STUDY ON FOCAL DEPTH OF THE MS5.4 CANGWU EARTHQUAKE IN GUANGXI

doi:10.3969/j.issn.0253-4967.2019.03.006

Abstract

Abstract: On July 31th, 2016, a magnitude 5.4 earthquake struck Cangwu Country, Guangxi Zhuang Autonomous Region, it was the largest earthquake recorded by Guangxi Seismological Network since it set up. The number of people affected by the earthquake had reached 20 000, and the direct economic losses caused by the earthquake were nearly 100 million Yuan.
After the earthquake, USGS provided a global earthquake catalog showing that the focal depth of Cangwu earthquake was about 24.5km. However, the result given by the Global Centroid Moment Tensor showed the focal depth of this earthquake was 15.6km. However, the result obtained by Xu Xiaofeng et al. using CAP method was 5.1km. It was clear that the focal depths of Cangwu earthquake given by different institutions were quite different from each other. However, accurate focal depth of the earthquake has important significance for exploring the tectonic mechanism near the epicenter, so it is necessary to further determine the more accurate depth of the Cangwu earthquake.
In order to further accurately determine the focal depth of Cangwu earthquake, we used the global search method for travel-time residual to calculate the focal depth of this earthquake and its error range, based on the regional velocity model, which is a one-dimensional velocity model of the Xianggui tectonic belt produced by the comprehensive geophysical profile. Then, we inverted the focal mechanism of this earthquake with the CAP method. Based on this, the focal depth of Cangwu M_S5.4 earthquake was further determined by the method of the Rayleigh surface wave amplitude spectrum and the sPL phase, respectively.
Computed results reveal that the focal depth of this earthquake and its error range from the travel-time residual global search method is about(13±3)km, the focal depth inverted by CAP method is about 10km, the focal depth from sPL phase is about 10km, and the focal depth from Rayleigh surface wave amplitude spectrum is about 9~10km. Finally, we confirmed that the focal depth of Cangwu M_S5.4 earthquake is about 10km, which indicates that this earthquake still occurred in the upper crust. In the case of low network density, the sPL phase and Rayleigh wave amplitude spectrum recorded by only 1 or 2 broadband stations could be used to obtain more accurate focal depth.
The focal depth's accuracy of Cangwu M_S5.4 earthquake in the USGS global earthquake catalog has yet to be improved. In the future, we should consider the error of the source parameters when using the USGS global earthquake catalog for other related research.

Key words: focal depth, sPL phase, Rayleigh wave amplitude spectrum, Guangxi Cangwu M_S5.4 earthquake

摘要： 2016年7月31日广西苍梧发生的M_S5.4地震是广西测震台网自建立以来所记录到的区域内最强地震。不同机构对于此次事件给出的震源深度结果相差较大，例如美国地质调查局（USGS）地震目录显示其深度为24.5km，而全球矩心矩张量研究中心（Global CMT）测定的深度为15.6km。为了进一步准确确定广西苍梧地震的震源深度，文中基于区域速度模型，首先利用走时残差全局搜索法初步得到此次地震震源深度及误差范围，然后使用CAP （Cut and Paste）方法反演苍梧地震震源机制，在此基础上，采用Rayleigh波振幅谱和sPL震相方法进一步约束了此次地震的震源深度。研究结果显示：利用走时残差全局搜索法获得的苍梧M_S5.4地震震源深度及误差范围为（13±3） km，CAP方法反演的深度为10km，Rayleigh波振幅谱测定深度结果为9~10km，sPL震相测定深度结果为10km，最终确定广西苍梧M_S5.4地震的震源深度约10km，这表明此次事件仍为发生于上地壳的地震。本文结果同时表明，USGS地震目录在研究区的震源深度精度有待提高。

关键词: 震源深度, sPL震相, Rayleigh波振幅谱, 广西苍梧M_S5.4地震

CLC Number:

P315.63

ZHAO Tao, CHU Ri-sheng, NI Si-dao, WANG Ying, ZHOU Yong, ZENG Xiang-fang. STUDY ON FOCAL DEPTH OF THE M_S5.4 CANGWU EARTHQUAKE IN GUANGXI[J]. SEISMOLOGY AND GEOLOGY, 2019, 41(3): 619-632.

赵韬, 储日升, 倪四道, 王莹, 周勇, 曾祥方. 广西苍梧M_S5.4地震震源深度[J]. 地震地质, 2019, 41(3): 619-632.

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