Bottom response to a tsunami earthquake: Submersible observations in the epicenter area of the 1993 earthquake off southwestern Hokkaido, Sea of Japan

Abstract

We conducted a research campaign of nine dives using the manned submersible vehicle Shinkai 6500 in 1995 beneath the central Okushiri Ridge along the eastern margin of the Sea of Japan. The main objective was to investigate coseismic response and bottom disturbance to the tsunamigenic 1993 Offshore Southwestern Hokkaido earthquake (Ms 7.8; 42°47′N, 139°12′E) in the northern aftershock area including the mainshock epicenter. The whole aftershock area occupies the western half of source area of the related tsunami. Between the Okushiri Ridge and the neighboring Shiribeshi Trough to the east, we found a gentle scarplet of the earthquake fault that bounds the first terrace and a zone of open gashes on the flat terrace. A number of sand blows, open cracks, and other slope failures occupied the slope of the Okushiri Ridge on the uplifted hanging wall side of the tsunami source area. Some fractures and holes whose activity had ceased soon after the earthquake began to be buried by the 2‐year accumulation of recent sediments. In contrast, the Shiribeshi Trough, on the footwall of the fault and in the subsided domain of the tsunami source area, suffered no remarkable disturbance other than gentle folding. Our submersible observations elucidated the seafloor response to tsunamigenic uplift of the hanging wall of a seismogenic thrust fault which displays the present time section of back‐thrusting with an embryonic subduction along the eastern margin of the Sea of Japan.