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

SAFOD钻孔的主要目标，也是NSF EarthScope计划的主要动机之一，是收集理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，钻孔本身将仅穿透圣安德烈亚斯断层带周围的一小部分地壳体积，并且将仅对地下存在的岩性子集进行采样。虽然SAFOD将提供一个主要板块边界断层的浅层孕震区的观测，其细节是前所未有的，但包含SAFOD和圣安德烈亚斯断层的三维体积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突出问题至关重要。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 3-D容积内的液体压力升高是否合理？3)地球物理观测，如低速度或电阻率如何与应力和流体压力的原位条件联系起来？4)浅层地下的热数据告诉我们关于断层能量收支的什么？我们正在对影响断层带力学行为和输运性质的过程和性质进行全面研究。这项研究包括对SAFOD岩心和露头样品进行实验室测量。这些测量的目的是从3-D SAFOD体积的变形过程和岩石的物理性质的特征。这项工作补充了正在进行的关于SAFOD样品的工作。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核心材料和邻近圣安德烈亚斯断层的主岩的摩擦强度和本构特性。- 检验从地震断层到地震断层的上部稳定性转变与断层泥和/或寄主岩石矿物学变化有关的假设。- 开发必要的实验约束条件，以测试1）圣安德烈亚斯断层在绝对和相对意义上都很弱的假设，以及2）长期孔隙压力生成和动态断层弱化的模型。- 提供与断层能量收支有关的过程的限制条件：包括摩擦生热、平流热输送和热折射。- 研究摩擦强度（包括愈合和稳态速度相关性）、地震波速度和渗透率之间的关系。- 研究断层带和围岩中P波和S波速度与应力和孔隙压力的相关性及其各向异性，以评估和改进孔隙压力、有效应力、摩擦强度、流体含量和其他根据钻孔测井和地面地震数据推断的属性的地震属性代用指标。这项研究将提供一个过程，管理的强度和稳定性的主要故障的理解。此外，我们将测量断层岩石的这些属性，确定遥感地球物理特征，这是重要的，更好地评估地震的危险性，并与基本物理过程的故障行为的观察。

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.