青藏高原地壳结构构造及其与地震的关系

THE STRUCTURE AND TECTONICS OF THE CRUST AND THEIR RELATION WITH EARTHQUAKES IN THE QINGHAI-XIZANG PLATEAU

本文对1977—1983年间在青藏高原所进行11条爆炸地震剖面进行了重新解释;归纳了高原地壳结构9个方面的特点;论述了高原深部构造特征及与表层构造的相似性;讨论了高原的地震分布与地壳结构构造的关系。

On the basis of the results of the two-dimensional explanation for the DSS date and the seismic reflection/refraction studies of the continental li-thosphere, a new model of three-layer crustal structure in the Qinghai-Xi-zang plateau has been made. The upper crust(Fp = 6.3-6.6km/s) has complicated velocity structure, which includs a gradient and a low-speed layar and the letteer(Vp = 5.6-5.7km/s)is at the bottom of the upper crust; The middle crust(Vp = 6.3-6.6km/s)has more uninform velocity structure} The lower crust(Vp = 6.8-7.4km/s)is mainly characterized by a gradient layer (Fp = 7.00-8.15km/s). The average velocity of the crust is 6.2-6.3km/s. In the upper mantle, Fp = 8.1-8.2km/s. The crustal thickness of the middle part of the plateau is generally 60 km with 75-85 km as its maxima. The crust on the both sides of the plateau is getting thiner. High Himalayas has no mountain root with the crust thickness only 55 km. The checking up with the broadside(fan)section shows that the Moho can be tracked in all the area, but it is uncontinous. The Moho is cut by a. set of strike thrust faults and imbricate thrust faults. The shallow and deep-seated structures show that the thickening of the crust, the rise and the longitudinal shortening of the earth's surface were all caused by the SN-horizontal com-pressive stress.The distribution of the shallow earthquakes are closely related with the depth of the low-speed layer. The distribution of the episentres is con-troled by different faults. The tectonic stress field and the focal stress field show that the present crust is still being compressed by the horizontal stress along the SN-direction.