Coherent crustal deformation in SE Tibet revealed by Rayleigh wave azimuthal anisotropy from ambient noise

The SE margin of the Tibetan Plateau, located in proximity to the assumed transition between the deformation regime of Tibet and that of Yunnan and south China, is a vital region to study evolution and deformation of the Tibetan plateau. In this study we present Rayleigh wave azimuthal anisotropy in the period range of 4-30 s in Yunnan and adjacent region, SW china. Ambient noise data recorded by 401 stations from 3 temporal seismic arrays and regional seismic network are used for analysis. The 1°×1° checkerboard model of azimuthal anisotropy can be well recovered. Our results reveal widespread low velocity zones (LVZs) in the middle and lower crust (12-30 s) of SE Tibetan plateau. The LVZs are located in Central Yunnan block and Eastern Yunnan block and obscured by high velocity zones in Panzhihua-Xichang rift and Nanpanjiang basin. The phase velocity map at 30 s is consistent with that of the Moho depth variation. Combined with receiver function h-k results, magnetotelluric soundings and heat flow data, we find the LVZs are not spatially correlated with the locations of high Poisson's ratio (>0.30), high conductivity and high heat flow. Our results do not support lower crustal flow with large areas in Yunnan. Lower crustal flow may only exist in local areas, such as the north and south end of Xiaojiang fault zone. The fast directions at different periods are predominately North-South orientation in central Yunanan and are consistent with the fast polarization direction of receiver function Pms splitting results, implying coherent deformation pattern in the whole crust in this region. Previous studies also indicate strong radial anisotropy in our study region. The azimuthal anisotropy and radial anisotropy characteristics in central Yunnan can be best explained by a titled hexagonally symmetric medium.