Measurement Of Turbidity Current Associated With 2003 Tokach-Earthquake Using Cabled Seafloor Observatory

A turbidity current due to the _g2003 Tokachi-oki Earthquake (M8.0)_h was successfully measured by a cabled seafloor observatory which was deployed off Kushiro, Hokkaido in 1999. The cabled seafloor observatory consists of three-component ocean bottom seismometers, precise pressure sensors and a cable end station. The cable end station consists of an ADCP, electro-magnetic current meter, CTD, hydrophone and heat flow temperature probes. It was deployed at the landward slope of the southern Kuril Trench at a depth of 2630 m (Hirata et al., 2002, IEEE J. Oceanic Engineering). A direct reading 150 kHz Broadband ADCP is mounted with keeping up-looking direction by a gimbals assembly. The ADCP is set to measure the vertical current profiles up to 380 m every half hour. Bottom current is measured by electro-magnetic current meter with 1 Hz sampling speed. Temperature, salinity and pressure data are measured by CTD. All measurement data from the underwater instruments are transmitted to Yokohama Institute for_@Earth Science of JAMSTEC in real-time for using submarine electro-optical cable and NTT digital lines. A south-southwestward strong bottom current of over 100 cm/sec was observed as the turbidity current about two hours after the mainshock at 22:01 (UTC) on September 25, 2003 (Mikada et al., 2004, Tectonophysics (submitted)). It had been continued for about seventeen hours. The current speed has reached over 140cm/sec at about two hours later. The temperature increased in 0.5 deg C at maximum and salinity decreased in 0.2 PSU simultaneously. The thickness of the turbidity current becomes about 60 m. According to acoustic scattering signals, the suspended particles from bottom sediments reached up to100 m above the seafloor. A counter current was observed with 300 m in thick for approximately twelve hours after the turbidity current was over. These phenomena indicate that some submarine landslides or collapses of gas hydrate layers were caused by the earthquake. The temperature difference shows that the source of turbidity current was shallower than 1700 m.