Stress fields and fault reactivation angles of the 2000 western Tottori aftershocks and the 2001 northern Hyogo swarm in southwest Japan

We estimated the stress fields of the aftershocks of the 2000 western Tottori earthquake (M_w 6.6) and the northern Hyogo swarm (max M_w 5.2) by a stress tensor inversion of moment tensor solutions reported from the National Research Institute for Earth Science and Disaster Prevention (Japan). The maximum principal stress direction of the western Tottori sequence was estimated as N107°E with a strike–slip regime. In the northern Hyogo swarm, the orientations of the principal stress directions could not be well constrained by the observed data, but after examining the detailed characteristics of the solution, we obtained a most probable solution of N113°E for the σ₁ direction. These solutions are consistent with the maximum horizontal directions roughly estimated from the strike directions of large earthquakes occurring geographically between these two seismic activities. We measured the angle between each fault–slip direction and maximum principal stress direction to investigate the frictional properties of earthquakes. The distribution of the angles was forward modeled to estimate the coefficient of friction and the stress ratio, assuming uniformly distributed fault orientations. For the western Tottori sequence, a homogeneous stress field with a coefficient of friction less than 0.4 was estimated. A high stress level was also suggested because very little change occurred in the stress field during the mainshock. For the northern Hyogo sequence, the coefficient of friction was estimated to be between 0.5 and 1.0.