Aftershock distribution of the M_S 8. 0 Wenchuan earthquake and three dimensional P-wave velocity structure in and around source region

3-D P-wave velocity structure in crust and uppermost mantle in and around Wenchuan earthquake source region was studied using long-term accumulated seismic travel time data in Sichuan-Yunnan region and aftershock data of Wenchuan earthquake. The result shows that shallow P-wave velocity structure has good correlation with surface geology. Longmenshan fault zone is imaged as high P-wave velocity region in 0-20 km depth. Pengguan complex and Baoxing complex are imaged as two local high velocity anomaly bodies. The upper crustal high velocity anomaly bodies in Longmenshan fault zone control the distribution of aftershocks. At the southern part of the aftershock zone, aftershocks occurred only in the northeast of the high velocity body related to Baoxing complex. In the middle part, the distribution of aftershocks seems to be controlled by the high velocity anomaly body corresponding to Pengguan complex to some extent; In the northeast part, the high velocity body around Ningqiang-Mianxian may prevent the further extension of aftershocks to the northeast. The existence of upper crustal high P-wave velocity zone in Longmenshan implies that the upper crust has relatively high strength, which may play an important role in obstructing the extrusion of Tibetan Plateau material to east, and are prone to accumulate energy in deep depth. Yangzi Block is characterized by high velocity region below 30 km depth, and its front extends to Tibetan Plateau with depth increasing and reaches to the west of Longmenshan fault zone in the lower crust and upper mantle.