Local Seismicity beneath Suiyo Seamount, an Active Submarine Volcano, on the Izu-Ogasawara (Bonin) Island-arc System

Several ocean bottom seismographic (OBS) observations were carried out to investigate seismic characteristics related to the volcanic and/or hydrothermal activity beneath Suiyo Seamount on the Izu-Ogasawara (Bonin) volcanic front. A preliminary seismic observation for 2 weeks using 4 OBSs in the caldera was carried out in Oct. 2000, and detected several kinds of seismic activities including long period events and seismicity just beneath the seamount. In 2001, we conducted a seismic refraction experiment to obtain a P wavespeed structural model of the upper crust across the seamount. In the 2002 full-dress investigation of the seismicity, we deployed 10 OBSs in and around the caldera of Suiyo Seamount region and observed more than two months from Aug. 5 to Oct. 9. The seismic activity was characterized by dominance of earthquakes with S-P times of 1-2 sec. We could locate 594 events with an RMS residual of P wave travel time of less than 0.2 s. The hypocenters concentrated at depths of 3.5-7.5 km beneath the seamount and revealed a conduit-like shape with a hollow, which indicated a relation to magma conduit. Furthermore, in the only 5 days observation during Dec. 5 to 9, 2003, we also obtained the same pattern of the hypocenter distribution. A remarkable burst detected on Dec. 6, 34 events occurred in 70 minutes, characterized the seismicity. The most events were S-P times of 0.7 s at the stations in the caldera and located just beneath the summit of Suiyo Seamount. A common characteristic in the 2002 and 2003 observation was a seismic gap at a depth around 3 km beneath the seamount. From these observations for resent 4 years, we conclude that rather stable seismic activity related to the magma conduit is maintained beneath the seamount, which may support the long-standing hydrothermal activity of Suiyo Seamount.