New Insights into the Coulomb Stress and Seismicity Rate Changes Induced by the 2008 Mw 7.9 Wenchuan Earthquake

Abstract:

Some new insights into the relationship between the Coulomb stress and seismicity rate changes after the Wenchuan earthquake are gained. Three improvements are made in the calculation including employing a new triangular dislocation element method,setting up a much more realistic model and resolving stress change on variable focal mechanisms. The calculated Coulomb stress changes indicate that not only the three major faults systems including east Kunlun,southern part of Xianshuihe and Northern Min Jiang, but southern rim of Qinling, parts of the Tazang and Pingwu-Qinchuan faults were encouraged by the Wenchuan earthquake. Eastern segment of Longriba are shown to be quite dangerous due to dramatic stress increase. Ya’an was rightly located in the southwestern triggering lobe of the main rupture.Spatial distribution patterns of events before and after the Wenchuan mainshock are shown to be quite different . The observed seismicity rate changes were also calculated and compared with the Coulomb stress changes. We found that besides the three major faults systems mentioned above, east Longriba, Daliangshan, Zemuhe ,two ends of the main rupture,and some cities including Ya’an , Chengdu,Chongqing underwent a seismicity rate increase. It is in agreement with positive coulomb stress changes in these regions. The results also indicate that high background seismicity rate will amplify the effect of static coulomb stress increase. It is noticeable that a burst of seismicity near the main rupture with stress drop clearly confirmed the existence of the dynamic waves which were induced by the earthquake and lasted for the first 1-3 months after the mainshock.To check whether an event is responsive to the Wenchuan earthquake, we resolved coulomb stress changes on the focal mechanisms after the Wenchuan earthquakes. We found that only some events with long distance from the main rupture were brought closer to failure. The events with no stress change near the main rupture are associated with dynamic stress triggering and the other ones with no or negative coulomb stress change are much likely unrelated to the Wenchuan earthquake. The focal mechanism for Lushan earthquake received a stress increase by about 0.1 bar which clearly proves that the Wenchuan earthquake has effectively promoted it.