三峡库区上地壳三维速度结构的双差层析成像研究

Three-dimensional velocity structure of upper crust in the Three Gorges Reservoir area derived from double-difference tomography

本文利用三峡水库诱发地震遥测台网22个子台在2009年初至2016年底期间记录的2093个地震事件直达P、S波到时数据,采用双差层析成像方法联合反演了三峡水库库区及近邻地区上地壳P波三维速度结构,并讨论了库水渗透对速度结构的影响和库区地震活动与速度结构的关系.研究结果表明三峡库区上地壳存在着明显的沉积盖层与结晶基底层的双层结构,两个层的深度与P波速度结构与前人的研究结果基本一致,黄陵背斜西侧当前仍然存在较明显的低速异常区.上地壳浅表层P波速度结构横向差异变化较大,0~5km深度层P波高速区主要分布在秭归盆地及周缘,8km深度层高速区主要分布在周家山—牛口断裂东侧至仙女山断裂中段西侧一带,8km内的高低速区分布与11km深度层比较存在明显差异.与三峡水库蓄水前与蓄水初期比较,当前库区上地壳沉积盖层内的P波速度结构受到蓄水渗透的影响较明显,主要表现为0~8km深度内秭归盆地及周缘的水库近岸区存在较大范围高速区,这一现象可能与库水长期渗透改变了上地壳沉积盖层内岩层孔隙裂隙的物理性质从而使地震P波速度增加有关.三峡库区地震事件重新定位后显示,较大地震事件主要分布在P波高速区或高低速区交界地带,而低速区内通常很少发生地震.

This work attempts to probe the three-dimensional velocity structure of the upper crust in the Three Gorges Reservoir and adjacent areas using the double-difference tomography method.We process the data of direct P and S wave travel times of 2093 earthquakes from early 2009 to the end of 2016 recorded by 22 sub-stations of the Three Gorges seismic network,and discuss the influence of reservoir water infiltration and the relationship between seismic activity and velocity structure.The results show that the upper crust of the Three Gorges Reservoir and adjacent areas have an obvious two-layer structure of the sedimentary cap and crystalline basement,which is largely the same as those of previous work.An obvious low-velocity anomaly remains west of the Huangling anticline.The prevail lateral anisotropy exists in the P wave velocity structure of the upper crust.High-velocity anomalies are mainly distributed in the Zigui basin and surrounding areasat depths of 0~5 km and in the west of the Zhoujiashan-Niucou fault to the west of the Xiannushan fault at depth of 8 km.Moreover,there is a significant difference in the distribution of the high and low velocity zones of 8 km and 11 km deep layers.Currently,the P wave velocity structure in the sedimentary cover is significantly influenced by the reservoir water infiltration,which is characterized by the existence of a large-scale high-velocity anomaly in the Zigui basin and surroundings at depths of 0~8 km.This phenomenon is related to the change of the pore physical property in the upper crustal sedimentary cover due to long-time infiltration of reservoir water.The results of the seismic relocation show that larger earthquakes are mainly distributed in high-velocity zones or high-low velocity junction zone,while few earthquakes occurred in low-velocity zones.