The structure of the base of the outer core inferred from seismic waves diffracted around the inner core

[1] We systematically searched for seismograms of waves diffracted around the inner core (PKPCdiff) from all the temporary seismic arrays with data currently available at the IRIS DMC, as well as some permanent regional seismic arrays including F-NET in Japan and GRF in Germany, to assemble the largest high-quality PKPCdiff database ever created. PKPCdiff waves preferentially sample the base of the outer core and so contain important clues about Earth structure in this region. We measured PKPDF-PKPCdiff differential traveltimes and PKPCdiff/PKPDF amplitude ratios in the distance range of 154 degrees-160 degrees and modeled the observations using grid searches and full wave theory synthetic seismograms. The optimum model found by fitting the differential traveltimes has relatively low velocity at the base of the outer core as in AK135, which is consistent with many previous traveltime studies. However, the optimum model found by fitting the amplitude ratios (PKPCdiff/PKPDF) does not exhibit this feature, and instead is closer to PREM. The discrepancy may be explained by two likely causes. One is that small-scale topography or roughness on the ICB tends to scatter energy away from PKPCdiff waves by generating trailing coda waves. The other is that there exists a thin layer with relatively low Q at the base of the outer core. This might be expected if there are suspended solid particles at the base of the outer core, as proposed decades ago. Both mechanisms could generate smaller PKPCdiff amplitudes without significantly affecting PKPCdiff traveltimes.