Initial model construction for intermediate-period full-waveform inversion of the contiguous US and surrounding regions

Tong Zhou , Min Chen , Ziyi Xi, Jiaqi Li 

Contiguous US is one of regions well instrumented with broadband seismic stations due to the deployment of the EarthScope Transportable Array. Previous studies have provided various 3D seismic wave speed models for the crust and upper mantle with improving resolution. However, discrepancies exist between these models due to the differences in both data sets and tomographic methods, which introduce the uncertainties of the imaged lithospheic structure beneath North America. A further model refinement using the best data coverage and advanced tomographic methods such as full-waveform inversion is expected to provide better seismological constraints. Initial models have significant impacts on the convergence of full-waveform inversions. However, how to select an optimal initial model is not well investigated. Here, we present a data-driven initial model selection procedure for the contiguous US and surrounding regions by examining waveform fitting and misfit functions between the observations and synthetics from candidate models. We use a benchmark data set of waveforms from 30 earthquakes recorded by 5,820 stations across the North America. The results suggest that the tested 3D models well capture long-period waveforms while have discrepancies in short-periods especially on tangential components, indicating that the smaller-scale heterogeneities and radial anisotropy in the crust and upper mantle are not well constrained yet. Based on the data fitting, a hybrid initial model combining S40RTS or S362ANI in the mantle and US.2016 for Vsv and CRUST1.0 for Vsh in the crust is optimal for future full-waveform inversions to achieve higher resolutions of the crust and upper mantle structure.