Long-term deep current measurements by JAMSTEC cabled observatories

Abstract

Japan Agency for Marine-Earth Science and Technology (JAMSTEC) operates three cabled observatories in the northwest Pacific around Japan. Each observatory has a cable end station which is composed of several environmental monitoring instruments to measure physical parameters on deep seafloor. The first observatory had consisted of only a cable end station. It was deployed in the Sagami Bay at a depth of 1172 m, 7 km off Hatsushima Island, central Honshu in 1993 and renewed in March 2000. The second cable end station was deployed at the landward slope of the Nankai Trough at a depth of 3572 m, 120 km off Cape Muroto, Shikoku, in March 1997. Third station was deployed at the landward slope of the southern Kuril Trench at a depth of 2630 m, 220 km off Kushiro, Hokkaido, in August 1999. ADCP (RDI BBADCP 150 kHz) and electro-magnet current meter are attached on the all of the cable end stations as the bottom current monitoring instrument. Measurement data of each observatory are transmitted to the land stations in real-time using a submarine electro-optical cable. Following phenomena were obtained as results and topics of the observation. 1) deep exchange flow, which was divided in the internal boundary, was observed in the Sagami Bay. 2) As the features of the deep currents, the southwestward current can be observed on the landward slope in the Nankai Trough. 3) A turbidity current was observed two hours after the mainshock of the 2003 Tokachi-oki Earthquake (M8.0) on September 26, 2003. The bottom current speed has reached over 140 cm/sec