Multi-scale,finite-frequency travel-time tomography of the crust and upper mantle structure in the Longmenshan region

Shu-Huei Hung(洪淑蕙),Biao Kuo,Qi-Yuan L iuand Jiu-Hui Chen

1)Department of Geosciences,Taiw an University,Taipei 10617 2)State Key Labo rato ry of Earthquake Dynamics,Institute of Geology,CEA,Beijing 100029

With an innovative technique of multi-scale,finite-frequency tomography,we present results of P-wave velocity structure of the crust and upper mantle in the Longmenshan region,where the devastating magnitude 7.9 Wenchuan earthquake occurred on 12 May,2008.Between 2006 and 2008,Institute of Geology of China Earthquake Administration deployed a broadband array of 175 densely-spaced stations(～4—50 km apart)in the study region which covered the high-relief Chuan-Dian and Songpan-Ganzi blocks in the eastern margin of the Tibetan Plateau and the flat western Sichuan basin.Relative P-wave traveltime residuals from recorded teleseismic events are used to construct 3-D variations in the speed of P wave(lnVP)down to the depths of 450 km.The resulting images show strong anomalies of high VP extensively underlies the Sichuan basin,suggesting that underneath exists a mechanically-competent,thermally-cold continental lithosphere.The cratonic lithosphere is about 200 km thick to the south of 30.5°N and gradually thinned to 150 km to the north.Strong low VP(lnVP＜ -3%)layers found in the lower crust and uppermostmantle(～50—100 km)beneath the Songpan-Ganzi and Chuan-Dian blocks are indicative of the channel-like lower crustal flow being likely p resent.The warm lithospheric mantle and ex-tensive crustal flow probably provide the dynamic support of up lift of the eastern margin of the Tibetan Plateau.P wave speeds in the lithospheric mantle under the NE margin of Chuan-Dian Block along the Xianshuihe fault appear to be faster than the surrounding region.This boundary fault zonemayact to accommodate the extrusion of the eastern Tibetan Plateau.A broad swath of fast anomalies exists extensively under the northern Chuan-Dian block and the S W edge of the Songpan-Ganzi block at the dep ths greater than 300 km.The feature may be associated with either the no rthw estw ard subduction of the Sichuan mantle lithosphere or delaminated Eurasian lithosphere.The crust and lithospheric mantle along the Longmenshan fault exhibit distinct lateral variations in VP across Wenchuan(31°N).To the S W,the velocity decreases abruptly to be relatively slow,imp lying that the deformed crustal rocks in the continental margin of the Songpan-Ganzi block may have been extruded into the Sichuan basin.To the NE,the crust consistently has very high VP and behaves brittle,resulting in that the Wenchuan earthquake ruptured toward northeast and the aftershocks were only clustered along the northern segment of the Longmenshan fault.