Stress Distribution Near the Seismic Gap Between Wenchuan and Lushan Earthquakes

Abstract

The Wenchuan M S 8.0 earthquake and Lushan M S 7.0 earthquake unilaterally fractured northeastward and southwestward, respectively, along the Longmenshan fault belt. The aftershock areas of the two earthquakes were separated by a gap with a length of nearly 60 km. We have determined the focal mechanisms of 471 earthquakes with magnitude M ≥ 3 from Jan 2008 to July 2014 near the seismic gap using a full waveform inversion method. Normal, thrust and strike-slip focal mechanisms can be found in northern segment. But in a significant contrast, focal mechanisms of the earthquakes in the southern segment are dominated by thrust faulting. Based on the determined source parameters, we further apply a damped linear inversion method to derive the regional stress field. The southern segment is characterized by an obvious thrust faulting stress regime with a nearly horizontal maximum compression that orients in SE–NW direction. The stress environment in the northern segment is a lot more complicated. The maximum compressional stresses appear to rotate around the ‘‘asperity’’ near west of the Dujiangyan city. Stress field also shows strong variation with time and depth. Before 2009, the seismic activities are more concentrated on the Pengxian–Guanxian fault and Yingxiu–Beichuan fault with dominant strike-slip faulting and normal faulting, while after 2009, the seismic activities are dominated by thrust faulting from north to south, while the activities are more concentrated on the Wenchuan–Maoxian fault in northern segment and Pengxian–Guanxian fault in southern segment. The maximum compressional stresses vary in different depths from north to south, thus may imply the decoupled movement in shallow and in depth.