Abstract
Great interplate earthquakes have repeatedly occurred in pairs along the Nankai trough. Ruptures started to the east of the Kii Peninsula and propagated to the west within a few years, and, especially for the last three events, the delay within pairs of earthquakes gradually increased as the magnitude and recurrence interval of events decreased. Recent numerical simulations of the earthquake cycles, based on a plane fault model with depth-dependent frictional properties and anomalous structure beneath the Kii Peninsula, have qualitatively reproduced the characteristics of earthquakes along the Nankai trough. We used a similar numerical simulation to examine the effect of the 3D geometry of the Philippine Sea (PHS) plate on the earthquake cycles. To the east of the Kii Peninsula, where the PHS plate is relatively flat, the stress increase rate is similar to that of the plane fault model. In contrast, the stress increase rate west of the Kii Peninsula is considerably lower; in this region there is marked flexure of the PHS plate. Furthermore, east-west gaps in the stress increase rates of the 3D plate model greatly increase the delay time (by∼60 years) between simulated Tonankai and Nankai earthquakes. However, we tested our 3D plate model with minor adjustments of the frictional parameters beneath the Kii Peninsula (reducing characteristic slip distance by only a few percent) and showed that the earthquake cycles thus simulated were nearly the same as those of the plane fault model. Therefore, frictional properties at the boundary between Tonankai and Nankai earthquakes have greater influence on earthquake occurrence patterns than stress changes related to 3D plate geometry.
