TECTONIC STRESS CHANGES AROUND THE EPICENTER BEFORE AND AFTER THE 2008 WENCHUAN EARTHQUAKE

The 2008 Wenchuan earthquake started at the intersection of the south and central segments of the Longmenshan Fault. Its aftershocks extended northward to the whole central and north segments of the Longmenshan Fault. The seismicity of the aftershocks along the central and north segments of Longmenshan Fault zone decreased very quickly. Within one year after the Wenchuan earthquake,the number of aftershocks per month with magnitude 3. 5 is greater than 10,indicating that there was a strong tectonic stress adjustment around the epicenter region during this period. However,after April of 2009,the number of aftershocks per month with magnitude 3. 5 is less than 10,indicating that the tectonic stress adjustment became more stable. Even after the Ya'an Lushan earthquake occurring in the south segment of the Longmenshan Fault in April,2013,the seismicity in the central and north segments does not increase obviously. In the present study,we firstly calculated focal mechanisms of micro-earthquakes from about one year of seismic observations recorded by 26 stations located around the main-shock epicenter. We then made an analysis on the tectonic stress changes before and after the main shock,based on our calculated focal mechanism solutions and previous results of focal mechanisms and in-situ stress measurements. The analysis results show that the maximum principal stress direction close to the epicenter changes from near E- W before the main shock to near N- S after the main shock. The focal mechanisms indicate that the Dujiangyan-Yingxiu-Gengda Fault,separating the middle segment with the south segment of the Longmenshan Fault zone,and the region to its north have a trend of northeastward movement. The northeastward movement may be one of the reasons triggering the Ya'an Lushan earthquake which occurred in the middle of the south segment of the Longmenshan Fault zone in 2013.