Stress Changes Caused By The2008Mw7.9Wenchuan Earthquake

Large earthquakes can change the stress state of their surrounding areas, the study of stress field is very important for the post-disaster reconstruction and hazard assessment. We study the stress field redistribution caused by the Mw7.9Wenchuan earthquake occurred on May12,2008in two aspects:①invert the maximum horizontal compressive stress of Longmen Shan region before and after the Wenchuan earthquake using focal mechanisms,②calculate the co-seismic and post-seismic coulomb failure stress changes on the surrounding faults using the slip-model of Wenchuan earthquake.The Wenchuan earthquake caused heavy casualties and loss of property, the stress state of Longmen Shan region has been extensively studied by researchers at home and abroad after the shock. A lot of results of the Longmen Shan stress field have indicated that some areas might be under earthquake risks after the stress redistribution. The Ya’an Mw6.6earthquake occurred on April20,2013, caused nearly200people deaths, it seems that the stress redistribution caused by Wenchuan earthquake did not end and still requires more researches.Based on numerous researches about the2008Wenchuan earthquake, we now have a more clear geological tectonic setting, more accurate fault parameters, higher resolution slip model and crust model about the post-shock Longmen Shan region. We hope to use these updated information to make a better assessment to the stress redistribution caused by the Wenchuan earthquake.We collect463and143> Ms3.0focal mechanisms occurred in the Longmen Shan region before and after the2008great Wenchuan earthquake respectively. After determining the fault plane according to the historic earthquakes and known fault information, we select the best-fit damping parameters of each three different grid, and inverted the maximum horizontal compressive stress field of the Longmen Shan region before and after the2008Wenchuan event. The whole stress orientations in our results are basically consistent with those of the background stress. However, the stress orientations at some sites rotate slightly after the shock. Near the Pengxian-Anxian fault, Yingxiu fault, North Xianshuihe fault and Mao-Wen fault, the stress orientations are toward NWW-SEE before the shock and E-W after the shock. Near the Mianyang city at northeast of Chengdu city and the junction of Minxian and Zhangxian, the stress orientations rotate counterclockwise significantly. The overall orientation tends to be more coherent after the shock than that before the event. These prove that Wenchuan earthquake has changed the tectonic stress field orientations at some sites.The calculation of coulomb stress change is a very mature method to study the stress redistribution and hazard assessment. In this paper, we study the impacts of the viscoelastic coefficient and friction coefficient to the coulomb stress change, test their sensitivities, and calculate the co-seismic and post-seismic coulomb stress change caused by the Wenchuan earthquake, using an updated high resolution slip model and a redefined receiver faults according to focal mechanisms and slip rate. The result shows:the southeast segment of the Xianshuihe fault has a stress load of10kpa, the Qingchuan, Lushan, Longquanshan and East Kunlun faults have a stress load of10-30kpa, the Longriba fault has a stress load of30-50kpa. Combining the previous aftershock analysis, we conclude that the Longriba, East Kunlun and Longquanshan faults have relatively higher risks of earthquake. Moreover, two major earthquakes occurred near the Longmen Shan region in2013, the Mw6.0Minxian-Zhangxian earthquake and the Mw5.1Mianyang earthquake. These two earthquakes happened in the stress shadow zones of our coulomb stress changes, and cannot be explained by the dynamic stress transfer, but the stress rotations existed in our previous inversion results demonstrate that the stress states at these areas have been changed. Therefore we speculate that the occurrences of these two earthquakes might be related to partial stress rotation caused by the Wenchuan earthquake.Usually, the studies on the static or dynamic stress transfer mainly focus on the stress state changes on local faults caused by large earthquake, ignoring the tectonic stress change. Moreover, the calculation of coulomb stress change can only indicate the relatively change of stress magnitude on certain fault. By combining the information of the stress orientation changes and the coulomb stress changes, we can study the stress field more comprehensively. This will improve our understanding of earthquake occurrence and earthquake hazard assessment.