Relationship between Stress Evolution and Aftershocks after Changning M 6.0 Earthquake in Sichuan on 17 June, 2019
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摘要: 2019年6月17日四川长宁县发生6.0级地震，该次地震余震活动频度高、强度大，其中超过5.0级的强余震就有4次，具有不同于以往6.0级地震的独特特征.余震活动与震后区域应力变化密切相关，为研究它们之间的关系，考虑区域主要活动构造、地表起伏和深部反演结果，建立长宁地区岩石圈三维黏弹性有限元模型.采用数值方法重建基本符合研究区GPS观测和最大水平主压应力方向测量结果的现今构造背景应力场.进而依次模拟了长宁6.0级地震和5.0级以上强余震序列.通过计算库仑破裂应力变化研究了震后应力演化与余震分布，以及主震和5.0级强余震序列之间的关系.研究表明，长宁6.0级地震的发生可能与区域内非构造加载因素有关，余震活动明显受震后区域应力变化的控制.长宁地震后，于滩-长宁背斜在10 km深度应力得到充分释放，库仑破裂应力明显减小；而在3 km深度库仑破裂应力明显增加，应力水平仍然较高.Abstract: On 17 June, 2019, an M 6.0 earthquake occurred in Changning County, Sichuan Province, with high aftershock activity frequency and intensity. And among the aftershocks, there are 4 strong ones with M≥5.0, which is different from the previous M 6.0 earthquakes. The aftershock activity is closely related to the regional stress change after the mainshock. In order to study the relationship between them, a 3D viscoelastic finite element model of the lithosphere in the Changning area is established. In the model, the main active structures, topographic relief and deep inversion results were considered. By using numerical method, the present tectonic background stress field in the study area is reconstructed to conform to the GPS observed value and the measured one of maximum horizontal principal compressive stress direction. Then the Changning M 6.0 earthquake and its strong aftershock sequence with M≥5.0 were simulated sequentially. The relationship between the stress evolution and aftershocks, as well as the mainshock and the strong aftershock sequence with M≥5.0 were studied by calculating the Coulomb failure stress. The results show that the occurrence of the Changning M 6.0 earthquake may be independent with the regional tectonic loading factors, and the aftershock activity is obviously controlled by the regional stress change after the mainshock. After the Changning earthquake, the stress at the depth of 10 km in the Yutan-Changning anticline was fully released, and the Coulomb failure stress decreased significantly. However, at the depth of 3 km, the Coulomb failure stress increases obviously and the stress level is still high.
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图 1 2019年6月17日长宁6.0级地震序列重定位结果及邻区的主要活动构造
主要活动构造数据由中国地震局地震预测研究所李文巧和熊仁伟提供，黑色实线为背斜，灰色实线为向斜，红色实线为活动断裂；长宁6.0级地震及5.0级以上余震重定位结果见易桂喜等（2019）；长宁6.0级地震余震重定位结果由中国地震局地球物理研究所杨婷提供（个人通讯）
Fig. 1. Relocation results of the Changning M6.0 earthquake sequence on June 17, 2019 and the main active structures in Changning area and its vicinity
图 2 长宁地区三维有限元模型
不同颜色代表不同的断层或地块
Fig. 2. Three-dimensional finite element model of the Changning area
图 4 三维模型在10~80 km深度范围内的弹性参数
a.杨氏模量；b.泊松比
Fig. 4. Elastic parameters of three-dimensional model in the depth range of 10-80 km
图 5 模拟值与观测值的比较
a.GPS；b.最大水平主压应力方向；红色五角星表示长宁6.0级地震的震中位置
Fig. 5. Comparison of simulated values and the observed values
图 6 长宁地区库仑破裂应力年变化
a.震源机制解取胡晓辉等(2020)的结果；b.震源机制解取易桂喜等(2019)的结果；红色五角星表示长宁6.0级地震的震中位置
Fig. 6. Annual variation of Coulomb failure stress in the Changning area
图 7 长宁地区断裂带上库仑破裂应力年变化
a.震源机制解取胡晓辉等(2020)的结果；b.震源机制解取易桂喜等(2019)的结果
Fig. 7. Annual variation of Coulomb failure stress on faults in the Changning area
