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

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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.