged in and below the source zone and extend to the upper surface of the subducting Pacific slab, most likely reflecting ascending fluids released by the slab dehydration. A high consistency between the fault plane and the low‐Vs and high Poisson's ratio anomalies indicates that the fluids may have entered the fault and affected the rupture nucleation. A high‐V (1–3%) anomaly is revealed in the fore‐arc mantle wedge and connects with the high‐V seismogenic zone, probably reflecting a lithospheric fragment and contributing to cool down the mantle wedge. Complex seismic anisotropy is revealed in the crust in and around the source area, which may reflect complicated stress regime and strong structural heterogeneities there. Key Points: The 2018 Eastern Iburi earthquake occurred at the edge of a high‐Vp seismogenic zoneAscending fluids from slab dehydration affected rupture nucleation of this earthquakeA high‐V anomaly exists below the source zone, which may cool down the mantle wedge [ABSTRACT FROM AUTHOR]

Seismic events characterize active hydrothermal and volcanic areas and may be due to magma/fluid migration, hydrothermal pressurization, gravitational instability, and local tectonics. On 21 August 2017, an Md 4.0 earthquake occurred at Ischia volcanic island (Italy), within an active hydrothermal system. We analyze seismic, Global Positioning System, and interferometric synthetic aperture radar data to shed light on the source mechanism of such an event. The low‐frequency content (2 Hz), the low stress drop (0.01 MPa), and a low S/P spectral ratio suggest the involvement of fluids in the source mechanism. The focal mechanism suggests a mixed shear‐tensile (opening) rupture with the P first arrivals showing up movements in the nearest stations. Geodetic data describe an E‐W elongated area of coseismic subsidence overlapping a WSW‐ENE fault bounding the hydrothermal reservoir at depth. The modeled deformation field is consistent with a two‐source model consisting of a WSW‐ESE striking, north dipping normal fault, and a closing subhorizontal crack. This closure immediately followed an initial opening related to a fluid pressurization event responsible for the earthquake. We show that moderate magnitude earthquakes in active hydrothermal areas may be associated with the pressurization/depressurization cycles of a hydrothermal reservoir due to self‐sealing processes and not to the arrival of new fluids from depth. Other events like that recorded at Ischia, which have affected the island in historical times, are not necessarily associated with 'volcanic unrest' episodes and imply the occurrence of fault‐valve mechanisms. Therefore, the dynamics of hydrothermal systems must be taken into account in the seismic hazard evaluation. Key Points: The Md=4.0, 2017 damaging event at Ischia volcanic island is characterized by low‐stress drop and S/P ratioThe source includes shear faulting and cracks opening followed by closure (subsidence)Hydrothermal, nonmagmatic earthquakes must be considered in the evaluation of volcanic hazard at active volcanoes [ABSTRACT FROM AUTHOR]

We use Sentinel‐1 radar imagery to explore the coseismic and postseismic surface displacements associated with the 2016 MW 6.2 Lampa earthquake in southern Peru. Based on coseismic interferograms, the preferred slip model links to a blind south southeast striking, south southwest dipping normal fault with a shallow dip (45.2°) and a peak slip of 0.71 m at depth ~5.3 km, which is consistent with seismic solutions. Postseismic interferograms, derived from two tracks of the Sentinel‐1A/B satellites using a small baseline subset method, show subsidence up to ~3 cm in the first year after the mainshock. The kinematic inversions of interferometric synthetic aperture radar (InSAR) observations imply that the postseismic surface displacements observed in 1 year after the earthquake are governed by afterslip occurring along the updip extension of the coseismic slip patches. To further improve the data fitting, we generate a fault with variable strike to refine the kinematic afterslip model. The stress‐driven afterslip forward modeling shows that the postseismic deformation is controlled by afterslip distributed at the edge of the compact coseismic slip area. The surface displacement predictions of the poroelastic rebound show subsidence of the hanging wall, but the magnitude of the displacements is small compared to the observed signal. We as well test a collection of viscoelastic relaxation models and find that the predicted surface displacements are not consistent with the observations. The InSAR results show that the strike of the seismogenic fault is quasi‐parallel to the Vilcanota normal fault system and both the fault associated with earthquake and Vilcanota normal fault dip in the same direction. Therefore, we suspect that the causative fault of this 2016 event may be a normal fault belonging to a "domino" faulting system. Key Points: Coseismic and postseismic deformation of the 2016 Lampa earthquake were derived from the InSAR observationsThe inversion results suggest that the seismogenic fault of the Lampa earthquake is a blind, southwest dipping normal faultInSAR time series analysis that shows postseismic deformation of the Lampa earthquake mainly locates just updip of the coseismic rupture [ABSTRACT FROM AUTHOR]

