基于模板识别研究龙门山断裂带精细地震活动与三维地壳各向异性

The Longmenshan Fault Zone (LFZ) is a major block boundary that separates the Songpan—Ganzi block within the Tibetan Plateau and Sichuan Basin. It marks the highest topographic change in the world, and is home to several large damaging earthquakes in recent years. Middle-lower crustal flow and crustal shortening are two end-member models to explain the uplifting along the LFZ. The disastrous 2008 Mw7.9 Wenchuan and 2013 Mw6.7 Lushan earthquakes occurred in middle and southern part of the LFZ, resulting in a 60-km seismic gap in between that have caught many researchers’ attentions. While some studies have estimated a potential of magnitude up to 7.5 in this gap, other proposed that the southeast movement of the Songpan—Ganzi block is likely accommodated by ductile, rather than brittle deformation in this gap. A more complete earthquake catalog along the LFZ can help to better understand seismotectonics and potential seismic hazard in this region. Here we propose to build a complete earthquake catalogues along the LFZ by using the waveform match technique with the continuous waveform from both permanent and temporary stations. With the new catalogue, we plan to examine spatial-temporal evolutions of seismicity in this region, focusing on the potential interactions between the Wenchuan and Lushan earthquakes, the seismic behaviors of the gap in between, and potentially static or dynamically triggered seismicity. Next, we plan to quantify shear-wave splitting with an automatic method for all possible event-station pairs. Finally, we will apply a start-of-the-art anisotropy tomography to obtain a better image of anisotropic structures in the crust and their potential spatio-temporal evolutions. Our systematic detections of seismic activity and measurements of crustal anisotropy could help to improve our knowledge of mountain building process and faulting behavior in this region.

龙门山断裂带是规模宏大的构造边界带，西抵松潘—甘孜块体，东临四川盆地。该带具有全球最高地形落差，中下地壳流与地壳缩短是断裂带隆升机制的两个端元模型。产生巨大震害的汶川与芦山地震相继发生在断裂带中段与南段，两者间长约60km“破裂空段”的构造成因与地震危险性备受关注。研究指出“破裂空段”具有M7.5以上地震可能，但亦有学者认为松潘—甘孜块体的东南向运动在此产生塑性形变，而非脆性破裂。完整地震目录是探讨地震构造成因与潜在震害的关键。因此，我们拟采用模板识别方法，综合断裂带及周缘固定与流动台数据，首先获取完整地震目录，继而开展断裂带地震活动时空演化、汶川与芦山地震关系、及断裂带应力触发研究。其次，依据新目录和剪切波分裂自动分析方法反演中上地壳各向异性。最后，采用各向异性成像方法，获取断裂带各向异性结构。我们对断裂带地震活动与各向异性结构的系统研究，将有助于理解龙门山断裂带造山过程。

紧密围绕立项目标，分别采用地震识别与地震各向异性方法，针对龙门山断裂带强震余震演化及其周缘地壳介质应力与动力学特征开展研究。近四年主要获取以下认识：.a) 全地壳介质地震各向异性揭示藏东动力学机制的多样性. 采用共享地震波形数据，基于Pms时差方法开展藏东全地壳各向异性研究，揭示藏东呈现多种地壳形变模式：松潘—甘孜地块沿边界断裂带呈现整体运移的模式，鲜水河—小江断裂带下方的中下地壳具有流动可能，龙门山断裂带的隆升源于该区的地壳缩短过程(Kong et al., EPSL, 2016)。.b) 中上地壳介质地震各向异性揭示藏东北缘中上地壳受到区域构造应力与活动断裂的双重约束. 基于青海省数字地震台网共七年的数据，获取研究区域内直达剪切波分裂参数，指出地处多个块体交汇部位的西宁地区及其周缘，地壳各向异性呈现两个优势偏振方位，表明该区域中上地壳应力环境由区域主压应力场和活动断层共同约束，较为复杂；玉树地震序列的地壳各向异性与此不同，其优势方位与区域主压应力场一致(钱旗伟等，地球物理学报，2017)。.c) 藏东龙门山断裂带南段2013Mw6.6芦山余震触发机制. 基于中国地震局固定台站与强震震后流动台阵数据，我们对2013年Mw6.6芦山地震早期三天的余震，开展基于模板匹配方法的识别研究，最终获取六倍于原始地震目录的事件，揭示了余震随时间的离逝逐渐向外迁移的现象，指出震后余滑是余震外延的可能机制(Wu et al., JGR, 2017)。.d) 龙门山断裂带新近余震演化特点与机制. 发现在汶川地震发生近十年后，其余震活动依旧活跃。汶川地震现今余震活动主要分布在10~25 km的深度区间，震源深度呈现西南段较东北段偏深的特征。此外，汶川近年余震分布相比早期余震偏深，破裂带西南段的余震活动有向深部迁移的趋势。对于芦山地震，其近期余震活动较弱，余震主要分布在10~15 km深度区间，比早期余震的分布区间偏浅。最新余震活动分布特征表明，余震活动随着时间的推移有迁移的现象。考虑到距离主震事件已分别有5~10年时间，余震迁移现象可能与以流体扩散方式为主的准静态应力机制触发相关(刘小梅等，地球物理学报，2019)。. 龙门山断裂带是长约400km的一条巨型断裂带，震后流动台阵布设时间有限，对该断裂带的长期余震观测与震后形变研究还有待推进。