2021年5月21日云南漾濞M_(S)6.4地震序列发震构造

Seismogenic structure of the 21 May 2021 M_(S)6.4 Yunnan Yangbi earthquake sequence

2021年5月21日云南省大理州漾濞县发生M_(S)6.4地震.地震序列表现为“前-主-余”型,余震频次较高.为获得此次地震序列的活动特征和震源区潜在的发震构造,本文首先计算2010年以来震源区地震b值随时间的变化.其次对漾濞地震序列进行精定位,并反演地震震源机制解和构造应力场.结果表明:(1)震源区地震b值自2020年12月逐渐降低.2021年5月18日起地震活动逐渐增强,b值短暂回升.M_(S)6.4地震发生前又迅速下降.目前b值已恢复至均值水平,意味着区域发生强震的危险性逐渐降低.(2)结合精定位和震源机制解结果发现,漾濞地震序列的发震断层以SE走向的高倾角右旋走滑兼正断型断层为主.兼有多条NE走向和NNE走向的左旋走滑兼正断型的高倾角次级断层.(3)震源区受控于NNW向水平挤压,NEE向水平拉张的构造应力作用.主震前构造应力控制作用增强.

An M S6.4 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 S6.4 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.