Micromechanics-Based Modeling of Dynamic Earthquake Rupture in a Structurally Complex Fault Zone

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。尽管地震通常被模拟为平面断层表面的摩擦不稳定性，但大多数自然断层具有更复杂的结构。大多数位移似乎集中在一个或多个相对较窄的岩心（毫米至厘米尺度）。由高度应变的颗粒状岩石组成，边缘是宽层（米到几十米）的破碎岩石。这些层被称为断层泥、角砾岩或粉状岩石，它们有一个重要特征：它们似乎很少或根本没有宏观剪切应变。这种低应变的破碎岩层提出了两个重要的问题：它们是如何形成的？它们是否会影响单个地震的动力学？长期以来，人们一直假设，断层泥和角砾岩层的形成是为了容纳几何障碍（弯曲和缓动），因为断层的总位移在演化过程中积累，当滑动局限于岩心时，它们就被抛弃了。然而，最近的理论、实验室和地震现场研究发现，地震破裂尖端的应力集中可以使距离断层核心几十米远的岩石破碎。这些研究提出了这样一种可能性，即断层泥、角砾岩和粉状岩石可能主要是在一系列地震的动态应力场中形成的，因此，断层带的结构可能包含有关过去地震事件的有用信息。基于实验室的断裂损伤材料破裂扩展的高速摄影观察也发现，非断层损伤会强烈影响破裂速度，即使在损伤没有因新裂缝的形成而增加的情况下也是如此。这些结果得到了动态破裂扩展的二维数值模型的支持，其中非断层损伤的影响已由莫尔-库仑塑性近似表示。然而，这些模型没有考虑到构成断层核周围预先存在的破坏的裂缝的大小或密度。研究人员提出开发新一代数值动力地震模型，其中断层外损伤的产生及其对破裂传播的影响采用Deshpande和Evans[2008]扩展并适用于数值模拟的微力学损伤力学模型进行建模。该模型与先前使用Mohr-Coulomb塑性甚至连续损伤力学的模型相比有了显著的改进，因为它考虑了介质中预先存在的损伤、断裂带中裂缝的摩擦损失以及新裂缝的成核和扩展。由于该模型能够准确地解释裂缝的大小和密度的变化，因此它所做出的预测既可以在现场进行测试，也可以在实验室进行验证。此外，地震破裂尖端裂缝密度的动态变化可能会对目前用于合理化低动态摩擦系数值的热压模型产生重大影响，以满足热流和其他岩石学对地震力学的约束。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)Although earthquakes are commonly modeled as frictional instabilities on planar fault surfaces, most natural faults have a more complex structure. Most displacement appears to be concentrated in one or more relatively narrow (mm to cm scale) ?cores? of highly strained granular rock which are bordered bywide layers (meters to tens of meters) of fragmented and shattered rock. Termed gouge, breccia, or pulverized rock, these layers share one important characteristic: they appear to have accommodated little or no macroscopic shear strain. Such low-strain layers of shattered rock raise two important questions:how were they formed and do they affect the dynamics of individual earthquakes?It has long been hypothesized that the gouge and breccia layers were formed to accommodate geometrical barriers (bends and jogs) as total displacement accumulated on an evolving fault, and were then abandoned when slip localized in the core. However, recent theoretical, laboratory and seismological field studies have found that the stress concentration at the tip of an earthquake rupture can shatter rock to distances of tens of meters from the fault core. These studies raise the possibility that gouge, breccia, and pulverized rock might form, primarily, in the dynamic stress fields of a sequence of earthquakes, and that the structure of a fault zone might therefore contain useful information about past events. Laboratory based high-speed photographic observations of rupture propagation in fracture damaged materials have also found that off-fault damage can strongly affect the rupture velocity, even incases where the damage is not increased by the formation of new fractures. These results are supported by 2D numerical models of dynamic rupture propagation where the effects of the off-fault damage have been approximated by Mohr-Coulomb plasticity. These models, however, do not take into account eitherthe size or density of fractures that constitute pre-existing damage surrounding the fault-core.The investigators propose to develop a new generation of numerical dynamic earthquake models in which the generation of off-fault damage and its effect on rupture propagation are modeled using a micromechanical damage mechanics model expanded and made suitable for numerical modeling by Deshpande and Evans [2008]. This model represents a significant improvement on previous models that use Mohr-Coulomb plasticity or even continuum damage mechanics in that it takes into account pre-existing damage in the medium, frictional loss on fractures in the fault zone, as well as the nucleation and propagation of new fractures. Because it specifically accounts for the evolution of the size and density of fractures, it makes predictions that can be tested in the field, and verified in the laboratory. Moreover, dynamic changes in fracture density at the tip of an earthquake rupture may have a significant effect on thermal pressurization models currently used to rationalize the low value of the coefficient of dynamic friction required to satisfy heat flow and otherpetrological constraints on the mechanics of earthquakes.