［Abstract］ 　The Plate Dynamics Modeling Research Group is conducting numerical modeling on the rupture processes of megathrust earthquakes occurring along plate subduction zones, using structural models based on seismic surveys and seismicity data. Our research mainly focuses on the Nankai Trough subduction zone and demonstrates (i) that there are several irregularities on the surface of the subducting Philippines Sea Plate and (ii) these irregularities may have significant impacts on the rupture process of the megathrust earthquakes occurring along the Nankai Trough. ［Surface Shape of Philippines Sea Plate］ 　Figure 1 shows the surface topography of subducting Philippines Sea Plate obtained in this study. The key features of the plate model are; (i) isolines of the surface topography are deflected landward near the Tosa-Bae basin off Muroto, (ii) the distribution of isolines is dense to the east of the Kii Peninsula, while it is sparse to the west of the peninsula. 　Figure 1: Model of surface topography of the subducting Philippines Sea Plate obtained by Baba et al. [2001]. This model is developed by the compilation of the results of seismic surveys [e.g. Kodaira et al., 2000] and hypocentral distributions of micro-earthquakes [Nakamura et al., 1997]. The thick red lines indicate the isolines of the depth to the surface of the subducting plate in units of km. The thin black lines indicate the survey track of seismic surveys used in the compilation of this study. ［Tear in the slab beneath the Kii Peninsula］ 　Figure 2 shows a plan view of the micro-earthquake activity beneath eastern Shikoku and the Kii Peninsula and two cross-sections along the direction of plate convergence. As can be seen from the figure; there is a clear abrupt change in the distributions of hypocenters along the direction of plate subduction, indicating a change in the apparent dip of the plate subduction, possibly a "kink" or "tear" in the slab along the line separating the two cross sections. Figure2: Plan view and vertical cross-sections of the distributions of the hypocenters of micro-earthquakes beneath the eastern Shikokuand the Kii Peninsula obtained by the Nankai Net [Nakamura et al.,1997]. ［Subducted seamount beneath Tosa-Bae Basin］ 　Figure 3 shows the result of the seismic survey by refraction tomography conducted along the KY9903 track. As can be seen from the figure; (i) there exists a bulge in the subducting oceanic crust beneath the Tosa-Bae basin, indicating a buried seamount in this region, (ii) the buried seamount is attached to the region of high P-wave velocity landward, which is thought to represent the island-arc crust. 　Figure 3: The P-wave velocity structure beneath the KR9907 track obtained by seismic tomography and its geological interpretation [Kodaira et al., 2000]. The region below the white broken line indicates the region which was not sampled by seismic rays. ［Rupture process of 1946 Nankaido Earthquake］ 　Figure 4 shows the rupture process of 1946 Nankaido Earthquakes obtained by our analysis of teleseismic [Cummins et al., 2001] and tsunami [Baba et al., 2001] records. Our analysis has demonstrated that the key features of the rupture propagation are; (i) the rupture did not propagate eastward of the Kii Peninsula over the "tear" in the subducting slab, (ii) westward of the subducted seamount, rupture occurred only in the landward part of the inferred seismic zone. 　Figure 4: The slip distribution along the plate boundary of 1946 Nankaido Earthquakes obtained by the analysis of teleseismic [Cummins et al., 2001] and tsunami [Baba et al., 2001] records. The thick, blue broken line indicates the location of the "tear" in the subducting plate, while the hatched region indicates the location of the subducted seamount.