Mechanics and Seismogenic Potential of Low Angle Normal Faults: A Field and Laboratory Investigation

该奖项是根据2009年美国恢复和再投资法案(公共法律111-5)资助的。了解控制主要构造断层上的应力状态和滑动性质的因素是地震物理和断层力学中的一个基本问题。特别是，许多主要断裂带，包括圣安德烈亚斯断层，几个研究得很好的俯冲带板块边界，以及低角度正断层，似乎在异常低的剪应力下滑动(即，它们的机械强度很弱)。最近的研究也对现代低角正断层发生地震滑动的可能性提出了相互矛盾的观点，这对地震危险性评估具有重要意义。最近和正在进行的许多工作都集中在通过钻井对活动断裂带进行采样和测量来确定导致断层软弱的机制。另一种方法是研究地球历史早期形成的暴露良好的出土断层，这些断层类似于活动断层。该项目主要研究中新世(2400万至600万年前)区域地壳伸展所形成的低角度正断层，该地区现在是加利福尼亚州和亚利桑那州的莫哈韦沙漠。这些低角度正断层倾角较浅，可容纳数十公里的滑动，似乎在严重错位的情况下滑动，(垂直)最大主应力几乎垂直于断裂面。随后的侵蚀从2-10公里深处挖出了断裂带，并提供了极好的暴露条件。这项研究项目将利用宾夕法尼亚州立大学岩石力学实验室的压力容器来表征这些出土断层的断层泥和断层岩的摩擦特性和稳定性，以解决有关低角度正断层的两个悬而未决的问题：(1)来自低角度正断层的天然断层泥的绝对强度是多少，以及弱粘土矿物的存在是否足以解释其明显的机械弱点？以及(2)断层岩的摩擦特性是否与这些构造上地震成核的可能性一致？这项工作的一个特殊特点是能够测试完整的断层泥样本，这些断层泥保存了它们独特的组构，并可能在控制它们的摩擦行为方面发挥关键作用。地震对美国大部分地区和全球人口稠密地区构成了重大危险。主要构造断层上的整体机械强度和滑动性质(无论是通过蠕变发生还是通过地震中的幕式破坏发生)，在很大程度上都取决于这些断层带内岩石和断层泥的物理性质。板块构造边界的许多主要断裂带似乎在异常低的应力下滑动，这意味着它们的机械强度很弱。低角度正断层是一类表现出这种明显的机械弱点的断层，这种断层在美国西南部很常见。这些断层上发生地震的可能性也是一个重要的辩论主题，主要是因为总体滑移率较低，潜在的重复时间较长，使得危险难以评估。本研究将通过野外填图、取样和详细的断层材料实验室研究，探讨控制低角度正断层强度和滑动行为的因素。该项目将为这些结构的力学提供新的见解，并将阐明机械弱断层的力学行为和稳定性。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Understanding the factors controlling the stress state and nature of slip on major tectonic faults is a fundamental problem in earthquake physics and fault mechanics. In particular, many major fault zones, including the San Andreas Fault, several well-studied subduction zone plate boundaries, and low angle normal faults appear to slip under anomalously low shear stresses (i.e., they are mechanically weak). Recent studies also provide conflicting views about the potential for seismic slip on modern low angle normal faults, which is of importance for earthquake hazard assessment. Much recent and ongoing work has focused on identifying the mechanisms causing fault weakness through sampling and instrumentation of active fault zones by drilling. Another approach is to study well-exposed exhumed faults that formed earlier in Earth's history, and which serve as analogs for active faults. This project focuses on low angle normal faults that formed in response to the regional crustal extension during the Miocene epoch (24 to 6 million years ago) in the area that is now the Mojave Desert of California and Arizona. These low angle normal faults dip shallowly, have accommodated tens of kilometers of slip, and appear to have slipped while severely misoriented, with the (vertical) maximum principal stress nearly perpendicular to the fault surface. Subsequent erosion has exhumed the fault zones from depths of 2-10 km, and has provided access to excellent exposures. This research project will characterize the frictional properties and stability of gouge and fault rock from these exhumed faults using a pressure vessel in the rock mechanics laboratory at Pennsylvania State University, in order to address two outstanding questions about low angle normal faults that bear on the underlying causes of fault weakness in general: (1) What is the absolute strength of natural fault gouge from low angle normal faults, and is the presence of weak clay minerals sufficient to explain their apparent mechanical weakness?; and (2) Are the frictional properties of the fault rock consistent with the possibility of earthquake nucleation on these structures? A particular feature of this work is the ability to test samples of intact fault gouge, which preserve their distinctive fabric and are likely to play a key role in governing their frictional behavior.Earthquakes pose a major hazard to populated regions in much of the United States and globally. Both the overall mechanical strength and the nature of slip (whether it occurs via creep or by episodic failure in earthquakes) on major tectonic faults depend, to a large extent, on the physical properties of rock and gouge within these fault zones. Many major fault zones at plate tectonic boundaries appear to slip under anomalously low stresses, implying that they are mechanically weak. Low angle normal faults are one class of faults that exhibit this apparent mechanical weakness, and which are common throughout the southwestern United States. The potential for earthquakes on these faults is also a subject of significant debate, owing mainly to overall low slip rates and potentially long recurrence times that make hazard difficult to assess. This study will investigate the factors that control the strength and slip behavior of low angle normal faults through field mapping, sampling, and detailed laboratory study of fault material. The project will provide new insight into the mechanics of these structures, and will shed light on the mechanical behavior and stability of mechanically weak faults in general.