中文摘要：

通过对甘肃岷县2013年7月3个M＞4.0地震震相的细致分析,认为在震中距大于250 km时四川数字地震台网部分台站可记录到较为清晰的sPn震相.采用sPn和Pn震相确定震源深度的方法分别对岷县3个地震的震源深度进行了重新计算,得到了较为精确的地震震源深度.

英文摘要：

Earthquake focal depth is an important parameter for research on seismology, the struc- tural study of earthquakes, seismic hazard assessment, and earthquake event recognition. As a basic seismic parameter of time and space, earthquake focal depth affecting the size of the earth- quake disaster and is one of the most difficult parameters to measure accurately because its deter- mination is related to source process fault structures and stress fields. Stations layouts are sparse in our country; therefore, the velocity model generally used to determine earthquake focal depth. The China Earthquake Networks Center detected a magnitude 6. 6 earthquake in Dingxi City, Gansu Province, on July 22, 2013, at 7：45 Beijing time in the border area between Minxian and Zhangxian with a focal depth of approximately 20kin. The epicenter was approximately 15 km from Minxian, 45 km from Zhangxian, 120 km from Dingxi City, and 170 km from Lanzhou Cit- y. As of July 30, 2013, 1066 aftershoeks were recorded, nine of which were greater than magni- tude 3.0. According to the analysis, the seismic phases of three M〉4.0 earthquakes occurred in Gansu Minxian in July 2013. The Siehan digital seismograph network recorded relatively clear sPn seis- mic phases of more than 250 km in the area near the epicenter. In this study, sPn and Pn seismic phase methods were used to calculate the seismic depth. The focal depths of the three Minxian earthquakes were recalculated to obtain a more accurate earthquake focal depth. The earthquake focal depth is an important focal shock parameter. Because the method of using more than one pair of sPn and Pn seismic phases to caleulate the seismic depth was adopted in this study, the main error source is the seismic phase and the Earth＇s crust model error; therefore, identification of sPn and Pn is crucial. By recalculating the focal depth of the three Minxian M~4.0 earth-quakes, it is concluded that focal depth of the M 6.6 principal earthquake was 15.4 km, the focal depth of the M 5.6 aftershock was 9.6