Calculation of focal depths of Shizuishan M_L4.4 earthquake sequence by using the first arrival P phases

In this paper，the focal depths of eleven events of the Shizuishan M_L4.4 earthquake sequence were calculated by using the data of first arrival P phases. The results show that the earthquake sequence was composed of shallow earthquakes，which is one reason for the earthquakes felt strongly. Then the focal depth of the sequence was calculated based on double-layer and four-layer velocity models by the method of first arrival P phase. And the above results were compared with the double difference location. Firstly，the comparison of the focal depth obtained by the double difference location method with that by the first arrival P phase method show that the two results are consistent with each other on the whole，but the depth got by the latter method is rela-tively deep. Secondly，the results based on double-layer and four-layer crustal velocity models are in good agreement. The depth of the main shock is 7--8 km，and the depth of the largest aftershock is 6 km. According to the velocity imaging results of Yinchuan basin，it is deduced that the main shock and the largest aftershock occurred in the lower part of the Yinchuan basin basement. In addition，the other aftershocks are small with depth concentrating in 3--5 km，and they occurred mainly on the top of the basement of Yinchuan basin or within overburden layer.