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

合作研究：断层粗糙度对岩石尺度相关强度的约束

• 批准号: 1624657
• 负责人: Emily Brodsky
• 金额: $ 25.82万
• 依托单位: University of California-Santa Cruz
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-08-15 至 2022-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1624657&HistoricalAwards=false
• 关键词: Collaborative; Research; Constraints; Fault; Roughness

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.

地壳岩石的强度是构造过程中的一个基本因素：断层运动、造山和地壳演化都影响着岩石强度，并受到岩石强度的影响。尽管地壳岩石强度至关重要，但很难在野外尺度上进行测量。实验室实验将强度限制在亚米尺度上，但这些结果表明强度与尺度有关：大岩石比小岩石弱。这个问题在断裂带尤为严重。对断层强度的理解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天然断层是断层和岩石力学的主要问题之一。这个项目探索了一种新的方法，基于断层表面粗糙度在广泛的尺度上提供强度估计的想法。这项研究涉及非常小规模的实验室测量，并结合计算机建模和对断层表面的直接观测。这一结果将提供对断层摩擦力的定量了解，可用于预测地球上发现的各种尺度和几何形状的断层摩擦力，这些信息对于改善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发展一支具有全球竞争力的STEM劳动力队伍。断层表面粗糙度与强度之间存在密切联系。粗糙度的屈服通过动态调整实际接触面积来控制表面摩擦力，以响应载荷。这种屈服过程可以控制断裂面的地形。该项目使用观测到的、保留的粗糙度来推断屈服标准。由于断层上的粗糙度在多个尺度上存在，因此可以推断不同尺度下的强度(破坏准则)。本研究的目的是建立断层粗糙度与散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测量具有观测到的粗糙度关系的断层表面样本的强度。特别是，研究人员的目标是了解脆性和塑性强度的尺度相关性，并了解随着长度尺度的减小从脆性变形到塑性变形的预期转变。为了实现这些目标，他们将使用压痕和纳米管实验相结合的天然断层样本，以获得一套稳健的强度测量。这些结果将与使用原子力显微镜测量相同样品上的粗糙度的可比尺度上的粗糙度进行比较。下一步是通过以下步骤在自然表面上建立不同尺度上的相关失效模式：(A)使用实验室数值预测相关尺度上的主要失效模式；(B)使用最小凹槽尺度的观测来分离分离失效模式的过程；以及(C)调查由材料的绝对强度确定失效模式的较小尺度。研究小组将通过使用在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结果对摩擦的影响，然后使用纳米实验得出的脆性破坏准则来计算变形表面运动所需的剪应力，并将结果与断层摩擦力的典型值进行比较。