2022年6月1日四川芦山MS6.1强震构造精细特征

2022年6月1日, 四川芦山发生M_S6.1(M_W5.9)强震, 龙门山断裂带南段的地震危险性再次引起广泛的关注和讨论.本次研究根据震源机制、余震重定位和深地震反射剖面, 详细分析了发震断层, 并刻画了芦山震区同震断层的三维结构和模型.此次地震为典型的逆冲型地震, 震源机制两组节面参数(走向/倾角/滑动角)分别为节面Ⅰ: 221.5°/44.4°/103.3°; 节面Ⅱ: 23.2°/47.1°/77.3°, 矩心深度为15.5 km.结合余震展布特征, 认为发震断层为一条倾向SE的反冲断层, 初始破裂深度约19 km, 倾角约50°.研究重新解译了芦山震区上地壳的地质构造, 认为龙门山断裂带南段存在两套重要的滑脱层, 具有分层滑脱变形(解耦)的特征: 浅部滑脱层之上为叠瓦构造, 深浅滑脱层之间为双重构造.2013年M_S7.0地震与2022年M_S6.1地震的发震断层, 均为发育在基底约20 km深度的滑脱层上的盲冲断层, 为早期先存构造并重新活化的产物.芦山M_S6.1地震的余震丛集相对独立, 与之前2013年芦山M_S7.0地震的发震断层并非相同的断层, 因此认为是一次相对独立的破裂事件.2008年汶川M_S8.0地震与2013年芦山M_S7.0地震之间的地震空区, 先存断裂众多, 就空区的断层规模而言可以产生7级左右强震.而深部基底盲断层和浅部盖层隐伏断层的广泛存在, 增加了该地区地震灾害风险评估的难度和不确定性.

Detailed structural characteristics of the 1 June 2022 MS6.1 Sichuan Lushan strong earthquake

On June 1, 2022, an MS6.1 (MW5.9) earthquake occurred in Lushan County, Sichuan Province. The strong earthquake risk in this area once again sparked widespread attention and debate. We analyzed the seismogenic faults and built a three-dimensional model of the coseismic faults in the Lushan area based on the focal mechanism of the mainshock and aftershock relocation. The 2022 Lushan MS6.1 earthquake is a typical thrust event, with the following nodal parameters of focal mechanism (strike/dip/slip angle): Nodal Ⅰ: 221.5°/44.4°/103.3°; Nodal Ⅱ: 23.2°/47.1°/77.3°, and the centroid depth is 15.5 km, respectively. It is hypothesized that the seismogenic fault is a SE-dipping recoil fault with an initial rupture that is approximately 19 km deep and a dip angle of around 50° when combined with the distribution patterns of relocated aftershocks. The geological structure of the upper crust in the Lushan area was reinterpreted in light of a deep seismic reflection profile that was compiled prior to 2015 in the Lushan earthquake area. In the southern Longmen Shan, there are two sets of significant detachment layers that demonstrate layered detachment or decoupling deformation. While the deep basement between the two detachment layers produces duplex structure, the sedimentary cover overlaying the shallow detachment layer develops imbricate structure. Blind thrust faults, which may have originated from reactivation of the pre-exisiting structures, converged into the deep detachment at a depth of about 20 km in the basement and are the seismogenic faults of the Lushan 2013 MS7.0 earthquake and the 2022 MS6.1 earthquake. The 2022 MS6.1 earthquake's aftershock cluster is spatially distinct from the 2013 MS7.0 earthquake's, and these two quakes did not originate from the same seismogenic fault. The 2022 MS6.1 event is therefore viewed as a rather independent rupture event. In the seismic gap between the 2008 Wenchuan MS8.0 earthquake and the 2013 Lushan MS7.0 earthquake, the study finds that there are numerous pre-existing faults, which obviously increase the possibility of occurring earthquakes with magnitudes more than 7.0 within this seismic gap. Furthermore, the prevalence of shallow concealed faults and deep blind faults in the basement makes it more challenging and ambiguous to assess the potential seismic hazard in the Longmen Shan area of eastern Tibetan Plateau.