The implications of regional microseismic activities: A lesson from 2008 Ms. 8.0 Wenchuan earthquake

The Ms. 8.0 Wenchuan earthquake has greatly perturbed the crustal stress in Longmenshan fault zone (LFZ) and its neighboring regions, and hence significantly changed the regional seismicity pattern. Convectional coulomb stress change model based on half-space dislocation source of layered elastic or viscoelastic crust can only provide brief reference to estimate the after-shock sequences roughly. In this study, we apply the Z-test statistical method to examine the earthquake catalogue five years before and after this event to detect the changes of regional seismicity intensity and their statistical significance in LFZ and its neighboring regions. We analyze changes of the Z values with respect to time and seismic magnitude in each region. Our results reveal that, after the Wenchuan earthquake, seismicity intensity has significantly increased in Eastern Kunlun fault, southern Longmenshan fault, Longquanshan fault and Huayingshan fault in Sichuan basin, but has significantly decreased in northern and central Longmenshan fault, vicinity of the Songpan-Minjiang fault, and Fubianhe fault in Bayankala block, respectively. Seismicity rate change in northern and central Anninghe-Zemuhe fault zone is not correlated with the Wenchuan earthquake. These results remain within 99% confidence level of the Z-test statistical method. We compare our statistical analysis results with calculations of coseismic Coulomb failure stress change due to the Wenchuan earthquake from previous studies. In most of the regions and fault zones, the sign of the seismicity rate change agrees with the sign of the Coulomb stress change.