Seismic Anisotropy Beneath Qinghai Province Revealed by Shear Wave Splitting

Abstract

AbstractShear wave splitting analyses have been carried out using teleseismic data from 30 broad‐band seismograph stations deployed at permanent locations in Qinghai province, and we determined fast polarization directions φ and delay times δt between fast and slow components using both the minimum energy method and rotation‐correlation method. Our shear wave splitting measurements show that the anisotropy parameters vary with propagation directions beneath most stations, which can be best explained by a two‐layer anisotropic model with horizontal symmetry axes. The optimal fast direction for the upper layer is in the range 65°~95° and is probably related to material flow in the middle and lower crust of this region. The fast direction for the lower layer ranges from 105° to 135° and can be attributed to the ongoing movement of Qilian block in NEE direction and the consequent lithospheric shortening. Our results also show that the fast directions beneath stations GOM and DWA are parallel with the trend of Kunlun fault. The correlation between anisotropy and the fault strike suggests that the Kunlun fault must be deep and probably extends into the mantle. At stations HTG and LEH lying close to Altyn‐Tagh fault, we can not determine any splitting parameters but observe many null events whose back‐azimuths are neither parallel nor normal to the strike of Altyn‐Tagh fault, suggesting that in this region the fault is a crustal structure.