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

美国国家科学基金会EAR奖编号0510193：地震动力学的基于物理的断层带本构响应和后果(2005年7月1日至2008年6月30日)詹姆斯·R·赖斯(首席研究员)，地球和行星科学系，工程和应用科学部，哈佛大学，剑桥，麻省，地震发生的原因是断层强度随着滑动或滑移率的增加而减弱。这项工作的目的是确定这种减弱背后的物理过程，并分析它们对地震破裂动力学的影响。重点放在能够产生大地震的成熟地壳断层上。最近的野外观测表明，个别事件的滑动主要发生在细粒(超碎裂)断层核心内的1-5毫米薄剪切带内。由于能量是在一个狭窄的区域内耗散的，因此，大型地壳事件中的相关减弱过程可能是热源的。我们为这项研究做的准备工作已经形成了一个强有力的例子，即在重大地壳地震期间，主要的减弱机制是热的，涉及以下内容：(1)由于断层泥的摩擦加热，驻留在(略有)多孔断层泥中的地下水的热压作用；(2)在高应力摩擦微接触处的闪热加热，在地震期间的高滑移率下，使它们甚至在滑出之前就失去了正常的高剪切强度。对这些机制的初步模拟受到最近确定的断层核心物质的孔弹性和输运性质以及最近的高速摩擦研究的限制。预测是，强度下降通常应该在1米量级的滑移时几乎完成，除了大滑动事件外，应该阻止在地震孕育区的大部分地区发生熔融。这与主要断层的低热量外流和快速再冷却留下的玻璃(假黄晶石)的稀缺性是一致的。一个更可定量检验的预测是，如果实际地震破裂受这些热机制控制，将隐含的剪切破裂能。对地震观测进行了处理，以便能够推断大型地壳事件的破裂能，包括其随事件中滑动的变化。结果表明，理论预测能较好地描述地震结果，从而支持这种热力减弱在地球上占优势的可能性。该项目的重点是巩固这些新进展，扩大基本物理模型，以便能够描述在自然事件高度可变的滑移率下的断裂带响应，并利用本构关系中体现的这一理解以及动态断裂模拟技术，重新探讨地震动力学中的一系列重要问题，包括地震的成核、传播和滞留。我们计划开发可行的动态断裂分析方法，将这些热机制包含在2D边界积分方程式中，并在显式动力学有限元公式中用于更强大的2D和3D应用。在其他稳定的粒状岩性中，剪切进入薄层的局部化问题将被解决，以及当滑移带试图穿透具有固有稳定、速率增强、摩擦响应的热区时，它与阻止破裂的关系。对于这种静强动态弱的故障响应模型，阐述了低总驱动应力下的故障运行原理。

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.