Anisotropic characteristics beneath Tibetan Plateau and its surrounding areas
This study conducted SKS shear-wave splitting measurements of 39 distant earthquakes with magnitudes above 5.8 and epi-central distances between 88° and 120° recorded by the monitoring stations of China Earthquake Networks Center in the Qinghai-Tibet region in the past five years.Based on these measurements,this study investigated the anisotropic and deformation characteristics of Ti-betan Plateau and its surrounding areas.Within the Qinghai-Tibet region,the fast axis of SKS shear-wave splitting measurements from west to east gradually shifted from NE-SW to E-W and then to NNW,roughly aligning with the direction of the GPS velocity field and changing in a clockwise direction.The reason for the measurement results in this region lies in that the westward retreating of the Burma plate during the eastward subduction may have caused circular deformation in the local lithosphere.The GPS data of the southern Si-chuan-Yunnan region reveal nearly ES-directed surface deformation and NW-SE-trending strike-slip faults,with the fast-axis direction of the Pms wave for crustal anisotropy being S-N or NNE-SSW.The SKS splitting measurement results of this study reveal the E-W-di-rected anisotropy of the upper mantle,which intersects at a high angle with or is perpendicular to the fast-axis direction of the Pms wave.This suggests that the mantle flow field of the deep asthenosphere in Yunnan is inconsistent with the deformation characteristics of the crust and surface,and the mantle and crust show distinct deformation mechanisms and anisotropy sources,resulting in decou-pling deformation.
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