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

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.