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的基本因素：断层运动、造山和地壳演化都影响岩石强度，也受岩石强度的影响。尽管地壳岩石的强度非常重要，但它很难在现场尺度上测量。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与尺度相关：大岩石比小岩石弱。这个问题在断层带中尤为严重。对断层强度的理解很大程度上基于实验室实验。将这些良好控制的实验室实验结果扩展到自然断层是断层和岩石力学的主要问题之一。该项目探索了一种新方法，其基础是断层表面粗糙度可提供大范围尺度的强度估计。该研究涉及非常小规模的实验室测量，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，可用于预测地球上发现的尺度和几何形状范围的断层摩擦，这些信息对于增进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。断层表面粗糙度与强度之间存在密切联系。粗糙度的屈服通过响应负载动态调整实际接触面积来控制表面摩擦。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、保留的粗糙度来推断屈服标准。由于断层上存在多个尺度的粗糙度，因此可以推断出各种尺度的强度（失效准则）。这项研究的目标是建立断层粗糙度和散装材料强度特性之间的联系。研究断层粗糙度和材料强度之间所提出的联系的第一步是直接测量具有观察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依赖性，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，他们将在自然断层样本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结果。将使用原子力显微镜将这些结果与可比尺度的粗糙度进行比较，以测量相同样品的粗糙度。下一步是通过以下方式建立自然表面上不同尺度的相关失效模式： (a) 使用实验室值预测相关尺度的主要失效模式； (b) 利用最小开槽规模的观察来隔离分离失效模式的过程； (c) 研究较小规模，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推断出的脆性破坏准则来计算运动所需的剪切应力变形表面并将结果与​​断层摩擦的典型值进行比较。