Collaborative EAGER Research: Mineral reactions during seismic slip and earthquake instability

尽管人们对地震的位置及其对社会的危害已经了解得很多，但我们对地震如何成核的基本物理知识，特别是对地震期间断层滑动区域扩张的物理过程的了解仍然很少。为了更好地预测地震震动的危险，更好地了解这些过程是必要的，人们希望有一天能够预测大地震。这个EAGER项目将使用实验研究和高分辨率电子显微镜来测试一个关于大陆地震如何发生滑动的新假设。据了解，地震是由两个截然不同的过程引发的：在地球内部几十公里的寒冷、低压环境中，地震通常是通过克服预先存在的断层上的静摩擦而开始的。然而，在俯冲的海洋岩石圈中，地震也会持续发生，深度接近700公里，那里的压力太高，不允许脆性破裂。实验表明，高压下的剪切破坏（断层）需要一种矿物反应，这种反应产生少量的“流体”，用于它们的起始和膨胀；“流体”可以是真正的流体(例如：水或二氧化碳)或在固体状态下表现出极低粘度的纳米晶体固体。该项目的两位pi之一（Reches）是摩擦滑动领域的领先专家，另一位（Green）是高压剪切破坏领域的世界领先专家。美国地质调查局（US Geological Survey）的两位主要地震物理学专家（D. Lockner和N. Beeler）也加入了他们的行列，这对该项目来说是免费的。这个EAGER项目将测试一个假设，即矿物反应引起的剪切不稳定过程，即高压下断层的机制，也可以在浅层地震中起作用，在浅层地震中，它是由地震滑动开始时发生的摩擦加热/拉伸激活的。研究小组设想了可能发生这种情况的两种主要方式：(1)断裂带中的粘土矿物或碳酸盐破裂，释放出一种流体（分别是水或二氧化碳），导致断层上滑动的阻力大幅下降；(2)在滑动开始过程中产生极小的颗粒，形成纳米晶固体，可以通过晶界滑动以孕震速度流动，正如高压断层已经证明的那样。地震滑动模型、实验和对断裂带的实地考察都有力地表明，在某些地震中会发生剪切加热引起的挥发脱失。高压实验观察表明，这种反应会导致剪切不稳定，这进一步表明，类似的过程可能会增强浅层地震。同样，最近至少有两个实验室的实验室工作得出结论，粉末润滑可能是故障扩展和润滑的关键部分。我们将在实验中测试的关键问题是，在浅地壳条件下，这种剪切加热引起的矿物反应是否会导致摩擦迅速下降和/或滑移增强。该小组将研究断层力学中产生“流体”的反应的作用。在俄克拉何马大学的实验室实验中，他们将通过在一定范围的正常应力下高速滑动来激活剪切诱导的脱挥发。然后，他们将通过加州大学河滨分校的高分辨率扫描和透射电子显微镜来表征这些实验中产生的断层泥和滑动表面的微观结构，并将这些微观结构与高压断层实验中产生的“超塑性”断层填充材料进行比较。碳酸盐的初步结果非常令人鼓舞。更广泛的影响：该项目汇集了来自两个大学校园和一个联邦政府实验室的科学家和研究生，这是一个EAGER项目，对加利福尼亚等地震多发地区的居民可能产生深远的影响。如果这项工作证明了脱挥发直接导致了摩擦下降和/或大地震的断层沟是弱纳米晶体固体，他们将打开一扇门，使人们更好地了解断层，并有可能更好地了解特定地区（如加利福尼亚）哪些断层的哪些部分是危险的，哪些不是。该项目的学生（一名在公开大学，另一名在UCR），以及可能增加的本科生助理，将接受有关最先进仪器的培训，并将参与其科学前沿的研究。

Although a great deal is known about the location of earthquakes and their danger to society, our knowledge of the underlying physics of how they nucleate and, especially, the physical processes operating as the slipped area on the fault expands during an earthquake is still poorly understood. Greater knowledge of these processes is necessary to better predict seismic shaking danger and, it is hoped, to one day enable prediction of major earthquakes. This EAGER project will use experimental studies and high-resolution electron microscopy to test a new hypothesis of how the slip on continental earthquakes occurs. Earthquakes are understood to initiate by two distinctly different processes: In the cold, low-pressure, environment of the upper few tens of km within the Earth, earthquakes generally begin by overcoming static friction on pre-existing faults. However, earthquakes also occur continuously to depths approaching 700 km in subducting oceanic lithosphere where the pressure is too high to allow brittle failure. Experiments show that shear failure (faulting) at high pressure requires a mineral reaction that yields a small amount of 'fluid' for their initiation and expansion; the 'fluid' can be either a true fluid (eg. H2O or CO2) or a nanocrystalline solid exhibiting an extremely low viscosity in the solid state. One of the two PIs of this project (Reches) is a leading expert in frictional sliding and the other (Green) is the world leader in high-pressure shear failure. They are joined by two leading experts on the physics of earthquakes of the US Geological Survey (D. Lockner and N. Beeler) at no cost to the project. This EAGER project will test the hypothesis that the process of mineral-reaction-induced shearing instability, the mechanism of faulting at high pressure, can also operate in shallow earthquakes where it is activated by the frictional heating/straining that occurs during initiation of earthquake slip. The team envisions two main ways in which this may occur: (1) Breakdown of clay minerals or carbonates in the fault zone releasing a fluid (water or CO2, respectively) that results in a large drop of the resistance to sliding on the fault; (2) generation of extremely small particles during initiation of sliding that form a nanocrystalline solid that can flow by grain-boundary sliding at seismogenic speeds, as has already been demonstrated for high-pressure faulting. Models of earthquake slip, experiments, and examination of fault zones in the field strongly suggest that shear-heating-induced devolatilization occurs in some earthquakes. The high-pressure experimental observations that such reactions lead to shearing instabilities further suggest that similar processes could enhance shallow earthquakes. Similarly, recent laboratory work in at least two laboratories concludes that powder-lubrication may be a critical part of fault propagation and lubrication. The key question we will test experimentally is whether such shear-heating-induced mineral reactions can lead to rapid drop in friction and/or enhancement of slip under shallow crust conditions. The team will investigate the role of 'fluid'-producing reactions in fault mechanics. They will activate shear-induced devolatilization in laboratory experiments at the University of Oklahoma by high-speed sliding under a range of normal stresses. They then will characterize by high-resolution Scanning and Transmission electron microscopy at UC Riverside the microstructure of gouge and sliding surface produced in these experiments and compare those microstructures with the 'superplastic' fault-filling materials produced in high-pressure faulting experiments. Preliminary results on carbonate that are very encouraging. Broader Impacts: This project brings together scientists and graduate students from two university campuses and a federal government lab for an EAGER project with potentially profound consequences for residents of earthquake-prone areas such as California. If this work demonstrates that devolatilization is directly responsible for friction drop and/or that fault gouges of large earthquakes are weak nanocrystalline solids, they will have opened a door that will lead to greater understanding of faulting and potentially will lead to a better understanding of which parts of which faults in a given area such as California are dangerous and which are not. The students of this project (one at OU and the other at UCR), with the likely addition of undergraduate assistants, will receive training on state-of-the-art instrumentation and will participate in research at the frontier of their science.