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

0545548托宾SAFOD钻孔的主要目标，也是NSF地球范围倡议的主要动机之一，是收集了解断层力学和地震所需的关键数据。通过采样、井下测量和长期监测，SAFOD实验将提供数据来测试有关长期断层强度、地震成核和断层滑动行为的关键假设。然而，钻孔本身只会穿透圣安德烈亚斯断裂带周围的一小部分地壳体积，并且只会对地下存在的一部分岩性进行采样。尽管SAFOD将提供对主要板块边界断层的浅层孕震带的前所未有的详细观测，但对包含SAFOD的3-D体积和圣安德烈亚斯断层的岩石物理性质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。这些问题包括：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的升高是可信的吗？3)低速或电阻率等地球物理观测如何与地应力和流体压力条件联系起来？4)浅层地下的热数据告诉我们关于断层能量收支的什么信息？我们正在对影响断裂带力学行为和运输性质的过程和性质进行全面研究。这项研究涉及对SAFOD岩芯和露头样品的实验室测量。这些测量旨在从三维SAFOD体积中表征岩石的变形过程和物理性质。这项工作是对SAFOD样品正在进行的工作的补充。我们收集的数据将为区域地质、水文和热力模型提供信息。我们的研究旨在解决以下主要目标：-确定SAFOD核心材料和与圣安德烈亚斯断层相邻的主岩的摩擦强度和本构特性。-检验从无地震到地震断裂作用的上部稳定性转变与断层泥和/或围岩的矿物学变化有关的假设。-开发必要的实验约束，以检验1)圣安德烈亚斯断层在绝对和相对意义上较弱的假设，以及2)长期孔隙压力产生和动态断层弱化的模型。-对与断层作用的能量收支相关的过程提供限制：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相关性)、地震波速和渗透率之间的关系。-研究断裂带和围岩中P波和S波速的应力和孔压相关性及其各向异性，评估和改进从井中测井和地面地震数据推断的孔压、有效应力、摩擦强度、流体含量和其他属性的地震属性替代物。这项研究将提供对支配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性和将断层行为的观测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.