S‐to‐Rayleigh Wave Scattering From the Continental Margin Observed at USArray

Abstract

We show examples of teleseismic S waves from western Pacific earthquakes converting to surface waves near the western U.S. coastline. Many of these events originate in the Tonga‐Samoa region. We observe these surface wave conversions at USArray stations at relatively long periods (>10 s). The amplitudes vary considerably from station to station and appear highly amplified in the Yellowstone region. Two‐dimensional spectral element simulations successfully generate scattered Rayleigh waves from incident SV waves and models with surface topography at the coastline and crustal thickness variations across the margin, although simple models cannot explain the large Rayleigh wave amplitudes (greater than the direct S wave amplitude) observed in some regions, suggesting that the wave train is amplified by local structure or 3‐D focusing effects.

Imaging Strong Lateral Heterogeneities Across the Contiguous US Using Body‐To‐Surface Wave Scattering

“…In the Pacific Border region, the geographic distribution of scatterers show a nice spatial correlation with large gradient in bathymetry along the continental slope (cf., Figures 10a and 10b). The result is consistent with previous findings in this region (Buehler et al., 2018; Yu, Zhan, et al., 2017). However, there is no detection of strong scatterers along the continental slope of Atlantic plains, where topographic gradients are also significant.…”

“…Figures 10a and 10b). The result is consistent with previous findings in this region (Buehler et al, 2018;Yu et al, 2017a). However, there is no detection of strong scatterers along the continental slope of Atlantic Plains, where topographic gradients are also significant.…”

“…Body-to-surface wave scattering, including both P/SV-to-Rayleigh wave scattering and SH-to-Love wave scattering, has been observed in many locations around the world (e.g. Bannister et al, 1990;Buehler et al, 2018;Furumura et al, 1998;Maeda et al, 2014;Yu et al, 2017a).…”

Imaging Upper‐Mantle Structure Under USArray Using Long‐Period Reflection Seismology

“…These arrivals, when interpreted as topside reflections, will produce an apparent eastward dipping structure. For a surface‐wave velocity of 3 km/s (the velocity observed for the scattered Rayleigh waves in USArray by Buehler et al, , and the approximate Rayleigh‐wave group velocity in the WUS from Moschetti et al, , at 10‐ to 25‐s period), the resulting image artifact is fairly sharp above 400 km but spreads and blurs below. Sharper artifacts at depth can be generated by assuming faster surface‐wave velocities, which in this case serve as a rough proxy for scattered S body waves as 3.9 km is a typical mantle Sn velocity.…”

“…Finally, there is the possibility that this dipping feature is some kind of artifact related to scattering of incoming S waves off the western continental margin, such as the surface-wave scattering observed by Yu et al (2017) and Buehler et al (2018), that is, a feature analogous to a diffraction hyperbola in reflection seismology. To test this idea, we generated a synthetic data set in which we replaced each of our observed seismograms with simple Ricker pulses at offset times from direct S that were derived from the sum of the predicted iasp91 source-to-scatterer time and a scatterer-to-receiver time computed from an assumed surface-wave velocity.…”