Micro-seismicity around the seaward updip limit of the 1946 Nankai earthquake dislocation area

We observed micro-seismicity around the seaward updip limit of the 1946 Nan kai earthquake dislocation area by using pop-up type ocean bottom seismogra phs deployed for three months in 1998. At the subduction zone, we must cons ider strong lateral heterogeneity of the seismic velocity structure to loca te earthquakes precisely. We constructed 3-D P- and S-wave velocity structu re models based on the results of airgun-OBS seismic surveys along the Nank ai Trough. We located 83 earthquakes using these 3-D models. These hypocent ers show two groups of seismicity. One seismicity group locates in oceanic layer 3 within a subducting seamount, These earthquakes may reflect the def ormation of the seamount caused by its subduction. The other seismicity gro up locates near the updip limit of the thermally modeled locked zone. These hypocenters are determined to be near the boundary between the accretionar y prism and the subducting oceanic crust.