Abstract: In this paper, the double difference seismic tomography method is applied to the phase arrival times of 7 465 seismic events to determine the hypocenter parameters of events as well as detailed 3D velocity structure at the northern segment of Xiaojiang Fault and its surrounding area. The data was recorded by 42 stations of the Jinshajiang River network from August 2013 to November 2016. At 2~6km, V_P and V_S present low velocity anomalies along the northern segment of Xiaojiang Fault, and the V_S anomaly is especially remarkable. On both sides of the Xiaojiang Fault, there also exist obvious P and S wave low velocity areas. These low velocity areas correspond to the terrain, lithology distribution and the watershed of Jinsha River at shallower layer in the study area. Starting from 6km, a NE-directed high V_P band along Zhaotong-Ludian and Huize-Yiliang Fault is formed on the eastern side of the northern segment of Xiaojiang Fault. V_S also shows the high value in the area bounded by Lianfeng Fault, Baogunao-Xiaohe Fault and Huize-Yiliang Fault. Above 10km depth, to the west side of the Xiaojiang Fault including the Ninghui Fault, V_P shows a significant low-velocity anomaly, while to the east side it presents high velocity feature. The Xiaojiang fault zone shows a significant low V_P from north to south in the study region, and the low velocity anomaly in the northern segment is relatively significant, especially the low velocity anomaly area reaches 15km deep around Qiaojia area. Beneath the Baihetan Dam, a significant low V_P area reaching to 5km deep is found. The earthquakes around the dam formed a strip from shallow to deep on the low-velocity area side. Whereas, a stable high-velocity area is found under the Wudongde Dam. The events relocation result shows that:all the focal depths in the study area are shallower than 20km, and the predominant focal depth is within 15km. Different from the NE-trending of the major faults in the study area, the relocated seismic events are obviously distributed nearly east-west along Matang Fault and Daduo Fault and the region around Huize. The focal depths of M_S6.5 Ludian earthquake sequences are shallower than 15km, and mostly less than 10km. The aftershocks within 2a after the Ludian M6.5 earthquake form two predominant bands of about 40km and 20km along near EW and SN direction, respectively.

Key words: northern segment of Xiaojiang Fault and its surrounding area, double difference seismic tomography, 3D velocity structure, Ludian M_S6.5 earthquake

摘要： 文中使用2013年8月—2016年12月金沙江下游水库台网42个台站记录到的7 465个地震事件的到时数据，采用双差层析成像方法，反演得到了金沙江下游水库区地震震源参数及周围地区分辨率达0.1°的地壳三维速度结构。在研究区2～6km深的浅层，P、S波沿小江断裂北段呈现低速异常特征，S波的低速异常尤其显著，在小江断裂带东、西两侧出现了明显的低速区，说明浅层低速区的分布与研究区的地形、岩性分布以及金沙江流经区域对应较好。自6km深处开始，P波在小江断裂北段东侧形成了沿昭通、会泽-彝良断裂的NE向高速条带，S波在莲峰断裂、包谷垴-小河断裂、昭通-鲁甸断裂和会泽-彝良断裂所围限的区域内呈现高速分布。以小江断裂为界，在深度10km以上的浅层，包括宁会断裂在内的小江断裂西侧，V_P呈现显著的低速异常，而小江断裂东侧则为高速。小江断裂带自北至南呈现显著的V_P低值，且北段的低速异常相对显著，在巧家附近低速异常区的深度达15km。白鹤滩大坝下方呈现显著的V_P低速分布，深度至5km。大坝周围的地震在低速一侧由浅至深形成了条带。乌东德大坝下方为稳定的高速分布。重定位结果显示：研究区内震源深度<20km的地震优势分布在15km以浅，在小江断裂带东、西两侧的麻塘断裂、大朵断裂以及会泽县附近的重定位事件出现了比较明显的近EW向分布，与该区域内主要大型断裂的NE走向不同。鲁甸地震序列的震源深度<15km，优势分布在10km以浅，地震序列的优势分布方向为近EW和SN向，在2个优势方向上2a内发生的余震分别形成了长约40km、20km的条带。

关键词: 小江断裂带北段, 双差层析成像, 三维速度结构, 鲁甸M6.5地震