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In the Okinawa Trough off southwestern Japan, hydrothermal circulation due to back‐arc rifting is active. Biogenic CH4 in discharging hydrothermal fluids at the Iheya North Knoll is derived from outside the knoll; however, the location of the gas reservoir has not been identified. Here, we applied automated velocity analysis to seismic reflection data to obtain a P wave velocity structure in high spatial resolution. The resulting profiles reveal large gas reservoirs as low‐velocity zones along the rifting axis (>5 km for axis direction) around the Iheya North Knoll. The main gaseous components in the reservoir could be CO2 and CH4. The heat flow inferred from seismic profiles (i.e., depth of reflector) indicates that CH4 hydrate could be trapping the gas. Furthermore, the heat flow is higher at the rifting axis and close to the knoll, reflecting the large gas reservoir feeding the hydrothermal fluids in the discharging area at the knoll. Plain Language Summary: Magmatic activity in the middle Okinawa Trough region southwest of Japan gives rise to intense hydrothermal activity. Gases in the hot fluids discharging from the hydrothermal field at Iheya North Knoll consist mainly of biogenic CH4 originating from microbial activity and CO2. By applying high‐resolution automated velocity analysis to seismic reflection data, this study revealed widely distributed gas reservoirs as inferred from their low seismic velocities. Furthermore, our estimates of heat flow suggest that these gas reservoirs are capped by CH4 hydrate. If the main gas component is CH4, the gas reservoir may be a potential natural resource. If the gas includes CO2, the distribution of similar reservoirs is important because CO2 is the main contributor to global warming. Key Points: High‐resolution P wave velocities in the middle Okinawa Trough demonstrate existence of large‐scale gas reservoir at the rifting axisWidespread gas reservoirs trapped by hydrates could influence hydrothermal fluids in the discharging area at Iheya North KnollEstimated heat flow is higher at the rifting axis and close to Iheya North Knoll, reflecting the presence of hydrothermal systems [ABSTRACT FROM AUTHOR]

Earthquakes are known to precede volcanic activity, including long‐period or very long period (VLP) volcanic seismicity. However, the relationships among earthquakes, VLP seismicity, and volcanic eruptions are not well understood. Here we present the locations of VLP seismicity at Aso volcano, Japan, between January 2015 and December 2016, a period that includes the Mw 7.0 Kumamoto earthquake and phreatomagmatic eruptions. By using a differential‐time backprojection method developed in this study to accurately locate VLP events, we clearly identified two distinct VLP clusters. Whereas the eastern cluster was active during eruptions, the western cluster displayed intense VLP seismicity only for a few months after the earthquake. The western cluster may be associated with opening of new fractures during the earthquake. This study explores the mechanisms that can relate earthquake to volcanic activities and provides a new approach to analyze the dynamic behaviors inside volcanoes that may yield useful information for hazard evaluation. Plain Language Summary: The relationship between earthquake and volcanic seismicity is studied by analyzing continuous seismic data recorded between January 2015 and December 2016, a period that includes the Mw 7.0 Kumamoto earthquake and volcanic eruptions at Aso. This study focuses on analyzing very long period (VLP) seismic event with a period of 15 s that can be explained by pressure fluctuation within hydrothermal systems. The intense activity of long‐period event could be an indication of impending eruptions. By using our developed localization method, we revealed two clusters of VLP sources, namely, eastern and western clusters. The eastern cluster was always active during an active period of Aso volcano. However, the western cluster showed intense VLP seismicity only for a few months after the earthquake. The western cluster could be associated with opening of new cracks triggered by the Kumamoto earthquake. This study suggests a strong relationship between distant large earthquake with volcanic seismicity that could be useful for disaster mitigation. Moreover, monitoring of temporal and spatial evolution of VLP sources may be considered as a tool to study the response of volcanic activity to ground shaking from large earthquake. Key Points: Backprojection technique based on differential‐time and phase weighted stacking is used to map clusters of very long period seismicityClusters of very long period seismicity indicate clear relationship between distant earthquake and volcanic activitiesThe 2016 Kumamoto earthquake suddenly induced very long period seismicity at west of Aso Caldera lasting for five months [ABSTRACT FROM AUTHOR]

