Global scale observation of scattered energy near the ICB: seismic constraints on the base of the outer-core

This talk will present recent and innovative observations of scattering that originates at the base of the outer-core. We have collected a global dataset of several thousands of high quality records of PKPbc, PKPbc-diff and PKPdf phase arrivals in the distance range 149-178 degrees. In almost 70% of this dataset, we have identified an energy packet that arrives 5-20 seconds after the PKPbc (or PKPbc-diff) and represents a coda that is not predicted by 1D reference seismic models. The origin of this scattered energy is unclear and may provide valuable information about structure in the core. We refer to this scattered energy the "M" phase. We used array analysis techniques to enhance the signal of theses scatterers and try and locate them. After exploration of possible location for these scatterers, we showed that their origin is most likely in the vicinity of the ICB. To assess our interpretation, we modelled synthetic seismograms and explore models of structures at the base of the outer-core and search for models that predict the relative travel times and amplitudes of the M to PKPdf without destroying the fit to PKPbc. Other possibilities for its origin, like shear in the outer-core or ICB topography will be discussed.