Plate Dynamics at the Eastern Margin of the Japan Sea from Long-term Seafloors Observation.

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The Japan Sea constitutes a back arc in the northern Japan plate subduction zone. How the subducting Pacific plate interacts with the overriding plate and the surrounding upper mantle is still not fully understood. This is partly due to the lack of key observational data from the surrounding deep seafloors. We have developed and emplaced a digital broadband seismometer in an ODP hole about 160 km west off NE Japan from the D/V JOIDES Resolution in Sep., 1989, in order to initiate a long-term seismic observation.Hole 794D is 733.5 m deep below seafloor (water depth: 2818 m). The borehole package including a 3-component feed-back type accelerometer and a digital data telemetry system was clamped near the bottom within solid basalt. The sensor is a ruggedized version of Guralp CMG-3 with leveling mechanisms for horizontal sensors. The seismic data are sent uphole in two gains at a rate of 80 Hz/ch, 16-bit each via the 7-conductor logging cable. A controlled source seismic experiment was carried out on R/V Tansei-maru for two days while recording the borehole data on the drill ship real-time. Then, the seafloor long-term seismic recording system including a battery package (24V-38AH x 20) and glass sphere buoys with an acoustic transponder attached to the end of a 6000 m rope was deployed from another vessel. The system can record 60 MB of data by time window/event trigger algorithms. Thus we believe we have initiated a broadband long-term seismic observation for the first time in a marine environment.