USArray observations of quasi‐Love surface wave scattering: Orienting anisotropy in the Cascadia plate boundary

Abstract

Quasi‐Love (QL) surface wave anomalies are observed along multiple great circle paths crossing the Pacific and through the mid‐ocean ridge and subduction zone associated with the Juan de Fuca plate. The long‐period QL waves observed on the USArray component of EarthScope arrive immediately after the fundamental Love wave arrival, suggesting that the QL waves were generated close to USArray and near the plate boundary. The dense USArray network allows the analysis of the azimuthal dependence of the QL surface wave scattering to determine the horizontal anisotropic axes of symmetry in Earth's upper mantle. We focus on the location, 42.5°N and 127.5°W just offshore of the southern Oregon coastline near the Gorda Ridge, and we analyze the QL scattering along 14 different great circle paths crossing through this location. We low‐pass filter the data to suppress overtones and crustal effects. QL amplitude and polarity constrains an anisotropic axis of symmetry that correlates well with the hot spot referenced Juan de Fuca plate motion and regional shear wave splitting. QL observations at multiple periods (50, 75, 100, 150, and 200 s) suggest a maximum Love‐to‐Rayleigh scattering at 100 s period, which is consistent with anisotropic lateral gradients in the asthenosphere beneath the Juan de Fuca and Gorda plates. Our observations do not follow the predictions of slab rollback and suggest the entrainment of asthenosphere with the overriding Juan de Fuca and Gorda plates.