Crustal shear-wave velocity structure of northeastern Tibet revealed by ambient seismic noise and receiver functions

Abstract

The Tibetan plateau is formed by the persistent convergence between the Indian and Eurasian plates. The northeastern Tibetan plateau is undergoing young deformation that has been noticed for a long time. We conduct a passive-source seismic profile with 22 stations in NE Tibet in order to investigate the crustal shear-wave velocity structure and its relationship with tectonic processes. In this paper we obtain the Rayleigh-wave phase velocity dispersion data among all station pairs within the period bandwidth of 5–20s from the method of ambient noise cross-correlations. Phase velocity variations correlate well with surface geological boundaries and tectonic features, for instance, low phase velocity beneath the Songpan–Ganzi block and the Guide basin. We also compute P-wave receiver functions based on the selected teleseismic events with similar ray parameters, and perform the joint inversion of surface wave dispersion data and receiver functions to obtain the 2-D crustal shear-wave velocity structure along the profile. The inversion results show that low shear-wave velocities beneath the Songpan–Ganzi block are widespread in the middle-to-lower crust. In together with high crustal Vp/Vs ratios and high temperature suggested by the P-wave velocities obtained from the active-source seismic study, we suggest that the low velocity zone beneath the Songpan–Ganzi block is probably attributed to partial melting. Across the North Kunlun fault, there is no crustal LVZ found beneath the Kunlun block. This structural difference may have already existed before the collision of the two blocks, or due to limit of the northward extension for the crustal LVZ across the North Kunlun fault.