新疆南天山地区地壳三维速度结构:来自近震层析成像的结果

Three-Dimensional Crustal Velocity Structure in the South Tianshan Mountains,Xinjiang:Results from Local Earthquake Tomography

利用新疆测震台网在南天山地区架设的固定和流动台站记录的近8 a的观测资料,通过近震走时层析成像技术获得地壳三维速度结构,并利用三维速度结构对该区域的近震事件进行重定位。重定位后走时残差均方根由1.29 s减小为0.64 s,震源位置的不确定性约为0.1 km。三维速度结构显示,区域内速度结构表现出明显的横向和纵向不均匀性,5 km深度处,阿图什背斜和柯坪断裂以北表现为高速异常,以南表现为低速异常,波速比整体上以高值为主;15 km和20 km深度处,研究区P波速度主要以高速异常为主;25~30 km深度处,研究区内P波高速异常转变为以近NE向低速异常为主,波速比也以近NE向低值异常为主。区域内地震活动性与地壳速度结构具有较强的对应性,震源主要集中在P波高低速交界处的高速区域,同时波速比也偏向高波速比一侧。震源位置集中区的下方存在低速条带,该条带可为应变能的积累和释放提供有利的环境基础。

This paper uses the observation data for the past eight years recorded by fixed and mobile stations of the Xinjiang seismic network in the south Tianshan area.We carry out local earthquake travel time tomography technology to obtain the three-dimensional crustal velocity structure,and use the three-dimensional velocity structure to relocate local earthquake events.After relocation,the root mean square of the travel time residual decreases from 1.29 s to 0.64 s,and the uncertainty of the source location is about 0.1 km.The three-dimensional velocity structure shows that the velocity structure in the region has obvious lateral and vertical inhomogeneities.At 5 km,it is a high-speed anomaly north of the Artux anticline and the Keping fault,a low-speed anomaly in the south.The velocity ratio is dominated by high values on the whole.At the depths of 15 km and 20 km,the P-wave velocity in the study area is dominated by high-speed anomalies.At the depth of 25-30 km,the P-wave high-speed anomalies in the area turn into obvious near north-east with low-speed anomalies,the velocity ratio is also dominated by the low value anomalies in the NNE direction.The seismic activity in the area has a strong correspondence with the crustal velocity structure.The source is mainly concentrated in the high speed region at the junction of high and low P wave velocity.The velocity ratio also tends to the side of the high velocity ratio.There is a low-speed band under the focal area,which provides a strong environmental foundation for the accumulation and release of strain energy.