Localized imaging of the uppermost mantle with USArray Pn data

Abstract

USArray has now provided several years of high-quality seismic data and improved ray coverage for much of the western United States. This allows increased resolution for regional studies of the lithosphere and deeper structure of the North American continent. In this study, we use Pn phases in the USArray data set to solve for velocity structure in the uppermost mantle in the western United States. This article focuses on localized imaging techniques that complement traditional Pn tomography analysis. We apply waveform cross-correlation to obtain inter-station travel times between the closely and uniformly spaced USArray stations. This allows us to use traces without phase picks and reduces errors associated with the picking. We obtain differential times that can directly be used to fit locally for slowness and, depending on the approach, for the direction and curvature of the incoming wavefront. The various measurements of incoming wavefronts at different sub-arrays provide constraints on azimuthal variations in velocity. The traditional tomography approach and the local fitting method reveal similar large-scale features. No regularization is applied with the local method, and the resulting velocity maps reveal smaller-scale structures than the tomographic images. © 2012. American Geophysical Union. All Rights Reserved.