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，其主要目标是调查圣安德烈亚斯断裂带附近和内部的脆性断裂、变形性、渗透性和地震各向异性，这是EarthScope科学计划的一部分。这项实验室研究是在圣安德烈亚斯深井断层观测站(SAFOD)孔中提取的岩芯上进行的。通过这项研究所表现出的地球物理特性对于了解圣安德烈亚斯断层上地震的发生机制是至关重要的。麦迪逊实验室提供的一种独特的真三轴仪器用于获得圣安德烈亚斯断层内和邻近岩石的经验强度标准，这些信息对于评估构造应力条件至关重要，当与沿该孔记录的钻孔崩落尺寸结合使用时。真三轴试验还提供了必要的力学性能信息，如弹性模数、体积应变、剪胀开始和脆性断裂的断层和围岩。这项研究试图回答以下问题：这些特性在断裂带和更远的地方是如何变化的；它们如何依赖于地应力、矿物学、孔压和温度等环境因素；岩心样品测量的力学特性与从钻孔记录和地面地球物理观测推断的特性如何比较。渗透率和地震各向异性测量是在石溪实验室进行的。在断裂带附近的岩心样品的提取和作为应力函数的渗透率的系统测量提供了关于圣安德烈亚斯断层结构的重要背景数据，以及关于孔压过剩和断层弱化的某些机制是否可以发挥作用的关键制约因素。以前还没有对断层岩心的地震各向异性进行系统的研究。虽然已有相当复杂的理论模型来描述应力引起的损伤和速度各向异性，但在受控应力和孔压条件下对这些速度的实验室测量以及对损伤的定量描述对于阐明这种现象的机理是必要的。对圣安德烈亚斯断裂带的断层物质和围岩的一些力学和水力性质的全面研究是实现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.