Ocean Bottom Seismometer Handled by Submersible Vessel and Its Observation Prior to the 1993 Hokkaido Nansei-Oki Earthquake

Abstract

Nowadays, most ocean bottom seismometers (OBS) are of the free-fall and pop-up type. For the observations, they are dropped from ships and self land on the flat sea floor where generally thick sediments cover basement rocks. However, soft sediment affects and distorts incoming seismic signals. If we could place ocean bottom seismometers on hard rock, it could produce high quality seismograms which contained more information on structure and earthquake sources. For this purpose, a submersible vehicle is necessary as it can carry and place an OBS properly on exposed sea floor hard rock. We designed a new type of OBS which can be handled by a submersible vehicle. This new OBS contains instruments within two aluminum cylinders with syntactic foam blocks outside the cylinders for buoyancy. The submersible vessel, Shinkai-6500, was used to set the OBS to observe seismicity in the Okushiri Ridge Area. It was placed on a relatively flat and hard basement outcrop at a depth of 3338m. The OBS was released to the sea surface through a self pop-up system after two days of observation. During this observation period, we noticed very high micro-earthquake activity around the Okushiri Ridge area where one year later the Hokkaido-Nansei-Oki earthquake of M7.8 took place. Such seismic activity could not have been detected by land-based seismic networks alone.