2022年9月5日四川泸定M_(S)6.8地震序列发震构造

Seismogenic structure of the 5 September 2022 Sichuan Luding M_(S)6.8 earthquake sequence

2022年9月5日在四川省泸定县磨西镇发生M_(S)6.8强烈地震,极震区烈度达Ⅸ度,共造成97人遇难,20人失联.本文利用截至2022年10月22日的震相数据和波形资料,对本次地震序列进行了重新定位,并利用CAP波形反演方法,计算了主震与M_(S)>3.0余震的震源机制解,初步分析了该序列的发震构造及其几何结构特征和震源区构造变形特征.序列重新定位结果表明,泸定M_(S)6.8地震余震区长轴沿鲜水河断裂带南段磨西段呈NNW向展布,余震密集区长约55 km,北端和南端大致止于泸定新店子北侧和石棉草科乡南侧.余震存在明显的分区丛集特征,且在南门关南侧和湾东附近各存在一个余震稀疏区,将余震区自然划分为北、中、南三段,北段和南段相对较窄、中段较宽.M_(S)6.8主震与10月22日泸定M_(S)5.0余震位于中段,9月7日石棉M_(S)4.5余震位于南段,北段无M_(S)4.0以上余震活动.CAP反演结果显示,主震矩震级MW6.44,震源机制解节面参数(走向/倾角/滑动角)为:节面Ⅰ75°/90°/155°,节面Ⅱ165°/65°/0°,矩心深度3.0 km;7次M_(S)>3.0余震的矩心深度在2.5~7.5 km之间,反映本次泸定主震及其3.0级以上余震能量释放主要发生在上地壳浅部.面应变As值揭示主震为纯走滑型,而余震震源机制解分段特征显著,南段和北段均为纯走滑型,中段为拉张(正断)型.基于余震空间分布、序列震源机制解与余震区构造展布,推测本次泸定M_(S)6.8地震的主发震断层为鲜水河断裂带南段NNW走向的磨西段(磨西断裂),左旋走滑,其断层面走向、倾角和倾向均呈现明显的分段差异,揭示主发震构造几何结构具有复杂性,但应变花形态揭示石棉草科乡以北的主发震断层构造变形模式具有很好的一致性,呈NE向拉张、NW向挤压等量的纯走滑变形模式.余震区中段主震西侧3次M_(S)>3.0正断型余震与2016年泸定震群4次M_(S)≥3.4纯正断和斜滑正断型地震应变花揭示该区域发震断层呈NE向拉张分量为主或NE向纯拉张的变形模式,主震西侧可能存在一系列走向SSE(NNW)的正断型微破裂(~2 km),震源机制解给出的两个节面倾向表明高海拔的贡嘎山山体在震间及同震过程中或存在垮塌现象.

On September 5,2022,an M_(S)6.8 earthquake hit the Moxi Town of Luding County in Sichuan Province,China,caused 97 deaths and 20 people missing.The intensity of its meizoseismic area was IX.To investigate the seismogenic structure of this M_(S)6.8 earthquake and its tectonic deformation characteristics,we relocated the Luding earthquake sequence and calculated the focal mechanism solutions of the M_(S)6.8 mainshock and its M_(S)>3.0 aftershocks using the CAP waveform inversion method,which is based on the seismic phase and waveform data recorded on and before October 22,2022.The spatial distribution of the relocated Luding earthquake sequence reveals a north-northwest(NNW)trending aftershock zone,with its long axis about 55 km length of dense aftershocks along Moxi segment,the southern segment of Xianshuihe fault zone.The northern and southern ends of the aftershock zone were near the north of Xindianzi in Luding County and south of Caoke in Shimian County,respectively.We observed an obvious zonal clustering feature of the aftershock activity,and there are sparse aftershock areas on the south side of Nanmenguan and near Wandong,respectively.The aftershock zone is therefore divided into three sections,as northern,middle,and southern ones.The middle section is relatively wider,but it becomes narrower both in the southern and northern sections.The M_(S)6.8 mainshock and the Luding M_(S)5.0 aftershock on October 22 occurred in the middle section,while the Shimian M_(S)4.5 aftershock on September 7 was located in the southern section,and no M_(S)≥4.0 aftershocks occurred in the northern section.The moment magnitude of the mainshock is MW6.44 from CAP inversion.The strike,dip and rake angles of the nodal planes I and II of the focal mechanism solution for the mainshock are 75°/90°/155°and 165°/65°/0°,respectively.The centroid depth of the mainshock is at 3.0 km,and the depth for seven M_(S)>3.0 aftershocks ranges from 2.5 to 7.5 km,which means most of the accumulated strain energy was released in shallow upper crust during the Luding mainshock and its M_(S)>3.0 aftershocks.The areal strains(As)exhibit the pure strike-slip faulting style for the mainshock and the aftershocks in the southern and northern sections,while the aftershocks in the middle section are of extensional(i.e.,normal faulting).Based on the spatial distribution of aftershocks,the focal mechanism solutions of the entire sequence and the structural trending in the aftershock zone,we conclude that the major seismogenic fault of the Luding M_(S)6.8 earthquake is the NNW-trending left-lateral strike-slip Moxi fault on the southern segment of Xianshuihe fault zone.The variations of the trend,dip angles and dip-orientations along the Moxi segment indicate the geometrical complexity of the major seismogenic fault.Nonetheless,the strain rosettes show the consistency in tectonic deformation along the seismogenic fault north of Caoke Town,Shimian County:all of them display a pure NE-trending extensional and NW-trending compressional strike-slip faulting.The three M_(S)>3.0 normal-faulting aftershocks in the middle section and the four M_(S)≥3.4 normal-faulting and oblique-normal-faulting events in the 2016 Luding earthquake swarm imply the possible existence of a series of SSE(NNW)-striking normal faults,some of which might contribute to the occurrence of aftershocks there.The strain rosettes of the seven events reveal that the seismogenic faults in the western part of the middle section exhibit dominantly or purely NE-trending extensional deformation.It is possible that a sequence of micro-ruptures(~2 km)with normal faulting sense are trending SSE(NNW)on the west side of the mainshock.The geometry of the two nodal planes from the focal mechanism solution indicates that the high-altitude Gongga Mountain was in a collapsing state during the interseismic and coseismic cycles.