Long-term seafloor and land-based se\x7fsismological monitoring at southwestern end of the Kurile subduction zone, based on more than six-year continuous observation

Around Japanese islands, most of subduction-related megathrust earthquakes occur offshore seismogenic zone, and are difficult targets in seismological and disaster mitigation studies. In June 1999, JAMSTEC installed a seismological/geophysical cabled observatory with length of approximately 240 km, off Kushiro-Tokachi, southeastern off Hokkaido, Japan. The system locates near southwestern end of the Kurile subduction zone, where megathrust earthquakes are taking place repeatedly. For in-situ seismological/geophysical monitoring at plate boundary, which is one of the leading hopes to investigate the subduction process and related phenomena in detail, we reviewed more than six-year automatic event-based seismological data since September 2000, when a continuous recording and related automatic event detection against cabled seismological data combined with land-based observation started. Above long-term observation includes onset of a megathrust earthquake (2003 Tokachi-oki earthquake, M8.0) just beneath our cabled system and the following intense aftershock activity, which we first experienced on the ocean floor. In this paper, we summarize several remarks based on the results, including experiences with our observation.