Stress field in the source region after the 2007 Mw 6.6 Niigataken Chuetsu-oki earthquake deduced from aftershock focal mechanisms: Implication for a pre-mainshock stress field

Stress fields in the source region before and after the 2007 Mw 6.6 Niigataken Chuetsu-oki earthquake were investigated using aftershock focal mechanism solutions. We have attempted to determine aftershock focal mechanisms using P-wave polarity data as well as body wave amplitudes because this earthquake occurred offshore, where observation station coverage is poor. This approach enabled us to obtain 76 well-determined aftershock focal mechanisms. Although the stress field in the studied area is known to be generally characterized by a reverse-faulting regime, the application of a stress tensor inversion method to the aftershock focal mechanisms revealed that the stress field spatially varied on a scale smaller than the fault dimension of the mainshock, with a mixture of strike-slip and reverse faulting regimes. The post-mainshock stress field estimated by the stress tensor inversion was compared with the theoretical stress field computed using an observed non-uniform slip distribution of the mainshock and variously assumed pre-shock stress fields. The results of this comparison suggest that the pre-mainshock principal stress in the WNW-ESE direction was dominant and that the magnitudes of the other two principal stresses were similar.