Seismological monitoring on the 2003 Tokachi-oki earthquake derived from permanent OBSs and land-based observation - a challenge in monitoring M8 earthquake on the ocean floor

In July 1999, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) installed a cabled geophysical observatory system off Kushiro, southeastern Hokkaido Island, Japan. This observatory system comprises three ocean bottom seismographs (OBSs), two tsunami gauges, and a cable-end environmental monitoring system, connected with a 240 km long fiber optical cable. Processing OBSs and land-based data together, and comparing magnitude common recorded with that determined by Japan Meteorological Agency (JMA), we found event detection level was improved down to magnitude 1.5, which is much lower than the previously designed as down to magnitude around 2. We compared detection level before and after installing OBSs, and found dramatic improvement on the earthquake detection level after installation of the cabled system. Four years and two months after the installation, a megathrust earthquake (The 2003 Tokachi-oki earthquake, MJMA 8.0) occurred just beneath the system. The system recorded clear unsaturated seismograms just at 28.6 km from the epicenter, which is the first observation on the ocean floor recording an earthquake with magnitude eight in the world. This paper reports hypocenter distribution derived from permanent cabled OBSs and land-based observation in the period from mainshock of the 2003 Tokachi-oki earthquake, to middle of May 2004. In the large slip area of the mainshock, a planar, with ten-degree dip, hypocenter distribution is obtained. Another deep planar seismic zone is found about 20 km depth from the plate interface. We think that the geophysical observations helps to understand the initiation process of the rupture of the 2003 Tokachi-oki earthquake and that observations including seismological, geodynamic, hydrogeological, and the other multidisciplinary observations would provide a clue to future understanding of seismogenic processes at southern end of the Kurile subduction zones