Full-Waveform Tomography of the African Plate Using Dynamic Mini-Batches

We present a seismic model of the Africa Plate, constructed with the technique of full-waveform inversion. The purpose of our model is to serve as a foundation for quantitative geodynamic and geochemical interpretation, earthquake-induced ground motion predictions, and earthquake source inversion. Starting from the first-generation Collaborative Seismic Earth Model, we invert seismograms filtered to a minimum period of 35 s and compute gradients of the misfit function with respect to the model parameters using the adjoint state method. We use dynamically changing mini-batches of the complete data set to compute approximate gradients at each iteration. This approach has three significant advantages: (a) it reduces computational costs for model updates and the inversion, (b) it enables the use of larger datasets without increasing iteration costs, and (c) it makes it trivial to assimilate new data since we can extend the data set without changing the misfit function. We invert data from 397 unique earthquakes and 184,356 unique source-receiver pairs. We clearly image tectonic features such as the Afar triple junction and low-velocity zones below the Hoggar, Air, and Tibesti Mountains, pronounced more than in earlier works. Finally, we introduce a new strategy to assess model uncertainty. We deliberately perturb the final model, perform additional mini-batch iterations, and compare the result with the original final model. This test uses actual seismic data instead of artificially generated synthetic data and requires no assumptions about the linearity of the inverse problem. Plain Language Summary In this paper, we contribute to the knowledge about geological structures under the African continent. For that, we use publicly available seismograms (records of the ground motion at seismic stations) measured in Africa and the regions around it. These we process with an advanced imaging method called full-waveform inversion (FWI). The result is a model of the speed of seismic waves propagating through the crust and upper mantle under Africa. We can use the model to generate maps of Earth's interior temperature anomalies in the given region. We can, for instance, clearly see hot structures beneath mountain ranges and volcanoes. Our FWI implementation benefits from several recent advances, allowing us to include a much higher number of seismograms than before, resulting in models with unprecedented resolution. We update the publicly available Collaborative Seismic Earth Model with those outputs. Finally, we propose a new strategy to assess the resolution and reliability of our resulting models.