RELOCATION AND FOCAL MECHANISMS OF YUNNAN TONGHAI EARTHQUAKE SEQUENCE OF AUGUST 2018

doi:10.3969/j.issn.0253-4967.2020.04.007

Abstract

Abstract: The August 2018 Tonghai earthquake sequence and focal mechanisms of the two main shocks about M_S5.0 were investigated through relocation of catalog data and inversion of event waveform recordings downloaded from the China National Seismic Network. The epicenter of 2018 Tonghai earthquake locates in the southern edge of Chuandian block, where the Xiaojiang Fault separates the Chuandian block from South China block in the east, and the Red River Fault separates the Chuandian block from the Indo-China in the southwest. These two faults blocked and absorbed the continuous southward movement of Chuandian block, significant tectonic stress has been built up in the southern tip of Chuandian block. As a seismicity active zone, Tonghai has been struck by a M7.0 strong earthquake in 1970. The August 2018 Tonghai earthquake is the major earthquake occurring in Tonghai and surrounding areas since the 1970 strong earthquake. Therefore, detailed focal mechanism study for 2018 Tonghai earthquake sequence is crucial for the earthquake relief effect for the Tonghai and surrounding area. In this study, we first relocate the epicenter of earthquake sequence by using the double difference relocation method, then we inverted focal mechanism for two main earthquakes with magnitude of ~5.0 by using the W-phase method. The relocated epicentral locations of Tonghai earthquake sequence show a NE-SW trend narrow band, predominant depth range of 5~10km and near vertical seismogenic fault. The focal parameters for the August 13 earthquake are: strike of 298.2°, a dip of 45.2°, a rake of -172.9° and strike of 203.2°, a dip of 84.9°, a rake of -45.0°, respectively, and magnitude of M_W=5.07; Focal parameters for the August 14 earthquake are: strike of 297.0°, a dip of 63.6°, a rake of -161.5° and strike of 198.5°, a dip of 73.5°, a rake of -27.7°, respectively, and magnitude of M_W=4.89. Combined analysis of the relocated epicentral locations and obtained focal mechanisms suggest that the seismogenic fault of the August 2018 Tonghai earthquake sequences could be related to the NE-striking Mingxing-Erjie Fault, and the temporal and spatial distribution characteristics of the earthquake sequence is consistent with the regional seismotectonic background.

Key words: the 2018 M5.0 Tonghai earthquake sequences, double-difference earthquake location, W-Phase, moment tensor inversion

摘要： 文中使用中国地震台网中心地震编目系统、中国地震台网数据备份中心提供的正式观测报告和区域事件的波形数据, 基于双差定位和W-Phase反演方法, 对2018年8月云南省通海地震序列进行了重定位并反演了2次M5.0主震的震源机制。 2018年8月云南通海地震序列发生在川滇块体的南部边缘地区。川滇块体东部, 小江断裂带分割了川滇块体与华南块体; 川滇块体的西南部, 红河断裂带分割了川滇块体与中南半岛。这2条主要断裂带阻挡并吸收了向S运动的川滇块体的大部分运动分量, 因此在川滇块体的南部边界积累了大量构造应力, 导致该地区的地震活动性较强。作为地震活动较强的地区, 通海在1970年曾发生过M7.0强震。 2018年通海地震序列中2次M5.0主震是1970年之后发生在通海及周边地区震级较大的地震。对通海地震序列的震源机制开展研究, 可为通海及周边地区的防震减灾工作提供重要的地震学证据支持。文中, 首先采用双差重定位方法对通海地震序列的震中进行定位; 在此基础之上, 采用W-phase波形反演方法反演获取了2次主震的震源机制。经过重新定位的地震震中分布显示, 通海地震序列以NE-SW优势方向呈条带状分布, 余震震源集中分布在深度5~10km的范围内, 且发震断层呈现高倾角的特征。震源机制反演结果显示, 2018年8月13日发生的M5.0地震的2个节面的走向、倾角和滑动角分别为298.2°、 45.2°、 -172.9°和203.2°、 84.9°、 -45.0°, 矩震级M_W5.07; 14日发生的第二次M5.0地震的2个节面的走向、倾角和滑动角分别为297.0°、 63.6°、 -161.5°和198.5°、 73.5°、 -27.7°, 矩震级M_W4.89。综合分析通海地震序列重定位及震源机制反演结果可知, 其发震断裂可能为NE走向的明星-二街断裂带, 且地震序列的时空分布特征及震源机制信息与区域地震构造背景一致。

关键词: 2018年通海M5.0地震序列, 双差重定位, W-Phase震相, 矩张量反演

CLC Number:

P315.2

LI Tong, GUO Zhi, GAO Xing. RELOCATION AND FOCAL MECHANISMS OF YUNNAN TONGHAI EARTHQUAKE SEQUENCE OF AUGUST 2018[J]. SEISMOLOGY AND GEOLOGY, 2020, 42(4): 881-892.

李通, 郭志, 高星. 云南通海2018年8月地震序列重定位及震源机制[J]. 地震地质, 2020, 42(4): 881-892.

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