Abstract
SUMMARY 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 United States. 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 per cent improvement in the signal-to-noise ratio 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° periodicity (2ψ) generally align with palaeo-spreading directions while 2ψ amplitudes mostly increase with lithospheric age, both displaying substantial variations with depth and age. Strong (>3 per cent) apparent anisotropy with 360° periodicity (1ψ) is observed at long periods (>50 s) surrounding the Cascade Range, probably caused by backscattering from heterogeneous isotropic structures.
Highlights
Large earthquakes (Mw ≥ 8) have recurred in Cascadia with a period of ∼500 years over the last 10,000 years (e.g. Atwater, 1987; Goldfinger et al, 2012), and the most recent one is dated to the 1700s (e.g. Nelson et al, 1995; Satake et al, 1996)
Helmholtz/eikonal tomography yields local estimates of uncertainties, which are useful to guide the comparison of different methods and are crucial for studies based on phase speed maps (e.g. 3-D inversions for both isotropic and anisotropic structures)
Our final product is a set of composite Rayleigh wave isotropic and azimuthally anisotropic phase speed maps from 10 s to 80 s period, constructed by combining earthquake (28–80 s) and ambient noise-based (10–40 s) data
Summary
Large earthquakes (Mw ≥ 8) have recurred in Cascadia with a period of ∼500 years over the last 10,000 years (e.g. Atwater, 1987; Goldfinger et al, 2012), and the most recent one is dated to the 1700s (e.g. Nelson et al, 1995; Satake et al, 1996). Our two principal purposes of this study are (1) to investigate the performance of three-station direct-wave interferometry and (2) to produce Rayleigh wave isotropic and azimuthal anisotropy observations from both earthquakes and ambient noise across the. The final product is a set of Rayleigh wave azimuthally anisotropic phase speed maps across the Cascadia combining ambient noise and earthquake observations. We present Rayleigh wave azimuthal anisotropy measurements and tomographic maps from ambient noise two- and three-station interferometry which, to the best of our knowledge, have not been produced offshore. About 44% (157) of land stations are from the USArray Transportable Array (TA), most of which are deployed from 2005 to 2008 and are asynchronous with the CI stations
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