四川长宁页岩气开采区上地壳各向异性及其变化特征

四川长宁页岩气开采区2018年12月16日和2019年1月3日相继发生了M_L5.7和M_L5.3地震, 两次中强地震给附近建筑物带来了严重的破坏.本文收集了长宁地区2018年11月至2019年2月的地震波形数据资料, 利用剪切波分裂方法获得了研究区内14个台站的剪切波分裂参数快波偏振方向和慢波延迟时间, 分析了长宁地区地壳各向异性特征, 以及地壳应力场的变化, 初步探讨了两次M_L5.0地震孕育的应力环境.结果显示, 多数台站快波偏振方向有两个优势方向, 北东向和北西向.研究区位于四川盆地南缘盆山结合地带, 构造环境复杂, 剪切波分裂参数较为离散.北西向的快波偏振方向与区域主压应力方向、震源机制解结果一致, 北东向快波偏振方向与之正交.整个研究区域内慢波延迟时间普遍较高, 均值为6.16 ms·km^-1.慢波延迟时间最大值来源于位于两次M_L5.0地震震中之间的XIC台站.离散的剪切波分裂参数以及较高的各向异性强度, 可能与页岩气开采压裂作业有关.水力压裂施工, 页岩地层中注入高压流体, 从而改变了岩体和断层的应力状态, 引起应力环境的改变.慢波延迟时间在M_L5.3地震发生之前表现出明显增加的趋势, 表明地震发生之前地壳应力的积累.M_L5.3地震发生之后观测到慢波延迟时间的降低, 反映了震后地壳应力的突然释放.

Seismic anisotropy and variation characteristics of the upper crust in Changning shale gas production area,Sichuan

The ML5.7 and ML5.3 earthquakes occurred successively on December 16, 2018 and January 3, 2019 in Changning shale gas production area, Sichuan Province. These two moderately strong earthquakes caused serious damage to the nearby buildings. In this paper, the seismic waveform data in Changning area from November 2018 to February 2019 were collected, and the shear wave splitting parameters which are the polarization direction of fast wave and the delay time of slow wave from 14 seismic stations in the study area were obtained by using the shear wave splitting method. The anisotropic characteristics and the changes of the stress field in the crust in Changning area were analyzed. And the stress environment of these two ML5.0 earthquakes was preliminary discussed. The results show that the polarization directions of fast shear wave have two dominant directions, northeast and northwest. The study area is located in the basin-range junction zone of the southern margin of Sichuan basin, with the complex tectonic environment. And the parameters of shear wave splitting are discrete. The northwest direction in the polarization direction of fast wave is consistent with the direction of regional principal compressive stress and focal mechanism results, which is orthogonal to the northeast direction in the polarization direction. The delay time of slow shear wave is generally high in the whole study area, and the average value is 6.16 ms·km-1. The maximum value is from the XIC station which is located between the epicenters of two ML5.0 earthquakes. The discrete parameters of shear wave splitting and high anisotropy intensity may be related to the hydraulic fracturing operation of shale gas production. During the hydraulic fracturing, the high pressure fluid is injected into the shale formation, which changes the stress state of the rock mass and fault, and causes the change of the stress environment. The delay time of slow wave increases significantly before the ML5.3 earthquake, indicating that the stress in the crust increases before the earthquake. After ML5.3 earthquake, the delay time of slow wave decreased. It reflects that the sudden release of stress in the crust after earthquake.