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什么？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影响。在这次地震学实验中，PI将在一条活动正断层上布设非常密集的EarthScope FlexArray台站，以构建追踪整个地壳的接收功能图像。地质填图和活动震源反射地震表明，上地壳存在不同的伸展模式，包括高角度平面断层、斜面断层和低角度滑脱断层。在纯剪切伸展中，断层以次水平滑脱的形式植根于中地壳，标志着从上地壳局部的脆性变形向下地壳普遍的韧性流动的转变，通常认为断裂是向其中弯曲的。将变质的中地壳岩石带到地表的低角度拆离断层的发现，导致了截然不同的简单剪切模式，即下地壳和上地幔沿狭窄的韧性剪切带发生伸展。然而，在表明平面、陡峭断层的大地震中，并没有观察到沿浅倾斜表面滑动的现象。一些大地震似乎破裂到下地壳。这是耐人寻味的，因为断裂进入下地壳可能会留下结构性印记。然而，高分辨率(高频)反射地震勘探提供了断层几何图形的最详细图像，仅穿透到最上面的地壳。这可能是由于随机散射体的能量损失，或者是断层带不够尖锐，无法用高频波成像。该项目的核心是在一个活跃的、大倾角的正断层--内华达州欢乐谷断层--上部署一个由33个三分量地震仪组成的线性阵列，为期一年，该断层于1915年在一次7-7.5级地震中破裂。利用较小的台站间距和来自天然震源的低频能量，PI将能够产生显示整个地壳的断层几何形状的接收函数图像，并解决有关大陆伸展的基本问题：1)脆性上地壳中的断层是平面的还是条状的，2)断层是作为局部剪切带延伸到下地壳还是起源于中地壳滑脱，以及3)地壳规模的断层活动是否偏转莫霍面并影响上地幔？解决大陆正断层的几何问题是一个具有深远科学和社会后果的全球问题。地壳尺度的断层几何学对于建立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风险，尤其与人口稠密的东部和西部盆地和山脉边界(里诺盐湖城)相关，并在全球范围内与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.