青海地区S波分裂研究

Seismic anisotropy beneath Qinghai province revealed by shear wave splitting

本文利用国家地震台网及中国地震局"十五"期间在青海布设的30个宽频带地震台站记录到的远震波形数据,分别采用最小能量法和旋转相关法对SKS、SKKS和PKS波震相进行了偏振分析,计算了台站下方介质的各向异性分裂参数:快波的偏振方向(ψ)和慢波延迟时间(δt).本文研究结果表明,研究区多数台站下方的地震各向异性参数都表现出随方位角变化而变化的特征,可以用双层各向异性模型来解释.其中上层各向异性的快波偏振方向位于N65°E～N95°E之间,可能与中下地壳物质的流动有关;而下层各向异性的快波偏振方向位于N105°E～N135°E之间,可能为祁连块体NEE向的推移及导致的岩石圈缩短有关.此外,我们还发现,与周边的台站下方各向异性分裂参数相比,昆仑断裂附近两个台站GOM和DAW的各向异性特征急剧变化,其快波方向都与该断层近乎平行,这很可能暗示昆仑断裂已经切穿整个岩石圈;阿尔金断裂附近两个台站(LEH和HTG)无效分裂事件的方位分布与阿尔金断裂走向缺乏相关性,我们推测研究区内阿尔金断裂可能为地壳尺度的断裂.

Shear 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 there are variations with propagation directions beneath most stations. Analysis of individual measurements from multi-station highlights variations of splitting parameters with event backazimuth and 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 65°~95° range which is related to crust flow in this region. The fast direction for the lower layer ranges from 105° to 135° and can consequently be attributed to the ongoing NEE direction 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 measurements and only observe many Nulls whose event backazimuths are neither parallel nor normal to the strike of Altyn-Tagh fault, suggesting that in this region the fault is a crustal scale structure.