Seafloor borehole broadband seismic observatories in the western pacific and performance of recovered seismic data

In 2000 and 2001, the seafloor borehole seismological observatories WP-1 and WP-2 in the northwestern Pacific were installed. The WP-1 site is in the west Philippine Basin, and the WP-2 observatory is situated on a normal oceanic Mesozoic crust in the northwestern Pacific Basin. The WP-1 observatory was activated in March 2002 using the ROV KAIKO and long-term observation was started. In June 2006, the new ROV KAIKO-7000II dived to the WP-1 (fourth visit) and recovered the data. At present, seismic records of 692-days (Mar. 2002 - Feb. 2004) have been obtained from the WP-1. The WP-2 observatory was activated in October 2000 using the KAIKO. In June 2005, the KAIKO-7000 made fourth visit to the WP-2 and recovered the data. Recording at the WP-2 has been suspended from the fourth ROV visit. In total, 436-days data (Oct. 2000 - Jan. 2001, Aug. 2001 - July 2002) were retrieved. The long-term variations of broadband seismic noise spectra (3 mHz -10 Hz) in oceanic basins were revealed. The stable ambient seismic noise levels enable us to record many teleseismic events uniformly in time. Especially, events with magnitudes greater than 6.5 were clearly recorded with a good signal to noise ratio.