图 8 长宁6.0级地震序列ML≥1.0地震的M-T和日频次N-T图
Fig. 8. M-T and daily frequency N-T diagrams of the Changning M 6.0 earthquake sequence with ML≥1.0
图 9 主震和强余震的同震库仑破裂应力变化与余震分布之间的关系
a.长宁6.0地震，长宁6.0级地震后至珙县5.1级地震前的余震；b.珙县5.1级地震，珙县5.1级地震后至长宁5.3级地震前的余震；c.长宁5.3级地震，长宁5.3级地震后至珙县5.4级地震前的余震；d.珙县5.4级地震，珙县5.4级地震后至珙县5.6级地震前的余震；e.珙县5.6级地震，珙县5.6级地震后至2019年7月17日的余震；f.珙县5.4级地震和珙县5.6级地震，珙县5.6级地震后至2019年7月17日的余震；图例中[1]表示采用胡晓辉等(2020)震源机制解，[2]表示采用易桂喜等(2019)震源机制解
Fig. 9. Relationship between the coseismic Coulomb failure stress change and aftershock distribution
图 10 主震和强余震的总库仑破裂应力变化与余震分布之间的关系
a.长宁6.0地震后，长宁6.0级地震后至珙县5.1级地震前的余震；b.珙县5.1级地震后，珙县5.1级地震后至长宁5.3级地震前的余震；c.长宁5.3级地震后，长宁5.3级地震后至珙县5.4级地震前的余震；d.珙县5.4级地震后，珙县5.4级地震后至珙县5.6级地震前的余震；e.珙县5.6级地震后，珙县5.6级地震后至2019年7月17日的余震；图例中[1]表示采用胡晓辉等(2020)震源机制解，[2]表示采用易桂喜等(2019)震源机制解
Fig. 10. Relationship between the total Coulomb failure stress changes and aftershock distribution
图 11 震后断裂带库仑应力变化
a.3 km深度；b.3 km深度；c.5 km深度；d.5 km深度；e.10.7 km深度；f.10.7 km深度；其中a，c，e采用胡晓辉等(2020)震源机制解，d，e，f采用易桂喜等(2019)震源机制解
Fig. 11. Coulomb failure stress changes in fault zones after earthquake
表 1 长宁地震序列震源机制解
Table 1. Focal mechanism solution of Changning earthquake sequence
序号 时间（年-月-日）/地点 经度(°) 纬度(°) 深度(km) 走向(°) 倾角(°) 滑动角(°) 震级(Ms) 震级(Mw) 资料来源 1 2019-06-17长宁 104.90 28.34 10.7 309 68 41 5.8 胡晓辉等(2020) 104.905 28.344 3 131 51 36 6.0 5.79 易桂喜等(2019) 2 2019-06-17珙县 104.77 28.43 2.4 309 73 56 5.0 胡晓辉等(2020) 104.805 28.418 2 311 60 45 5.1 4.99 易桂喜等(2019) 3 2019-06-18长宁 104.87 28.38 5.8 133 34 90 4.4 胡晓辉等(2020) 104.869 28.368 3 164 24 111 5.3 4.74 易桂喜等(2019) 4 2019-06-22珙县 104.77 28.43 4.2 191 25 110 5.3 胡晓辉等(2020) 104.793 28.424 2 170 31 71 5.4 5.08 易桂喜等(2019) 5 2019-07-04珙县 104.74 28.41 8.2 129 37 90 5.1 胡晓辉等(2020) 104.74 28.41 7 165 41 79 5.6 5.02 易桂喜等(2019) 
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表 2 长宁6.0级地震主震及强余震破裂参数及模拟结果
Table 2. Rupture parameters and simulation results of mainshock and strong aftershocks of the Changning M 6.0 earthquake
序号 时间(年-月-日) 地点 震级(Ms) 地表下破裂长度(km) 平均位错(km) 模拟值(km) 1 2019-06-17 长宁 6.0 11.482 0.550 0.549 0 2 2019-06-17 珙县 5.1 3.451 0.466 0.467 0 3 2019-06-18 长宁 5.3 4.508 0.483 0.483 7 4 2019-06-22 珙县 5.4 5.152 0.492 0.492 0 5 2019-07-04 珙县 5.6 6.730 0.511 0.513 4
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表 3 长宁6.0级主震和5.0级强余震序列震中位置之间的距离
Table 3. The distance between epicenters of the Changning M 6.0 mainshock and the strong aftershock with M≥5.0
地震序号 震中位置之间的距离（km） 2 3 4 5 1 12.93 4.47 14.27 17.95 2 8.47 1.37 6.50 3 9.81 13.63 4 5.48
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表 4 长宁6.0级主震和5.0级强余震序列前序地震在后继地震位置的库仑破裂应力变化
Table 4. Coulomb failure stress changes at the epicenter of the strong aftershock with M≥5.0
后继地震 前序地震导致的在后继地震位置的库仑破裂应力变化(MPa) 震源机制数据 1长宁6.0级 2珙县5.1级 3长宁5.3级 4珙县5.4级 总库仑应力 2珙县5.1级 -0.057 7 -0.057 7 胡晓辉等（2020） -0.166 3 -0.166 3 易桂喜等（2019） 3长宁5.3级 0.342 9 -0.022 6 0.320 3 胡晓辉等（2020） 0.616 6 -0.071 3 0.545 3 易桂喜等（2019） 4珙县5.4级 -0.058 6 -0.057 4 -0.066 5 -0.182 5 胡晓辉等（2020） -0.067 5 -0.102 0 -0.080 2 -0.249 7 易桂喜等（2019） 5珙县5.6级 -0.006 8 0.253 0 -0.002 8 0.429 8 0.673 2 胡晓辉等（2020） 0.047 0 0.492 1 0.058 4 0.739 2 1.336 7 易桂喜等（2019）
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