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 analysed,
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, 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 that arrive with different slowness
vector properties and times. The mantle structure beneath the contiguous
Unites States has been thoroughly analysed by seismic studies and is
data-rich, making it an excellent testing ground to analyse mantle
structure with our approach and compare with other techniques. We apply
an automated array-analysis technique to an SKS dataset to create the
first continent-scale dataset of multipathing and slowness vector
measurements. We analyse the divergence of the slowness vector deviation
field to highlight seismically slow and fast regions in our data. 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
\citeA{bedle_continental_2021} 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.