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 degrees to 160 degrees. Differential travel times between PKPBC (Cdiff) and PKPDF (T-BC-T-DF) are about 0.5 sec shorter than those computed from PREM at distances from 146 degrees to 152 degrees and increase with distance to nearly the same as those from PREM at 155 degrees. Differential travel rimes between PKPCD and PKPDF (T-CD-T-DF) at distances from 130 degrees to 143 degrees are approximately 0.2 sec shorter than those of PREM. Amplitudes of PKPBC (Cdiff) relative to PKPDF (A(BC)/A(DF)) at distances 150 degrees to 156 degrees 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 (similar to 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.