In the Nankai Trough, southwest Japan, the Philippine Sea plate descends beneath the Eurasian plate. The locking on the plate interface causes stress change in the seismogenic lithosphere of the Eurasian plate. This interplate locking does not always result in stress accumulation but can cause stress release. The present study employs shear strain energy to determine whether the stress is accumulated or released in the seismogenic zone. While most studies employed a shear stress assuming a fault plane, the shear strain energy is more feasible for the stress accumulation/release mapping because the assumption of the fault plane is not required. First, we estimated the directions of the principal stress axes and the stress ratio of the background stress field by analyzing ~8,000 focal mechanisms. In the Chugoku region, the maximum compression axis is in the E-W direction. We then calculated the stress change due to the slip deficit along the Nankai Trough. The stress change showed that the maximum compression is in the N-S direction in the Chugoku region. By using both the background stress and the stress fluctuation, we estimated the shear strain energy change caused by the slip deficit. The shear strain energy is decreased due to the interplate locking in the Chugoku region. We also statistically compared the shear strain energy change with the seismicity in southwestern Japan and found that the seismicity is high where the strain energy increases.
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