Earthquake nucleation on rate and state faults: Theory (mostly) and some observations

速率和状态断层的地震成核：理论（主要）和一些观察

• 批准号: 0538156
• 负责人: Allan Rubin
• 金额: $ 19.4万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-12-01 至 2009-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0538156&HistoricalAwards=false
• 关键词: Earthquake; nucleation; rate; state; faults

Earthquakes do not begin at a single point in space, but via accelerating slip over a region of finite size termed the nucleation zone. The nucleation process is important because it determines whether or under what conditions detectable precursory signals may be produced, as well as the response of the fault to a suddenly-applied stress step, such as from a nearby large earthquake. Despite the fact that the "rate and state" formulation for fault friction has been widely used for over two decades, there is currently no general theory for determining the size of the nucleation zone on natural (elastically deformable) faults. In a recent numerical and analytical study, the PI and post-doc Jean-Paul Ampuero mapped out two regimes of earthquake nucleation on faults governed by the "aging law" (the more popular of two commonly used empirical laws for the evolution of the "state variable" part of "rate and state" friction). In one regime, the nucleation zone spontaneously evolves to a state of accelerating slip on a patch of fixed length. For laboratory values of the relevant constitutive parameters "a" (pertaining to "rate" effects) and "b" (pertaining to "state" effects), on the other hand, the nucleation zone takes the form of an expanding crack that asymptotically approaches a length that for a/b=0.95 is 100 times larger. This would lead to nucleation events that might sometimes be detectable using surface seismometers. However, these large sizes are a direct consequence of the rapid increase in effective fracture energy with increasing slip speed that the aging law implies. In fact, such a rapid increase has little experimental support, suggesting that despite its popularity the aging law is a poor descriptor of nucleation. In contrast, and in better agreement with relevant experiments, under the "slip law" the effective fracture energy of nucleation increases more slowly with slip speed. Preliminary calculations show that for laboratory values of a/b, nucleation under this law takes the form of a unidirectional slip "pulse" that is most active over a much smaller region.The PI is currently investigating, numerically and analytically, earthquake nucleation under the slip law and other classes of evolution laws, much as was done for the aging law. Because current laws for the evolution of the "state variable" are strictly empirical, trying to obtain a fundamental understanding of what distinguishes different classes of laws is quite important. Although this study is primarily theoretical, the results are also being applied to the analysis of several clusters of small repeating earthquakes that were activated by the M6.2 Morgan Hill, California, earthquake and that "turned off" within a year. These earthquakes may have occurred on nearly velocity-neutral (a/b~1) patches of the Calaveras fault with a size close to the nucleation length. The results of this study have already proven useful for the design of laboratory experiments that can distinguish between different proposed evolution laws, and should be useful for interpreting observations expected to come from two major NSF-sponsored projects to image earthquake nucleation at seismogenic depths - the SAFOD borehole along the San Andreas fault, and the NELSAM experiment in deep South African mines. The study will also support a first-year graduate student, Yue Tian to work

地震不是从空间的一个点开始，而是通过在有限大小的区域上加速滑移，称为成核区域。 成核过程很重要，因为它决定是否可以或在哪些条件下产生可检测到的前保留信号，以及断层对突然施加的应力步骤的响应，例如来自附近的大地震。 尽管事实是，断层摩擦的“速率和状态”公式已被广泛使用了二十年，但目前尚无一般理论来确定天然（弹性变形）断层的成核区域的大小。 在最近的一项数值和分析研究中，PI和DOC后的Jean-Paul Ampuero绘制了两个受“老化定律”管辖的断层的地震核定状态（在两个常用的经验定律中更受欢迎，用于“状态变量”部分“速率和状态”的“状态变量”部分的进化）。 在一个制度中，成核区域自发发展到固定长度片上的加速滑移状态。 对于相关本构参数的实验室值，“ A”（与“速率”效应）和“ b”（有关“状态”效应）的实验室值，另一方面，成核区域采用了渐近裂纹的形式，该裂纹渐近地接近了A/B = 0.95的长度，该长度大于100倍。这将导致成核事件有时使用表面地震米可以检测到。但是，这些大尺寸是随着衰老法律所暗示的下滑速度的提高，有效断裂能量迅速增加的直接结果。 实际上，如此迅速的增长几乎没有实验支持，表明尽管它流行了，但老化定律还是成核的描述较差。 相反，在“滑移定律”下，与相关实验更好地吻合，有效的成核断裂能随着滑移速度而增加。 初步计算表明，对于A/B的实验室值，该法律下的成核采用单向滑动“脉冲”的形式，该形式在一个较小的区域中最为活跃。当前在数值和分析上，在数值和分析上，地震核定在地震定律和其他类别的进化法中，对Aging Agan Aging Law进行了许多类别。 由于“状态变量”进化的当前法律是严格的经验，因此试图获得对区分不同规律的基本理解非常重要。 尽管这项研究主要是理论上的，但结果也应用于对M6.2 Morgan Hill，California，Garreatquake，一年内“关闭”的几个小型重复地震群的分析。 这些地震可能发生在几乎速度 - 中性（A/B〜1）的Calaveras断层斑块上，其大小接近成核长度。 这项研究的结果已被证明可用于设计可以区分不同提议的进化法的实验室实验的设计，并且对于解释预期的观察值应该是有用的，这些观察结果将来自两个主要的NSF赞助的项目，以对地震深度的地震核定形象形象地震核定作品 - Safod Borehole -Safod Borehole沿Safod Borehole沿San Andreas the San Andreas and Nelsam and nelsam inter Ampecrimans in Deep Anspecrimens in Deep Anspecimens in Deep Anspeciments in safod Borehole。该研究还将支持一年级研究生Yue Tian工作