Abstract: A M_W6.4(M_S7.0)earthquake occurred in Gonghe, Qinghai on 26 April 1990. The Gonghe area is located on the northeastern margin of the Qinghai-Tibet Plateau. The geological tectonic movement in this area is mainly affected by the uplift of the Qinghai-Tibet Plateau. There has been no earthquakes larger than moderate strength in the Gonghe Basin since the historical records, and there are no large-scale active faults on the surface of the epicenter area, so the earthquake has aroused great concern. No major earthquakes have occurred in the Gonghe area since 1995, but the data of small earthquakes is very rich, which ensures the completion of the research. The TomoDD method combines the double-difference relocation method with seismic tomography, and solves two problems at the same time, one is the problem of fine positioning of the earthquake, and the other is the calculation of the 3D velocity structure of the earth’s crust. In this paper, we collected 63872 P and S wave arrival time data in Gonghe and surrounding area recorded by Qinghai, Gansu seismic networks and temporary seismic array from January 2009 to January 2019. The 3D crustal velocity structure and source position parameters of the region are inversed. The relationship between the geological structure setting of the main shock and the velocity structure and seismicity of the region was analyzed. The results show that the crustal velocity structure in the Gonghe area shows lateral inhomogeneity. The Gonghe mainshock is located in the low-velocity anomaly directly below the Gonghe Basin, close to the high-low-velocity anomaly boundary. There is an obvious high-speed anomaly in the southwest of the mainshock, which thrusts from underground to near-surface in the northeast direction. It is estimated that the Wayuxiang-Lagan concealed fault is located at 35.95°N, the dip of the fault is about 45° at the deep part. It is inferred that the occurrence of the Gonghe main shock is caused by the sliding of the Wayuxiangka-Lagan Fault whose strike is NWW and dip is SSW under the action of horizontal tectonic stress. The high-velocity anomaly is about 5~40km deep underground in the northeast direction of the Riyueshan Fault, and a large number of small earthquakes occurred around the high- and low-velocity transition zone. It is presumed that under the action of the near-horizontal NE-directed tectonic stress, the high- and low-velocity zones were further interacted to generate faults and ground folds, and a large number of small earthquakes occurred during the fusion process.

Key words: Gonghe earthquake, double-difference tomography, 3D velocity structure, earthquake relocation

摘要： 文中收集了2009年1月—2019年1月青海、 甘肃固定地震台网及野外流动观测台阵记录的青海共和及周边地区的P波和S波到时数据, 应用双差层析成像方法反演了该地区地壳的三维速度结构和震源位置参数, 分析了共和1990年4月26日M_W6.4地震孕育发生的地质构造背景与该地区速度结构和地震活动性之间的关系。 结果显示, 共和地区的地壳速度结构呈现出明显的横向不均匀性, 共和主震位于共和盆地正下方区域的低速异常体内, 主震的SW侧为高速异常, 该异常从地下向NE向上逆冲至近地表处, 推测35.95°N处即为哇玉香卡-拉干隐伏断裂。 共和主震发生在水平NE向构造应力作用下, 由走向NWW、 倾向SSW的隐伏断层的滑动造成。

关键词: 共和地震, 双差层析成像, 三维速度结构, 地震重定位