Imaging the Earth's small-scale structure using full-waveform inversion: Theory and application to the imaging of mantle plumes

Abstract

Within the last decade, increasing computational power and theoretical advances have initiated the transition from approximation-based tomography to iterative full-waveform tomography in regional and global seismology. Such full-waveform methods allow for the exploitation of complete seismograms to constrain the 3-D structure of the Earth, which potentially leads to much higher resolution compared to approximation-based methods. The method used in this thesis relies on numerical wavefield simulations with the spectral-element method, which honours the full complexity of seismic wave propagation in realistic 3-D media. Gradients are constructed with the adjoint method. Full-waveform models are affected by a number of subjective inversion choices, such as the type of misfit and the regularisation of the gradient. The complexity of seismic wave propagation together with the non-linearity of the problem make it a challenging task to determine optimum inversion strategies. For this thesis, we first performed synthetic experiments to investigate and optimise the capability of full-waveform tomography to constrain small-scale structures in the Earth's mantle. These experiments were focused on the tomographic detection of possible mantle plumes, which are a challenging target for seismic tomography due to their small lateral extent and their extension deep into the lower mantle. Using a misfit based on the time-continuous and amplitude-independent instantaneous phase difference, we succeeded to fully recover an idealised narrow mantle plume. The accurate measurement and inclusion of diffracted waves arriving in the body-wave coda was identified as essential for the recovery in the lower mantle. We further confirmed that simple cross-correlation traveltime misfits do not permit the recovery of plumes in the lower mantle, wavefront healing largely conceals the acquired time delays. We applied these results to the construction of a high-resolution 3-D model of the S-wave speed beneath the North Atlantic region, extending to a depth of 1200 km. Unprecedented details are revealed, giving new insight into the complex tectonics and dynamics of the North Atlantic region. The resolution of particularly interesting features was validated with resolution tests based on Hessian kernels. Striking features of the new model include individual low-velocity conduits beneath the Iceland and Jan Mayen hotspots, extending into the lower mantle. This observation strongly supports the existence of mantle plumes as a feature of mantle convection, which has been debated passionately over the last decades due to a lack of conclusive seismological evidence. Our model is furthermore the first to clearly resolve two separated hotspots associated with Iceland and Jan Mayen. A further highlight is the observation of a wide-spread layer of low-velocity plume material beneath the lithosphere of much of the North Atlantic ocean. Two fingers of this layer extend beneath the continental lithosphere of parts of the British Isles and of southern Norway, providing an explanation for the considerable post-rift uplift that these regions experienced in Neogene times. Gravity-based estimates of present-day dynamic support agree very well with the location of the low-velocity layer.