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Investigating Lateral Variations in Frequency-Dependent Rayleigh Wave Phase Velocity in the Contiguous U.S.

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Abstract

Surface waves are a subset of seismic waves that travel along the surface of Earth. These waves can encounter different subsurface structures depending on their azimuth, which can alter their velocity. This directional dependence of velocity on azimuth is called azimuthal anisotropy. We examine this property along with the velocity itself to gather insight into the structure of the lithosphere and the deformation of the mantle. We used Rayleigh waveform data from the USArray Transportable Array (TA), a ~70 km grid spaced seismic network that migrated across the contiguous United States from 2004-2015. Using these stations, we applied a sub-array technique that consists of a target station and its four nearest neighboring stations. We then determined the average frequency-dependent phase velocity within the sub-array for each individual earthquake from a large list of suitable earthquakes to ensure quality of data and gain understanding of the azimuthal dependence. The azimuthally varying data were then fitted with an equation relating phase velocity to its dependence on azimuth to azimuthal anisotropy, namely the strength of anisotropy and the fast direction. This procedure is repeated for all potential sub-arrays to obtain phase velocity measurements covering the contiguous U.S. We generate maps of the azimuthally averaged phase velocity for frequencies between 10-22 mHz for most of the contiguous U.S. that we compare to the statistically averaged phase velocity maps. Our resulting phase velocity maps are compared with previous studies to validate our methods.

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This item is under embargo until July 8, 2025.