Rheology of Nanocrystalline Materials: Clarifying the Sliding Mechanism of Earthquakes

纳米晶材料的流变学：阐明地震的滑动机制

• 批准号: 1345130
• 负责人: Javier Garay
• 金额: $ 27万
• 依托单位: University of California-Riverside
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-08-01 至 2018-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1345130&HistoricalAwards=false
• 关键词: Rheology; Nanocrystalline; Materials; Clarifying; Sliding

Earthquakes occur from just below the surface of Earth to almost 700km where they stop abruptly. Near the surface, earthquakes begin by overcoming friction on pre-existing faults but at depths greater than about 40km the pressure is so high that overcoming friction is impossible; the elevated temperatures at such depths allow rocks to flow at stresses lower than they could slip suddenly and cause an earthquake. Then why are there any deeper earthquakes? Twenty-five years ago the investigator discovered the physical mechanism by which faults (and by inference deep earthquakes) initiate and slide at high pressure. The key is formation of a "nanocrystalline" sliding zone that is weak because the crystals in the zone are so tiny-a few millionths of a millimeter in diameter. Recent high-speed friction experiments simulating shallow earthquakes have raised major questions concerning how shallow earthquakes actually slide. These experiments show that immediately after sliding begins, there is strong heating caused by the two sides of the fault rubbing against each other. The temperature rises rapidly until it initiates a mineral reaction that produces a very weak nanocrystalline sliding zone that looks just like the high-pressure fault zones. The PIs propose that this nanocrystalline zone slides by exactly the same physical process as the high-pressure faulting and that most earthquakes slide by this mechanism also. To test this hypothesis, they and a graduate student, with undergraduate help, will conduct experiments to determine the stresses needed to make nanocrystalline materials flow over a broad range of temperature, pressure, crystal size and sliding rate. If successful, they will have established for the first time a physical understanding of earthquake sliding. This physical understanding will enable more sophisticated modeling of predicted shaking damage caused by earthquakes and move us one step closer to the elusive goal of predicting earthquakes. Another result that should come from these studies is deeper understand-ding of flow of nanocrystalline ceramics that will provide valuable basic information for attempts to develop high-speed forming processes for a variety of ceramic products in industry. This project will provide a graduate student and undergraduate students with very valuable interdisciplinary experiences bridging between earthquake physics and materials science. UCR is the most ethnically diverse campus of the University of California and one of the most ethnically diverse universities in the USA. The institution supports the pursuit of state-of-the-art research including involvement of undergraduates making a vital contribution to their education. As a consequence, minority students at UCR graduate at the same rate as white students and at a much higher rate than that of "peer institutions".Based upon high-pressure faulting and high-speed friction experiments, the PIs propose a universal sliding mechanism for most earthquakes. They hypothesize that in the first second or so of most earthquakes, a low-viscosity, nanocrystalline, "gouge" is generated, enabling sliding with a very low frictional resistance. They propose to test this hypothesis by synthesizing nanocrystalline materials and measuring their rheology as a function of grain size, strain rate, temperature and pressure. Deformed specimens will be analyzed by SEM and TEM accompanied by Electron Back-Scattered Diffraction (EBSD), selected-area electron diffraction, and energy-dispersive chemical analysis. The rheological experiments are straightforward and the PI and CoPI are experts in such procedures and have the necessary equipment in their laboratories. The critical step is in synthesizing fully-dense composites of controlled grain sizes. The CoPI is an expert in this step as well and has demonstrated success over the last 8 years. Therefore, the experimental program is essentially assured of success. There is already fragmentary existing knowledge suggesting that the materials chosen will flow by grain-boundary sliding at low viscosities under a subset of the conditions that will be examined. By creating an extensive data set, the PIs will be able to demonstrate whether or not high-speed friction experiments slide by this mechanism under some conditions, all conditions, or no conditions. Application to sliding of real earthquakes will be through comparison to the high-speed friction experiments and microstructures of deeply eroded faults analogous to the San Andreas Fault (SAF) of California. It is expected that these experiments will provide a quantitative mechanistic explanation of why friction falls precipitously very shortly after sliding initiates, and rises again as sliding slows. Additionally, the study will also demosntrate why the SAF shows no thermal anomaly (the so-called San Andreas heat flow paradox) and why pseudotachylytes are rare.

