Laboratory Study of Phase III SAFOD Core: Physical Properties and Mechanical Behavior of the Active San Andreas Fault Zone

我们正在努力了解地壳中大规模构造断层的物理性质，特别是加利福尼亚州的圣安德烈亚斯断层。作为圣安德烈亚斯深部断层观测站(SAFOD)的一部分，钻探最近穿透了约2.5公里深的断层。我们正在研究在这次钻探(SAFOD第三阶段)中回收的岩心，以及合成断层泥、在SAFOD钻孔附近地表收集的样品，以及在断层内和周围钻探的岩屑。从断层的活动链中只找到了一小部分岩心；因此，我们正在使用其他材料来扩展我们的数据集，并调查断层内的物理过程。我们的工作重点是：1)对样品的摩擦特性和渗透率进行详细测量，2)开发基于过程的模型以控制断层强度和稳定性，3)开发创新方法来研究断裂带组构对摩擦稳定性和物理特性的作用，以及4)将实验室结果与流体流动和热传输模型相结合，这将使对明显的断层软弱的假设能够进行批判性评估。我们正在解决的突出问题包括：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的原地条件联系起来？4)断裂带是否作为区域和局部流体流动的屏障？4)在包含活动SAF的物质中是否发生了显著的断层修复？如果是的话，声学(地震)特征是什么？5)断裂带的摩擦、弹性和运输属性与周围原岩的那些属性是如何联系的(换句话说，原岩成分如何影响断裂带的性质)？我们的工作补充了SAFOD样品的其他小组的研究，以及宾夕法尼亚州立大学专注于其他断裂带钻探项目的大量工作，如NantroSEIZE。拟议的实验结果将通过提高对断层力学和地震物理学的理解而产生社会影响，并将其应用于地球上研究最好、仪器最好的板块边界断层。

We are working to understand the physical properties of large-scale tectonic faults in Earth's crust, with particular focus on the San Andreas Fault in California. As part of the San Andreas Fault Observatory at Depth (SAFOD) drilling recently penetrated the fault at a depth of ~ 2.5 km. We are studying core recovered during this drilling (SAFOD phase III) along with synthetic fault gouge, samples collected at the surface in the vicinity of the SAFOD borehole, and cuttings from drilling in and around the fault. Only a small amount of core was recovered from the active strand of the fault; therefore, we are using the other materials to expand our data set and investigate physical processes within the fault. Our work is focused on: 1) detailed measurements of frictional properties and permeability of the samples, 2) development of process-based models for controls on fault strength and stability, 3) development of innovative methods to study the role of fault zone fabric on frictional stability and physical properties, and 4) integration of laboratory results with models for fluid flow and heat transport, which will allow critically evaluation of hypotheses for apparent fault weakness. Outstanding questions that we are addressing 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) Does the fault zone act as a barrier for regional and local fluid flow? 4) Does significant fault healing occur in materials comprising the active SAF, and if so, what is the acoustic (seismic) signature, if any? 5) How are fault zone frictional, elastic, and transport properties linked with those of the surrounding protolith (in other words, how does protolith composition influence fault zone properties)?Our work complements studies by other groups working on SAFOD samples as well as a substantial body of work at Penn State focused on other fault zone drilling projects, such as NanTroSEIZE. Results of the proposed experiments will have societal impact through improved understanding of fault mechanics and earthquake physics, applied to the best-studied and best-instrumented plate boundary fault on Earth.