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

Abstract

Stress fields in the source region before and after the 2007 M w 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.

Depth-dependent stress field in and around the Atotsugawa fault, central Japan, deduced from microearthquake focal mechanisms: Evidence for localized aseismic deformation in the downward extension of the fault

“…In the present study, we determined focal mechanism solutions of microearthquakes using absolute P and SH wave amplitudes as well as P wave polarity. The same approach was used by Imanishi et al [2006a, 2006b] and Imanishi and Kuwahara [2009], and was shown to be effective for microearthquakes. We analyzed earthquakes having at least eight P wave polarities.…”

Crustal Deformation Process in the Mid‐Niigata Region of the Niigata‐Kobe Tectonic Zone as Observed by Dense GPS Network Before, During, and After the Tohoku‐Oki Earthquake

“…If a shallow aseismic fault is the only continuation of such a deep fault to the shallow portion, the shallow creeping can continue. This idea is also supported by seismological observation indicating no significant change in the earthquake source mechanisms before and after the 2004 Chuetsu, the 2007 Chuetsu‐oki, and the 2011 Tohoku‐oki earthquakes (Imanishi & Kuwahara, ; Kato et al, ; Yoshida et al, ). The only apparent inconsistency is the degree of elastic heterogeneity.…”