利用尾波差分成像方法研究实验室断层失稳全过程应力场时空演化

Nearly all aspects of earthquake rupture are controlled by the friction along the fault that progressively increases with weakly but continuously tectonic stress loading. Inferring the frictional state on faults, and monitoring where a fault is within its seismic cycle, is extremely challenging due to the stress sensitivity limitation of the current observation techniques. .In this project, we propose (1) to optimize an innovative coda wave differential tomography in terms of of three qualitative metrics including increase stress sensitivity as well as promote temporal and spatial resolution. (2) to perform stick-slip experiments under different loading conditions on the large scale granite specimens (>1m) in a laboratory setting that closely mimic rupture of the heterogeneous Earth faulting, meanwhile obtain a series of 4D images of velocity changes and micro-structural changes of fault by the optimized coda wave differential tomography at each key stage of the fault instability including nucleation, coseismic, postseimic and interseimic phase, (3) to characterize velocity changes and micro-structural changes at each stage of the fault instability as well as to examine the possible factors that influence the stress evolution of the fault instability during the experiments. These results could facilitate the understanding of fundamental stress evolution of the fault system. Similar approaches using the coda waves may yield new insight into fault physical characteristics either in laboratory or in Earth in future.

应力状态是断层滑动失稳产生强震的主要控制因素之一。由于与地震过程相关的断层内部应力变化非常微弱且深度深，受观测技术限制，目前对地震过程相关的断层内部精细的应力时空场变化缺乏系统认识，为解决这一问题，本项目拟（1）继续发展尾波差分成像方法，使其具备高应力敏感性、高时间分辨率、高空间分辨的四维成像特点；（2）采用非均匀特征明显的大型花岗岩样品(>1米)，在实验室开展不同加载条件下断层摩擦粘滑实验模拟断层失稳产生强震，并利用改进的尾波差分成像方法反演震前(成核)、同震、震后和震间等关键阶段演化波速变化和微裂隙变化四维图像；（3）通过获取的细观尺度四维图像研究断层应力场在地震周期四个阶段演化特征，总结规律及其影响因素。本研究不仅有助于在方法上为室内试验与野外观测对孕震机制进行融合研究提供思路。更有利于加深断层应力场演化规律这一科学问题的认识。

本工作改进并发展的尾波（多重散射波）差分成像方法，形成了以高灵敏度的多重散射波波速变化和微裂隙变化开展介质损伤演化的监测的系统。 开展了从大尺度岩石断层的摩擦试验到野外边坡的观测，对诸如实验室断层摩擦失稳、滑坡损伤时空演化特征进行了研究，获得了从实验室尺度的岩石断层到野外尺度的岩石边坡等脆性介质在失稳以及地震加载等关键阶段演化的关键时空场信息..主要研究成果如下：.（1）发展了从超声波尺度到地震波尺度的尾波差分成像方法，提出了幅度调制超声散射波来获取介质高阶弹性模量的监测方法，形成了基于尾波差分成像的高灵敏度，高时空分辨率的跨尺度介质时空演化观测能力；.（2）具备了开展最大1.5 米尺度的岩石断层摩擦试验并开展包括连续多通道的超声，声发射、应力—应变等综合观测的能力， 开展了不同加载速率下的实验室断层摩擦实验，实现了利用尾波差分成像方法以时间分辨率20 ms，空间分辨率25 cm的时空精度获得了震前、同震、震后等关键阶段演化的时空场演化信息，观测到了岩石断层失稳前存在局部前兆波速下降区域，讨论了尾波幅度变化与断层接触条件等因素的对滑动的影响。.（3）具备了开展边坡尺度介质损伤时空演化观测的能力，形成了一套基于多重散射波差分成像的新型地质灾害监测系统，开展了典型岩石边坡的高时间分辨率、高精度的原位观测，将波速变化的时间分辨率提升到20分钟的同时将观测误差控制在10-5，获得了在局部滑坡失稳发生前40分钟观测到可信的前兆式波速下降；获得了地震加载前后的边坡时空演化特征，讨论了地震加载、降雨气温等环境因素的影响机制。..本研究不仅有助于在方法上为室内试验与野外观测对孕震、失稳等机制进行融合研究提供思路。更有利于加深介质应力场时空演化规律这一科学问题的认识。