Simulation for Inland Stress accumulation due to Interseimic Coupling in the Southwest Japan Arc

In the Southwest (SW) Japan arc, the Philippine Sea plate descends from the south at the Nankai trough. According to Japanese historical documents, M8-class interplate earthquakes repeatedly occurred with the recurrence interval of 100-200 years. It was also suggested that M7-class inland earthquakes were activated from 50 years before the interplate earthquakes. Considering that 70 years has passed since the last interplate rupture in 1944 and 1946, and recent frequent occurrence of M7 damaging earthquakes in inland areas of SW Japan, we have to reveal the relation between interseimic coupling at the Nankai and stress accumulation in SW Japan to result in activation of inland earthquakes before an interplate earthquake. To this end, we construct a 3-D finite element model (FEM) considering the heterogeneities in viscoelastic structure. Our FEM considers a region of 3700 km x 4600 km x 700 km, incorporating the Pacific and the Philippine sea slabs by interpolating models for the Northeast (NE) and SW Japan arcs, as well as the Kuril, Ryukyu and Izu-Bonin arcs. We incorporated complex geometry of the Philippine Sea slab with the large bend due to the subduciton of the Kyushu-Palau ridge, which could significantly influence modeling of stress field in SW Japan. We utilized the large, land-based Japan GPS array to constrain the model. In particular, the effect of slab rollback is important, which is observed along the Ryukyu trench. The model region is divided into about 1000,000 tetrahedral elements with dimension ranging from 5-100 km. Using a slip distribution from previous studies, we calculated coseismic stress change due to an interplate earthquake, subsequent viscoelastic stress relaxation and interseimic stress accumulation rate. Initial results suggest the following scenario: (1) Immediately after the interplate earthquake, stress that favors to induce inland earthquake abruptly decrease. (2) In the early stage of the earthquake cycles, viscoelastic relaxation dominates to suppress the inland earthquake. (3) In the late stage of the earthquake cycles, stress in favor of inducing inland earthquakes gradually accumulates.