用天然地震探测青藏高原中部地壳、上地幔结构

从西藏南部的定日、嘎拉至青海铜铁山的天然地震探测剖面，实际路线长约２０００ｋｍ，布设了约１１０台便携地震仪，记录了数百次远震和近震事件，采用多种方法进行了资料处理与解释．依据ＳＫＳ，ＰＫＳ，ｐｓ等横波分裂特征计算的青藏高原中部上地幔的地震各向异性表明：研究区各构造单元内的地震各向异性有明显变化，发现上地幔各向异性快速波的偏振方向与造山带的走向不完全一致．在雅江缝合线、崩错－嘉黎、唐古拉山口、昆仑山口几条断裂带处南、北各向异性出现显著的差异，而金沙江缝合线和班公－怒江缝合线的南、北则没有明显的各向异性变化．由Ｐ波走时残差，利用层析技术反演了４００ｋｍ深度内的速度图像，可以看出近地表１００ｋｍ范围内速度的不均匀变化与地表划分的构造单元很吻合，进一步佐证了青藏高原是由不同时期的微板块拼合而成的认识．在青藏高原中部１５０ｋｍ深度以下发现了多处低速区．在金沙江缝合带下方约２００ｋｍ深度处有一长２５０ｋｍ以上、延伸１５０ｋｍ的低速体，推测可能是一地幔柱．利用ＰＳ转换波划分的界面，显示出青藏高原北部具有低速层和高速层交替出现的地壳结构．

THE STUDY ON THE STRUCTURE OF CRUST AND UPPER MANTLE WITH NATURAL EARTHQUAKES IN CENTRAL TIBETAN PLATEAU

A seismic array,consisting of 110 stations and extending for about 2000km.was deployed from Tingri in southern Tibet to Xitieshan in Qinghai.Hundreds of remote and local events were recored,the data has been processed and Interpreted by various methods. The anisotropic map for the lithospheric mantle beneath the Central tibetan Plateau ,based on the splitting character of SKS,PKS and PS waves,Shows prominent variation among tectonic blocks along the profile.thus.disclosing an important fact that the direction of anisotropy of the lithospheric mantle doesn't coincide with that of orogenic belt.Apparent disparity in anisotropy can be seen across the Yarlungzangbo suture,Pengcuo-Jiali,Tanggula pass and Kunlun pass.faults,but not for the Jinshajiang suture and Bangong-Nujinag suture.The velocity image for the depth of 400km and above,produced by travel time tomography inVersion,gives a clear picture that in the scope of near-Surface to the depth of 100km,the varying velocities are in good agreement with the surface tectonic units,there fore further evidencing the idea that the Plateau is an assemblage of blacks with different ages and histories.There are low velocity zones at every level of depth,especially beyond 150km.At the depth of 200km beneath Jinshajing suture,a body of low velocity with width of 250km and 150km downward reaching,is presumablya mantle plume.The crustal structure with alternate high and low velocity layers in norther Tibet is suggested from PS comverted waves.