地震发生在地球表面以下近700公里处，然后突然停止。在地表附近，地震开始是通过克服先前存在的断层上的摩擦力，但在深度超过40公里的地方，压力是如此之高，以至于克服摩擦力是不可能的;在这样的深度，高温使岩石在低于它们可能突然滑动并引起地震的应力下流动。那为什么会有更深层次的地震呢 25年前，研究人员发现了断层（以及由此推断的深地震）在高压下启动和滑动的物理机制。关键是形成一个“纳米晶体”滑动区，这个滑动区很弱，因为区域中的晶体非常小，直径只有百万分之一毫米。 最近的高速摩擦实验模拟浅源地震提出了关于浅源地震实际上如何滑动的主要问题。 这些实验表明，在滑动开始后，断层两侧相互摩擦引起强烈的加热。 温度迅速上升，直到引发矿物反应，产生非常弱的纳米晶滑动区，看起来就像高压断层带一样。 PI提出，这种纳米晶带的滑动与高压断层的物理过程完全相同，大多数地震也是通过这种机制滑动的。 为了验证这一假设，他们和一名研究生将在本科生的帮助下进行实验，以确定使纳米晶材料在广泛的温度、压力、晶体尺寸和滑动速率范围内流动所需的应力。 如果成功的话，他们将首次建立对地震滑动的物理理解。这种物理上的理解将使预测地震造成的震动破坏的模型更加复杂，并使我们更接近预测地震的难以捉摸的目标。这些研究的另一个结果是对纳米晶陶瓷流动的更深入的了解，这将为工业上开发各种陶瓷制品的高速成形工艺提供有价值的基础信息。本计画将提供一位研究生与一位大学生在地震物理学与材料科学之间的跨学科经验。 UCR是加州大学中种族最多元化的校园，也是美国种族最多元化的大学之一。该机构支持追求最先进的研究，包括本科生的参与，为他们的教育做出重要贡献。 因此，加州大学的少数民族学生的毕业率与白色学生相同，而比“同龄机构”的毕业率高得多。基于高压断层和高速摩擦实验，PI提出了大多数地震的通用滑动机制。他们假设，在大多数地震的第一秒左右，会产生一种低粘度的纳米晶体“断层泥”，使滑动具有非常低的摩擦阻力。 他们建议通过合成纳米晶材料并测量其流变性作为晶粒尺寸，应变速率，温度和压力的函数来测试这一假设。 变形样本将通过SEM和TEM进行分析，并伴有电子背散射衍射（EBSD）、选区电子衍射和能量色散化学分析。 流变学实验非常简单，PI和CoPI是此类程序的专家，并且在其实验室中拥有必要的设备。 关键的一步是合成完全致密的复合材料的控制晶粒尺寸。 CoPI也是这方面的专家，在过去8年中取得了成功。 因此，实验计划基本上是有成功保证的。 已经有零碎的现有知识表明，所选择的材料将流动的晶界滑动在低粘度下的一个子集的条件下，将被检查。 通过创建广泛的数据集，PI将能够证明高速摩擦实验在某些条件下、所有条件下或无条件下是否会受到该机制的影响。 应用于真实的地震的滑动将通过与高速摩擦实验和类似于加州的圣安德烈亚斯断层（SAF）的深侵蚀断层的微观结构进行比较。 预计这些实验将提供一个定量的力学解释，为什么摩擦福尔斯急剧下降后不久，滑动开始，并再次上升，滑动减慢。 此外，该研究还将演示为什么SAF没有显示出热异常（所谓的圣安德烈亚斯热流悖论）以及为什么伪玄武质是罕见的。