We observed micro-seismicity around the seaward updip limit of the 1946 Nankai earthquake dislocation area by using pop-up type ocean bottom seismographs deployed for three months in 1998. At the subduction zone, we must consider strong lateral heterogeneity of the seismic velocity structure to locate earthquakes precisely. We constructed 3-D P- and S-wave velocity structure models based on the results of airgun-OBS seismic surveys along the Nankai Trough. We located 83 earthquakes using these 3-D models. These hypocenters show two groups of seismicity. One seismicity group locates in oceanic layer 3 within a subducting seamount. These earthquakes may reflect the deformation of the seamount caused by its subduction. The other seismicity group locates near the updip limit of the thermally modeled locked zone. These hypocenters are determined to be near the boundary between the accretionary prism and the subducting oceanic crust. ¿ 2001 American Geophysical Union