SPiRaL: a multiresolution global tomography model of seismic wave speeds and radial anisotropy variations in the crust and mantle

Abstract

Summary SPiRaL is a joint global-scale model of wave speeds (P and S) and anisotropy (vertical transverse isotropy) variations in the crust and mantle. The model is comprised of >2.1 million nodes with five parameters at each node that capture velocity variations for P- and S-waves traveling at arbitrary directions in transversely isotropic media with a vertical symmetry axis (VTI). The crust (including ice, water, sediments, crystalline layers) is directly incorporated into the model. The default node spacing is approximately 2° in the lower mantle and 1° in the crust and upper mantle. The grid is refined with ∼0.25° minimum node spacing in highly sampled regions of the crust and upper mantle throughout North America and Eurasia. The data considered in the construction of SPiRaL includes millions of body wave travel times (crustal, regional, and teleseismic phases with multiples) and surface wave (Rayleigh and Love) dispersion. A multi-resolution inversion approach is employed to capture long-wavelength heterogeneities commonly depicted in global-scale tomography images as well as more localized details that are typically resolved in more focused regional-scale studies. Our previous work has demonstrated that such global-scale models with regional-scale detail can accurately predict both teleseismic and regional body wave travel times, which is necessary for more accurate location of small seismic events that may have limited signal at teleseismic distances. SPiRaL was constructed to predict travel times for event location and long-period waveform dispersion for seismic source inversion applications in regions without sufficiently tuned models. SPiRaL may also serve as a starting model for full-waveform inversion (FWI) with the goal of fitting waves with periods 10–50 seconds over multiple broad regions (thousands of kilometers) and potentially the globe. To gain insight to this possibility, we simulated waveforms using SPiRaL and independent waveform-based models for comparison. The performance of the travel-time-based SPiRaL model is shown to be on par with regional 3-D waveform-based models in three regions (western United States, Middle East, Korean Peninsula) suggesting SPiRaL may serve as a starting model for FWI over broad regions.

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

“…A more quantitative measure of model comparison is based on the waveform misfit metrics considered above. Figure 16 compares the misfit reduction of five models relative to the SPiRaL (Simmons et al., 2021) starting model (similar to Figures 5c and 5d). Here again, positive/negative values indicate that a model fits the waveforms better/worse than SPiRaL.…”

“…(f–j) Cross‐sections of $${\xi }_{\mathrm{S}}$$ along the five latitudes shown in Figure 13a. Note the sharp contrast of the Moho separating crust and mantle which was present in the SPiRaL (Simmons et al., 2021) starting model.…”

“…Figure 6 demonstrates how waveforms and windows for one particular path are impacted by progressive model updates. Here, we illustrate the improved waveform fits with plots of the observed and simulated waveforms for the SPiRaL (Simmons et al., 2021) starting model and WUS256. It is impossible to visualize all (>94,000) waveforms used to create WUS256.…”

“…We used the SPiRaL model (Simmons et al., 2021) as the starting model. SPiRaL is a global model representing radially anisotropic wavespeeds from the crust to the core.…”

“…Figure16. Event-averaged time-frequency phase (TF, upper) and normalized amplitude differences (NL2, lower) misfit reductions for (a) inversion and (b) validation data sets and five models (colors) relative to the SPiRaL model(Simmons et al, 2021). Misfits are sorted by the TF misfit for WUS256 and plotted by event index.…”