南海西沙石岛地震台下的地壳结构研究

为加强海上天然地震观测和岩石圈结构研究，我们在南海西沙群岛建立了石岛流动地震台，进行了为期一年多的观测试验.观测结果表明，虽然岛礁区的地震数据受热带气旋影响期间背景噪声较大，但仍能记录到MW 6级以上的地震.本文对具有清晰P波波形的远震记录进行了接收函数处理和计算模拟，得到台站下简单的地壳结构模型，其莫霍面深度为28 km，上地壳顶部有一层2 km厚的低速层，横波速度只有23 km/s，向下逐渐过渡到横波速度为38 km/s的下地壳.与已有的研究结果比较，发现石岛台与琼中台的地壳结构模型是西沙海槽剖面的自然延伸，西沙地块的地壳结构属减薄型陆壳，可与华南地块的正常陆壳对比.

The crustal structure beneath the Shidao Station on Xisha Islands of South China Sea

We installed a temporary earthquake station at Shidao on Xisha Islands of South China Sea to study the seismicity and lithospheric structure of the region,and carried out the observation experiment for more than one year.Although the seismic data in reef and island areas have relatively large background noise,especially during the passage of a tropical cyclone,earthquakes above M-W 6 can still be recorded.In this paper,those teleseismic records with clear P waveforms are chosen to carry out receiver function analysis and modeling.A simple crustal structure model beneath the station is obtained,in which the Moho depth is 28 km.On the top of the upper crust is a 2 km-thick low velocity layer with a shear wave velocity of only 2.3 km/s.The shear wave velocity increases gradually with depth to 3.8 km/s in the lower crust.We compared this model with previous results and discovered that the models of Shidao Station and Qiongzhong Station(QIZ) are the natural extensions of the Xisha Trough section.The crustal structure of Xisha Block consists of thinned continental crust,with a velocity structure that is similar to the normal continental crust of South China Block.