Collaborative Research: Experiments and Simulations at the Nexus of Geophysics, Chemistry, Materials Science and Mechanics to Determine the Physical Basis for Rate-State Friction

合作研究：结合地球物理学、化学、材料科学和力学来确定速率状态摩擦的物理基础的实验和模拟

• 批准号: 1951462
• 负责人: David Goldsby
• 金额: $ 25.88万
• 依托单位: University of Pennsylvania
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-03-01 至 2023-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1951462&HistoricalAwards=false
• 关键词: Collaborative; Research; Experiments; Simulations; Nexus

This project aims to identify the physical processes underlying rock friction. It has strong implications for our understanding of earthquakes and associated hazards. Earthquakes occur periodically; their recurrence is due to the "stick-slip" behavior of large fractures in the Earth’s crust, called faults. A fault "sticks" in the time periods between earthquakes and "slips" during earthquakes. The stick-slip motion arises from the interaction of the elastic (spring-like) behavior of relatively cold rocks and the frictional behavior of faults. Rock friction has been extensively studied in the laboratory because of its relevance to earthquakes. Empirical equations – that is, derived from experimental data rather than based on known mechanisms – describe how friction varies with time and sliding velocity. Computer models use these equations to reproduce a wide range of earthquake-related phenomena. However, the physical and/or chemical processes underlying these equations are still largely unknown; identifying and quantifying them is critical for applying laboratory results to geological faults. Here, the research team investigates these processes at the microscale and nanoscale of the asperities (bumps) on the fault surface where fault rocks are in actual contact. They use atomic force microscopy and nanoindentation to mimic the behavior of single asperities and to measure their behavior at small scales. Feeding the results of experiments into computer simulations that incorporate larger scales, they model and predict the frictional behaviors of rock surfaces. Ultimately, the researchers aim to develop new equations that better capture the behavior of earthquake faults and improve hazard assessment. This project also provides support to two graduate students and a postdoctoral associate. It fosters training for undergraduate students and outreach to high-school students and teachers, notably from underrepresented groups in science.Empirical rate-and-state friction laws, which describe the frictional sliding behavior of faults, are commonly used in earthquake models. Their physical basis is largely unknown, particularly for the equations that describe the evolution of the "state" of a frictional interface. This renders the extrapolation of laboratory results to geological faults fraught with uncertainty. A common explanation of frictional "state" is that it represents the true area of contact on a fault surface; this area evolves (increases) with time or slip due to asperity yielding and creep. An emerging alternative is that contacts strengthen due to chemical bonding at contact junctions. The team previously demonstrated – using single-asperity atomic force microscopy and coordinated with computer simulations, and with nanoindentation experiments - that both mechanisms may contribute to the increase of friction with time (or slip), an effect termed frictional aging. A unifying hypothesis is that asperity creep and chemical bonding occur simultaneously at asperity contacts, but that aging is due primarily to chemical bonding. In this scenario, contact area and chemical bonding are inextricably linked, with asperity yielding and creep providing the contact area upon which chemical bonding occurs. Here, the research team will conduct novel experiments and simulations at the nexus of geophysics, chemistry, materials science, and mechanics to unveil the physical basis for rate-and-state friction. Specifically, they seek to 1) elucidate the roles of yielding and creep of asperities in friction, 2) explore the influences of temperature and fluid chemistry on surface aging and 3) elucidate the roles of slip versus time in state evolution. They employ specimens made of silica and quartz and, for the first time, amorphous alumina, sapphire and feldspar. Results from single-asperity experiments are integrated into simulations which describe the behavior of rough rock surfaces in contact via multiscale modeling. The models incorporate asperity yielding and creep with chemical bonding effects for the first time, allowing new insights into rate-and-state friction behavior of rock surfaces and faults. This project may lead to a paradigm shift with transformative implications for understanding earthquake nucleation, and for the assessment of earthquake hazards and associated risks.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

该项目旨在确定岩石摩擦的物理过程。它对我们理解地震和相关灾害有很大的影响。地震是周期性发生的;它们的重复发生是由于地壳中大裂缝（称为断层）的“粘滑”行为。断层在两次地震之间的时间段内“粘”在一起，在地震期间“滑”在一起。粘滑运动是由相对较冷岩石的弹性（类似弹簧）行为和断层摩擦行为的相互作用引起的。由于岩石摩擦力与地震的相关性，它在实验室中得到了广泛的研究。经验方程--也就是说，从实验数据中得出，而不是基于已知的机制--描述了摩擦力如何随时间和滑动速度变化。计算机模型使用这些方程来再现各种与地震有关的现象。然而，这些方程背后的物理和/或化学过程在很大程度上仍然是未知的;识别和量化它们对于将实验室结果应用于地质断层至关重要。在这里，研究小组在微观和纳米尺度上研究了这些过程，这些过程发生在断层岩石实际接触的断层表面上。他们使用原子力显微镜和纳米压痕来模拟单个微凸体的行为，并在小尺度上测量它们的行为。将实验结果输入到包含更大尺度的计算机模拟中，他们对岩石表面的摩擦行为进行建模和预测。最终，研究人员的目标是开发新的方程，更好地捕捉地震断层的行为，并改善灾害评估。该项目还为两名研究生和一名博士后助理提供支持。它促进对本科生的培训，并推广到高中学生和教师，特别是来自科学界代表性不足的群体。经验速率和状态摩擦定律描述了断层的摩擦滑动行为，通常用于地震模型。它们的物理基础在很大程度上是未知的，特别是对于描述摩擦界面“状态”演化的方程。这使得将实验室结果外推到地质断层充满了不确定性。摩擦“状态”的一个常见解释是它代表了断层表面上的真实接触面积;该面积随着时间或由于粗糙屈服和蠕变而滑动而演变（增加）。一种新兴的替代方案是，由于在接触结处的化学键合，接触加强。该团队先前使用单粗糙原子力显微镜并与计算机模拟和纳米压痕实验相协调证明，这两种机制都可能导致摩擦力随时间（或滑动）的增加，这种效应称为摩擦老化。 一个统一的假设是，粗糙蠕变和化学键合同时发生在粗糙接触，但老化主要是由于化学键合。在这种情况下，接触面积和化学结合是不可分割的联系，粗糙屈服和蠕变提供了接触面积，化学结合发生。在这里，研究小组将在物理学、化学、材料科学和力学的结合点上进行新的实验和模拟，以揭示速率和状态摩擦的物理基础。具体而言，他们试图1）阐明屈服和蠕变的粗糙在摩擦中的作用，2）探索温度和流体化学对表面老化的影响，3）阐明滑动与时间在状态演变中的作用。他们使用了由二氧化硅和石英制成的标本，并首次使用了无定形氧化铝、蓝宝石和长石。从单粗糙实验的结果被集成到模拟，通过多尺度建模来描述粗糙的岩石表面接触的行为。该模型首次将粗糙屈服和蠕变与化学键合效应相结合，使人们对岩石表面和断层的速率和状态摩擦行为有了新的认识。该项目可能会导致一个范式的转变，对理解地震成核，地震灾害和相关风险的评估具有变革性的影响。该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估的支持。