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

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。了解主要构造断层上控制应力状态和滑动性质的因素是地震物理学和断层力学中的一个基本问题。特别是，许多主要的断层带，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，以及低角度的正断层似乎在非常低的剪切应力下滑动（即，机械性能差）。现代低角度正断层的地震滑动潜力对地震危险性评估具有重要意义，但近年来的研究对这一问题的认识存在分歧。许多最近和正在进行的工作都集中在通过钻探活动断层带的采样和仪器来确定导致断层薄弱的机制。另一种方法是研究在地球历史早期形成的暴露良好的折返断层，它们可以作为活动断层的类似物。该项目的重点是在中新世时期（2400万至600万年前），在现在的加州和亚利桑那州的莫哈韦沙漠地区，由于区域地壳伸展而形成的低角度正断层。这些低角度的正断层倾向浅，已经容纳了几十公里的滑动，似乎已经滑动，同时严重错误的方向，与（垂直）最大主应力几乎垂直于断层面。随后的侵蚀使断层带从2-10公里深处露出，并提供了良好的暴露。本研究项目将使用宾夕法尼亚州立大学岩石力学实验室的压力容器，描述这些出土断层的断层泥和断层岩的摩擦特性和稳定性，以解决与一般断层软弱的根本原因有关的低角度正断层的两个突出问题：（1）来自低角度正断层的天然断层泥的绝对强度是多少，弱粘土矿物的存在是否足以解释其明显的机械弱点？断层岩的摩擦性质是否与地震在这些构造上成核的可能性相一致？这项工作的一个特点是能够测试完整的断层泥样品，这些断层泥保留了其独特的结构，并可能在控制其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重大危险。主要构造断层的整体机械强度和滑动的性质（无论是通过蠕变发生还是通过地震中的偶发性破坏发生）在很大程度上取决于这些断层带内岩石和断层泥的物理性质。许多位于板块构造边界的主要断裂带似乎在异常低的应力下发生滑动，这意味着它们的力学性质很弱。低角度正断层是表现出这种明显的机械弱点的一类断层，在美国西南部很常见。在这些断层上发生地震的可能性也是一个重大辩论的主题，主要是由于整体滑动率低，可能复发时间长，使危险难以评估。本研究将通过野外测绘、取样和对断层材料的详细实验室研究，调查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，并将揭示一般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.