Full-waveform inversion of the African Plate

We present a full-waveform inversion (FWI) of the African plate. Starting from the Collaborative Seismic Earth Model, we invert seismograms that are filtered to 35 s and compute gradients using the adjoint state method. This FWI uses a novel approach that we introduced earlier with the name Evolutionary FWI.

In contrast to conventional waveform inversion, our approach uses dynamically changing mini-batches (subsets of the full dataset) that approximate the gradient of the larger dataset at each iteration. This has three major advantages, (1) We exploit redundancies within the dataset, which results in a reduced computational cost for model updates, (2) The size of the complete dataset does not directly impact the computational cost of an iteration, thereby enabling us to work with larger datasets, and (3) The nature of the algorithm makes it trivial to assimilate new data, as the new data can simply be added to the complete dataset from which the mini-batches are sampled.

The aforementioned advantages enable us to extend the boundaries of what was previously possible for a given computational budget. We perform more than 80 mini-batch iterations and invert waveforms from over 400 unique earthquakes. This has the same cost as 8 iterations with all data. Our latest model clearly images tectonic features such as the Afar triple junction as well as slow zones below areas with dynamic topography, such as the Tibesti and Hoggar mountain ranges.