摘要：We studied the effect of the oceanic water layer on strong ground motion simulations. Source faults of
subduction zone earthquakes, such as the Nankai-Tonankai earthquake, West Japan, are situated in the offshore
area, under a thick water layer. The necessity of including the oceanic water layer in the velocity model for
simulations employing the finite difference method is debated by many researchers, and consideration given to
the possibility of neglecting by this layer, which would reduce the computation time and stabilize the calculations.
Although the oceanic water layer has a low velocity and density, it can affect surface wave generation. In this
study, for demonstration purposes, we calculated and compared strong ground motions from three source fault
models, placed into the boundary between the crust and subducting plate, where source rupture of the Tonankai
earthquake is expected. Simulations were made for two realistic three-dimensional velocity models: without and
with the oceanic water layer. The model without the oceanic water layer was constructed simply by subtracting
the depth of the oceanic layer from the depth of all velocity interfaces under the ocean. This procedure keeps
the thickness of the layers (oceanic sediments, surface low-velocity layer, upper crust, and lower crust) the same
as in the model with the oceanic layer and reduces simulation errors. Simulations were made for the set of sites
on a line across the subduction zone and directed to the Osaka basin. The results show that the water layer has
a strong effect on the fundamental mode of the Rayleigh wave, which can be generated by the shallow (approx.
5 km) source. Considering that all asperities of the expected Nankai-Tonankai earthquake are deep (>10 km), we
conclude that the effect of the water layer can be neglected for ground motions at land sites.
