延怀盆地及其邻区地壳上地幔速度结构的宽频带地震台阵研究

BROADBAND SEISMIC ARRAY STUDY OF THE CRUST AND UPPER MANTLE VELOCITY STRUCTURE BENEATH YANHUAI BASIN AND ITS NEIGHBOURING REGION

在沙城以东的延庆盆地及其邻近区域内布设了由GDS－1000宽频带数字地震仪组成的流动地震台阵，利用台阵记录的宽频带远震P波波形数据和非线性接收函数反演方法获得了延怀盆地内0-80km深度范围的地壳、上地幔S波速度结构．利用计算机三维彩色剖分显示技术研究了台阵下地壳、上地幔速度结构的横向非均匀变化。结果表明，研究区域内的地壳厚度为40km左右，壳幔界面有4km左右的上下起伏．地表沉积盖层在延庆盆地中心附近厚度约1km，而在向盆地外围延伸的方向上相对变薄．研究区域内上地壳S波速度结构较复杂，而下地壳与上地幔则相对比较均匀．其上地壳最突出的特点是在10km深度附近有明显的S波低速层．在延庆盆地下方，它延伸到6-20km的深度范围．在延庆盆地南侧，该低速层有从西往东逐渐减弱的趋势．研究区域内的地震基本上都发生在延庆盆地下方上地壳低速体外围．据此推断，延庆盆地及其临近区域内的地震活动与该区域地壳内的热状态有密切关系．

A portable broadband seismic array consisted of 15 GDS-1000 seismic stations was deployed in the Yanhuai basin and itS neighboring region tO investigate the fine velocity struCtUre of the crust and upper mantlee beneath the arry. The spacing between stations is about 5-10km. By means of the nonlinear receiver function inversion technique (Liu etC. 1996) the S wave velocity structures of the crust and upper mantle beneath the stations have been obtained from the broadband teleseismic P waveforms recorded by our seismic array. In this study the 3-D crustal velocity model beneath the array is also given by using the computer tomographic display technique. Our results show that the crust is about 40 km thick in the Yanhuai basin and its neighbouring region. The crust-mantle boundary has a up-down undulation of about 4 km. The sedimentS is about I kin thick in the center of the Yanqing basin,and it becomes thinner along the directions to the basin's borders. The upper crust structute beneath the investigated area is quite complicated and the lower crust and upper mantle are relatively simple. An obvious feature of the upper crust is that at the depth of 10 tri exists a low velocity layer, which extends to the depth from 6 to 20 fan under the Yanqing basin. In the southern side of the basin this low velocity layer becomes weak along the direction from the west tO the east. The locations of earthquakes occurred in the investigated area from January of 1985 to September of 1990 show that these eventS are distributed mainly around the low velocity body of the upper crust and its neighbouring region. It could be inferred from thes Phenomenon that the seismic activity of the Yanqing basin is related closely to the thermic status within the crust.