Vp/Vs Structure of the Crust and Reflection Fabric of the Upper Mantle beneath the Southern Appalachians using Teleseismic and Global Phases

这项工作的目的是为了更好地了解美国东南部从阿巴拉契亚山脉到海岸的深层地质结构。通过利用遥远地震产生的地震波，研究人员能够对300公里（180英里）深处的地壳和地幔进行回声探测。这种方法部分地改编自石油工业使用的勘探技术。这项工作的结果，当与其他研究人员的平行分析相结合时，将允许构建地下的三维模型。这个模型将帮助我们解开导致阿巴拉契亚山脉及其邻近地形和那里发现的矿产资源形成的地质事件序列。这项研究将为追求地球科学事业的研究生提供财政支持和有价值的技术培训。拨款也将为本科生提供宝贵的研究经验，并将支持我们对4-6年级学生的持续教育/推广计划。截至2017年10月，首席研究员与2150多名中小学生（每次15-20人一组）一起进行了板块构造和地震学的户外实践练习。目前的计划是将这些活动扩展到乔治亚州的其他学区。“美国阵列”穿过美国东部，对大陆岩石圈的精细结构产生了根本性的新见解，并有助于重新激起人们对古老山脉带的兴趣。拟议的工作将使用体波（Ps, SsPmp, ppppmp）的组合。以及由PKIKP/ PKIKP产生的反射，以增强我们对阿巴拉契亚山脉南部部分地区的地壳和上地幔的图像，这些地区位于SESAME阵列站点下方。本研究的目标是解决以下问题：1)在大西洋沿岸平原，在极低速沉积物中的混响往往会淹没来自莫霍盆地的Ps转换，是否可以获得更可靠的平均地壳Vp/Vs估计，以及这些Vp/Vs值如何告诉我们这个高度扩展区域的基底在中生代裂陷期间被基性侵入所改变的程度？2)小波长反射结构是否存在根本差异，可用于区分岩石圈和软流圈？岩石圈中的应变是以分布剪切的形式出现的，还是典型地沿着明确的剪切带定位？4)我们能否区分地幔中不同的流动模式——例如垂直与水平流动，如果可以，我们能否提高SKS/PKS/SKKS到达的垂直分辨率，从而对负责观测剪切波分裂的深度范围施加更强的约束？首先利用SsPmp和ppppmp到达建立地壳厚度和平均地壳纵波速度的局域模型，从而得到Vp/Vs的改进估计。由此产生的地壳厚度和地壳平均纵波速度的局部估计集将用于帮助约束部署在沿海平原的台站对Vp/Vs的网格搜索。对于上地幔，我们将使用全球地震相位PKIKP （PKPdf）作为虚拟源来构建与地壳主动源反射剖面相似的正入射反射剖面。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The purpose of this work is to gain a better understanding of the deep geologic structure of the southeastern U.S., from the Appalachian Mountains to the coast. By using seismic waves generated by distant earthquakes the investigators are able to conduct echo soundings of the Earth's crust and mantle to a depth of 300 km (180 miles). The methodology is adapted in part from exploration techniques used by the petroleum industry. The results from this work, when added to parallel analyses by other researchers, will allow the construction of a three-dimensional model of the subsurface. This model will help us to unravel the sequence of geologic events that led to the formation of the Appalachian mountains and adjacent terrain and the mineral resources found there. The research will provide financial support and valuable technical training for graduate students pursuing careers in the geosciences. The grant will also provide valuable research experience for undergraduate students and will support our ongoing education/outreach program for students in grades 4-6. As of October 2017, the principal investigator had worked with over 2150 elementary and middle-school students (in groups of 15-20 at a time) in outdoor, hands-on exercises in plate tectonics and seismology. The current plan is to expand those activities to additional school districts across the state of Georgia.The passage of USArray across the eastern United States has resulted in fundamentally new insights into the fine-scale structure of the continental lithosphere and has helped to rekindle interest in old mountain belts. The proposed work will use a combination of body waves (Ps, SsPmp, PpPmp. and reflections generated by PKIKP/PKiKP) to enhance our images of the crust and upper mantle across a portion of the southern Appalachians, beneath stations of the SESAME array. The goal of this work is to address the following questions: 1) Can more reliable estimates of average crustal Vp/Vs be obtained across the Atlantic Coastal Plain, where reverberations within very low-velocity sediments tend to overwhelm Ps conversions from the Moho, and what do those values of Vp/Vs tell us about the degree to which the basement in this highly extended region was modified by mafic intrusions during Mesozoic rifting? 2) Are there fundamental differences in small-wavelength reflection fabric that might be used to distinguish lithosphere from asthenosphere? 3) Does strain in the lithosphere occur as distributed shear, or is it typically localized along well-defined shear zones? 4) Can we distinguish between different flow patterns in the mantle - e.g., vertical versus horizontal flow, and if so, can we improve on the vertical resolving power of SKS/PKS/SKKS arrivals to place stronger constraints on depth ranges responsible for observations of shear-wave splitting? Improved estimates of Vp/Vs will be obtained by first constructing localized models of crustal thickness and average crustal P-wave velocity using SsPmp and PpPmp arrivals. The resulting set of localized estimates of crustal thickness and average crustal P-wave velocity will be used to help constrain the grid search for Vp/Vs for stations deployed in the Coastal Plain. For the upper mantle, we will use the global seismic phase PKIKP (PKPdf) as a virtual source to construct normal-incidence reflection sections that are analogous to those produced by active-source reflection profiling of the crust.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.