Fine seismic structure under the Longmenshan fault zone and the mechanism of the large Wenchuan earthquake

In this study we used a large number of P-wave arrival times recorded by the Sichuan permanent and portable seismic stations during January 2001 and June 2008 to obtain a fine threedimensional P-wave tomographic model of the crust around the Longmenshan fault zone. The results show that there exist obvious differences between south and north of the Wenchuan mainshock in the seismic structure. North of the Wenchuan mainshock there exist much more heterogeneities, where many aftershocks occurred. These results have significantly improved over the previous results suggesting that the Longmenshan fault zone is only a transition zone between different blocks. The source area of the Wenchuan mainshock is underlain by obvious low velocity anomalies, suggesting that there exist fluids within the Longmenshan fault zone. These fluids may affect the occurrence of the large Wenchuan earthquake. The present results provide significantly seismological evidence for the upward intrusion of the lower crustal flow along the Longmenshan fault zone.