A passive and active seismic experiment near the Boso triple junction in the far northwestern part of the Pacific plate

The Pacific Plate subducts beneath northeastern Japan along the Japan Trench and beneath the Izu-Bonin-Mariana arc along the Mariana Trench. The Boso triple junction is located at between the Japan Trench and the Mariana Trench, and the southeastern end of the Sagami Trough where the Philippine Sea Plate subducts beneath northeastern Japan. It is thus a trench-trench-trench type triple junction. For the purpose of understanding the interaction between three plates and its effect to the Pacific Plate, we have performed a passive and active seismic experiment near the Boso triple junction in the far northwestern part of the Pacific plate. We deployed 10 Ocean Bottom Seismometers (OBSs) equipped with a three-componet 1Hz geophone mounted on gimbabl systems on KH09-3 cruise of R/V Hakuho-maru on July 2009, and recovered the OBSs by using M/V Shinchou-maru on October 2010.During the KH09-3 cruise, we shot by using an airgun array (6000 cubic inch in total) during 18 hours on three profiles. We obtained 442days' seismic data from July 29, 2009 to October 13, 2010 in the experiment. More than 2000 earthquakes were detected, and the foci form some clusters.