Seismic Velocity changes in the Longmenshan before and after Wenchuan earthquake

To investigate seismic velocity changes in the seismogenic layer of Longmenshan area, a new 2D tomography method (Pei and Chen, 2012, BSSA) was applied to Pg travel time data before and after 12 May 2010 Ms=8.0 Wenchuan earthquake. A high-resolution two-dimensional (horizontal) seismic velocity model was obtained using this simple tomography method including both station and event depth corrections. Uncertainty of event depth can be removed efficiently in this method and therefore the tomographic results should be more accurate. The results show clear velocity decrease before and after Wenchuan earthquake in the upper crust. The average Pg velocity decrease can reach 2% from 5.88km/s to 5.77km/s in entire Longmenshan area. It is much larger than velocity changes obtained from ambient noise cross correlation method. The tomographic results show the details of velocity change distribution. Large velocity decrease exists in the southern part of Longmenshan fault zone, which probably resulted from broken rocks in the fault zone and rocks dilatancy after relaxation of stress. In contrast, clear velocity changes do not exist in northern part of the fault zone, but two sides of the fault. Seismic velocity decreases in the east side and increases in the west side of northern Longmenshan fault, maybe because of right-lateral strike-slip motion here which results in extension in the east side and compression in the west side.