Deep Earth seismic structure and earthquake source processes from long period waveform modelling
We model long-period seismic waveforms to investigate both the deep Earth velocity structure as well as earthquake source parameters. We utilize a normal mode-based perturbation approach to model and invert a global dataset of 3 component long-period seismic waveforms. The approach, which has been used for modelling isotropic velocity structure, is extended for modelling radial anisotropy, which describes an anisotropic medium with a vertical axis of symmetry. A model for shear velocity anisotropy near the core-mantle boundary is developed, and the stability and significance of the fit to the data is analyzed. The model has important implications for relations between flow and observable seismic anisotropy in this important thermal, chemical, and mechanical boundary layer. This modelling approach is extended to a multiple iteration inversion appropriate for a non-linear problem, and the anisotropic structure of the whole mantle is examined. Tests of the stability of the model and the influence of assumptions made in the modelling process are examined. Relations between mantle flow and seismic anisotropy are examined for a variety of depth ranges. We also perform inversions for earthquake source parameters using the same waveform modelling approach, using the improved velocity model. Small, but systematic, relocations of events are observed, as well as small perturbations to the orientation of the mechanisms, and the updated sources result in significant improvement in fit to the data. We also use a finite-difference approach to model regional, rather than tele-seismic, long-period waveforms to determine the significance of observed volumetric components of earthquakes in a volcanic region of eastern California.