Focal Mechanisms of Wenchuan Aftershocks and Stress Field around Longmenshan Fault Zone

The May 12, 2008, Wenchuan earthquake (Mw=8.0) generated many moderate sized aftershocks throughout the source region of the mainshock. These aftershocks were well-recorded by densely deployed portable instruments immediately following the earthquake, which provide excellent data for the determination of source focal mechanisms and for the investigation of the seismogenic environment and regional stress field. In this study, we use broadband records of the Wenchuan aftershocks at regional distances and determine their source depths and focal mechanisms by fitting the waveforms of body and surface waves. The focal mechanism results are then used in a regional linear stress inversion to estimate the orientation of the stress field around the Wenchuan earthquake source area along the Longmenshan fault zone. Results show clearly that the Wenchuan aftershocks are dominated by thrust faulting with occasional strike-slip faulting towards the two ends of the fault plane of the main shock. The maximum compressional stress in Wenchuan earthquake source area is largely oriented in east-west direction, with a slightly clockwise rotation from south to north.