Seismic structure near the inner core-outer core boundary

A model of P-wave velocity of the Earth's core 500 km above and below the inner core boundary (ICB) beneath North America is constructed from travel time analysis and broad-band waveform modeling of core phases at distances from 130° to 160°. Differential travel times between PKP_BC (Cdiff) and PKP_DF (T_BC-T_DF) are about 0.5 sec shorter than those computed from PREM at distances from 146° to 152° and increase with distance to nearly the same as those from PREM at 155°. Differential travel times between PKP_CD and PKP_DF (T_CD-T_DF) at distances from 130° to 143° are approximately 0.2 sec shorter than those of PREM. Amplitudes of PKP_BC (Cdiff) relative to PKP_DF (A_BC/A_DF) at distances 150° to 156° are about 40% larger than those from PREM. These observations require a smaller P-wave velocity gradient (0.0005 sec^-1) in the lowermost 300 km of the outer core than that from PREM (∼0.00065 sec^-1), a slower P-wave velocity (10.96 km/sec) at the top of the inner core, and a larger velocity gradient (0.0005 sec^-1) in the uppermost 300 km of the inner core.