Spatio-temporal relationship between source parameters such as stress drop and significant earthquakes in Changning-Xingwen area
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摘要: 基于中国地震台网提供的震相观测报告，利用双差方法对长宁—兴文地区2019—2020年1 948个ML≥2.0地震事件进行了重定位，重定位后的1 823个ML≥2.0地震事件分布显示长宁—兴文地区北区地震序列沿NW—SE方向背斜展布，而南区地震分布较分散。计算了四川长宁—兴文地区2019年6月17日至2020年12月31日208个ML≥3.0地震的地震矩和应力降等震源参数，分析了2019年6月17日长宁MS6.0地震后研究区内地震应力降的演化特征，结果显示长宁—兴文地区中强地震发生前ML3.0—3.9地震的应力降显著升高，且ML≥4.0地震多发生在ML3.0—3.9地震的应力降高值区。Abstract: A MS6.0 earthquake with a focal depth of 16 km occurred with the epicenter of （28.34°N, 104.90°E） in Changning county, Yibin city, Sichuan Province on June 17, 2019. There were three strong aftershocks above MS5.0 within five days after the Changning earthquake and another MS5.6 earthquake occurred in Gongxian County on July 4. This paper relocated 1 948 ML≥2.0 earthquakes in Changning-Xingwen area during 2019 to 2020 by the double difference method based on the seismic phase observation report provided by the China Seismic Network. The distribution of 1 823 earthquakes above ML2.0 after relocation shows that the aftershock sequence in the northern area of Changning spreads along the NW-SE direction anticline. Moreover, the earthquakes in southern area is more scattered. We calculated source parameters such as seismic moment and stress drop based on 208 earthquakes with magnitude above ML3.0 in Changning, Sichuan from January 2019 to December 2020, and analyzed the evolution characteristics of seismic stress drop in the region before and after the Changning MS6.0 earthquake on June 17, 2019. The results show that the stress drop of ML3.0−3.9 earthquakes before the strong aftershocks significantly increased in Changning-Xingwen area, and most of the earthquakes with magnitude above ML4.0 occurred in the high stress drop area of ML3.0−3.9 earthquakes in the study area.
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Key words:
- Changning MS6.0 earthquake /
- relocation /
- stress drop
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图 1 双差定位中使用的台站、主要活动断层及研究区地震分布
Figure 1. Distribution of the stations used in double difference positioning，active faults and earthquakes in the studied area
图 2 相对定位前震源距-到时曲线及震相数据筛选
Figure 2. Focal distance versus travel time curves before relative location and seismic phase data screening
图 3 重定位后的震中分布
（a） 重定位震中分布；（b，c） 重定位后沿经度、纬度方向深度分布；（d） 重定位后深度分布统计
Figure 3. Distribution of relocated earthquake events
（a） Relocation of epicenter distribution；（b，c） The depth distribution along longitude and latitude，respectively；（d） Statistics of depth distribution after relocation
图 4 拐角频率拟合过程示例
L1表示用上述方法求得的Ω0和fc代入式（4）得到的频谱曲线，L2为实际观测频谱曲线
Figure 4. The example of corner frequency fitting process
L1 represents the spectrum curve obtained by substituting Ω0 and fc obtained by the above method into Equation （4），and L2 is the observation spectrum curve
图 5 研究区震源参数与震级ML的关系
Δσ为应力降，fc为拐角频率，M0为地震矩，r为震源半径，ML为震级，R为相关系数，下同
Figure 5. The relationship between source parameters and magnitude ML in the studied area
Δσ is stress drop，fc is corner frequency，M0 is seismic moment，r is source radius，ML is magnitude，R is correlation coefficient，the same below
图 6 北区（a，b）、南区（c，d）震源参数地震矩M0、震源半径Y与震级的关系
Figure 6. The relationship between source parameters seismic moment M0，source radius r and ML in the northern region （a，b） and southern region （c，d）
图 7 应力降∆σ随时间（a）和震级ML（b）的变化
Figure 7. Variation of the stress drop ∆σ with time （a） and magnitude ML （b）
图 8 （a） 北区和南区ML3.0—3.9应力降随时间变化及强余震；（b） 2019年10月至2020年12月高应力降地震及其强余震的空间分布
Figure 8. （a） Stress drop changes of ML3.0—3.9 earthquakes and strong aftershocks with time in the northern region and the southern region；（b） Spatial distribution of high stress drop earthquakes and strong aftershocks from October 2019 to December 2020
图 9 ML3.0—3.9应力降网格均值分布（网格大小0.05°×0.05°）
黑点表示本文用于计算应力降均值的地震，空心圆圈表示2019年6月17日至2020年12月31日期间长宁地区ML4.0以上地震
Figure 9. Mean grid distribution of stress drop of ML3.0—3.9 （grid size 0.05°×0.05°） distribution Black dots denote the earthquakes used for calculating mean stress drop，white circles denote the earthquakes above ML4.0 in Changning area from 17 June 2019 to 31 December 2020
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