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

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 in more than a thousand records 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 in the vicinity of the inner-core boundary. We refer to this scattered energy the "M" phase.

To assess our interpretation, we model 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. We show that a ~300km thick layer with a larger gradient than in AK135 over a ~50km lower velocity layer above the ICB provides a good fit to the differential travel times. We also consider the possibility that lateral variations in the thickness or velocity of this layer and ICB topography could produce a M phase.