粘滑失稳前断层预滑时空演化及力学机制的实验研究

Evolution of preslip prior to fault instability includes two phases: a slow quasi-static phase followed by a fast quasi-dynamic phase which accelerates to the failure of the fault. The two phases are the key physical process during the transition of the fault from stable to unstable states. Thus, it will be of great significance in both theory and reality for fault instability risk analysis to study and identify the condition under which the evolution of preslip reaches a transition from quasi-static to quasi-dynamic phases. Based on the previous experimental studies on fault preslip, two issues, which are likely to be ignored, were stressed in the present proposal. One is the existence of multiple preslip zones which expend and interact with each other simultaneously during the quasi-static phase. The other issue is the occurrence of a catastrophic change during the transition from the quasi-static to the quasi-dynamic phases of preslip. Multiple measurements will be used to obtain an accurate and full-field spatiotemporal evolution of preslip with high temporal and spatial resolutions in this proposal. By doing this, this proposal is going to deal with the following scientific questions: (1) The mechanism which cause the occurrence of multiple preslip zones. (2) The manner and mechanism of the interaction among different preslip zones during the evolution of multiple preslip zones. (3) The mechanic factors that controls the transition from quasi-static to quasi-dynamic phase during the evolution of preslip.

断层失稳前的预滑包括缓慢的准静态阶段以及向失稳加速演化的准动态阶段，它是断层从稳定向失稳转变的关键物理过程，因此，分析预滑从准静态阶段向准动态阶段转变的力学机制并识别其转变的标志，对于分析断层失稳的危险性具有重要的理论意义与现实意义。本项目基于前人实验研究结果，从两个问题出发（即准静态阶段多预滑区并存及同时扩展的现象，以及预滑区从准静态扩展向准动态扩展的转变存在突变模式），拟通过综合利用多种观测手段，实现高时空分辨率的实验观测，从而试图解决以下科学问题：（1）引起断层出现多预滑区并存的力学机制。（2）多预滑区并存条件下各预滑区之间相互作用的方式与机制。（3）预滑区从准静态扩展向准动态扩展转变的力学机制。

断层失稳前的预滑包括缓慢的准静态阶段以及向失稳加速演化的准动态阶段，是断层从稳定向失稳转变的关键物理过程，本项目针对准静态阶段多预滑区并存及同时扩展，以及预滑区从准静态向准动态扩展的转变存在突变模式的现象，开展了断层粘滑实验，结合应变、位移、声发射、高速相机等手段对样品的变形进行了观测，分析了实验前后断层面接触应力分布和几何形貌变化。研究取得以下进展：（1）断层波纹度控制的毫米级接触应力分布是影响预滑时空分布的主要因素，毫米级接触应力分布越均匀，预滑区数量越多。（2）预滑区从准静态向准动态扩展过渡的标志，在不同的过渡模式下有不同的特征：对于渐变过渡模式，标志是多预滑区之间相互连接贯通及由此引起的贯通后预滑区的加速扩展；对于突变过渡模式，标志往往是预滑区到达断层端部。（3）预滑区从准静态向准动态扩展过渡的突变模式主要受控于断层波纹度控制的毫米级接触分布非均匀性及样品端部效应，即在毫米级接触应力非均匀分布的条件下，预滑受强接触部位抑制而受弱接触部位促进，并在弱接触部位的促进下到达断层端部。在这种非均匀性的约束下，预滑区因连接贯通所引起的加速扩展起不到主导作用，无法使预滑区在空间上连续地从准静态转变为准动态扩展，而是到达断层端部后再反向加速扩展。（4）预滑区准静态扩展速度与加载速率成正比，预滑区由准静态向准动态扩展转变的过渡模式与加载速率的关系不大。（5）震颤式声发射事件的优势频率随断层预滑加速而上升的特征可以作为识别断层临近失稳加速滑动的前兆信号。（6）预滑区扩展与凹凸体破坏之间形成了正反馈的相互作用，促使断层加速向动态失稳发展。（7）具有稳定几何形态的断层拐折因不会在预滑过程中受到破坏而可以与预滑进行反复多次的相互作用。上述进展所揭示的预滑从准静态向准动态扩展转变的机制及其转变的标志，对于分析断层失稳的危险性具有重要的意义。