| Summary: | International audience Constraining the topography of the inner core boundary (ICB) is important for studies of core-mantle coupling and the generation of the geodynamo. We present evidence for significant temporal variability in the amplitude of the inner core reflected phase PKiKP for an exceptionally high quality earthquake doublet, which occurred in the South Sandwich Islands within a ten year interval (1993/2003). This doublet was observed post-critically at the short period Yellowknife seismic array (YK). The analysis of PKIKP/PKiKP amplitude ratios indicates that the PKiKP phases for the 1993 event - but not the 2003 event - are significantly defocused by structure near the inner core boundary (ICB). This observation cannot be explained by small differences in the eearthquake source, interference with another local, regional or teleseismic event, or different scattering from local heterogeneities near the stations or the sources. Combined with data from several other doublets, we infer the presence of topography at the inner- core boundary, with a horizontal wavelength of about 10 km. Such topography could be sustained by small scale convection at the top of the inner core, and is compatible with a rate of super-rotation of the inner core of ~0.1-0.15 deg/year. In the absence of inner core rotation, decadal scale temporal changes in the ICB topography would provide an upper bound on the viscosity at the top of the inner core.
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