Structure of the Crust and Mantle Down to 700 km Depth beneath the Longmenshan From P Receiver Functions

Abstract

Since the Ms 8.0 Wenchuan earthquake occurred in 2008 in the Longmenshan at the eastern margin of Tibet, this region has attracted much attention. In order to investigate the deep structure of the region, an analysis of P receiver functions was carried out, using data from an array of 80 broadband seismic stations operated in 2012 and 2013, and straddling the Longmenshan fault zone around the epicenter of the Wenchuan earthquake. Positive Ps conversions from 5 to 10 km depth indicate the base of the Mesozoic sediments in the Sichuan basin and the base of the Songpan-Ganzi flysch deposits in Tibet. Later positive Ps conversions represent the Moho, which occurs at 40–50 km depth below the Sichuan basin and deepens to the NW to 55–70 km depth under Tibet. An important finding of this study is that whereas in the NE, the Moho deepening from SE to NW is smooth, in the middle and SW of the region the Moho deepening is abrupt with, in some places, a possible overlap. Negative Ps conversions at stations in Tibet represent the top of low shear velocity zones in the middle and lower crusts below Tibet. Beneath the Sichuan basin, a negative phase about 1 s earlier than the Moho Ps conversion is to a large extent, almost certainly due to multiples from discontinuities in the upper crust. Finally, positive Ps conversions can be observed from the 410 and 660-km discontinuities at the top and bottom of the mantle transition zone, respectively.