The Existence, Source, and Mechanical Significance of Hierarchical Fractal Structures in Fault Zones

9902901Sammis 最近对加利福尼亚州帕克菲尔德的小地震进行的一项研究发现，应力降随着震级的增加而减小（Nadeau 和 Johnson，1998）。 可以解释这一观察结果的一个假设是，断层面被粗糙体的分形阵列锁定，使得每单位面积的粗糙体数量随着面积的增加而减少，从而减少了较大事件的表观应力降。这一假设得到了震源配对相关函数的支持，该函数被发现与二维康托微凸体尘埃一致（Sammis 等人，1998）。该提案将进一步研究断层带中分层分形结构的几何形状、来源和力学意义。 该项目将进一步分析帕克菲尔德地震数据，以确定作为震级函数的空间和时间分布是否也与康托尘假说一致。 关于分形层次的来源，我们提出了三个假设：1）断层被具有层次分形结构的粗糙体锁定，2）断层被断层中具有层次分形结构的“粒桥”锁定，3）地震发生在嵌套的剪切局部层次上。 计划进行一系列实验室调查来检验每个假设。 主要目标是尝试确定剪切变形中出现离散分层分形结构的条件。

9902901SammisA recent study of small earthquakes at Parkfield, CA, has found that the stress drop decreases with increasing magnitude (Nadeau and Johnson, 1998). One hypothesis that can explain this observation is that the fault plane is locked by a fractal array of asperities, such that the number of asperities per unit area decreases with increasing area thereby reducing the apparent stress drop for larger events. This hypothesis is supported by the pair correlation function of the hypocenters, which were found to be consistent with a 2-D Cantor dust of asperities (Sammis et al., 1998). This proposal will further investigate the geometry, source, and mechanical significance of hierarchical fractal structures in fault zones. The project will further analyze Parkfield seismic data to determine if the spatial and temporal distributions as a function of magnitude are also consistent with a Cantor dust hypothesis. As to the source of the fractal hierarchy, we have formulated three hypotheses 1) the fault is locked by asperities which have a hierarchical fractal structure, 2) the fault is locked by "grain bridges" in the gouge zone which have a hierarchical fractal structure, and 3) earthquakes occur on a nested hierarchy of shear localizations. A series of laboratory investigations are planned to test each hypothesis. A primary objective is to try to identify the conditions under which discrete hierarchical fractal structure emerges in shear deformation.