The anatomy of uppermost mantle shear-wave speed anomalies in the western U.S. from surface-wave amplification

Abstract

We build SWUS-amp, a three-dimensional shear-wave speed model of the uppermost mantle of the western U.S. using Rayleigh wave amplification measurements in the period range of 35-125 s from teleseismic earthquakes. This represents the first-ever attempt to invert for velocity structures using Rayleigh wave amplification data alone. We use over 350,000 Rayleigh wave amplitude measurements, which are inverted using a Monte Carlo technique including uncertainty quantification. Being a local seismic observable, Rayleigh wave amplification is little affected by path-averaged effects and in principle has stronger depth resolution than classical seismic observables, such as surface wave dispersion data. SWUS-amp confirms shallow mantle heterogeneities found in previous models. In the top 100 km of the mantle, we observe low-velocity anomalies associated with Yellowstone and the Basin & Range province, as well as a fast-velocity anomaly underneath the Colorado

Rayleigh Wave Attenuation and Amplification Measured at Ocean‐Bottom Seismometer Arrays Using Helmholtz Tomography

“…One study that we are aware of has inverted Rayleigh wave amplification measurements for shear velocity structure, and this too was carried out in the western U.S. (Schardong et al., 2019). The sensitivities of Rayleigh wave amplification to shear velocity ($${V}_{S}$$), compressional velocity ($${V}_{P}$$), and density ($$\rho $$) are complementary to that of phase velocity and may be used to refine models of 3‐D Earth structure (Bowden et al., 2017; F. C. Lin et al., 2012; Schardong et al., 2019). Amplification displays opposite sensitivities to shear and compressional velocities at shallow depths, implying that $${V}_{P}/{V}_{S}$$ may be especially well resolved by amplification measurements (F. C. Lin et al., 2012).…”

“…The sensitivities of Rayleigh wave amplification to shear velocity ( 𝐴𝐴 𝐴𝐴𝑆𝑆 ), compressional velocity ( 𝐴𝐴 𝐴𝐴𝑃𝑃 ), and density ( 𝐴𝐴 𝐴𝐴 ) are complementary to that of phase velocity and may be used to refine models of 3-D Earth structure (Bowden et al, 2017;F. C. Lin et al, 2012;Schardong et al, 2019). Amplification displays opposite sensitivities to shear and compressional velocities at shallow depths, implying that 𝐴𝐴 𝐴𝐴𝑃𝑃 ∕𝐴𝐴𝑆𝑆 may be especially well resolved by amplification measurements (F. C. Lin et al, 2012).…”

“…C. Lin et al, 2012), and these studies all used data from the USArray. One study that we are aware of has inverted Rayleigh wave amplification measurements for shear velocity structure, and this too was carried out in the western U.S. (Schardong et al, 2019). The sensitivities of Rayleigh wave amplification to shear velocity ( 𝐴𝐴 𝐴𝐴𝑆𝑆 ), compressional velocity ( 𝐴𝐴 𝐴𝐴𝑃𝑃 ), and density ( 𝐴𝐴 𝐴𝐴 ) are complementary to that of phase velocity and may be used to refine models of 3-D Earth structure (Bowden et al, 2017;F.…”

“…While TPW provides a simple approach for measuring 1-D Rayleigh wave attenuation in the presence of weak or moderate multipathing, it may suffer in complex regions such as near the coastlines, where the wavefield may not be well approximated by two interfering plane waves. Furthermore, we are unaware of any previous reports of Rayleigh wave site amplification in the oceans, despite having sensitivity to elastic structure that complements that of phase velocity (F. C. Lin et al, 2012;Schardong et al, 2019). Helmholtz tomography offers a promising approach that can simultaneously constrain attenuation and site amplification while accurately accounting for wavefield focusing/defocusing.…”

Rayleigh Wave Attenuation and Amplification Measured at Ocean-Bottom Seismometer Arrays using Helmholtz Tomography

“…C. Lin et al, 2012), and these studies all used data from the USArray. One study that we are aware of has inverted Rayleigh wave amplification measurements for shear velocity structure, and this too was carried out in the western U.S. (Schardong et al, 2019). The sensitivities of Rayleigh wave amplification to shear velocity (𝑉 𝑆 ), compressional velocity (𝑉 𝑃 ), and density (𝜌) are complementary to that of phase velocity and may be used to refine models of 3-D Earth structure (Bowden et al, 2017;F.…”