Seismogenic structure of the 21 May 2021 Ms6. 4 Yunnan Yangbi earthquake sequence

An M(s)6. 4 earthquake hit the Yangbi County in Dali Prefecture, Yunnan Province on May 21, 2021. The earthquake sequence is of "foreshock-mainshock-aftershock" type, with high aftershock frequency. In the current paper, aiming to obtain the seismicity characteristics of the earthquake sequence and potential seismogenic structure in the focal area, we first calculate the time-dependent b-value in the focal area since 2010. Then we relocate the Yangbi earthquake sequence, and invert focal mechanism solutions and tectonic stress field. The main conclusions are as follows : (1) The b-value in the focal area gradually decreased since December 2020. The seismicity gradually increased since May 18,2021 , and the b-value shortly rebounded, followed by a rapid decrease until the M(s)6. 4 Yangbi earthquake occurred. Now the b-value recovers to the average level, indicating that the risk of strong earthquake is gradually reduced. (2) By combining the results of relocation and focal mechanism solutions, we find that the seismogenic fault of this earthquake sequence is dominated by a SE-trending high-dip right-lateral strike-slip fault and with several NE or NNE-trending high-dip left-lateral strike-slip secondary faults. (3) The focal area is controlled by a tectonic stress field with the NNW-trending horizontal compression and NEE-trending horizontal tension, and it is strengthened before the mainshock occurs.