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

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具有重要意义。地震周期性地发生；它们的复发是由于地壳中被称为断层的大裂缝的“粘滑”行为。断层在地震之间的时间段“粘滞”，而在地震期间“滑动”。粘滑运动是由相对较冷的岩石的弹性（弹簧样）行为和断层的摩擦行为相互作用产生的。由于岩石摩擦与地震的相关性，在实验室中进行了广泛的研究。经验方程——即从实验数据而不是基于已知机制推导出来的方程——描述了摩擦如何随时间和滑动速度变化。计算机模型利用这些方程再现了一系列与地震有关的现象。然而，这些方程式背后的物理和/或化学过程在很大程度上仍然是未知的；识别和量化它们是将实验室结果应用于地质断层的关键。在这里，研究小组在断层岩石实际接触的断层表面的凹凸不平（凸起）的微尺度和纳米尺度上研究了这些过程。他们使用原子力显微镜和纳米压痕来模拟单个凸起的行为，并在小尺度上测量它们的行为。他们将实验结果输入到包含更大尺度的计算机模拟中，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进危险评估。本项目还资助2名研究生和1名博士后。它促进对本科生的培训，并向高中学生和教师伸出援手，特别是来自科学领域代表性不足的群体。经验速率-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，特别是描述摩擦界面“状态”演变的方程。这使得实验室结果外推到地质断层充满了不确定性。对摩擦“状态”的一种常见解释是，它代表了断层表面的真正接触面积；这个区域随着时间的推移或由于粗糙、屈服和蠕变而发生滑动而演变（增加）。一种新出现的替代方法是由于接触连接处的化学键而使接触加强。该团队先前通过使用单尖原子力显微镜，配合计算机模拟和纳米压痕实验证明，这两种机制都可能导致摩擦随时间（或滑移）的增加，这种效应被称为摩擦老化。一个统一的假设是，在粗糙接触处，粗糙蠕变和化学键同时发生，但老化主要是由化学键引起的。在这种情况下，接触面积和化学键是不可分割地联系在一起的，粗糙屈服和蠕变提供了化学键发生的接触面积。在这里，研究小组将在地球物理学、化学、材料科学和力学的联系上进行新颖的实验和模拟，以揭示速率和状态摩擦的物理基础。具体来说，他们试图1)阐明摩擦中凹凸不平的屈服和蠕变的作用，2)探索温度和流体化学对表面老化的影响，3)阐明滑移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。单粗糙度实验的结果被整合到模拟中，通过多尺度建模来描述粗糙岩石表面在接触中的行为。该模型首次将粗糙度、屈服和蠕变与化学键效应结合在一起，为岩石表面和断层的速率和状态摩擦行为提供了新的见解。这个项目可能会导致范式的转变，对理解地震成核以及地震灾害和相关风险的评估具有变革性的影响。该奖项反映了美国国家科学基金会的法定使命，并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，被认为值得支持。

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.