The accretionary prism in the subduction zone, which consists of thick low-velocity oceanic sediments, significantly affects the propagation of seismic waves for shallow, offshore earthquakes, including large interplate earthquakes. In order to simulate long-period (> 5 s) ground motions in the Nankai subduction zone, we constructed a three-dimensional (3D) seismic velocity structure model of the accretionary prism by interpolation/extrapolation of local S-wave velocity structures beneath 46 oceanfloor seismic stations (DONET), which are deployed just above the accretionary prism off the southern Kii and eastern Shikoku regions. We modeled local S-wave velocity structures using a simple two-parameter depth-varying velocity function. To investigate the effects of the accretionary prism on ground and seafloor motions, we conducted numerical simulations of seismic wave propagation for three local earthquakes that occurred in southwestern Japan. The simulations reasonably reproduced the observed seismograms, not only for the period ranges of the moment tensor inversion (~ 50 s), but also for the strong, long-period ground motions in the sedimentary basins (~ 5 s), especially in the region where DONET stations are densely deployed. Since depth-varying, local S-wave structures significantly improve the reproducibility of long-period ground motions, our modeling procedure is useful for modeling long-period ground motions of local and regional offshore subduction zone earthquakes.
