Local stress concentration in the seismic belt along the Japan Sea coast inferred from precise focal mechanisms: Implications for the stress accumulation process on intraplate earthquake faults

Spatial changes in the azimuth of the maximum principal stress (sigma(1)) axis are found in the intraplate region of southwest Japan. It is inferred from stress inversions of numerous precise focal mechanisms that the azimuths of the sigma(1) axis in the seismic belt along the Japan Sea coast are oriented in N110 degrees E-N130 degrees E directions, while in the surrounding region they are aligned in almost the EW direction, N90 degrees E-N100 degrees E. Maximum horizontal compressional stresses in an EW direction are widely observed in the shallow crust in the inland plate along the Nankai Trough. However, the maximum horizontal stress in a WNW-ESE direction is seen only in the seismic belt. This spatial change in the stress field in and around the seismic belt is explained by the deformation of an aseismic fault or a ductile fault zone in the lower crust beneath the seismic belt.