Micro-seismicity off Cape Muroto Along the Nankai Trough Seismogenic Zone Detected by a Series of OBS Observations

Micro-seismicity observation is a one of the way to monitor a state of stress in the seismogenic zone. We observed micro-seismicity off cape Muroto at the Nankai trough seismogenic zone using pop-up type ocean bottom seismographs (OBS). The recording period of our OBS's are limited to about two months. We repeated installing and retrieving OBS's. A series of OBS observation have been carried out for nine months in total from 1998. In our observation area, a cable-linked OBS system is installed by JAMSTEC. This system has two seismometers. The data recorded by this system was also used in the analysis. Generally, the seismic velocity structure has a large lateral variation along the subduction zone. We must consider the realistic velocity structure to obtain precise hypocenters. We used 3-D P- and S-wave velocity structure models to locate earthquakes. These models are based on OBS-airgun seismic survey results along the Nankai trough [e.g. Takahashi et al., 1998; Kodaira et al., 2000]. The models represent the characters of the Nankai trough subduction zone, such as subducting seamount off cape Muroto and high Poisson's Ratio in the accretionary prism. The obtained micro-seismicity seems to be classified into two activities; one is at the plate interface and the other is in the uppermost mantle. Seismicity near the plate interface makes several clusters. Although a seismic-aseismic transition zone has not been clearly observed because of the coverage of our OBS array, these clusters may be caused by the stress concentration as a result of the transition of the plate interface state from unlocked to locked. A partially serpentinized mantle is implied by a low P-wave velocity beneath the subducting seamount [Kodaira et al., 2001]. Along their profile, the upper most mantle seismicity seems to be active at the seaward part of this low velocity mantle. The seismicity in the upper most mantle is probably related to the structure and nature of the down going oceanic mantle.