Study on the effect of the oceanic water layer on strong ground motion simulations

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.