基于损伤粘弹模型的震后应力松弛过程模拟研究

Aseismic deformation and aftershocks are important processes following large earthquakes. It has been found that the strain energy produced by postseismic activities is sometime comparable to or even higher than the main event itself. Therefore, the study on postseismic stress relaxation can improve our understanding on stress accumulation and release in the whole seismic cycle, and enrich our knowledge of long-term earthquake forecast. The research on interaction between aseismic deformation and aftershocks is helpful for our understanding aftershock mechanics, and is further helpful for aftershocks forecast. Seismological and geological reports have presented that there exist a volumetric fault zone (with widths of hundreds to thousands meters) around main rupture plane of the large earthquake. The fault zone is characterized by dense distribution of microcracks. The microcracks-related damage changes dynamically with the occurrences of large earthquakes. According to rock experiments, the microcracks (damage) change the mechanical property of the fault zone, and significantly decrease the rock rigidity. Meanwhile, the damaged rock appears obvious inelastic behavior. In this project, we utilize a damage rheology model, whose construction includes a damage zone (fault zone) and viscoelastic lower crust/upper mantle, to simulate both postseismic deformation and aftershocks. The numerical calculation of the damage rheology model is realized based on FLAC algorithm, which has been applied widely in engineering and earthquake scince. In this project, we specifically study the difference of postseismic activities following strike-slip event, thrust event and normal-faulting event, and take the 1992 M7.2 Landers earthquake, the 2001 M8.0 Qinghai earthquake, the 2008 M8.0 Wenchuan earthquake, the 2011 M9.0 Tohoku earthquake and the 2008 M7.3 Yutian earthquake as study cases.

震后应力松弛造成的非弹性形变和余震是地震活动周期中的重要过程。研究发现，震后活动所释放的应变能，会接近甚至高于主震。因此,对震后松弛的研究,有助于理解地震周期的应力累积和释放过程，有益于长期地震预测；对非弹性形变和余震相互作用的研究，有助于余震机理的理解和余震预测。地质、地震学家的研究报告均指出，断层并不是简单滑动面，而是个上千米的断层带。在断层带上，存在着微破裂，微破裂造成的损伤程度是随大地震的发生而改变的动态过程。岩石实验表明，介质损伤显著降低断层带的弹性刚度，且造成断层带非弹性特征，那么，损伤性松弛在震后可能影响显著。另一方面，地壳深部的粘弹性松弛是震后应力调整的重要方面。因此，本研究基于损伤力学和粘弹理论，构建包括断层带损伤特性和地壳深部粘弹特性的损伤粘弹模型，利用FLAC方法进行数值模拟，来研究震后应力松弛行为，并针对性地研究几种地震类型（走滑、逆冲、拉张）的震后松弛差异。

地震从孕育、发生和震后应力调整经历了间震期、主震和震后期。在间震期，构造加载的作用使得应变在断层强度高的地方长时间积累；间震期所积累的应变，很大一部分在主震发生过程中释放；近些年基于GPS和InSAR观测的地表形变数据的研究表明，震后期所释放能量，有时接近甚至高于主震所释放能量。因此，研究震后活动的物理机制对于理解地震物理过程有重要意义。本项目针对震后应力松弛过程，考虑了断层带介质损伤引起的非弹性松弛、蠕滑、脆性破裂（余震）和深部粘弹效应，基于地震学和地表形变观测，理解震后活动的物理机制。.针对震后损伤性应力松弛，本项目建立三维有限元模型，研究了不同损伤流变参数的主导作用，并对1992年7.2级Landers地震，基于区域地质、地球物理、地震学和大地测量资料，构建了区域损伤粘弹模型 (Wang et al., 2013)。同时，为了研究震后应力松弛与主震和间震期应变积累的关系：（1）本项目提出一种根据形变资料确定地震应力降的方法（Wang et al., 2014b）（2）根据间震期、主震、余震和震后余滑的力学耦合关系，构建模型来理解地震的物理过程（Wang et al., 2014a； Cattania et al., 2014）；（3）提出定量研究地震周期中应变/滑动平衡的方法，并针对几个实际震例的研究结果表明，震后应力松弛是主震和间震期应变积累共同作用的结果，主要的应力松弛发生在主震造成应力增加，并且主震之后间震期剩余应变仍然显著的区域（Wang et al., 2015a; Wang et al., 2015b）。.在自然基金的资助下，项目的开展对震后物理过程，及其与间震期和主震的动力学关联，取得了很多新的发现；所提出的新的建模方法，具有普适性，可以应用到其他震例当中。.