Seismic Anisotropy in the upper crust beneath the Sanjiang lateral collision zone in the southeastern margin of the Tibetan Plateau revealed by S wave splitting from a temporary array

Abstract

The Sanjiang lateral collision zone is a key region to understand the Tibetan Plateau’s tectonic structure and the Tethys-Himalayan’s tectonic evolution. Complex tectonic structures, intense crustal deformation, frequent seismicity, and abundant metal deposits are all present. With the seismic data recorded by a temporary array (SJ-Array) and permanent stations (Nov. 2018 ~ Dec. 2020), this paper adopts the S wave splitting technique to obtain the essential properties of upper crustal anisotropy. In the interested area, it is shown that the dominant polarization of the fast S wave is NNW, with a mean polarization direction of 167.9°. In addition, the study area can be divided into three subzones from the west to the east: A, B, and C, according to the various mean polarizations varying from NNW, NS to NNE. The mean normalized time delay between the two split S waves is 4.0 ms/km, and the range of time delay is from 2.0 to 6.3 ms/km. The largest time delay is located at the east side of the western boundary of the Sichuan-Yunnan rhombus block. Furthermore, there is a strip area of strong anisotropy stretching along the western segment of the Lijiang-Xiaojinhe fault. These all demonstrate the local tectonic differences and indicate that the crustal structure may be strongly controlled by the fault and block boundary strike.