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.
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