基于台阵的青藏高原东北缘海原-六盘山断裂带及邻区地壳结构研究

Research on Crustal Structure of the Haiyuan-Liupanshan Fault Zone and Its Adjacent Areas in Northeastern Qinghai-Tibetan Plateau Based on Seismic Array

通过收集青藏高原东北缘海原-六盘山断裂带及邻区的喜马拉雅地震科学台阵112个台站的远震P波资料,并用时域迭代反褶积提取接收函数,用H-k扫描方法得到了各个台站下方的地壳厚度及波速比,且计算了其泊松比。结果表明,在天景山断裂带附近,地壳厚度存在急剧变化的陡变带,该陡变区域最大落差可达14km;研究区内泊松比值存在明显的横向不均一性,但是整体上都沿着GPS观测到的速度场方向递减。据此推测,在青藏高原东北缘可能存在一条以海原-六盘山断裂带为主的小规模的局部物质东流带,由于遇到稳定块体的阻挡,海原-六盘山构造区呈现为典型的挤压构造及持续的地壳缩短,这使得该构造区的地壳相对于研究区其他地区较厚,其青藏高原部分物质东移可能在天景山断裂带附近明显停滞。另外,研究区内的青藏高原东北缘弧形构造区的地壳泊松比陡变区域与5级以上的中强震对应关系较好。研究区的地壳厚度、泊松比变化特点与该区域的GPS速度场、应变场以及构造变形的变化特征有较好的对应关系。

Through collecting the teleseismic P-wave data from 112 stations of the Himalaya seismic array in the Haiyuan-Liupanshan fault zone and its adjacent areas in northeastern QinghaiTibetan Plateau,we extract receiver functions by time-domain iterative deconvolution and then extract the crustal thickness and velocity ratios of these stations by the H-k stacking method,and calculate the poisson ratios. The results show that the crustal thickness has obvious blocking characteristic which is thick in southwest and thin in northeast. Near the Tianjinshan fault zone,there is a steeper variation belt in the crustal thickness of which the maximal drop can reach 14 km.In the research area,the poisson ratios present on the obvious horizontal heterogeneity,but generally are decreasing in the direction of velocity field from the GPS observations. This paper further suggests that in northeastern Qinghai-Tibetan Plateau there may be an eastward partial mass flow area which is mainly beneath the Haiyuan-Liupanshan fault zone,and there are typical compressional tectonics and continuous crustal shortening within the Haiyuan-Liupanshan tectonic region owing to the obstacles of stable block,making the region thicker than the others. The eastward partial mass flow of Qinghai-Tibetan Plateau may be in apparent stagnation near the Tianjinshan fault zone. Besides,in the arc tectonic zone of northeastern Qinghai-Tibetan Plateau,the steeper variation belt of poisson ratios are in good relation to the strong earthquakes with M≥5.0. There are a good causality between the characters of variation of crustal thickness and poisson ratios,the direction of the velocity and strain field,and change features of tectonic deformation in the research area.