Variation of b-value before and after Yangbi four MS≥5.0 earthquakes on May 21，2021
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摘要: 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MS≥5.0地震，分别为MS5.6，MS6.4，MS5.0和MS5.2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MS≥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。Abstract: Four MS≥5.0 earthquakes occurred successively in Yunnan Yangbi from 21:21 to 22:31 on May 21, 2021, which are MS5.6, MS6.4, MS5.0 and MS5.2, 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 earthquake swarm. It may indicate that the strength of crustal medium was close to the critical state of fracture. Secondly, the four MS≥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 earthquakes in the abnormal areas in the future.
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Keywords:
- Yangbi earthquake /
- b-value /
- the maximum likelihood method /
- seismic risk
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图 1 2015年1月1日至2021年6月4日研究区震中分布
Figure 1. Distribution of epicenters from January 1，2015 to June 4，2021 in the studied area
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图 2 研究区ML≥0.0地震M-t图（a）和震级频次图（b）
Figure 2. M-t （a） and magnitude-frequency （b） diagrams of the ML≥0.0 earthquakes from January 1，2015 to June 4，2021 in the studied area
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图 3 研究区ML≥1.5地震密度分布图
Figure 3. Distribution of seismic density with ML≥1.5 for the studied area
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图 5 b值空间扫描时不同扫描半径所占节点数的比例
Figure 5. Proportion of nodes with different search during b value space scanning
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图 6 b值及标准差随截止震级的变化
Figure 6. b-value and its standard deviation changes with cut-off magnitude
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图 7 研究区地震震级-频度关系曲线（2015年1月1日—2021年6月4日）
Figure 7. Magnitude-frequency curve of earth-quakes from January 1，2015 to June 4，2021 in the studied area
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图 8 研究区不同时段b值空间分布
Figure 8. Spatial distribution of b-value in different periods in the studied area
① 2016-05-18 00:48 MS5.0；② 2017-03-27 07:55 MS5.2；③ 2021-05-21 21:21 MS5.6； ④ 2021-05-21 21:48 MS6.4；⑤ 2021-05-21 21:55 MS5.0；⑥ 2021-05-21 22:31 MS5.2 （a） 2015-01-01—2021-05-17；（b） 2015-01-01—2021-05-21 21:00；（c） 2015-01-01—2021-06-04
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图 10 沿断裂方向的地震深度剖面图（a）和b值剖面分布图（b）（地震资料时段为2015-01-01—2021-05-21 21:00）
Figure 10. Depth profile （a） and b-value profile （b） along the fault （The seismic data is recorded from January 1，2015 to 21:00，May 21，2021）
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