Physically-Based Fault Zone Constitutive Responses and Consequences for Earthquake Dynamics

基于物理的断裂带地震动力学本构响应和后果

• 批准号: 0510193
• 负责人: James Rice
• 金额: $ 11.2万
• 依托单位: Harvard University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-07-01 至 2008-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0510193&HistoricalAwards=false
• 关键词: Physically; Based; Fault; Zone; Constitutive

AbstractNSF EAR Award number 0510193: Physically-based fault zone constitutive responses and consequences for earthquake dynamics(1 July 2005 to 30 June 2008)James R. Rice (principal investigator)Department of Earth and Planetary Sciences and Division of Engineering and Applied SciencesHarvard University, Cambridge, MAEarthquakes occur because fault strength weakens with increasing slip or slip rate. The aim of this work is to identify the physical processes underlying that weakening, and to analyze their consequences for the dynamics of earthquake rupture. The focus is on mature crustal faults, capable of producing large earthquakes. Recent field observations suggest that slip in individual events then occurs primarily within a thin shear zone, 1-5 mm, within a finely granulated (ultracataclastic) fault core. Since energy is dissipated in a narrow zone, the relevant weakening processes in large crustal events might, therefore, be expected to be thermal in origin. Our preparatory work for this study has assembled a strong case that primary weakening mechanisms during significant crustal earthquakes are thermal, and involve the following: (1) Thermal pressurization of groundwater that is resident within the (slightly) porous fault gouge, due to frictional heating of the gouge, and (2) Flash heating at highly stressed frictional micro-contacts which, at high slip rates like during earthquakes, causes them to lose their normally high shear strength even before they have slid out of existence. Elementary modeling of these mechanisms has been constrained with recently determined poroelastic and transport properties of fault core materials, and with recent high-speed friction studies.Predictions are that strength drop should often be nearly complete at slip of order 1 m, and that the onset of melting should be precluded over much of the seismogenic zone, except in large slip events. These are qualitatively consistent with low heat outflow from major faults and a scarcity of glass (pseudotachylyte) that would be left from rapid re-cooling. A more quantitatively testable prediction is of the shear fracture energies that would be implied if actual earthquake ruptures were controlled by those thermal mechanisms. Seismic observations have been processed to allow inference of the fracture energy of large crustal events, including its variation with slip in an event. It is found that the seismic results are plausibly described by the theoretical predictions, thus supporting the possibility that such thermal weakening prevails in the earth. This project focuses on consolidating those new advances, on expanding the modeling of the underlying physics to allow characterization of fault zone response under the highly variable slip rates of natural events, and on applying that understanding, embodied in constitutive relations, along with techniques of dynamic fracture simulation, to revisit a set of important problems in the dynamics of earthquakes, including their nucleation, propagation and arrest. We plan to develop viable methodologies of dynamic rupture analysis which incorporate these thermal mechanisms, in a 2D boundary integral equation formulations and, for more robust 2D and 3D applications, in an explicit-dynamics finite-element formulation. The problem of localization of shearing into a thin zone in otherwise stable granular lithologies will be addressed, as well as its relation to arrest of rupture as the slip zone attempts to penetrate hot regions of inherently stable, rate-strengthening, frictional response. Principles of fault operation under low overall driving stress are to be elaborated for such models of statically strong but dynamically weak fault response.

美国国家科学基金会（NSF）第0510193号奖：基于物理的断层带本构响应和地震动力学的后果（2005年7月1日至2008年6月30日）。赖斯（首席研究员）地球与行星科学系和工程与应用科学部哈佛大学，剑桥，马萨诸塞州地震的发生是因为断层强度随着滑动或滑动率的增加而减弱。这项工作的目的是确定物理过程的基础上，削弱，并分析其后果的动力学地震破裂。 重点是能够产生大地震的成熟地壳断层。最近的现场观察表明，在个别事件的滑动，然后主要发生在一个薄的剪切带，1-5毫米，在一个细颗粒（ultracatactic）的故障核心。由于能量是在一个狭窄的区域内耗散的，因此，大规模地壳事件中的相关弱化过程可能是热成因的。我们为这项研究所做的准备工作已经收集了一个强有力的案例，即在重大地壳地震期间的主要弱化机制是热的，并涉及以下内容：（1）地下水的热加压，这是居住在（轻微）多孔断层泥，由于断层泥的摩擦加热，以及（2）在高应力摩擦微接触处的闪热，在地震期间的高滑动速率下，导致它们甚至在滑出存在之前就失去其通常高的剪切强度。这些机制的基本建模已被约束与最近确定的孔隙弹性和输运性质的断层核材料，并与最近的高速摩擦study.Predictions是，强度下降往往应接近完成在1米的顺序滑动，并开始融化应被排除在大部分的孕震区，除了在大的滑动事件。这些都是定性一致的低热量流出的主要故障和稀缺的玻璃（pseudotachylyte），将离开快速再冷却。如果实际地震破裂是由这些热机制控制的话，那么就意味着剪切断裂能的预测是更可定量检验的。对地震观测进行了处理，以推断大地壳事件的断裂能，包括断裂能随事件滑动的变化。结果表明，地震结果与理论预测相符，从而支持了这种热弱化在地球中普遍存在的可能性。该项目的重点是巩固这些新的进展，扩大基本物理学的建模，以便能够描述自然事件的高度可变滑动率下断层带的反应，并应用这种理解，体现在本构关系中，沿着动态断裂模拟技术，重新审视地震动力学中的一系列重要问题，包括其成核、传播和停止。我们计划开发可行的方法动态断裂分析，将这些热机制，在一个二维边界积分方程制剂，更强大的2D和3D应用程序，在一个显式动力学有限元制剂。在稳定的粒状岩性中，剪切局部化到一个薄的区域的问题将得到解决，以及它与破裂的关系，因为滑动带试图穿透固有稳定的、速率强化的、摩擦响应的热区。对于静态强而动态弱的故障响应模型，需要详细阐述低总驱动应力下的故障运行原理。