Abstract: In this paper, a new method for small event detection named Match & Locate(M&L)is used to detect and locate the small earthquakes that are missing in the catalogue of the February 28, 2014 Shizuishan M_L4.4 earthquake swarm. A total of 34 earthquakes were detected, which is nearly 3 times as much as the number(13)of earthquakes from Ningxia seismic network. The number of earthquake swarm sequence is greatly increased. Then, it provides the possibility for the fine study of the earthquake swarm activity and seismogenic fault. The best double couple solution of the main shock obtained by the cut and paste method is strike 354°, dip 70° and slip 166° for nodal plane I, and strike 89°, dip 77° and slip 21°for nodal plane Ⅱ. The main shock is a dextral strike-slip earthquake with a small amount of thrust component. And, the depth of the main shock is 7~8km, which is a shallow earthquake, derived from the results of the double difference relocation and the best fitting depth of focal mechanism. Together with the results of deep 3-D seismic tomography of the Yinchuan Basin, our results show that the main shock and the largest aftershock more likely occurred in the upper crust, and the rest of earthquakes mainly occurred at the bottom of sedimentary layer or on the top of the upper crust crystallization basement. We find some interesting phenomena on the pattern of time-space evolution of the earthquake swarm. The distribution of earthquake swarm is in the near north-south direction. Aftershocks are mainly concentrated in the north region of the main shock, which show an obvious trend of extending gradually from the south to the north. Also, the result shows the general trend of shallower focal depth with the development of aftershocks to the north. The results of distribution and depth profile of the earthquake swarm and the focal mechanism of the main shock all show that the sequence probably occurred in the fault at the east foot of Helan Mountain with an eastward dip and a larger dip angle. Surface projection image of the earthquake sequence shows that the epicenter distribution extends northward from the northern end of the fault. This may suggest that the deep part of the fault is likely to extend northward.

Key words: Match and Locate, double-difference location, focal depth, Shizuishan earthquake swarm, seismogenic fault

摘要： 采用新发展的匹配定位（M&L）方法检测与定位2014年2月28日宁夏石嘴山M_L4.4震群地震目录中遗漏的微小地震事件，共检测定位34次地震，是宁夏台网定位地震数目（13次）的近3倍；地震数明显增多，地震目录完整性明显改善，为更科学合理地研究震群活动过程和发震构造提供了可能。利用CAP方法反演得到的主震最佳双力偶解为节面Ⅰ：走向354°，倾角70°，滑动角166°；节面Ⅱ：走向89°，倾角77°，滑动角21°；矩震级为M_W3.9；震源以走滑错动为主带有少量逆冲分量。结合主震双差定位结果和震源机制最佳拟合深度，认为主震深度为7~8km，属于浅源地震。结合银川盆地深地震三维成像研究成果，认为主震及最大余震发生于上地壳内的可能性较大，其余地震则主要发生于沉积层底部或者是上地壳结晶基底的顶部。震群活动时空演化图像显示，震群整体分布呈现近SN向展布，余震主要集中于主震以北区域，并呈现较明显的由南向北逐步延伸的态势，随着余震向北发展震源由深部向浅部迁移。地震深度剖面投影图像及主震震源机制解结果均显示发震断层E倾，倾角较大，表明石嘴山震群发生于贺兰山东麓断裂的可能性较大。地震地表投影图像显示震中分布延展至贺兰山东麓断裂北端终止点以北，由此推测贺兰山东麓断裂存在继续向北延伸的可能。

关键词: 匹配定位, 双差定位, 震源深度, 石嘴山震群, 发震构造