Shallow velocity structure and seismogenic environment in the Zigui section of the Three Gorges Reservoir region of China
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摘要: 收集了湖北省秭归地区24个流动地震台站2020年6—7月期间的垂直分量连续波形记录，利用背景噪声互相关得到了各台站之间的经验格林函数，提取了0.6—5 s的瑞雷波群速度频散曲线，并反演获得了该区域近地表6 km以内的三维Ｓ波速度模型。结果显示：秭归盆地及其南部邻区S波速度明显低于东侧断裂区S波速度，与研究区不同构造地块的构造演化和沉积特征一致；2014年MS4.2地震发生在研究区垂向高低速交界区；断裂区在长江区域附近S波速度明显降低，表明长江水渗透到断层区，因此秭归地区地震频发与三峡库水荷载和水渗透作用有关。Abstract: After the impoundment of the Three Gorges Reservoir in 2003, shallow earthquakes occurred frequently, which had a great impact on local productions and lives. The study of shallow velocity structure is of great significance for shallow seismic disaster assessment and disaster prevention and reduction in Zigui area. In this paper, based on the vertical component continuous waveform records of 24 mobile seismic stations in Zigui area from June to July of 2020, the empirical Green’s functions between stations are obtained by cross-correlating of the ambient seismic noise, the Rayleigh wave group velocity dispersion curves of 0.6−5 s are extracted, and the inversion for the three-dimensional S-wave velocity model is obtained within 6 km near the surface of the region. The results show that the S-wave velocity of Zigui basin and its south adjacent area is significantly lower than that of the eastern fault area, which is consistent with the structural evolution and sedimentary characteristics of different structural blocks in the studied area; an earthquake with MS4.2 occurred on the vertical high-velocity and low-velocity junction area of the study area in 2014. The S-wave velocity decreases obviously near the Yangtze River region in the fault zone, indicating that the Yangtze River water penetrates into the fault area. Therefore, the frequent occurrence of earthquakes in Zigui area is related to the water load and water infiltration of the Three Gorges reservoir.
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图 1 2009—2021年期间地震和本文所用地震台站的分布
红色六角表示近些年研究区域所发生的MS＞4.0地震位置,蓝色圆点代表2009—2021年间该区域发生的ML＜4.0中小地震位置（郑秀芬等，2009）；黑色三角代表台站位置；天蓝色实线代表长江；Ⅰ. 秭归盆地；Ⅱ. 黄陵背斜（盆地和背斜区域划分参考李小勇等，2014）
Figure 1. Distribution of earthquakes in the period of 2009−2021 and seismic stations used in the study
The red hexagonal stars indicate the location of Ms＞4.0 earthquakes in the study area in recent years，blue dots represent the location of ML＜4.0 small and moderate earthquakes in the region from 2009 to 2021 （Zheng et al，2009）；black triangles represent the stations used in this study，and red dotted line represents Xiannvshan and Jiuwanxi fault zones；Ⅰ. Zigui basin，Ⅱ. Huangling anticline （Basin and anticline regional division refer to Li et al, 2014）
图 2 截取长度不同时的信噪比变化对比图
实线表示数据有50%的重叠，虚线表示没有重叠。不同颜色的曲线代表不同周期
Figure 2. Comparison of SNR variation with different interception lengths
Solid lines represent 50% overlapping, while dashed lines represent no overlapping. Colors denote different period （1−5 s）
图 3 （a） 台站对互相关函数；（b） 进行正反向反序求平均后的互相关函数
Figure 3. （a） Noise cross-correlation function （NCFs） for all stations pairs；（b） NCFs after averaged over positive and negative lag
图 4 （a） S04-S21台站对叠加模型预测的瑞雷波群速度色散和模态谱振幅，图中黑色小方块为频散点，红色区域为最大振幅区域；（b）所有台站对的群速度频散曲线，红色虚线表示平均值
Figure 4. （a） Rayleigh wave group velocity dispersion and modal spectrum amplitude predicted by the superposition model for the station-pair S04-S21. The small black square is the dispersion point，and the red area is the maximum amplitude area；（b） Group velocity dispersion curves for all station-pairs where the red dotted line represents the mean value
图 6 不同周期下的检测板测试恢复结果和射线覆盖
Figure 6. Checkerboard test results and ray coverage with different periods
图 7 周期为1.2 s时基阶瑞雷波初始残差与反演模型走时残差直方图
Figure 7. Histogram of initial （gray） and inversion （red） travel time residuals of base order Rayleigh wave with period of 1.2 s
图 8 不同周期T的群速度图像
多边形区域表示可信度较高的反演区域；Ⅰ为秭归盆地，Ⅱ为黄陵背斜；紫色星形为典型地震位置，黑色三角形为台站位置，红色虚线为断层，F1：九畹溪断裂；F2：仙女山断裂
Figure 8. Group velocity images of different periods T
Polygonal regions represent inversion regions with high reliability；regions Ⅰ is Zigui basin and Ⅱ is Huangling anticline；the purple star is the typical earthquake location，the black triangles is the station，and the red dashed line is fault，F1：Jiuwanxi fault；F2：Xiannüshan fault
图 9 Ｓ波反演模型及其频散曲线的拟合图
绿线为最终模型，蓝线为初始模型； 红线为最终模型正演计算的理论频散，黑色三角为测量频散点（a） 无高低速异常的S波速度结构；（b） 含低速异常S波速度结构；（c） 含高速异常S波速度结构
Figure 9. S-wave inversion model and its dispersion curve fitting diagram
The green line is the final model and the blue line is the initial model；the red line is the theoretical dispersion calculated by the forward modeling of the final model，and the black triangle is the measured dispersion point. （a） S-wave velocity structure without low and high velocity anomalies；（b） S-wave velocity structure with low velocity anomalies；（c） S-wave velocity structure with high velocity anomalies
图 10 秭归地区三维Ｓ波速度结构
（a—f） 不同深度的S波速度，紫色星形为典型地震位置，红色虚线代表断裂，F1：九畹溪断裂；F2：仙女山断裂；黑色实线为AA′、BB′以及CC′剖面位置；（g—i） AA′，BB′以及CC′剖面0—6 km深度的S波速度结构，菱形方块为地震位置
Figure 10. Maps of S-wave velocities at different depths in Zigui area
（a−f） S-wave velocities at different depths. The purple stars is the typical earthquake location，and the red dotted line represents fault，F1：Jiuwanxi fault；F2：Xiannüshan fault. The black solid lines indicate the location of the sections AA′，BB′and CC′’；（g−i） S-wave velocity structure at the depth of 0−6 km along the sections AA′，BB′ and CC′ respectively，and the diamond is the seismic location
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