The Crustal Poisson Ratio and S‐Wave Velocity Structure Beneath the NE Tibetan Plateau Inferred from Receiver Function and Their Implications

Abstract

AbstractWe selected broadband teleseismic waveform data recorded by the Gansu Digital Seismic Network to study the crustal and upper mantle structure beneath the Northeastern Tibetan Plateau. The crustal thickness and VP/VS ratio are estimated by the grid‐search method using crustal conversion and multiples, assuming an average P‐wave velocity in the crust. Teleseismic P waveform modeling is carried out to investigate the S‐wave velocity structure, assuming the fixed VP/VS value inferred from the grid‐search method. Our results show that the crust is 50~55km thick beneath Qilian terrane, and about 45 km beneath the Hexi Corridor and Qaidam terrane. The VP/VS ranges from 1.66~1.85, and the Poisson's ratio is about 0.215~0.294. Our results also show that the Moho interface is sharp beneath the Northeastern Tibetan Plateau. Both of the low‐to‐normal Poisson's ratio and the sharp Moho do not support the argument of significant partial melting in the upper‐to‐middle crust and underplating beneath the lower crust of the Northeastern Tibetan Plateau. The existence of anticorrelation between VP/VS and the crustal thickness strongly suggests that the crust is mainly composed of felsic mineral in the northeastern Tibetan crust. The seismic evidence may indicate that the thickened crust, mostly in its upper part, is probably due to the successive intracrustal thrusts.