Abstract: Small earthquakes have been recorded in Yibin area, Sichuan Province since 1970, the frequency and intensity of seismicity have shown an increasing trend in recent ten years, and the earthquakes are distributed mainly in Changning, Gongxian and Junlian areas. Based on the seismic data from January 2008 to May 2015 recorded by Sichuan and Yunnan regional networks and Yibin local network, seismicity analysis, precise location and velocity structure inversion for earthquakes in Yibin area are carried out, the three-dimensional spatial distribution of seismic activity and the velocity structure at different depths in this region are investigated, trying to analyze the seismic activity law and seismogenic mechanism in Yibin area.
The earthquake relocation result shows that the spatial cluster distribution of earthquakes is more obvious in Yinbin area, the earthquakes are concentrated in Changning-Gongxian and Gongxian-Junlian regions. The seismic activity presents two dominant directions of NW and NE in Changning-Gongxian region, and shows asymmetric conjugate distribution, the long axes of NW-trending and NE-trending seismic concentration area are about 30km and 12km respectively, and the short axes are about 5km. There is a seismic sparse segment near Gongxian, the frequency and intensity of seismicity in the southeast side are obviously higher than that in the northwest side, and the earthquakes with larger magnitude are relatively deep, the focal depth is gradually shallower with the distance away from Gongxian. Seismic activity is sparse in the west and dense in the east in Gongxian-Junlian region, the predominant direction of earthquakes in the seismic dense area of the eastern segment is NE. Seismic activity extends in opposite direction in the easternmost part of the two earthquake concentrated area.
The P-wave velocity structure at different depths in the study area is obtained using joint inversion method of source and velocity structure. In view of the predominant focal depth in this region, this paper mainly analyzes the velocity structure of the upper crust within 10km. Within this study area, the P-wave velocity of earthquake concentration areas is relatively high within 10km of the predominant focal depth, especially in the northwest of Gongxian and eastern Junlian area, the P-wave velocity on the southeast of Gongxian increases gradually with depth, especially at 6km depth. These high-velocity zones are generally related to brittle and hard rocks, where the stress is often concentrated.
Comparing earthquake distribution and velocity structure, seismic activity in this area mainly occurs in high-low velocity transition areas, the inhomogeneity of velocity structure may be one of the factors controlling earthquake distribution. The transition zone of high and low velocity anomalies is not only the place where stress concentrates, but also the place where the medium is relatively fragile, such environment has the medium condition of accumulating a large amount of strain energy and is prone to fracture and release stress.

Key words: Yibin area, earthquake location, joint inversion, velocity structure

摘要： 文中基于2008年1月-2015年5月四川、云南区域台网及宜宾地方台网记录的地震观测资料，对四川宜宾的地震密集区开展了地震定位和速度结构反演工作，研究该区域地震活动的三维空间分布和不同深度的速度结构特征。重新定位结果显示，宜宾地区的地震活动具有明显的空间丛集性，主要集中分布在长宁-珙县和珙县-筠连，2个地震密集区均呈现不对称的共轭分布，地震活动在密集区最东段出现相向扩展的现象。长宁-珙县的地震活动区位于长宁背斜核部，在珙县附近存在稀疏段，其东南侧的地震活动频次和强度明显高于西北侧，且震级较大的地震震源深度相对较深，远离珙县的地震震源深度逐渐变浅。P波速度结果显示，地震密集区在10km的优势深度范围内均表现出相对较高的P波速度，尤其在珙县北西侧和筠连以东地区，珙县东南侧的P波速度呈现随深度增大逐渐增加的特征，特别是在深6km处呈现相对较高的波速。这些高速区一般与地下较脆、较强的岩体有关，往往是应力最为集中的区域。地震活动主要发生在高、低速过渡的区域，速度结构的不均匀性可能是控制该区地震分布的因素之一。

关键词: 宜宾地区, 地震定位, 联合反演, 速度结构