Utilizing existing telecommunication cables for Distributed Acoustic Sensing (DAS) experiments has eased the collection of seismological data in previously difficult-to-access areas such as the ocean bottom. To assess the potential of submarine DAS for monitoring seismic activity, we conducted an experiment from mid-October to mid-December 2021 using a 45 km long dark fiber extending from the Greek island of Santorini along the ocean bottom to the neighboring island of Ios. This region is of great geophysical and public interest because of its historical and recent seismic and volcanic activity, especially along the Kolumbo volcanic chain. Besides recording anthropogenic noise and around 1000 seismic events, we observe the primary and secondary microseisms in the submarine section, the latter inducing Scholte waves in a sediment layer where the cable is well-coupled. By using the spectral element wave propagation solver Salvus, we compute synthetic strains for earthquakes with varying degrees of model complexity. Despite including topography, a water layer, and a heterogeneous velocity model, we are unable to reproduce the lack of coherence in our observed earthquake waveforms. Backpropagation simulations for four observed earthquakes indicate that clear convergence of the wavefield, and thus the ability to constrain a source region, is only possible when all model complexities are considered. We conclude that, despite the promising emergence of DAS, monitoring capabilities are limited by often unfavorable cable geometries, cable coupling, and the complexity of the medium. Interrogating multiple cables simultaneously or jointly analyzing DAS and seismometer data could help improve future monitoring experiments.

Geological interpretations, earthquake source inversions and ground motion modelling, among other applications, require models that jointly resolve crustal and mantle structure. With the second generation of the Collaborative Seismic Earth Model (CSEM2), we present a global multi-resolution tomographic Earth model that serves this purpose. The model evolves through successive regional- and global-scale refinements. While the first generation aggregated regional models, with this study, we ensure consistency between all individual submodels, resulting in a model that accurately explains wave propagation across scales. Recent regional tomographic models were incorporated, comprising continental-scale inversions for Asia and Africa, as well as regional inversions for the Western US, Central Andes, Iran, and Southeast Asia. Across all regional refinements, over 793,000 unique source-receiver pairs contributed. Moreover, the long-wavelength Earth model (LOWE) introduces large-scale structures outside of pre-existing local refinements. A global full-waveform inversion over a total of 194 iterations with a minimum period of 50 s on a large data set of 2,423 earthquakes and over 6 million source-receiver pairs ensures that regional updates in the crust and uppermost mantle correctly translate into updates of deeper, global-scale structure. To test the performance of CSEM2, we evaluate waveform fits between observed and synthetic seismograms at 50 s for an independent data set on the global scale, and on the regional scale for lower periods. We show that we can accurately simulate waveforms within and across the regional refinements, maintaining the original resolution of the submodels embedded in the global framework.