2021年5月21日漾濞4次M_(S)≥5.0地震前后b值变化

Variation of b-value before and after Yangbi four M_(S)≥5.0 earthquakes on May 21,2021

2021年5月21日21时21分至22时31分,云南大理州漾濞县接连发生4次M_(S)≥5.0地震,分别为M_(S)5.6,M_(S)6.4,M_(S)5.0和M_(S)5.2地震。选取2015年1月1日至2021年6月4日漾濞地震震源区及其附近区域记录的地震资料,利用极大似然法计算研究区b值,通过分析漾濞地震前后不同时段的b值空间分布特征,得到以下主要认识:①漾濞震群发生后,震源区b值出现快速下降,可能预示着局部地壳介质强度接近临界状态;②漾濞4次M_(S)≥5.0地震均沿剖面低b值异常区的边缘分布;③漾濞地震发生后,震源区b值明显回升,预示着地壳应力得到释放,短期强震危险性降低;④漾濞地震西北40 km和70 km处存在低b值异常区,未来存在发生中强地震的风险。

Four M_(S)≥5.0 earthquakes occurred successively in Yunnan Yangbi from 21:21 to 22:31 on May 21,2021,which are M_(S)5.6,M_(S)6.4,M_(S)5.0 and M_(S)5.2,respectively.Based on the earthquakes recorded in focal area of Yangbi earthquake and its vicinity from January 1,2015 to June 4,2021,we estimated the b-value by the maximum likelihood method.Then,we analyzed the spatial distribution of b-value before and after Yangbi earthquakes and got the views below.Firstly,an earthquake swarm occurred three days before the Yangbi earthquakes,then,the b-value near the focal area of Yangbi earthquake decreased rapidly after the earth-quake swarm.It may indicate that the strength of crustal medium was close to the critical state of fracture.Secondly,the four M_(S)≥5.0 earthquakes distribute along the edge of the abnormal area with low b-value.Thirdly,the b-value of the focal area increased obviously after the Yangbi earthquakes.It indicates that the regional stress has been released and the risk of strong earthquake is reduced in the short term.Fourthly,there are two b-value abnormal areas 40 km and 70 km northwest of the Yangbi earthquakes.It means that there is a risk of strong earth-quakes in the abnormal areas in the future.