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

地壳岩石的强度是构造过程中的一个基本因素：断层运动、造山运动和地壳演化都影响岩石强度，并受岩石强度的影响。尽管地壳岩石强度具有核心重要性，但在野外尺度上很难测量。实验室实验将强度限制在亚米尺度，但这些结果意味着强度是依赖于尺度的：大岩石比小岩石更脆弱。这个问题在断层带尤为严重。对断层强度的认识主要基于实验室实验。如何将这些实验结果推广到天然断层，是断层力学和岩石力学的主要问题之一。该项目探索了一种新的方法，该方法基于断层表面粗糙度在很大范围内提供强度估计的想法。这项研究涉及非常小尺度的实验室测量，结合计算机建模和对断层表面的直接观察。结果将提供一个定量的了解断层摩擦，可用于预测断层摩擦的范围内的尺度和几何形状在地球上发现，信息的地震力学的理解，提高必不可少的。该项目的其他预期社会成果包括通过研究生博士后研究员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存在密切联系。凹凸体的屈服通过响应于负载动态地调整接触的真实的面积来控制表面摩擦。这种屈服过程可以控制断层面的地形。该项目使用观测到的保留粗糙度来推断屈服准则。由于粗糙度发生在多个尺度上的故障，在各种尺度的强度（故障标准）可以推断。本研究的目的是使故障粗糙度和散装材料的强度性能之间的联系。研究断层粗糙度和材料强度之间的联系的第一步是直接测量具有所观察到的粗糙度关系的断层表面样品的强度。特别是，研究人员的目标是了解脆性和塑性强度的尺度依赖性，并了解随着长度尺度的减小，从脆性到塑性变形的预期过渡。为了实现这些目标，他们将在自然断层样本上使用压痕和纳米柱实验的组合，以获得一组强大的强度测量值。将这些结果与使用原子力显微镜测量相同样品粗糙度的可比尺度粗糙度进行比较。下一步是通过以下方式在自然表面上建立各种尺度的相关失效模式：（a）使用实验室数值预测相关尺度下的主要失效模式;（B）使用对最小尺度的开槽观察来隔离分离失效模式的过程;以及（c）调查较小尺度，其中失效模式由材料的绝对强度决定。研究小组将通过使用样品上测量的硬度值模拟粗糙断层的弹塑性变形来探索测量对摩擦的影响，然后使用从纳米柱实验中推断出的脆性破坏标准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。

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.