The upper mantle of Europe and Western Asia: Exposing potentials and limits of regional full-waveform inversion
We present EUWA310, a seismic velocity model obtained by full seismic waveform inversion (FWI) model of the upper mantle beneath Europe and Western Asia. Inferred via 310 quasi-Newton updates from more than 260$\,$000 three-component seismograms at 18 s minimum period. It increases the number of iterations and the amount of data by an order of magnitude, compared to previous FWI studies in the region. The data include recordings from two dense regional arrays: IberArray on the Iberian Peninsula and in Northern Africa, and AlpArray in the wider Alpine region. The construction of EUWA310 became computationally tractable through the use of dynamic mini-batch optimization and the adoption of wavefield-adaptive spectral-element meshes from the global to regional scales. We also demonstrate that EUWA310 may be used to reduce uncertainties of moment tensor estimates by inverting waveforms with decreasing minimum period until an optimal minimum period of 22 s, below which no additional improvements is achieved. This limit is connected to the termination of the FWI at 18 s period, which did not result from limited computational resources, but from failure to explain data at even shorter periods. Since other continental-scale arrays -- such as USArray or ChinaArray -- are comparable in density and coverage, this observation suggests that we may have reached the regime where data coverage instead of compute power is the main bottleneck in regional FWI.