Mechanochemical Processes dictating Calcite's Frictional Characteristics

地震是反复发生的毁灭性事件。然而，要准确预测其发生和规模仍然很困难。这是因为引发地震的断层破裂的机制尚未完全了解。水在断层动力学中起着重要的作用，然而，断层界面的复杂性阻碍了识别影响断层行为的具体机制。在这里，研究人员研究了方解石-一种在断层泥中发现的矿物-在水环境中的摩擦强度。为了研究富水流体的影响，他们使用了一个理想化的晶粒界面，该界面由两个1 mm的单晶体组成，它们之间有或没有流体。界面包含粗糙接触，其特征在于在微观尺度上的原子力显微镜。这项研究是相关的天然断层，因为施加到接口的应力和滑动速率是可比的地质设置。使用一种新技术来量化位移特性，该技术测量位移的精度为1埃（百万分之一毫米）。这允许调查所涉及的机制，如晶体溶解/重结晶的接触。该项目的成果改进了断层摩擦模型，有助于改进对存在反应性流体的地震危险的评估。该项目促进对研究生的支持，对本科生的培训和对K-12学生的推广，特别是来自科学代表性不足的群体。该团队遵循两条主要研究路线。在实验层面上，该项目推进了机械化学界面反应及其对方解石摩擦特性的影响的知识。它通知机制的压力解决在晶界。不同因素的影响进行了评估：流体化学，表面电位，层状硅酸盐或封闭的流体膜的存在。滑动速度，应力，温度，接触时间和地形，对摩擦和粘附的影响也被量化。这项工作的关键是对表面力装置进行修改。新的发展允许测量蠕变在湿方解石接触精度为0.1纳米，以及界面力（分离压力，粘附力和摩擦力）。这提供了在广泛的条件下压力-溶液蠕变与静态和动态摩擦之间的关系的见解。实验的简单几何形状允许研究宏观摩擦的微观机制，并测试描述它们的理论。在理论层面上，在单一和多粗糙接触的摩擦模拟剪切辅助热激活滑移理论的基础上，占接触老化由于压力溶液蠕变。新的模型参数进行了比较，并测试对速率和状态的摩擦本构方程及其经验参数。 因此，该项目填补了水环境中断层摩擦的微观和宏观尺度之间的差距。该奖项由极端事件预测和恢复力（PREEVENTS）计划共同资助。该奖项反映了NSF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。

Earthquakes are recurring and devastating events. To predict accurately their occurrence and magnitude remains, however, difficult. This is because the mechanisms underlying the rupture of earthquake-generating faults are not fully understood. Water plays an important role in fault dynamics; yet, the complexity of fault interfaces has hindered identifying the specific mechanisms influencing fault behaviors. Here, the researchers investigate the frictional strength of calcite - a mineral found in fault gouges - in aqueous environment. To study the effects of water-rich fluids, they use an idealized grain interface consisting of two 1-mm single crystals sliding against each other, with or without fluids in between them. The interface contains asperity contacts characterized at the microscale by atomic force microscopy. The study is relevant to natural faults because the stresses and slip rates applied to the interface are comparable to those of geological settings. Frictional characteristics are quantified using a new technique which measures displacements with an accuracy of 1 angstrom (0.1 millionth of a mm). This allows investigating the involved mechanisms, such as crystal dissolution/recrystallization on contacts. The project's results improve models of fault friction, which contributes improving the assessment of seismic hazards in the presence of reactive fluids. This project promotes support for a graduate student, training for undergraduate students and outreach to K-12 students, notably from groups underrepresented in Science. The team follows two main lines of research. At the experimental level, the project advances the knowledge of mechano-chemical interfacial reactions and their influence on calcite's frictional characteristics. It informs the mechanisms underlying pressure-solution at grain boundaries. The influence of different factors is evaluated: fluid chemistry, surface electric potential, the presence of phyllosilicates or confined fluid films. The effects of sliding velocity, stress, temperature, contact time and topography, on friction and adhesion are also quantified. Essential to this work is a modification of the Surface Forces Apparatus. The new development allows measuring creep deformation at wet calcite contacts with a precision of 0.1 nm, as well as interfacial forces (disjoining pressure, adhesion and friction). This provides insight on the relationship between pressure-solution creep and static and dynamic friction over a wide range of conditions. The simple geometry of the experiments allows to investigate the microscopic mechanisms responsible for macroscopic friction, and to test the theories that describe them. At the theoretical level, friction at single- and multi-asperity contacts are modeled based on the shear-assisted thermally activated slip theory, accounting for contact aging due to pressure-solution creep. The new model parameters are compared to and tested against the rate-and-state friction constitutive equations and their empirical parameters. The project, thus, fills the gap between the microscopic and the macroscopic scales in the understanding of fault friction in aqueous environment.This award is co-funded by the Prediction of and Resilience against Extreme Events (PREEVENTS) program.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.