Abstract: On July 31^st, 2016, an earthquake of M_S5.4 occurred in Cangwu County, Guangxi Zhuang Autonomous Region, which is the first M_S ≥ 5.0 earthquake in coastal areas of southern China in the past 17a. The moderate earthquake activities have come into a comparatively quiet period in coastal areas of southern China for decades, so the study about the Cangwu M_S5.4 earthquake is very important. However, differernt research institutions and scholars have got different results for the focal depth of the Cangwu M_S5.4 earthquake. For this reason, we further measured the focal depth by using CAP method and sPL phase method.
sPL phase was first put forward by Chong in 2010. It is often observed between P and S wave of continental earthquakes with epicentral distance of about 30km to 50km. The energy of sPL phase is mainly concentrated on the radial component. Arrival time difference between sPL phase and direct P wave is insensitive to epicentral distancs, but increases almost linearly with the increase of focal depth. Based on these characteristics and advantages, sPL phase method is chosen to measure the focal depth of Cangwu M_S5.4 earthquake in the paper.
First of all, we selected the broadband waveform data through seismic stations distributed mainly in Guangxi and adjacent provinces from Data Management Centre of China National Seismic Network and Guangxi Earthquake Networks Center. And an appropriate velocity model of Cangwu area was constructed by the teleseismic receiver function method. Then, the focal mechanism and focal depth of Cangwu M_S5.4 earthquake were determined by using the CAP(Cut and Paste)method. Next, we compared the synthetic waveforms simulated by F-K forward method of different focal depth models with the actual observed waveforms. According to the difference of arrival times between sPL and Pg phases, we finally obtained the focal depth of Cangwu earthquake. The results show that the focal depth is 11km measured by CAP method and 9km by sPL phase method. Based on the focal mechanism solution, isoseismal shapes, aftershocks distributions and investigation on spot, we conclude that the Cangwu M_S5.4 earthquake is a left-lateral strike-slip earthquake which occurred in the upper crust. Our preliminary analysis considers that the seismogenic structure of Cangwu earthquake is a north-northwest branch fault, and the control fault of this earthquake is the Hejie-Xiaying Fault.

Key words: Cangwu M_S5.4 earthquake, sPL phase, focal depth, focal mechanism

摘要： 2016年7月31日广西苍梧县发生了M_S5.4地震，该地震打破了华南沿海地区近17a的5级地震平静期，对华南沿海地区中强地震活动具有指示意义，然而不同研究机构和学者对此次地震震源深度的认识存在较大差异。基于此，文中使用CAP （Cut and Paste）方法和近震深度震相sPL进一步测定了本次地震的震源深度。结果显示，CAP方法测定的震源矩心深度为11km，sPL震相测定的地震深度为9km。结合震源机制解、等震线形态、余震分布和现场地震地质调查等资料分析认为，苍梧M_S5.4地震是发生在上地壳的左旋走滑性质的地震。

关键词: 苍梧M_S5.4地震, sPL震相, 震源深度, 震源机制解