Relating Earthquake Mechanics to Fault-Zone Structure

观测研究表明，断层是一种复杂的结构，其“核心”由细粒碎裂岩组成，边缘是断层泥和角砾岩层，夹在从核心延伸数百米的受损围岩带之间（Biegel和Sammis， 2004）。需要回答的关键问题是：(1)这个断裂带结构是如何形成的，(2)它是否影响地震破裂的力学？Rice和Sammis最近的理论研究表明，断层泥和角砾岩层可以在地震破裂尖端附近的动应力场中形成，Andrews已经证明，损伤带的能量损失可以显著地贡献断裂能，从而影响地震的破裂速度。通过建立一个动态滑动脉冲分析模型，并将其与希顿的观测数据进行拟合，Rice和Sammis能够证明，在传播的地震破裂周围的断层应力场能够产生数十米宽的泥砾-角砾岩层和数百米宽的破坏带。虽然该模型可以预测断裂速度对离断层应力的影响，但不能预测离断层损伤对破裂速度的控制程度。在本研究中，研究者将在实验室中探索主动损伤对破裂速度的反馈。他们将在两个弹性板之间的界面上形成一系列动态II型剪切破裂，并使用高速数码摄影来测量破裂速度并观察相关的非断层破坏模式。这些实验将在他在加州理工学院的实验室里与阿瑞斯·罗萨基斯教授合作进行。他在光活性Homalite上的实验没有产生非断层破坏，因为Homalite上没有可以成核的缺陷。pi将通过制造具有均匀分布的启动缺陷的样品来扩展Rosakis实验，以允许非故障损坏的成核。这些实验的结果将与滑移脉冲模型的预测结果进行比较，其中将使用Ashby和Sammis（1990）的微力学损伤模型来计算理论应力场中的非断层损伤程度。这种比较将作为对理论模型的现实检验，并允许定量评估主动断层破坏对动态地震破裂传播的反馈。更广泛的影响包括为教育目的制作图像和支持新的PI，以及促进南加州大学和加州理工学院航空工程实验室之间的合作。

Observational studies have shown that faults are complex structures with a "core" of fine-grained cataclasites bordered by layers of gouge and breccia sandwiched between zones of damaged wall rock extending for hundreds of meters from the core (Biegel and Sammis, 2004). Key questions to answer are: (1) how does this fault zone structure form, and (2) does it affect the mechanics of the earthquake rupture? Recent theoretical studies by Rice and Sammis have shown that gouge and breccia layers can form in the dynamic stress field near the tip of an earthquake rupture, and Andrews has demonstrated that energy loss in a damage zone can significantly contribute to the fracture energy, thereby affecting the rupture velocity of an earthquake. By formulating an analytical dynamic slip-pulse model and fitting it to observational data from Heaton, Rice and Sammis were able to show that the off-fault stress field around a propagating earthquake rupture is capable of producing a gouge-breccia layer tens of meters wide and a damage zone hundreds of meters wide. While the model can predict the effect of rupture velocity on off-fault stresses, it cannot predict the extent to which off-fault damage controls the rupture velocity. In this study, The investigators will explore the feedback of active damage on rupture velocity in the laboratory. They will nucleate a series of dynamic mode II shear ruptures on the interface between two elastic plates, and use high-speed digital photography to measure rupture speed and observe the pattern of associated off-fault damage. These experiments will be carried out in collaboration with Professor Ares Rosakis in his laboratory at Caltech. His experiments in photoactive Homalite did not generate off-fault damage because there are no flaws in Homalite from which it can nucleate. The PIs will extend the Rosakis experiments by fabricating samples that have a homogeneous distribution of starter flaws to allow the nucleation of off-fault damage. The results of these experiments will be compared with predictions from the slip pulse model in which the micromechanical damage model of Ashby and Sammis (1990) will be used to calculate the extent of off-fault damage in the theoretical stress field. This comparison will serve as a reality check on the theoretical models, and allow a quantitative assessment of the feedback of active off-fault damage on the propagation of a dynamic earthquake rupture. Broader impacts include producing images for educational purposes and the support of a new PI, as well as fostering a collaboration between USC and Aeronautical Engineering Labs at Caltech.