2019年6月17日四川长宁MS6.0地震序列震源机制解与发震构造分析

2019年6月17日，在青藏高原东缘四川盆地南缘宜宾市长宁县发生M_S6.0地震，其后5天内相继发生了珙县M_S5.1、长宁M_S5.3和珙县M_S5.4强余震；7月4日，在珙县珙泉镇再次发生M_S5.6地震.因灾害叠加，本次地震序列导致13人死亡，200多人受伤，大量房屋受损，造成了重大的人员伤亡和财产损失.本文基于四川区域地震台网提供的地震资料，采用多阶段定位方法，对长宁M_S6.0地震序列早期（2019年6月17日至22日）余震进行了重新定位，同时，利用CAP波形反演方法，获得了序列中截止至7月4日的16次M_S≥3.6地震的震源机制解与震源矩心深度，对该序列的发震构造进行了初步分析.长宁M_S6.0地震序列重新定位后的610次M_L≥1.5地震分布显示余震区呈NW-SE向展布，长约25 km，宽5 km；序列震源深度在0~10 km区间，深度均值约3.2 km，但空间上呈西深东浅的分布特征.长宁M_S6.0地震位于余震区的东南端，具单侧破裂特征.CAP波形反演结果显示长宁M_S6.0地震序列以逆冲和逆冲兼走滑型地震为主；16次M_S≥3.6地震的震源矩心深度在1~7 km范围，平均深度3.5 km，与定位结果一致，揭示本次长宁地震序列发生在上地壳浅部.根据序列空间分布、震源机制解及震区构造特征，推测本次长宁M_S6.0地震序列的发生可能与长宁—双河复式大背斜中白象岩—狮子滩背斜和双河场褶皱及其伴生断层活动有关，位于余震区西北段的6月17日珙县M_S5.1、22日珙县M_S5.4及7月4日珙县M_S5.6地震应为6月17日长宁M_S6.0地震触发白象岩—狮子滩背斜伴生断层活动所致.序列发震构造整体呈NE-SW向挤压为主、兼具一定NW-SE向拉张分量的构造变形特征，与南侧2018年12月16日兴文M_S5.7和2019年1月3日珙县M_S5.3地震所呈现的NW-SE向挤压、NE-SW向拉张构造变形特征具有显著差异，揭示四川盆地南缘地带处于构造变形模式的转换区域，所处构造环境的变化导致本次长宁地震序列震源区及附近区域发震构造变形特征具有复杂性.

Focal mechanism solutions and seismogenic structure of the 17 June 2019 MS6.0 Sichuan Changning earthquake sequence

On June 17, 2019, an M_S6.0 earthquake hit the Changning County in Yibin City, the southern margin of Sichuan Basin in the eastern Tibetan Plateau. During the following five days, strong aftershocks of M_S5.1, M_S5.3 and M_S5.4 occurred successively. On July 4, 2019, another M_S5.6 earthquake occurred at Gongquan Town in Gongxian. During these events, 13 people and more than 200 people were killed and injured, respectively. A large number of houses were damaged, resulting in heavy casualties and property losses. Based on the seismic data provided by Sichuan Regional Seismic Network, we relocated the early events of the M_S6.0 Changning earthquake sequence from June 17 to 22, 2019 by using the multi-stage location method. Meanwhile, the focal mechanism solutions and centroid depths of 16 M_S ≥ 3.6 earthquakes before 4 July 2019 were also calculated by using the CAP inversion method. Finally, the seismogenic structure of the sequence is preliminarily analyzed. The distribution of relocated 610 M_L ≥ 1.5 events shows that the aftershock area is 25 km long and 5 km wide in a NW-SE trending. The focal depths range from 0 to 10 km and the mean depth is about 3.2 km, the total sequence is deep in the west and shallow in the east. The M_S6.0 Changning earthquake is located at the southeastern end of the aftershock zone, showing its unilateral rupture behavior. The focal mechanism solutions show that the seismogenic structure system of the sequence was dominated by thrust and oblique thrust faulting mode; the centroid depths of 16 M_S ≥ 3.6 earthquakes are within 1~7 km, and the average depth is about 3.5 km, which is consistent with the relocation result. This observation reveals that the Changning earthquake sequence occurred in the shallow part of the upper crust. Based on the spatial distribution of the sequence, focal mechanism solutions and structural characteristics of the seismogenic area, it is inferred that the occurrence of the M_S6.0 Changning earthquake sequence may be related to the Baixiangyan-Shizitan anticline and the Shuanghechang anticline and their associated fault activities. This structure is belonging to the complex Changning-Shuanghe anticlines. The three Gongxian events with M_S5.1 on 17 June, M_S5.4 on 22 June, and M_S5.6 on 4 July which are located along the northwestern segment of the aftershock area, should be caused by the associated fault activity of the Baixiangyan-Shizitan anticline triggered by the M_S6.0 Changning earthquake. The tectonic deformation of the seismogenic structure for the Changning earthquake sequence generally presents the characteristics of a large NE-SW component of compression with a small NW-SE extensional component, significantly different from that in the southern area with the NW-SE compression and NE-SW extension characteristics derived from the 16 December 2018 M_S5.7 Xingwen and the 3 January 2019 M_S5.3 Gongxian events. Our study reveals that the southern margin of the Sichuan Basin is in a tectonic deformation transition region, the change of tectonic environment leads to the complexity of tectonic deformation characteristics in the seismogenic area of the Changning earthquake sequence and its adjacent regions.