应力时空演化揭示出的汶川地震前亚失稳过程

doi:10.6038/cjg2018M0225

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摘要近年来从岩石实验研究针对应力水平进入峰值强度后至断层快速失稳前的短临阶段变形物理场的时空演化过程提出了亚失稳阶段的概念，该阶段的基本特征为应力由峰值前的积累为主转为峰值后稳定释放并加速至最终快速失稳，这一过程伴随着其他变形物理场的相应转变.把握该阶段应力场的转变特征对认识未来地震危险区及其变形阶段至关重要.以2008年汶川地震为例，运用基于区域多个地震震源机制反演定量应力张量的方法对强震前是否存在可观测的亚失稳应力状态进行了研究.获得的震源附近紫坪铺水库区域临震前应力状态时空演化过程显示，水库测震台网开始运行的2004年8月至2007年6月期间该区域归一化有效平均应力水平相对较低且均匀分布，随时间无明显变化；而2007年6月后至汶川主震发生前应力水平局部显著增强.基于不同时段的定量应力进一步获得了随时间演化的增量场，显示2007年6月后除了汶川附近和彭县附近为应力增强区，其他大部分区域表现为幅值相对较低的应力释放，表明研究区在2007年6月后应力水平明显弱化；而在2007年6月之前的应力增量场却显示区域应力随时间以积累为主.根据应力演化特征及分时段的应力增量场可推断孕震区在2007年中期达到强度极限，峰值前区域应力随时间平稳变化并以积累为主，而峰值后则转变为明显的应力释放.该现象显示汶川地震前区域应力存在由积累为主转为释放为主的可观测阶段，符合实验研究中断层由稳态进入亚失稳状态的应力演化特征.结合其他研究结果，推测汶川地震前震源断层及附近区域存在联动加速变形过程，与震前亚失稳应力状态相关.

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关键词 ：亚失稳, 汶川地震, 震源机制, 定量应力, 应力增量场

Abstract：In recent years, the concept of sub-instability stage was proposed from the study of rock experiments. The basic characteristics of this stage are the stress release evolution after the stress peak in comparison with the stress accumulation evolution before the stress peak and the corresponding deformation fields. Stress tensor evolution before the Wenchuan earthquake is investigated by using the method of Cataclastic Analysis based on multiple focal mechanisms, to study whether there is measurable sub-instability stage before strong earthquakes. The results show that the maximum compressive stress axis of the Longmenshan fault zone is near horizontal, the maximum tensile stress axis is near vertical, the stress state is consistent with the subsequent dynamic mechanism of the Wenchuan earthquake and supports the significant dextral strike slip component of the fault in this earthquake. The temporal and spatial evolution of the stressfield is obtained around the focal area by using the mechanism data measured by the Zipingpu Reservoir Network for more than 3 years before the Wenchuan earthquake. It shows that the apparent stress change occurred in June 2007, and a new stress increasing area was added in the northeastern corner of the reservoir area near the main shock. The stress increment distribution over the two periods before and after June 2007 shows that the stresses increase near Wenchuan and Pengxian but decrease in most of the other locations, which indicates that the stress level of the study area is weakened significantly after June 2007. In comparison, the stress increment field at different time points before June 2007 shows that regional stresses increases with time in most areas. The stress changes are in accordance with the stress evolution characteristics of the fault from the steady state to the sub-instability state in rock experiments. With the results of previous studies such as cross-fault measurements, regional seismic activity and borehole strain, we infer that accelerated eastward movement happened in the east part of the Bayankela block in the middle of 2007, and it is directly related to the sub-instability state before the Wenchuan earthquake.

Key words： Sub-instability The Wenchuan earthquake Focal mechanism Quantitative stress Stress increment distribution

收稿日期: 2018-03-19

PACS:	P313
	P542

基金资助:国家自然科学基金（41474162）和中央公益性科研院所基本科研业务专项（IGCEA1605）联合资助.

作者简介: 王凯英,女,1976年生,中国地震局地质研究所副研究员,从事地震构造变形与应力场研究.E-mail:wangky@ies.ac.cn

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http://manu39.magtech.com.cn/Geophy/CN/10.6038/cjg2018M0225 或 http://manu39.magtech.com.cn/Geophy/CN/Y2018/V61/I5/1883

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