2021年云南漾濞M_(S)6.4地震亚失稳阶段的地震活动性特征分析

Seismic Characteristics in the Meta-instability Stage of the 2021 Yangbi, Yunnan M_(S)6.4 Earthquake

使用亚失稳台阵和固定台站组成的密集台网观测资料,对2021年漾濞M_(S)6.4地震前滇西北地区完备震级以上地震的频度、断层总面积和b值等地震活动性参数进行时空扫描分析,发现研究区4级以上地震失稳前均有较为明显的增频、增总面积、低b值等共性特征,漾濞M_(S)6.4地震前以小震活动为指针的断层协同化演化进程大致可分为3个阶段:2020年5—12月为应变释放点产生阶段,小震频次缓慢增加;2021年1月至5月17日为应变释放点扩展和增多阶段,小震频次逐步增多,3级以上地震释放点明显增强,且有向主震震中迁移现象,后期成带状扩展;2021年5月18-21日为应变释放连接段,中小地震活动快速增长并最终连接成片,甚至重叠,且4级以上地震集中释放。而亚失稳过程则应是从2021年1月至5月17日为准静态阶段,2021年5月18-20日为准动态阶段。最后,讨论了出现明显的亚失稳过程的原因和断层协同演化进程划分存在的问题和不足。

In this paper,using the observation data of the dense network composed of meta-instability array and 10 fixed stations,we carry out the temporal and spatial scanning of seismic parameters such as frequency,total fault area,and b-value of the complete magnitude before the Yangbi M_(S)6.4 earthquake in northwest Yunnan province.We find that there are obvious common characteristics such as increasing frequency,increasing total fault area and low b-value before the instability of the M>4.0 earthquakes in the study area.The coordinated evolution of faults with small-earthquake activity as the indication before the Yangbi M_(S)6.4 earthquake can be roughly divided into three stages:(1)the period from May to December 2020,in which strain-release points appeared,and the frequency of small earthquakes increased slowly;(2)the period from January to May 17,2021,in which the strain-release points increased and extended,and the frequency of small earthquakes increased,and the number of M>3.0 earthquakes increased significantly,and their epicenters migrated to the epicenter of the main earthquake.At the end of the stage,the strain-release points expanded into a belt.(3)the period from May 18 to 21,2021,in which the strain-release points linked together and even overlapped.And small and medium earthquakes increased rapidly,and the M>4.0 earthquakes concentrated.We conclude that the meta-instability process should consist of the static phase(from January to May 17,2021),and the dynamic phase(from May 18 to 20,2021).Finally we discuss the causes of the meta-instability and the questions in the division of the meta-instability process.