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特性，特别关注加州的圣安德烈亚斯断层。作为圣安地列斯断层深度观测站（SAFOD）的一部分，最近钻探穿透了断层约2.5公里的深度。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井眼附近的地表收集的样本，以及在断层内部和周围钻探的岩屑。只有少量岩心从断层的活动链中恢复；因此，我们正在使用其他材料来扩展我们的数据集，并研究断层内的物理过程。我们的工作重点是：1)详细测量样品的摩擦特性和渗透率；2)开发基于过程的模型来控制断层强度和稳定性；3)开发创新方法来研究断层带结构对摩擦稳定性和物理特性的作用；4)将实验室结果与流体流动和热传输模型相结合，这将允许对明显断层弱点的假设进行批判性评估。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)断裂带是否对区域和局部流体流动起到屏障作用？4)在构成有源SAF的材料中是否发生明显的断层愈合，如果是，如果有，声学（地震）特征是什么？5)断裂带的摩擦、弹性和输运性质如何与周围原岩的性质联系起来（换句话说，原岩组成如何影响断裂带的性质）？我们的工作补充了其他研究小组对SAFOD样品的研究，以及宾夕法尼亚州立大学对其他断层带钻探项目（如NanTroSEIZE）的大量工作。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，应用于地球上研究最好、仪器最好的板块边界断层，从而产生社会影响。

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.