基于GPS、水准和强震动观测资料联合反演2013年芦山7.0级地震同震滑动分布

为了更好理解2013年四川芦山M_S7.0级地震的发生过程及其与发震构造和地表多种观测资料的动力学关联,本文综合重新定位的余震分布与地质、地球物理信息构建3D发震构造模型,采用水平层状介质模型,并以震区GPS、水准、强震动等同震位移/形变观测资料为约束,联合反演了芦山主震的同震滑动分布.其中,断层解译结果表明震源区包含5条相关断层F1-F5,通过对所有可能的断层组合模型进行反演分析,显示采用F1+F3+F4+F5的组合模型反演效果相对最好,是最可能的发震断层模型.反演得到的芦山主震矩震级为M_W6.5,其中同震滑动主要分布在NW倾的主断层F1的断坡周围,最大值为0.86 m,滑动角92.88°,纯逆冲型;F1上方反倾的次级断层F3上最大滑动量为0.37 m,滑动角119.92°,表现出以逆冲为主兼右滑的斜向反冲作用;而沿另一条反倾的次级断层F4的最大滑动量为0.40 m,滑动角97.98°,几乎为纯逆冲作用.此外,震区还存在一个NW缓倾深度为5~8 km的浅部滑脱面F5,它分隔了浅部沉积盖层与深部变质基底,限制了其下方F1、F3及F4等断层的同震破裂继续向更浅部扩展.主震时深部F1和F3断层夹持的冲起构造发生了上冲运动,除了使浅层和地表产生响应运动及变形外,还引起冲起构造顶面即F5底面的NE段和SW段分别产生了NE和SWW向调节滑动,最大值0.25 m.总之,基于文中构建的F1+F3+F4+F5的发震断层模型,反演结果能很好拟合地表多种观测资料,还能解释地表GPS观测的同震"左旋"运动与地震学观测的震源断层逆冲运动的"不协调性".

Joint inversion for coseismic slip of the 2013 MS7.0 Lushan earthquake from GPS,leveling and strong motion observations

In this paper, we inverted the coseismic slip distribution of the Lushan M_S7.0 earthquake of April 20th, 2013, to further understand its generating process and the relationship with the seismogenic structure and various surface observation data. A 3D fault model was built by integrating the relocated aftershocks distribution and the information of geology and geophysics. Employing the horizontal layered crust-mantle model, we used the near-field deformation data such as GPS, leveling and strong motion records to constrain the inversion. Our interpretation result of fault geometry shows that there exist 5 relevant faults named F1 to F5. We tested all of the possible combination models of different faults and the inversion results indicated that the combination of faults F1, F3, F4 and F5 can fit the observation data best, and may be the most probable seismogenic fault model. The inverted geodetic moment of Lushan mainshock in our research is about M_W6.5, and most of the coseismic slip are distributed around the ramp of fault F1, which is dipping to the NW, and the maximum value is 0.86 m, as a thrust faulting with the rake of 92.88°. The slip on fault F3, which is one of the back-thrust secondary faults located above fault F1, is dominated by thrust motion with a slight dextral component, of which maximum value is 0.37 m with the rake of 119.92°, while the slip on the other one named fault F4 is almost pure thrusting with the max value of 0.40 m and the rake of 97.98°. A shallow décollement named F5 in this paper is identified existing in the seismogenic zone at the depths of 5 to 8 km, dipping to the NW gently. It separates the shallower sedimentary cover from the deeper metamorphic basement, and prohibits the coseismic rupture of F1, F3 and F4 spreading to the shallower layer. When the mainshock occurred, the pop-up structure, which is a wedge-shape rock restricted by the faults F1 and F3, moved upward, leading to the responding movement and deformation of the shallower layer and ground surface, and also caused the layer under the NE and SW segments of fault F5 to slip to the NE and the SWW, respectively, with the maximum value of 0.25 m. In conclusion, the inversion result based on the combined seismogenic model of faults F1, F3, F4 and F5 in this paper can fit various kinds of the surface observation data very well, and also explain the "incompatibility" between the "sinistral" motion observed by GPS and the pure thrust faulting confirmed by seismological outcomes.