WUS256: An Adjoint Waveform Tomography Model of the Crust and Upper Mantle of the Western United States for Improved Waveform Simulations

Abstract

We report a new model (WUS256) of radially anisotropic seismic wavespeeds of the crust and upper mantle of the western United States (WUS) obtained from adjoint waveform tomography for the purpose of improving synthetic waveform fits to observed data. WUS256 is based on inversion of over 94,000 waveforms from 72 earthquakes recorded by nearly 3,400 stations. We started with the SPiRaL global model (Simmons et al., 2021, https://doi.org/10.1093/gji/ggab277) and waveforms in the period band of 50–120 s. We followed a conservative multiscale inversion approach with eight stages and 256 total inversion iterations which enabled monotonic misfit reduction to 20‐s minimum‐period waves. WUS256 relied on time‐frequency (TF) phase misfits and a trust region limited memory Broyden–Fletcher–Goldfarb–Shanno (L‐BFGS) optimization. Hessian‐vector products were used to qualitatively assess model resolution. Results indicate that WUS256 has good coverage of the continental regions to depths of about 150 km and is able to resolve features on lateral scales of about 200 km. We quantify waveform fits by the reduction in TF and normalized amplitude difference misfits between WUS256 and the SPiRaL starting model. WUS256 significantly improves waveform fits with misfit reduction ≥ $\ge $64% for both inversion and validation data sets compared to the SPiRaL starting model and shows even better fits compared to other models. Waveform fits illustrate that WUS256 reproduces body‐waves, fundamental mode surface waves as well as late arriving dispersed and/or scattered short period surface waves. The improvement in waveform fit indicates that WUS256 can be used to reproduce path effects on regional complete waveforms and moment tensor inversions.

SASSIER22: Full‐Waveform Tomography of the Eastern Indonesian Region That Includes Topography, Bathymetry, and the Fluid Ocean

“…This has traditionally limited its usefulness, but ongoing access to more powerful computing resources has enabled its successful application in earthquake seismology across the scales (e.g., Blom et al, 2020;M. Chen et al, 2015;Chow et al, 2022;Fichtner et al, 2009;Gao et al, 2021;Lei et al, 2020;Rodgers et al, 2022;Tape et al, 2010).…”

“…This makes the method much more computationally expensive than ray‐based imaging methods; the computational cost of adjoint waveform tomography is independent from the number of receivers but scales linearly with the number of events used and to the fourth power of the highest frequency in 3‐D (e.g., Tromp, 2020). This has traditionally limited its usefulness, but ongoing access to more powerful computing resources has enabled its successful application in earthquake seismology across the scales (e.g., Blom et al., 2020; M. Chen et al., 2015; Chow et al., 2022; Fichtner et al., 2009; Gao et al., 2021; Lei et al., 2020; Rodgers et al., 2022; Tape et al., 2010).…”

“…Instead, one common approach for dealing with this issue is to approximate the fluid ocean by the weight of its water column (“ocean load,” e.g., Lei et al., 2020; Rodgers et al., 2022) but this is only a valid assumption when the minimum wavelength considered is long compared to the water layer thickness (Komatitsch & Tromp, 2002; Zhou et al., 2016). Several studies suggest that the effects of the fluid ocean layer become noticeable at minimum periods between 10 and 20 s (e.g., An et al., 2017; Fernando et al., 2020; Yue et al., 2017).…”

SASSIER22: Full-waveform tomography of the eastern Indonesian region that includes surface topography and the fluid ocean

“…This has traditionally limited its usefulness, but ongoing access to more powerful computing resources has enabled its successful application in earthquake seismology across the scales (e.g. Fichtner et al, 2009;Blom et al, 2020;Gao et al, 2021;Chow et al, 2022;Rodgers et al, 2022).…”

“…Instead, one common approach for dealing with this issue is to approximate the fluid ocean by the weight of its water column ("ocean load", e.g. Rodgers et al, 2022) but this is only a valid assumption when periods are long compared to the water layer thickness (Komatitsch & Tromp, 2002;Zhou et al, 2016). Several studies suggest that the effects of the fluid ocean layer become noticeable at periods between 10 and 20 s (e.g.…”