SKS splitting beneath the eastern United States from Transportable Array data

Abstract:

Previous studies of SKS splitting beneath the eastern United States have produced evidence for complex and laterally variable anisotropy, including regions that are dominated by null SKS arrivals and those that include multiple layers of anisotropy. One key question is to what extent SKS splitting reflects anisotropy due to present-day flow in the asthenospheric mantle, fossilized anisotropy in the mantle lithosphere due to past deformation processes, or a combination of the two. The broadband station coverage in the eastern US has historically been sparse, but the arrival of the EarthScope USArray Transportable Array (TA) in the eastern US provides an unprecedented opportunity to probe upper mantle anisotropy in detail. Here we present a set of ~3800 SKS splitting measurements at ~380 TA stations east of ~85°W. We identify predominantly null (that is, non-split) SKS arrivals at eastern US stations, with many stations (particularly in the southeastern US) exhibiting clear null arrivals over a wide range of backazimuths. Stations located in the Appalachian Mountains tend to exhibit well-resolved splitting, with typical delay times of ~1 sec and fast directions that are generally parallel to the strike of the mountain range. In the northern part of the study area, we observe more split arrivals, with laterally variable fast directions. Splitting patterns at many individual stations exhibit backazimuthal variations that are consistent with multiple layers of anisotropy, suggesting contributions from both the lithosphere and asthenosphere. Comparisons between fast splitting directions and indicators such as topography, magnetic and gravity anomalies, upper mantle structure derived from tomography, and absolute plate motions reveal that the relative contributions from lithospheric and asthenospheric anisotropy likely vary laterally beneath the eastern US.