Seismic Anisotropy Across the Longmen Shan Mountain Range From a Passive Seismological Survey.

G. Herquel¹,  A. Robert², J. Vergne¹, J. Zhu³.

1. E.O.S.T. , 67000  Strasbourg 
2. Laboratoire de géologie de l'ENS, 75231 Paris Cedex
3. Institute of Technology of Chengdu, China

Located between the eastern margin of the Tibetan plateau and the Yang Tse craton, the Longmen Shan mountains range is a key area for understanding mechanisms that control the deformation and the eastward extrusion of the Tibetan plateau. This context motivated several French institutes and the University of Chengdu to set up a seismic network across the Longmen Shan to determine the patterns of the lithospheric deformation in this region. The profile, composed of 36 stations with a mean inter-station spacing of 10km, was deployed in several phases from november 2005 to april 2007 and ran from the Sichuan basin, across the Longmen Shan belt, the flyschs of the Songpan Ganze and up to the Xianshuehe fault..

Here, we present first results from this experiment about the anisotropy within the lithosphere based on shear wave splitting measurements. 41 clear SKS and SKKS phases from 23  teleseismic events were recorded during the two periods of deployment and selected through visual inspection. We used the cross-correlation method to calculate the splitting parameters, that is, the azimuth of the past polarization direction and the delay time between the split phases arrivals. We show that 1) The polarization directions are coherent in all the studied zone. They are compatible with previous observations, GPS measurements and with the main known surface features like the Xianshuehe strike-slip fault.. Strikingly, no significant change is observed between the Longmen Shan region and the eastern part of the Yang-Tse craton; 2) The measured delays are small and don’t reflect important asthenospheric flow. Some measurements of S splitting near the Beichuan-Weichuan fault system show normal anisotropy for the crust (around 0.05 to 0.1s).