The granular physics contribution to rate- and state-dependent fault friction

摩擦在许多涉及社会利益的领域发挥着关键作用，包括交通和制造业。在地球科学中，理解摩擦对于更好地理解与地震和滑坡相关的危害至关重要。材料的摩擦特性已经研究了几个世纪，但其时间依赖性背后的物理和化学仍然不清楚。然而，地震和山体滑坡期间摩擦特性的微小波动会对这些事件的规模和速度产生巨大影响。在诸如构造断层之类的滑动界面上的摩擦通常用所谓的“速率和状态相关摩擦”定律来描述。这些经验定律解释了滑动速度（“速率”）和被称为“状态”的界面的演化特性；后者是滑移历史的一个函数，很难直接观察到。速率-状态框架被广泛用于模拟摩擦滑动。但是，相应的定律不能准确地描述与地震有关的一系列条件的实验室观测结果。在这里，研究小组的目标是更好地了解岩石摩擦特性背后的物理原理。研究人员使用计算机模拟来模拟沿构造断层存在的被称为断层泥的细磨岩石颗粒层的行为。目的是测试岩石摩擦及其时间依赖性是否在晶粒尺度上受晶粒间相互作用的支配。模拟结果受到实验观察的限制：在许多情况下，它们比最成功的速率和状态摩擦定律描述得更好。因此，研究小组逐渐揭示了地震断层行为背后的物理原理。除了具有很强的社会相关性外，该项目还为早期职业科学家提供支持，并为本科生提供培训。为了模拟断层的行为，研究人员采用离散元法模拟。他们使用几何模型和加载条件来模拟标准的岩石摩擦实验，例如“速度-步骤”和“滑动-保持-重新滑动”协议。他们测试了一个假设，即在实验室中观察到的岩石摩擦是由颗粒-颗粒接触尺度上的时间无关特性决定的。这种创新的方法不同于传统的方法，传统的方法认为，时间依赖的可塑性或微观接触的化学键是摩擦的速率和状态依赖的来源。颗粒模拟与滑动协议中最成功的速率和状态相关的摩擦方程一致，这些方程准确地描述了实验（“速度-步进”和“滑动保持”协议）。在这些方程失效的情况下，它们更好地匹配滑动协议的实验室数据（例如，遵循“滑动保持”协议的幻灯片）。此外，颗粒模拟的输出允许研究模型中依赖于速率和状态的类摩擦行为的来源。研究小组发现，如果泥颗粒的动能被围压适当地归一化，它就会产生一个与模拟一致的速度依赖估计，并且在实验室数据的大致范围内。研究人员继续探索颗粒流模型，将其与现有方程无法很好解释的更广泛的滑动协议进行比较（例如，“滑动-保持-重新滑动”和“正常应力步骤”实验）。他们还将模拟中断层泥层的压实/膨胀与实验观察进行了比较；目的是评估孔隙度对断层泥滑动行为的影响。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。

Friction plays a critical role in many areas of societal interest, including transportation and manufacturing. In Earth Sciences, understanding friction is critical for a better understanding of the hazards associated with earthquakes and landslides. The friction properties of materials have been studied for centuries, but the physics and chemistry underlying their time dependence remain obscure. Yet, small fluctuations in friction properties during earthquakes and landslides can have tremendous effects on the size and speed of these events. Friction on sliding interfaces such as tectonics faults are usually described by the so called "rate- and state-dependent friction" laws. These empirical laws account for the sliding speed ("rate") and for the evolving properties of the interface termed "state"; this latter, a function of the slip history, is difficult to observe directly. The rate-and-state framework is widely used to model frictional sliding. But the corresponding laws fail to accurately describe laboratory observations for a range of conditions relevant to earthquakes. Here, the team aims to better understand the physics underlying the frictional properties of rocks. The researchers use computer simulations to model the behavior of granular layers of finely-ground rock, called gouge, that are present along tectonic faults. The goal is to test whether rock friction and its time dependence is governed at the grain scale by grain-to-grain interactions. The simulation outputs are constrained by experimental observations: in many cases they describe them better than the most successful rate-and-state friction laws. The team, thus, gradually unveils the physics underlying the behavior of earthquake-generating faults. In addition to its strong societal relevance, this project provides support for an early career scientist as well as training for undergraduate students. To model the behavior of the gouge, the researchers employ Discrete Element Method simulations. They use model geometries and loading conditions designed to mimic standard rock-friction experiments, such as "velocity-step" and "slide-hold-reslide" protocols. They test the hypothesis that rock friction as observed in the laboratory is governed by time-independent properties at the grain-grain contact scale. This innovative approach differs from more traditional ones which assume that time-dependent plasticity or chemical bonding at microscopic contacts are the source of the rate-and-state dependence of friction. The granular simulations are consistent with the most successful rate-and-state-dependent friction equations for sliding protocols where those equations accurately describe experiments ("velocity-step" and “slide-hold” protocols). They better match laboratory data for sliding protocols where those equations fail (e.g., the reslides following "slide-hold" protocols). Furthermore, output of the granular simulations allows investigating the source of the rate-and-state-dependent friction-like behavior of the model. The team finds that if the kinetic energy of the gouge particles is suitably normalized by the confining pressure, it produces an estimate of the velocity dependence that is consistent with the simulations and within the ballpark of laboratory data. The researchers continue exploring the granular flow model by comparing it to a wider range of sliding protocols that are not well explained by existing equations (e.g., "slide-hold-reslide" and "normal-stress-step" experiments). They also compare the compaction/dilation of the gouge layers in the simulations to experimental observations; the goal is to evaluate the role of porosity on the gouge sliding behavior.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.