Seismogenic Structure around the Epicenter of the May 12, 2008 Wenchuan Earthquake from Micro‐seismic Tomography

A three‐dimensional local‐scale P‐velocity model down to 25 km depth around the main shock epicenter region was constructed using 83821 event‐to‐receiver seismic rays from 5856 aftershocks recorded by a newly deployed temporary seismic network. Checkerboard tests show that our tomographic model has lateral and vertical resolution of ～2 km. The high‐resolution P‐velocity model revealed interesting structures in the seismogenic layer: (1) The Guanxian‐Anxian fault, Yingxiu‐Beichuan fault and Wenchuan‐Maoxian fault of the Longmen Shan fault zone are well delineated by sharp upper crustal velocity changes; (2) The Pengguan massif has generally higher velocity than its surrounding areas, and may extend down to at least ～10 km from the surface; (3) A sharp lateral velocity variation beneath the Wenchuan‐Maoxian fault may indicate that the Pengguan massif's western boundary and/or the Wenchuan‐Maoxian fault is vertical, and the hypocenter of the Wenchuan earthquake possibly located at the conjunction point of the NW dipping Yingxiu‐Beichuan and Guanxian‐Anxian faults, and vertical Wenchuan‐Maoxian fault; (4) Vicinity along the Yingxiu‐Beichuan fault is characterized by very low velocity and low seismicity at shallow depths, possibly due to high content of porosity and fractures; (5) Two blocks of low‐velocity anomaly are respectively imaged in the hanging wall and foot wall of the Guanxian‐Anxian fault with a ～7 km offset with ～5 km vertical component.