Isotropic and Azimuthally Anisotropic Rayleigh Wave Dispersion Across the Juan de Fuca and Gorda Plates and U.S. Cascadia from Earthquake Data and Ambient Noise Two- and Three-Station Interferometry

We use data from the Cascadia Initiative (CI) amphibious array and the USArray Transportable Array to construct and compare Rayleigh wave isotropic and azimuthally anisotropic phase speed maps across the Juan de Fuca and Gorda Plates extending onto the continental northwestern U.S. Results from both earthquakes (28--80~s) as well as ambient noise two- and three-station interferometry (10--40~s) are produced. Compared with two-station interferometry, three-station direct wave interferometry provides $> 50\%$ improvement in the signal-to-noise ratio (SNR) and the number of dispersion measurements obtained particularly in the noisier oceanic environment. Earthquake and ambient noise results are complementary in bandwidth and azimuthal coverage, and agree within about twice the estimated uncertainties of each method. We, therefore, combine measurements from the different methods to produce composite results that provide an improved data set in accuracy, resolution, and spatial and azimuthal coverage over each individual method. A great variety of both isotropic and azimuthally anisotropic structures are resolved. Across the oceanic plate, fast directions of anisotropy with $180^\circ$ periodicity ($2\psi$) generally align with paleo-spreading directions while $2\psi$ amplitudes mostly increase with lithospheric age, both displaying substantial variations with depth and age. Strong ($> 3\%$) apparent anisotropy with $360^\circ$ periodicity ($1\psi$) is observed at long periods ($> 50$~s) surrounding the Cascade Range, probably caused by backscattering from heterogeneous isotropic structures.