Rapid changes of the aftershock P axes 3 years after the 1995 Hyogo-ken Nanbu (Kobe) earthquake

Aftershock focal mechanisms of the 1995 Hyogoken Nanbu earthquake (M-w=6.9) are analyzed to detect temporal variations in the stress field on the Noji ma fault and the Rokko fault system, which ruptured during the mainshock. F ollowing the mainshock, we observe temporal changes in the P axes of afters hocks. Around the Nojima fault, P-axis orientations of aftershocks which oc curred in 1996 and. 1997 are almost normal to the fault. In contrast, after shocks which occurred after 1998 have P axes consistent with the regional s tress. Around the central part of the Rokko fault system, P-axis directions of aftershocks which occurred in 1996 with focal depths less than 10 km ar e almost normal to the faults. In contrast, aftershocks which occurred afte r 1997 have P axes consistent with the regional stress. These results indic ate that the Nojima fault and the Rokko fault system started to accumulate significant shear stress in mid-1998, only 3 years after the mainshock.