Crustal Structure Of South Longmenshan Fault Zone From Teleseismic Receive Functions

The worldwide study on the Wenchuan M 8.0 Earthquake (May 12, 2008) has great improved our understanding about the Longmenshan fault zone, although there are still many problems to be further investigated. For example, the rupture process of the Wenchuan earthquake propagated northeastward from its hypocenter, and its aftershocks were mainly occurred at the middle and north segment of the Longmenshan fault zone, with very sparse seismicity observed in the south segment of the fault zone. This study uses receiver functions from a transportable seismic linear array deployed in the south segment of the Longmenshan fault zone after the Wenchuan earthquake to investigate the crust structure. The Moho depths and the Vp/Vs ratios of the study area are determined using H-Kappa searching and CCP (Common Conversion Point) stacking methods.Our results suggest: 1) The Songpan-Ganzi block and the Yangzi block converged near vertically around the Pengguan fault. The Yangzi block shows clear interfaces among upper crust, middle crust and lower crust, but the Songpan-Ganzi block is not well layered. 2) Moho is shallower (~50 km) in the middle and the south parts along the array, and increases to around 70 km in the north part, which is consistent with the Moho vaiations observed in the middle and north segment of Longmenshan fault zone. 3) Relatively high Vp/Vs ratios are observed in the collision zone of the Songpan-Ganzi block and the Yangzi block. On average, the Vp/Vs ratios of the Songpan-Yangzi block are around 1.75, and that of the Yangzi block are around 1.65. 4) The lower seismic activities in the south segment of the Longmenshan fault zone maybe attribute to the hard crust of the Yangzi block and the convergent pattern between the Songpan-Ganzi and the Yangzi blocks in this area where is not easy to accumulate stress.