We present a newworkflowfor the localization of seismic eventswhich is based on a diffraction stacking approach. In order to address the effects from complex source radiation patterns, we suggest to compute diffraction stacking from a characteristic function (CF) instead of stacking the original waveform data. A new CF, which is called in the following mAIC (modified from Akaike Information Criterion) is proposed. We demonstrate that both P- and S-wave onsets can be detected accurately. To avoid cross-talk between P and S waves due to inaccurate velocity models, we separate the P and S waves from the mAIC function by making use of polarization attributes. Then, the final image function is represented by the largest eigenvalue as a result of the covariance analysis between P- and S-image functions. Results from synthetic experiments showthat the proposed diffraction stacking provides reliable results. Theworkflow of the diffraction stacking method was finally applied to local earthquake data from Sumatra, Indonesia. Recordings from a temporary network of 42 stations deployed for nine months around the Tarutung pull-apart basin were analysed. The seismic event locations resulting from the diffraction stacking method align along a segment of the Sumatran Fault. A more complex distribution of seismicity is imaged within and around the Tarutung basin. Two lineaments striking N-S were found in the centre of the Tarutung basin which support independent results from structural geology. [ABSTRACT FROM AUTHOR]

The Indo-Australian plate subducts obliquely beneath the Sunda plate leading to a slip partitioning into pure thrust and strike-slip motion. Just in the last 5 yr, three pure thrust earthquakes of have occurred along this subduction interface. The Great Sumatra Fault, traversing the Sumatra continental block, takes up a significant part of the strike-slip motion, but the Mentawai Fault bounding the NE margin of Mentawai Islands has been suggested to accommodate a part of the strike-slip motion. Although the great Sumatra Fault is active, no seismicity has been observed along the Mentawai fault. Using a combination of high-resolution seismic reflection and bathymetry data, here we show that the Mentawai Fault seems to be characterized by active SW dipping backthrusts. The presence of recent steeply dipping thrust earthquakes suggests that these faults should be active. Combined with results from north in 2004 earthquake region and south of this study area, our results suggest that backthrusting should play an important role in forearc evolution SW of Sumatra. We also observed several mass wasting sites at NE margin of the Mentawai Islands, which could be erosional features or landslides triggered by earthquake activities. Localized uplift along the steeply dipping backthrusts at the NE margin of Mentawai Islands in the fully locked region could pose serious seismic and tsunami risks to the SW coast of Sumatra in the future. [ABSTRACT FROM AUTHOR]

• Propose an alternative workflow to estimate location of many tectonic tremors. • Tremor could be in shear zone or underthrust sediments on the plate boundary. • Thickness of the shear zone or underthrust sediments could control tremor epicentre. • Pore pressure diffusion is a likely mechanism for tremor migration. • Identify migration speed of tectonic tremors and estimate permeability of fault. Episodes of tectonic tremor are observed in the Nankai accretionary prism close to the trough axis; however, their non-impulsive signals and the distortion of seismic signals in the accretionary prism make it challenging to accurately estimate their locations. Reliable tremor locations would help to characterize geological conditions favourable for tremor generation and could offer insights into the up-dip limit of coseismic ruptures. Here we report a new approach to pinpoint tremor locations based on seismic phase polarization. The estimated epicentres of tremor distribution mostly coincide with low-velocity shear zone or thick underthrust sediments where high pore pressures are inferred, and the distribution of tremor appears to be controlled by characteristics of the shear zone. A tremor episode occurred a couple days after the off-Mie earthquake clearly shows migration at a velocity similar to that of very low frequency earthquakes. Migration of tremor initiated around the outer ridge where the estimated pore pressure is greatest. The spatio-temporal tremor distribution indicates that pore pressure relaxation could trigger the tremor migration. The occurrence of tremor thus may indicate a weak shallow plate interface that is prone to slip following large earthquakes. Based on the information of tremor migration, the permeability along the fault can be estimated as 3.7 × 10 − 12 m2. [ABSTRACT FROM AUTHOR]