2019年四川长宁MS6.0地震的地质构造成因——区域性构造节理贯通、破裂结果

根据野外地质调查结果并结合区域地质资料, 对2019年长宁MS6.0地震震区的地质背景和新构造运动特征进行分析后, 讨论了此次地震的发震构造并给出该地区孕震模型。基于区域地质与地震资料, 结合地表调查结果发现, 该区的新构造运动具有间歇性整体弱隆升、断裂晚第四纪活动不明显以及地震活动相对微弱等特征。此次强震的仪器震中位于长宁双河大背斜的西段, 该背斜经历了古老的NE向挤压应力场和顺时针旋转应力场的双重叠加构造作用。在大背斜的核部和翼部普遍发育了两组NW向节理和一组NE向节理, 并显示出区域性分布特征, 构成了具一定规模的构造破碎带(软弱带)。地震记录显示, 此次强震的地震序列呈现出明显的NW向线性条带, 震源机制解主要指示了NEE—NE向水平挤压应力作用的结果。结合地表构造特征可进一步推断, 长宁地震的震源破裂型式是以沿NW向破裂面发生以逆冲为主兼具左旋走滑分量的破裂过程为特征。综合分析认为, 2019年长宁MS6.0地震可能是双河背斜中的赋存超压环境叠加NE向古构造应力场和旋转应力场作用, 导致区域性NW向纵向节理发生构造强化、贯通, 并进一步破裂的结果。

The Geological Genesis of the 2019 Changning MS6.0 Earthquake in Sichuan: Connecting and Rupturing of Regional Structural Joints

According to field geological survey results and regional geological data, the authors analyzed geological background and Neotectonic characteristics in Changning MS6.0 earthquake area, discussed the seismogenic structure of this earthquake, and built the seismogenic model of this area. Based on regional geological and seismic data, combined with the surface survey results, the authors have found that the neotectonic movement in this area has the characteristics of intermittent overall weak uplift, no obvious Late Quaternary activity along the faults and relatively weak seismic activity. The epicenter of the strong earthquake is located in the west section of Shuanghe anticline, which experienced the double superimposition of the old NE-trending compressive stress field and clockwise rotation stress field. Two sets of NW-trending and one set of NE-trending joints are widely developed in the core and wing of the great anticline, which show regional distribution characteristics and have formed a certain scale of structural fracture zone (weak zone). The seismic records show that the seismic sequence of this strong earthquake presents obvious NW-trending linear stripe, and the focal mechanism solution mainly indicates the result of NEE-NE-trending horizontal compression stress. Combined with the characteristics of the surface structure, it can be further inferred that the source fracture pattern of the Changning earthquake is characterized by the fracture process of thrusting as the main part and left lateral strike slip component along the NW-trending fracture. The comprehensive analysis shows that the 2019 Changning MS6.0 earthquake may be the result of structural strengthening, transfixion and further fracture of regional NW longitudinal joints caused by overpressure environment in Shuanghe anticline superimposed with NE direction paleotectonic stress field and rotation stress field.