Collaborative Research: Laboratory Study of the Mechanics and Physical Properties of the San Andreas Fault and 3D SAFOD Volume

[05:45548]托宾：SAFOD钻孔的主要目标，也是国家科学基金会EarthScope计划的主要动机之一，是收集了解断层力学和地震所需的关键数据。通过取样、井下测量和长期监测，SAFOD实验将提供数据来验证有关长期断层强度、地震成核和断层滑动行为的关键假设。然而，钻孔本身只能穿透圣安德烈亚斯断裂带周围地壳体积的一小部分，并且只能取样地下存在的一小部分岩性。虽然SAFOD将以前所未有的细节提供主要板块边界断层的浅层孕震带观测，但包含SAFOD和圣安德烈亚斯断层的三维体积岩石物理特性的额外表征对于解决断层力学和地震物理学中几个最重要的悬而未决的问题至关重要。其中包括：1)是什么导致了断层滑动行为和地震活动的空间变异性？2) SAFOD三维体积内流体压力升高是否合理？3)地球物理观测（如低速或电阻率）如何与应力和流体压力的原位条件联系起来？4)浅层地下热数据对断层能量收支有何启示？我们正在对影响断裂带力学行为和运移性质的过程和性质进行全面的研究。这项研究包括对SAFOD岩心和露头样品的实验室测量。这些测量旨在从3-D SAFOD体积中描述岩石的变形过程和物理性质。这项工作是对正在进行的SAFOD样品工作的补充。我们收集的数据将为区域地质、水文和热模型提供信息。我们的研究旨在解决以下关键目标：-确定SAFOD岩心材料和圣安德烈亚斯断层附近的宿主岩石的摩擦强度和本构性质。-验证从地震断层到地震断层的上部稳定性转变与断层泥和/或宿主岩石的矿物学变化有关的假设。-制定必要的实验约束，以验证1)圣安德烈亚斯断层在绝对和相对意义上是弱的假设，以及2)长期孔隙压力产生和动态断层减弱的模型。-提供与断层能量收支相关的过程约束：包括摩擦热产生、平流热传输和热折射。-研究摩擦强度（包括愈合和稳态速度依赖）、地震波速度和渗透率之间的关系。-研究断裂带和围岩中P波和S波速度及其各向异性对应力和孔隙压力的依赖关系，以评估和改进孔隙压力、有效应力、摩擦强度、流体含量以及从井眼测井和地面地震数据推断的其他属性的地震属性代理。这项研究将提供对控制主要断层的强度和稳定性的过程的理解。此外，我们将测量那些决定遥感地球物理特征的断层岩石的属性，这对于更好地评估地震危险和将断层行为观察与基本物理过程联系起来非常重要。

0545548TobinThe principal goal of the SAFOD borehole, and one of the main motivations for the NSF EarthScope initiative, is to gather critical data needed to understand fault mechanics and earthquakes. Through sampling, down-hole measurements, and long-term monitoring, the SAFOD experiment will provide data to test key hypotheses regarding long-term fault strength, earthquake nucleation, and fault slip behavior. However, the borehole itself will penetrate only a small part of the crustal volume surrounding the San Andreas Fault zone, and will sample only a subset of lithologies present in the subsurface. Although SAFOD will provide observations of the shallow seismogenic zone of a major plate bounding fault in unprecedented detail, additional characterization of rock physical properties for the 3-D volume containing SAFOD and the San Andreas Fault are critical for addressing several of the most important outstanding questions in fault mechanics and earthquake physics. These include: 1) What causes spatial variability in fault slip behavior and seismicity? 2) Are elevated fluid pressures within the SAFOD 3-D volume plausible? 3) How are geophysical observations such as low velocity or resistivity linked to in situ conditions of stress and fluid pressure? 4) What do thermal data in the shallow subsurface tell us about the fault energy budget? We are conducting a comprehensive study of the processes and properties that affect mechanical behavior and transport properties of fault zones. The research involves laboratory measurements of SAFOD core and outcrop samples. These measurements are designed to characterize the deformation processes and physical properties of rocks from the 3-D SAFOD volume. This work complements ongoing work on SAFOD samples. The data we collect will inform regional geologic, hydrologic, and thermal models. Our research is designed to address the following key objectives: - Determine the frictional strength and constitutive properties for SAFOD core material and host rock adjacent to the San Andreas Fault. - Test the hypothesis that the upper stability transition from aseismic to seismic faulting is associated with a change in mineralogy of fault gouge and/or host rock. - Develop experimental constraints necessary to test 1) the hypothesis that the San Andreas Fault is weak in an absolute and relative sense, and 2) models of long-term pore pressure generation and dynamic fault weakening. - Provide constraints on processes relevant to the energy budget of faulting: including frictional heat generation, advective heat transport, and thermal refraction. - Investigate the relationship between frictional strength (including healing and steady-state velocity dependence), seismic wave speed, and permeability. - Investigate the stress and pore pressure dependence of P and S wave speeds and their anisotropies in fault zone and wall rock, to evaluate and improve seismic-attribute proxies for pore pressure, effective stress, frictional strength, fluid content, and other properties inferred from borehole log and surface seismic data. This research will provide an understanding of processes that govern the strength and stability of major faults. In addition, we will measure those properties of fault rock that determine remotely sensed geophysical signatures, which is important for better assessment of earthquake hazard and for linking observations of fault behaviors with fundamental physical processes.