Abstract:Earthquake focal depth is an important parameter for research on seismology,the structural 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 earthquake disaster and is one of the most difficult parameters to measure accurately because its determination 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 6.6earthquake in Dingxi City,Gansu Province,on July 22,2013,at 7:45Beijing time in the border area between Minxian and Zhangxian with a focal depth of approximately 20km.The epicenter was approximately 15km from Minxian,45km from Zhangxian,120km from Dingxi City,and 170km from Lanzhou City.As of July 30,2013,1066aftershocks were recorded,nine of which were greater than magnitude 3.0.According to the analysis,the seismic phases of three M>4.0earthquakes occurred in Gansu Minxian in July 2013.The Sichan digital seismograph network recorded relatively clear sPn seismic phases of more than 250km 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 calculate 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 >4.0earthquakes,it is concluded that focal depth of the M6.6principal earthquake was 15.4km,the focal depth of the M 5.6aftershock was 9.6km,and the focal depth of the M4.1aftershock was 9.2km.The focal depth of the two aftershocks is obvious shallow;these results show that the rupture process of the M 6.6principal earthquake was from deep to shallow regions.An accurate focal depth was obtained by deep seismic phases to provide basic data for earthquake prediction and earthquake hazard analysis.