Seismic anisotropy in the top 400 km of the inner core beneath the "eastern" hemisphere

Depth variation in seismic anisotropy at the top 400 km of the inner core beneath the "eastern" hemisphere is investigated by comparing the differential travel times of PKP phases recorded at distances from 127° to 160°. At distances less than about 150°, there is no noticeable difference in differential travel times for PKP phases sampling both the polar and the equatorial paths. At greater distances, however, PKIKP phases arrive systematically earlier (about 0 ∼0.5 second) in the polar path than in the equatorial path. These observations suggest that, in the "eastern" hemisphere, seismic anisotropy exists only in depths greater than about 200 km below the inner core boundary (ICB) with seismic velocities along the polar paths (an average of 27.6° from the earth's spin axis) being 0.4% faster than those along the equatorial paths.