Crustal-Scale Geometry of Active Continental Normal Faults

适应大陆伸展的断层和剪切带的几何形状是怎样的？这是一个简单但尚未解决的问题，对伸展过程的性质具有重要的地球动力学意义。在这个地震学实验中，pi将在一个活跃的正断层上部署非常密集的EarthScope FlexArray站，以构建一个通过整个地壳追踪断层的接收函数图像。地质填图和主动源反射地震表明，该区存在不同的上地壳伸展模式，包括高角度平面断裂、表状断裂和低角度滑脱断裂。在纯剪切伸展中，断层根植于地壳中部的亚水平凹陷处，标志着上地壳局部脆性变形向下地壳普遍韧性流动的转变，断层通常被认为是弯曲进入其中的。低角度拆离断层的发现将变质的中地壳岩石带到了地表，形成了截然不同的简单剪切模式，下地壳和上地幔的伸展沿着狭窄的韧性剪切带进行。然而，在大地震中没有观察到沿浅倾斜表面的滑动，这表明有平面的陡倾斜断层。一些大地震似乎会破裂进入地壳下部。这很有趣，因为断裂进入下地壳可能会留下结构印记。然而，高分辨率（高频）反射地震调查，提供了断层几何形状的最详细图像，只渗透到最上层的地壳。这可能是由于随机散射体的能量损失，或者断层带不够锐利，无法用高频波成像。该项目的核心是在一个活跃的大断层上部署33个三分量地震仪的线性阵列，该断层于1915年在7-7.5级地震中破裂。利用小的测量站间隔和来自自然资源的低频能量，pi可以产生一个接收函数图像，显示整个地壳的断层几何形状，并解决有关大陆延伸的基本问题。1)脆性上地壳的断裂是平面的还是扁平的；2)断裂是作为局部剪切带延伸到下地壳，还是扎根于地壳中滑脱；3)地壳尺度的断裂是否使莫霍断裂带偏转并影响上地幔？解决大陆正断层的几何形状是一个具有深远科学和社会影响的全球相关问题。地壳尺度的断层几何对于构造真实的大陆伸展地球动力学模型具有重要意义。断层几何形状强烈影响地震危险，尤其与人口稠密的东部和西部盆地和山脉边界（盐湖城，里诺）以及全球范围内的意大利和整个爱琴海地区有关。

What is the geometry of faults and shear zones that accommodate continental extension? This is a simple, yet unresolved question with important geodynamic implications about the nature of extensional processes. In this seismological experiment, the PIs will deploy very densely spaced EarthScope FlexArray stations across an active normal fault to construct a receiver function image that traces the fault through the entire crust.Geologic mapping and active-source reflection seismics indicate the presence of different modes of upper crustal extension that include high-angle planar faults, listric faults and low-angle detachment faults. In pure shear extension, faults are rooted in the mid-crust at subhorizontal decollements that mark the transition from localized brittle deformation in the upper crust to pervasive ductile flow in the lower crust and often faults are assumed to curve into them. The discovery of low-angle detachment faults that brought metamorphic mid-crustal rocks to the surface led to the very different simple shear model where extension in the lower crust and upper mantle occurs along narrow ductile shear zones. Slip along shallow dipping surfaces, however, has not been observed for large earthquakes, which indicate planar, steeply dipping faults. Some large earthquakes seem to rupture into the lower crust. This is intriguing because rupture into the lower crust likely leaves a structural imprint. However, high-resolution (high-frequency) reflection seismic surveys, which provide the most detailed images of fault geometry, have only penetrated into the uppermost crust. This may be due to loss of energy from random scatterers or that the fault zone is not sharp enough to be imaged by high-frequency waves. The centerpiece of this project is a one-year deployment of a linear array of 33 three-component seismometers across an active, large-throw normal fault -- the Pleasant Valley, NV, fault that ruptured in 1915 in a magnitude 7-7.5 earthquake. The use of small station spacing and lower frequency energy from natural sources will allow the PIs to produce a receiver function image showing the fault geometry across the entire crust and address basic questions about continental extension: 1) are faults in the brittle upper crust planar or listric, 2) do faults extend into the lower crust as localized shear zones or do they root in a mid-crustal decollement, and 3) is crustal-scale faulting deflecting the Moho and affecting the upper mantle?Resolving the geometry of continental normal faults is a globally relevant question with far reaching scientific and societal consequences. Crustal-scale fault geometry is important to construct realistic geodynamic models for continental extension. Fault geometry strongly affects earthquake hazard and is particularly relevant for the densely populated eastern and western Basin and Range boundaries (Salt Lake City, Reno) and globally for places like Italy and the entire Aegean region.