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In the Okinawa Trough off southwestern Japan, hydrothermal circulation due to back‐arc rifting is active. Biogenic CH4in 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 Pwave 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 CO2and CH4. The heat flow inferred from seismic profiles (i.e., depth of reflector) indicates that CH4hydrate 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. 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 CH4originating 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 CH4hydrate. 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 CO2is the main contributor to global warming. High‐resolution Pwave 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

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 Mw7.0 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. 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 Mw7.0 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. 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