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

基于微观力学的结构复杂断层带动态地震破裂建模

• 批准号: 0838263
• 负责人: Charles Sammis
• 金额: $ 14.55万
• 依托单位: University of Southern California
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-07-15 至 2011-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0838263&HistoricalAwards=false
• 关键词: Micromechanics; Based; Modeling; Dynamic; Earthquake

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.

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。大多数位移似乎集中在一个或多个相对狭窄（毫米至厘米尺度）？核心？一种高度应变的粒状岩石，其边界是宽层（几米到几十米）的破碎岩石。这些地层被称为断层泥、角砾岩或岩石粉，它们有一个共同的重要特征：它们似乎只承受很少或根本不承受宏观剪切应变。这种低应变的破碎岩石层提出了两个重要的问题：它们是如何形成的，它们是否影响了单个地震的动力学？长期以来，人们一直假设，当总位移在不断发展的断层上累积时，断层泥和角砾岩层是为了适应几何障碍（弯曲和凹凸）而形成的，然后当滑动局限于核心时被放弃。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数十米的岩石破碎。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，因此断层带的结构可能包含有关过去事件的有用信息。基于实验室的高速摄影观察断裂损坏的材料中的破裂传播也发现，断层外的损坏可以强烈地影响破裂速度，即使在损坏没有增加的情况下形成新的裂缝。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近似莫尔-库仑塑性。然而，这些模型，没有考虑构成断层核周围预先存在的损伤的裂缝的大小或密度。研究人员建议开发新一代的数值动态地震模型，其中断层外损伤的产生及其对破裂传播的影响使用Deshpande和Evans [2008年]。该模型是一个显着的改进，以前的模型，使用莫尔-库仑塑性，甚至连续损伤力学，它考虑到预先存在的介质中的损伤，断裂带中的断裂摩擦损失，以及新的裂缝的成核和传播。因为它特别说明了裂缝的尺寸和密度的演变，它可以在现场进行测试，并在实验室中进行验证。此外，在地震破裂的尖端断裂密度的动态变化可能有显着的影响，目前使用的热加压模型，以合理化的低值所需的动摩擦系数，以满足热流和其他岩石学约束的力学地震。