地震前准动态失稳形成的动力条件和过程

Meta-instability is the last stage between stress peak and fault instability t on stress-time curve, and also the conversional stage of stress state from main accumulation to dominating release. The deformation of specimen in the stage of meta- instability includes quasi-static release in the prophase and quasi-dynamic release in the anaphase, and then instability occurs. The conversional moment from quasi-static to quasi-dynamic is just the appearance moment of accelerated synergism of fault and rapid cascade interaction along a fault. The studies of dynamic condition of conversion from quasi-static to quasi-dynamic and process of quasi-dynamic to instability are of great significance both in theory and application to analysis critical period, location and intensity of impeding earthquakes. The focuses of the project are: to study the evolution characteristics of multi-physical fields in the conversional stages of quasi-static to quasi-dynamic and quasi-dynamic to instability in laboratory on the basis of improved temporal and spatial resolutions; to discuss in theory the condition and process that the decentralized regions of strain release causes cascade reaction and leads to eventual instability by identifying the differences between insular fault growth and fault interactions; to explore the methods determining possible location and intensity of earthquakes by the synergism of strain release along fault, taking the strong earthquake-prone area locating in western part of southern Tianshan in Xinjiang as an example; to investigate the dynamic condition and process of quasi-dynamic instability before earthquake through the observations in meso-scale anthropogenic seismic site and retrospective studies of earthquake cases.

亚失稳是峰值应力后失稳前的最后阶段，也是区域应力状态由积累为主转变为释放为主的阶段。亚失稳阶段标本变形经历了两个阶段，前期为准静态释放阶段和后期为准动态释放阶段，随后失稳发生。由准静态向准动态阶段转变的时刻正好是多物理场发生加速协同化的时刻，也是断层带发生快速级联反应的时刻。研究由准静态向准动态转变的动力条件以及从准动态到失稳的变化过程对于分析地震危险时段、发震地点和强度有重要的理论意义和应用意义。拟在提高时空分辨率的基础上在实验室研究断层由准静态向准动态阶段以及由准动态向失稳阶段转变的多物理场的演化特点；通过研究孤立断层扩展和断层间相互作用的区别，从理论上探讨分散的应变释放区如何引起级联反应导致最终失稳的条件和过程；以新疆南天山西段强震多发区为例，在应变释放协同化的断层带上探讨预测发震位置和强度的方法；通过中尺度地震实验场观测研究和野外震例回溯，探讨地震前准动态失稳的动力条件与过程。

地震前兆机理及其特征识别一直是地震学界十分关心的前沿课题。亚失稳研究聚焦断层失稳前的最后一个阶段，整体区域应力由累计为主转变为释放为主，将主导断层（或结构）稳定性，对于研判断层失稳具有重要的临震意义。在前一基金研究的基础上，进一步完善了亚失稳理论，按结构稳定性将地震全过程划分为稳态、亚稳态、亚失稳态和是稳态4个应力状态，以利于今将相关研究统一亚失稳框架之内。经过自主研发或商业合作的升级改造，搭建起一套全新的复杂构造加载与多物理场观测系统，着重通过实验研究了断层由准静态释放阶段转变为准动态释放阶段的特征表现，尝试将亚失稳从实验研究推向野外观测，在川滇、新疆等区域建立高频微震仪与遥测地温实验性观测台网，对部分区域开展了震例回溯研究。实验研究表明：亚失稳阶段中应变的积累区与未来的震源区有关，而断层协同化过程中应变释放区域愈长, 应变积累区的应变愈高, 可能发生的震级会愈高；伴随断层局部卸载而出现的应变局部化加速和和断层局部升温区的扩展是进入亚失稳准静态阶段的近场判据；准动态阶段的应变场以逐点应变波动式加速往复传递为表现形式，且各子段间存在准备期，这可能有助于临震预测的识别；断层临时存在震颤式事件的优势频率随预滑加速而升高，其扩展与预滑区扩展在时空上具有一致性。部分震例回溯研究表明：通过南天山西段的Benioff应变特征分析，可识别出应应变释放段，进而根据断裂释放段的协同化程度判定其是否处于亚失稳阶段，寻找失稳部位；汶川地震前区域应力存在由积累为主转为释放为主的可识别阶段，发震前震源断层及附近区域存在联动加速变形过程，这一变化可能与该断裂带进入亚失稳状态相关。野外观测研究仍处于数据累计时段，后续研究结果有望为判定断层的应力状态提供新的理解与新研究方法。

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