Noise correlation functions from two seismic arrays deployed in the vicinity of the Pengguan complex in the Longmenshan, eastern margin of the Tibetan plateau, China

Understanding the lithospheric structure of and the processes operating at plateau margins is important as these margins can host destructive earthquakes e.g. the May 2008 Wenchuan earthquake with a magnitude of about 8, in the Longmenshan at the eastern margin of the Tibetan plateau. From the deployment of 125 seismographs, including a dense profile of 45 seismographs, in the vicinity of the epicentre of the Wenchuan earthquake, 3-D compressional- and shear-wave velocity models of the crust in this part of the Longmenshan have been derived. These models show that the old Pengguan complex which cores this part of the Longmenshan is about 20 km thick and displays high velocities and low Poisson’s ratios. However, along the dense profile, the complex is underlain between 20 and 40 km depth, by low shear-wave velocities which likely extend out beneath the Sichuan basin in a 10 km thick basal crustal layer. These zones of low shear-wave velocities have high Poisson’s ratios and can be explained as a result of the push exerted by the high plateau, causing processes such as ductile shearing and fluid/melt infiltration to occur.

研究高原边缘的岩石圈结构与活动过程具有重要意义，因为此类边缘区域极易孕育破坏性地震——例如2008年5月发生于青藏高原东缘龙门山地区的约8级汶川大地震。本研究在汶川地震震中附近布设了125台地震仪，其中包含一条由45台仪器组成的密集观测剖面，据此反演得到了龙门山该区域地壳的三维纵波与横波速度模型。模型结果显示，构成龙门山该区域核部的古老彭灌杂岩（Pengguan complex）厚度约20km，表现出高波速与低泊松比特征。但沿该密集观测剖面，该杂岩下方20~40km深度范围内存在低横波速度异常体，该异常体可能以10km厚的下地壳层形式延伸至四川盆地下方。此类低横波速度异常区具有高泊松比特征，其成因可归结为青藏高原高海拔块体的挤压作用，引发了韧性剪切、流体/熔体渗透等地质过程。