Finite-Difference Simulation of Long-Period Ground Motion for the Nankai Trough Megathrust Earthquakes

Abstract

We evaluated long-period ground motions for the anticipated Nankai Trough megathrust earthquake in southwest Japan. To understand a variation of longperiod ground motions caused by the uncertainty of the source model, we performed a finite difference simulation using 104 source models, assuming various possible source parameters, including rupture area, asperity configuration, and hypocenter location. For the variety of rupture areas, we included scenarios that have extremely huge rupture areas, as was proposed by the Japan Central Disaster Management Council of Cabinet Office after the 2011 Tohoku-Oki earthquake. We also included scenarios that have large slip areas near the trough following the lessons learned from the 2011 event. Simulated waveforms and response spectra show a large variation at a site. However, by grouping the simulation results with respect to the source area, we determined that scenarios with wider rupture areas have a larger peak ground velocity and velocity response than those with smaller rupture areas. The influence of the large slip near the trough causes later phases to be large and long. However, the later phases are decreased by using a boxcarlike slip velocity time function instead of a Kostrovlike function and by decreasing rupture velocity. The spatial distribution of the simulated peak ground velocity and velocity response show that the long-period ground motions are amplified particularly on sedimentary basins, where big cities have been established. It is important to consider how to account for the large variation of the simulation results in the seismic hazard assessment.