In this study, on the basis of absolute first-arrival times of 84756 P- and S-waves from 6085 earthquakes recorded at 56 fixed stations in Yibin and surrounding areas in China from January 2009 to January 2019, focal parameters and three-dimensional (3D) body-wave high-resolution velocity structures at depths of 0–30 km were retrieved by double-difference tomography. Results show that there is a good correspondence between the spatial distribution of the relocated earthquakes and velocity structures, which were concentrated mainly in the high-velocity-anomaly region or edge of high-velocity region. Velocity structure of P- and S-waves in the Yibin area clearly shows lateral inhomogeneity. The distribution characteristics of the P- and S-waves near the surface are closely related to the geomorphology and geologic structure. The low-velocity anomaly appears at the depth of 15–25 km, which is affected by the lower crust current. The Junlian-Gongxian and Gongxian-Changning earthquake areas, which are the two most earthquake-prone areas in the Yibin region, clearly differ in earthquake distribution and tectonic characteristics. We analyzed the structural characteristics of the Junlian-Gongxian and Gongxian-Changning earthquake areas on the basis of the 3D body-wave velocity structures in the Yibin region. We found that although most seismicity in the Yibin area is caused by fluid injection, the spatial position of seismicity is controlled by the velocity structures of the middle and upper crust and local geologic structure. Fine-scale 3D velocity structures in the Yibin area provide important local reference information for further understanding the crustal medium, seismogenic structure, and seismicity.

中文翻译：

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宜宾地震区三维速度结构与构造特征

本研究基于2009年1月至2019年1月在宜宾及周边地区56个固定台站记录的6085次地震的84756次P波和S波的绝对首次到达时间，震源参数和三维（ 3D）通过双差断层扫描技术检索了0-30 km深度的体波高分辨率速度结构。结果表明，搬迁地震的空间分布与速度结构有良好的对应关系，速度结构主要集中在高速异常区或高速区边缘。宜宾地区纵波和横波的速度结构清楚地显示出横向不均匀性。地表附近纵波和横波的分布特征与地貌和地质结构密切相关。低速异常出现在15-25 km的深度，这受地壳电流较低的影响。宜宾地区最容易发生地震的两个地区-lian连-巩县和巩县-长宁地震区，其地震分布和构造特征明显不同。我们根据宜宾地区的3D体波速度结构，分析了lian连-巩县和巩县-长宁地震区的结构特征。我们发现，虽然宜宾地区的地震活动多数是由流体注入引起的，但地震活动的空间位置却受中，上地壳的速度结构和局部地质构造的控制。宜宾地区的精细3D速度结构为进一步了解地壳介质提供了重要的当地参考信息，