Full waveform inversion of the African crust and mantle: strategies for accelerating model convergence

Master of Science in Applied Geophysics at Delft University of Technology, ETH Zürich and RWTH Aachen University. - The continent of Africa is one of the most geophysically interesting regions on the planet. More specifically, Africa contains the Afar Depression, which is the only place on Earth where incipient sea-floor spreading is sub-aerially exposed, along with other anomalous features such as the unexplained topography in the south. Despite its geophysical significance, relatively few tomographic images exist of Africa. This stems mainly from the sparse distribution of seismic monitoring stations on the continent, which itself is a result of the political instability and general geographical remoteness. As a result, the debate on the geophysical origins of Africa's anomalies is rich and ongoing. In this project a tomographic image is produced using the technique of elastic Full Waveform Inversion (FWI). To our knowledge this is the first attempt at performing a continental-scale FWI of the region. Data recorded from 100 earthquakes has been used as input for the inversion. The adjoint method was used to iteratively update the initial model, which was extracted from the Collaborative Seismic Earth Model. Forward and adjoint modelling were performed in the regional version of SPECFEM3D GLOBE, a global wave propagation solver based on the spectral element method. Over the course of ten iterations, the time-frequency phase misfit decreased satisfactorily, and additional details were added to the starting model. The final model was validated by evaluating the change in misfit for ten earthquakes not previously used in the inversion. All of these events showed a decrease in misfit.