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，尽管地震通常被建模为平面断层上的摩擦不稳定，但大多数天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芯(毫米到厘米尺度)？高应变的粒状岩石，其边界是碎裂和破碎的岩石的宽层(几米到几十米)。这些层被称为泥、角砾岩或粉碎岩，有一个重要的特征：它们似乎容纳了很少或根本没有宏观剪切应变。这种低应变碎裂岩层提出了两个重要的问题：它们是如何形成的，它们是否影响了个别地震的动力学？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屏障(弯曲和凸起)，因为总的位移在不断演变的断层上累积，然后当滑动定位在地心时，它们就被遗弃了。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将岩石破碎到距离断层核心数十米的地方。这些研究提出了一种可能性，即断层泥、角砾岩和粉碎的岩石可能主要是在一系列地震的动态应力场中形成的，因此断裂带的结构可能包含关于过去事件的有用信息。基于实验室的对断裂损伤材料中破裂传播的高速摄影观察也发现，即使在新裂缝的形成不会增加损害的情况下，断层外的破坏也会强烈地影响破裂速度。这些结果得到了动态破裂传播的二维数值模型的支持，其中断层外损伤的影响已被Mohr-Coulomb塑性近似。然而，这些模型也没有考虑到裂隙的大小或密度，这些裂隙构成了断层核心周围的预先存在的损伤。研究人员建议开发新一代动态地震数值模型，其中断层外损伤的产生及其对破裂扩展的影响是使用由Deshpande和Evans[2008]扩展并适用于数值模拟的细观损伤力学模型来模拟的。该模型比以往使用摩尔-库仑塑性甚至连续损伤力学的模型有了很大的改进，因为它考虑了介质中已有的损伤、断裂带中裂缝的摩擦损失以及新裂缝的形核和扩展。因为它专门考虑了裂缝大小和密度的演变，所以它做出的预测可以在现场测试，并在实验室得到验证。此外，地震破裂尖端裂隙密度的动态变化可能会对目前所用的热增压模型产生重大影响，这些模型用于合理地确定满足热流和其他岩石学约束的地震力学所需的动摩擦系数的低值。

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.