Topography of Earth's inner core boundary from high-quality waveform doublets

Precise determination of the topography of a major internal boundary of the Earth is difficult because of the trade-off with the unknown velocity structure above it. However, the discoveries of the inner core (IC) rotation and high-quality teleseismic waveform doublets make the precise mapping of the topography of the inner core boundary (ICB) possible. Here we examine IC refracted (PKP-DF) and reflected (PKP-CD) waves recorded at the Yellowknife Array and other global stations from 14 high-quality doublets, among a large collection of doublets in S. Sandwich Islands that are recently discovered. Our results show clear evidence for spatial and temporal variations of IC reflections in traveltimes and in waveforms. If the time separation (dT) between the two members of the doublet is less than 3 yr, the IC arrivals show little temporal change in traveltimes or waveforms. If dT is greater than about 6 yr, some doublets show large variations but some others do not. The ICB regions beneath Atlantic and Indian Oceans show little temporal change. The regions that show large variations are beneath Africa and the Central America, which coincide with large seismic anomalies at the core-mantle boundary (CMB). Inside these two ICB regions, there are fine-scale (km scale) variations. The largest temporal changes of IC reflections are about 0.10 to 0.14 s, corresponding to a topographic variation of up to about 3.7 to 5.2 km. The results suggest ICB topography of a few kilometres on fine to regional scales. The geographical coincidence of the ICB and CMB anomalies may suggest strong thermal coupling of the mantle and the core. Dynamic models include a bumpy ICB rotating with the IC itself or a transient slurry boundary containing a mixture of molten materials and solidified patches of iron crystals, which is rapidly modified by the turbulence at the base of the convecting outer core. If the former is the main cause of the observed temporal change of the topography, the IC rotation is required to be an oscillation, whose half period has an upper limit of a few hundred years.