A real-time observation network of ocean-bottom-seismometers deployed at the Sagami trough subduction zone, central Japan

We installed a real-time operating regional observation network of Ocean-Bo ttom-Seismometers, connected to an electro-optical fiber communication cabl e, at the Sagami trough subduction zone, just south of the Tokyo metropolit an area, central Japan. The network, called ETMC, has six seismic observati on sites at approximately 20 km spacing. In addition, there are three tsuna mi observation sites along the ETMC network to monitor the propagation proc ess of tsunamis around the Sagami trough region. The on-line data from the ETMC has been improving the detection capability of smaller-magnitude earthquakes even at areas close to the margin of the t rough. The ETMC data analyzing system, which has a function of real-time di gital filtering for each seismic channel, can read the arrival times of P- and S-waves precisely, constraining well the automatic on-line hypocenter l ocations. The network has been providing useful information regarding the b ending and downgoing process of the Philippine sea plate at the Sagami trou gh subduction zone. The pressure sensors of the installed network have a detection capability o f tsunami wave trains with an amplitude of less than 1 cm. For example, the sensors recorded the full time history of tsunami wave trains, with mm ord er resolution, originating from a 'tsunami earthquake' with 5.7 M-W and the tsunami magnitude of 7.5 occurred near Tori Shima (Tori Is.) of the Izu-Bo nin Is, are on September it, 1996. The maximum amplitude of the tsunami sig nals on the trough-floor was approximately 1 cm (P-P), in contrast with app roximately 20 cm (0-P) at a coastal site on Izu-Oshima, near the trough. Al so, the pressure sensors observed tsunamis due to a large tsunami earthquak e (7.1 M-W) at the northern New Guinea, on July 17, 1998.