Global Scale Observation of Scattered Energy from the Core: Seismic Constraints on the F-Layer

Abstract:

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. Within this collection, 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 use array analysis techniques to enhance the signal of theses scatterers and try and locate them. 

Our results show that the scattered energy originates along the great-circle path in a consistent range of arrival times and narrow range of ray parameter. There are no obvious variations with source or station location, in particular the depth of the source. After exploration of possible location for these scatterers, we show that their origin is most likely at the base of the outer-core, in the F-layer. 

To assess our interpretation, we model synthetic seismograms and test velocity profiles in the F-layer. We suggest that such a layer may be responsible for the scattering in the PKP coda as observed in the data. The detection and observation of this scattered energy enables us to constrain physical properties of the F-layer that play an important role in the investigation of the geodynamo modelisation and core evolution.