Structural heterogeneity of the Longmenshan fault zone and the mechanism of the 2008 Wenchuan earthquake (Ms 8.0)

We determined detailed 3-D images of P and S wave velocity (Vp, Vs) and Poisson's ratio (sigma) in and around the Longmenshan (LMS) fault zone by using a large number of P and S wave arrival times from the aftershocks of the 2008 Wenchuan earthquake (Ms 8.0) and other local events. Our results show that the structure of the LMS fault zone north of the Wenchuan main shock is very different from that south of the main shock. The southern section of the LMS fault zone contains a broad zone with low-Vp, low-Vs, and high-sigma anomalies, while the northern segment exhibits more scattered heterogeneities, corresponding to most of the aftershocks which occurred there. A prominent low-Vp, low-Vs, and high-sigma anomaly exists directly beneath the Wenchuan main shock hypocenter, suggesting that in addition to compositional variations, fluid-filled fractured rock matrices may exist in the LMS fault zone, which may have influenced the generation of the large Wenchuan earthquake. Our tomographic results provide sound seismic evidence for the hypothesis that an upward intrusion of lower crustal flow occurred along the LMS fault zone. In addition, most small earthquakes before the 2008 Wenchuan main shock occurred around the Guanxian-Jiangyou fault, while the Wenchuan aftershocks were mainly concentrated on the Yingxiu-Beichuan fault, suggesting that the rupture process of the Wenchuan earthquake may belong to an out-of-sequence thrusting event, a suggestion which is in good agreement with the results from geological surveys and also quite similar to the rupture processes of the 1999 Chi-Chi earthquake (M 7.5) and the 2005 Kashmir earthquake (M 7.6). A few aftershocks occurred close to the blind Guangyuan-Dayi fault in the Sichuan basin, suggesting that this blind fault was also ruptured by the Wenchuan earthquake, consistent with geological surveys.