Radial anisotropy in the crust of SE Tibet and SW China from ambient noise interferometry

We use Rayleigh and Love wave Green’s functions estimated from ambient seismic noise to study crustal structure and radial anisotropy in the tectonically complex and seismically active region west of Sichuan Basin and around the Eastern Himalaya Syntaxis. Seismic noise records are from 372 stations of 3 seismic arrays: MIT and Lehigh arrays (Nov 2003- Aug 2004), and CEA array (Jan 2007 - Dec 2007), with average inter-station spacing from 10km to 70 km. The 3-D variations of shear wave speed inferred from Rayleigh and Love waves are generally consistent. In agreement with previous studies, low velocity zones are ubiquitous in the mid-lower crust, with substantial variations both laterally and vertically. Discrepancies between 3-D shear velocity from either Rayleigh (V_SV) or Love (V_SH) waves are examined both in view of non-uniqueness of tomographic solutions and radial anisotropy. Low shear wave speed and radial anisotropy with V_SH > V_SV are most prominent in mid-lower crust in the Lhasa block, the southern Songpan-Ganze block and the western part of Yangtze block. Furthermore, in the middle crust we infer a (weak) negative correlation between V_SV and radial anisotropy, with low V_SV and strong radial anisotropy (V_SH > V_SV). Our results are consistent with flow-induced horizontal mica fabric in crustal zones of relatively low mechanical strength.