Collaborative Research: Constraints From Fault Roughness on the Scale-dependent Strength of Rocks

地壳岩石强度是构造过程的一个基本因素：断层运动、造山和地壳演化都影响并受岩石强度的影响。尽管地壳岩石强度非常重要，但它很难在野外尺度上测量。实验室实验限制了亚米尺度的强度，但这些结果表明强度与尺度有关：大岩石比小岩石弱。这个问题在断层带尤为严重。对断层强度的认识在很大程度上是基于实验室实验。将这些控制良好的实验室实验结果推广到自然断层是断层和岩石力学的主要问题之一。该项目探索了一种基于断层表面粗糙度在大范围尺度上提供强度估计的新方法。这项研究包括在非常小的尺度上进行实验室测量，结合计算机建模和断层表面的直接观察。结果将提供对断层摩擦的定量理解，可用于预测地球上各种尺度和几何形状的断层摩擦，这对提高对地震力学的理解至关重要。该项目的其他预期社会成果包括通过研究生博士后培训培养具有全球竞争力的STEM劳动力。断层表面粗糙度和强度之间有着密切的联系。凸起的屈服通过动态调整实际接触面积来控制表面摩擦，以响应负载。这种屈服过程可以控制断层表面的地形。该项目使用观察到的、保存的粗糙度来推断屈服标准。由于断层的粗糙度在多个尺度上发生，因此可以推断出不同尺度下的强度（破坏准则）。本研究的目的是建立断层粗糙度与块体材料强度特性之间的联系。研究断层粗糙度与材料强度之间的联系的第一步是直接在具有观察到的粗糙度关系的断层表面样品上测量强度。特别是，研究人员旨在了解脆性和塑性强度的尺度依赖性，并了解随长度尺度减小而从脆性变形到塑性变形的预期转变。为了实现这些目标，他们将在自然断层样品上结合使用压痕和纳米柱实验来获得一组可靠的强度测量。这些结果将与使用原子力显微镜测量相同样品的粗糙度在可比尺度上的粗糙度进行比较。下一步是在自然表面上建立不同尺度下的相关破坏模式，方法是：(a)利用实验室值预测相关尺度下的主导破坏模式；(b)使用最小开槽尺度的观察来隔离分离失效模式的过程；(c)研究较小的尺度，其中破坏模式由材料的绝对强度决定。研究小组将通过测量样品的硬度值来模拟粗糙断层的弹塑性变形，然后使用纳米柱实验推断的脆性破坏准则来计算变形表面运动所需的剪切应力，并将结果与断层摩擦的典型值进行比较，从而探索摩擦测量的含义。

The strength of crustal rocks is a fundamental factor in tectonic processes: fault motion, mountain building and crustal evolution all affect and are affected by rock strength. Despite its central importance, crustal rock strength is difficult to measure at field scales. Laboratory experiments constrain strength at sub-meter scales, but those results imply that strength is scale-dependent: large rocks are weaker than small ones. This problem is particularly serious in fault zones. Understanding of fault strength is largely based on laboratory experiments. Extending these well-controlled laboratory experimental results to natural faults is one of the major problems of fault and rock mechanics. This project explores a new approach based on the idea that fault surface roughness provides strength estimates at a wide range of scales. The study involves laboratory measurements at very small scales combined with computer modeling and direct observations of fault surfaces. Result will provide a quantitative understanding of fault friction that can be used to predict fault friction for the range of scales and geometries found in the Earth, information essential for the improved understanding of earthquake mechanics. Additional desired societal outcomes of the project include development of a globally competitive STEM workforce through graduate student post-doctoral fellow training.There is an intimate link between fault surface roughness and strength. The yielding of asperities controls surface friction by dynamically adjusting the real area of contact in response to a load. This yielding process can control the topography on the fault surface. This project uses the observed, preserved roughness to infer the yield criteria. Since roughness occurs on multiple scales on faults, the strength (failure criterion) at a variety of scales can be inferred. The goal of this research is to make the link between fault roughness and bulk material strength properties. The first step in investigating the proposed connection between fault roughness and material strength is to measure strength directly on fault surface samples that have the observed roughness relationship. In particular, the researchers aim to understand the scale dependence of both brittle and plastic strength, and to understand the expected transition from brittle to plastic deformation with decreasing length scale. To accomplish these goals, they will use a combination of indentation and nanopillar experiments on natural fault samples to obtain a robust set of strength measurements. These results will be compared to roughness at comparable scales using Atomic Force Microscopy to measure roughness on the same samples. The next step is to establish the relevant modes of failure at various scales on natural surfaces by: (a) predict the dominant failure mode at relevant scales using the laboratory values; (b) use the observation of the minimum scale of grooving to isolate the process that separates failure modes; and (c) investigate smaller scales where the failure mode is determined by the absolute strength of the material. The research team will explore the implications of the measurements for friction by simulating the elastoplastic deformation of a rough fault using the hardness values as measured on the samples and then use the brittle failure criterion inferred from the nanopillar experiments to calculate the shear stress required for motion of the deformed surface and compare the results to typical values of fault friction.