汶川和芦山地震之间地震空区综合性研究

The Longmenshan Fault System hosted the great 2008 May 12th Wenchuan earthquake and the 2013 April 20th Lushan earthquake in a short time span of 5 years. The seismic activity of the nearly 60km-long seismic gap between the two earthquake sequences is subject to continuing hot debates. Chen et al(2013) believed that earthquakes with Magnitude of 7.2-7.3 are still possible based on seismogenic analysis; while Pei et al (2014) rejected the possibility because they found the gap is largely located in a ductile deformation area. This study is trying to integrate data from historic seismicity, microseismicity, focal mechanisms as well as 3D fine structures to attack this puzzle. The first phase of observation including 32 seismometers started from April 2015. The preliminary results have shown very complicated stress field around the seismic gap. Significant differences in crust and Moho boundaries are also observed between two profiles passing Wenchuan earthquake and Seismic gap, respectively. In the second phase of observation we plan to deploy about 30 broadband seismometers over and around the seismic gap to collect data for noise inversion, traveltime inversion, microseismic locating and focal mechanism inversion. Ultimately we can obtain a 3D model of Vs, Vp, microseismic distributions as well as local stress field. With those parameters combined, a thorough and comprehensive discussion of the seismicity in the gap may be possible and meaningful.

汶川地震和芦山地震及余震序列之间的长达60多公里的未破裂段的断层活动性一直是一个争论激烈的科学问题。陈运泰等（2013）基于孕震环境等因素指出该段仍存在7.2-7.3级地震的危险性；而Pei et al (2014) 根据速度分布认为该地震空区位于以塑性变形为主的剪切带上，所以发生大地震的可能性不大。本研究试图从微地震监测，震源机制和精细速度结构反演等几个方面展开对这一问题的探索。2015年4月以来研究小组展开了第一期观测。初步分析揭示了空区周围非常复杂的应力场。分别穿过空区和汶川地震的两条剖面下方的壳幔结构也存在显著的差异。本研究计划在2017年-2018年间开展第二期观测，在地震空区及周边布设约30台宽频带地震计。通过噪声反演，走时反演，微地震位置和震源机制联合反演等手段得到该区域的3D速度结构，微地震分布以及应力分布特征。综合分析这些物理参量以期对地震空区的地震活动性有更深刻的认识。

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，首先获取完整目录，继而开展断裂带地震活动特时空演化、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。