Upper mantle structure beneath the contiguous US resolved with array observations of SKS multipathing and slowness vector perturbations

Abstract

Continent-scale observations of seismic phenomena have provided multi-scale constraints of the Earth's interior. Of those analyzed, array-based observations of slowness vector properties (backazimuth and horizontal slowness) and multipathing have yet to be made on a continental scale. Slowness vector measurements give inferences on mantle heterogeneity properties such as velocity perturbation and velocity gradient strength and quantify their effect on the wavefield. Multipathing is a consequence of waves interacting with strong velocity gradients resulting in two arrivals with different slowness vector properties and times. The mantle structure beneath the contiguous Unites States has been thoroughly analyzed by previous seismic studies and is data-rich, making it an excellent testing ground to both analyze mantle structure with our approach and compare with other imaging techniques. We apply an automated array-analysis technique to an SKS data set to create the first continent-scale data set of multipathing and slowness vector measurements. We analyze the divergence of the slowness vector deviation field to highlight seismically slow and fast regions. Our results resolve several slow mantle anomalies beneath Yellowstone, the Appalachian mountains and fast anomalies throughout the mantle. Many of the anomalies cause multipathing in frequency bands 0.15–0.30 and 0.20–0.40 Hz which suggests velocity transitions over at most 500 km exist. Comparing our observations to synthetics created from tomography models, we find model NA13 (Bedle et al., 2021, https://doi.org/10.1029/2021GC009674) fits our data best but differences still remain. We therefore suggest slowness vector measurements should be used as an additional constraint in tomographic inversions and will lead to better resolved models of the mantle.

Key Points

We present the first continent-scale analysis of slowness vector deviations, slowness vector divergence, and multipathing

We resolve seismically slow and fast structures such as slabs and hotspots and find tomography model NA13 fits our data best

Slowness vector measurements are vital to study the Earth's mantle and we argue should be included in tomography inversions

Plain Language Summary

Observations of many phenomena such as reflections and scattering of waves generated by earthquakes have been analyzed on a continental scale. These observations have led to great progress in our understanding of the Earth's structure and dynamics. What has yet to be analyzed on such a large scale are the deflections of seismic waves at the boundaries of structures such as mantle upwelling. Analyzing these deflections can give information on the boundary structure of mantle structures. In this study, we present the first continent-scale data set of measurements quantifying the extent waves have been perturbed from their path between the earthquake and recording station. From this new data set, we infer the extent of the deflection is dependent on the spatial scale we are sensitive to. When analyzing our data set spatially, we find evidence for several potential mantle upwellings and fragmented subducted crust beneath the US. Our observations support previous models of the upper mantle beneath the US but differences remain suggesting observations of direction and velocity could improve the creation of these models. Our observations show mantle structure can be well resolved with measurements of wavefield deflection and that these measurements will be crucial for furthering our understanding of Earth structure and dynamics.