Collaborative Research: Brittle fracture, deformability, permeability, and seismic anisotropy adjacent to and inside the San Andreas Fault Zone through SAFOD

威斯康星大学麦迪逊分校和纽约州立大学石溪分校正在进行一项合作研究项目，其主要目标是研究圣安德烈亚斯断裂带附近和内部的脆性断裂、可变形性、渗透率和地震各向异性，这是“地球范围”科学计划的一部分。这项实验室研究是对从圣安德烈亚斯断层观测站（SAFOD）洞中提取的岩心进行的。通过这项研究表征的地球物理特性对于理解导致圣安德烈亚斯断层地震的机制是至关重要的。麦迪逊实验室有一种独特的真三轴仪器，用于获得圣安德烈亚斯断层及其附近岩石的经验强度标准，当与沿孔记录的钻孔突破尺寸结合使用时，这些信息对于评估构造应力条件至关重要。真三轴试验还提供了挖掘断层和乡村岩石的弹性模量、体积应变、扩容起始和脆性断裂等力学特性的必要信息。这项研究试图回答诸如这些属性在断层带内外如何变化等问题；它们如何依赖于环境因素，如地应力、矿物学、孔隙压力和温度；在岩心样品上测量的力学性质如何与从钻孔测井和地面地球物理观测推断的性质进行比较。渗透率和地震各向异性测量是在石溪实验室进行的。对断裂带附近岩心样本的检索和渗透率作为应力函数的系统测量为圣安德烈亚斯断层结构提供了重要的背景数据，同时也为某些孔隙压力过剩和断层弱化机制是否可行提供了关键约束。以往没有对断层核地震各向异性进行系统的研究。虽然有相当复杂的关于应力诱发损伤和速度各向异性的理论模型，但在控制应力和孔隙压力条件下对这种速度的实验室测量以及损伤的定量表征对于阐明这种现象的机制是必要的。对圣安德烈亚斯断裂带断层材料和岩石的一些力学和水力特性的全面研究是实现SAFOD了解断裂带过程和地震物理的总体目标的先决条件。

A collaborative research project is underway at the University of Wisconsin-Madison and the State University of New York-Stony Brook, the principal objective of which is to investigate brittle fracture, deformability, permeability and seismic anisotropy adjacent to and inside the San Andreas Fault Zone, as part of the EarthScope science program. This laboratory study is being carried out on core extracted from the San Andreas Fault Observatory at Depth (SAFOD) hole. The geophysical properties that are characterized through this study are fundamental in efforts to understand the mechanisms that bring about earthquakes along the San Andreas Fault. A unique true-triaxial apparatus available in the Madison laboratory is used to obtain empirical strength criteria for the rocks in and adjacent to the San Andreas Fault, information that is crucial to assessing the tectonic stress conditions when used in conjunction with the dimensions of borehole breakouts logged along the hole. The true triaxial tests also provide necessary information on mechanical properties like the elastic moduli, volumetric strain, dilatancy onset, and brittle fracture of the exhumed fault and country rocks. The study seeks to answer questions such as how these properties vary across the fault zone and beyond; how they depend on environmental factors like situ stress, mineralogy, pore pressure, and temperature; how mechanical properties measured on core samples compare with properties inferred from borehole logs and surface-based geophysical observations. Permeability and seismic anisotropy measurements are conducted in the Stony Brook laboratory. The retrieval of core samples in the vicinity of the fault zone and the systematic measurement of permeability as a function of stress provide significant background data on the San Andreas Fault architecture, as well as critical constraints on whether certain mechanisms for pore pressure excess and fault weakening can be operative. No previous systematic studies of seismic anisotropy on fault cores have been conducted. While fairly sophisticated theoretical models relating stress-induced damage and velocity anisotropy are available, laboratory measurements of such velocities under controlled stress and pore pressure conditions as well as quantitative characterization of the damage are necessary to elucidate the mechanics of such phenomena. This comprehensive study of some mechanical and hydraulic properties of the fault material and country rock in the San Andreas Fault Zone is a prerequisite in accomplishing the SAFOD general goal of understanding fault zone processes and earthquake physics.