We used 1‐D and 3‐D velocity models to determine focal mechanism solutions (FMSs) of 349 crustal earthquakes (M 2.7–7.3) and stress tensors in the source area of the 2016 Kumamoto earthquake (M 7.3) that occurred on the Futagawa‐Hinagu fault zone in Kyushu, Southwest Japan. There are some differences in the FMSs determined with the 1‐D and 3‐D velocity models. The use of the 3‐D velocity model leads to better results of stress tensors, which are determined by inverting the FMSs. The orientation of the minimum stress (σ3) axis is more accurately determined, which trends NNW‐SSE to N‐S nearly horizontally. In contrast, the axes of the maximum and intermediate stresses (σ1 and σ2) trend WSW‐ENE to E‐W with wide ranges. Significant spatiotemporal variations of the stress field are revealed in the Kumamoto source zone, indicating a small magnitude of deviatoric stress. The friction coefficient of the faults is estimated to be relatively small (~0.4), indicating that the seismogenic faults in central Kyushu are weak. The fault weakening may be caused by fluids beneath the source area and arc magma under the nearby Aso active volcano. Key Points: Focal mechanism solutions and stress tensors in the 2016 Kumamoto earthquake area are determined with a 3‐D velocity modelThe 3‐D velocity model leads to better results of stress tensorsSeismogenic faults in the source area are weak due to the effect of arc magma and fluids [ABSTRACT FROM AUTHOR]

Recent earthquakes have demonstrated that rupture may propagate through geometrically complex networks of faults. Ancient exhumed faults have the potential to reveal the details of complex rupture at seismogenic depths. We present a new set of field observational criteria for determining which of a population of pseudotachylyte fault veins formed in the same earthquake and apply it to map rupture networks representing single earthquakes. An exceptional exposure of an exhumed ancient strand of the Norumbega Shear Zone preserves evidence of multistranded earthquake rupture in the deep seismogenic zone of a continental transform fault. Individual fault strands slipped at least 2–18 cm, so significant slip is represented by each rupture network. Our data show that synchronously slipped faults intersect at angles of 0 to ∼55°, with the opening angles of fault intersections directed toward the dilational quadrants for dextral slip. Multistranded rupture on a fault network instead of rupture of a single fault may result in greater and/or more variable slip and cause slip rake to vary spatially and temporally. Slip on intersecting faults unequivocally means that there will be motion perpendicular to the average fault plane. Modern earthquakes displaying non‐double‐couple components to focal mechanism solutions and spatially varying rake, slip, and anomalous stress drop may be explained by rupture across fault networks that are too close (spatially and temporally) to be resolved seismically as separate events. Plain Language Summary: Earthquake faults are often treated as simple planar features, but some recent earthquakes have shown that networks of faults can rupture together and produce very complicated patterns of deformation. We found an exposure of an ancient fault zone in Maine, USA, which preserves the pattern of ancient ruptures in the form of fossil earthquake slip surfaces. We mapped the networks of faults that slipped in individual earthquakes to show the geometry of complex earthquake ruptures at middle‐crustal depths and predict what seismologists would observe if similar earthquakes are happening today. Key Points: Networks of synchronous pseudotachylyte fault veins preserve multistranded earthquake rupturesSynchronous fault strands intersect at angles less than 55 degrees, usually in extensional quadrants for dextral slipGeometric complexity of rupture networks is consistent with anomalous seismic characteristics of some earthquakes [ABSTRACT FROM AUTHOR]