Uppermost Inner Core Heterogeneity from Differential Travel Times of PKIKP vs. PKP-Bdiff and PKP-Cdiff Phases

摘要

We evaluate the average P-wave velocity perturbations of the Earth's uppermost inner core (IC) by analyzing two pairs of core phases: PKIKP-PKP-Bdiff (456 observations in the distance range of 138.5A degrees-142A degrees) and PKIKP-PKP-Cdiff (1215 observations at 155A degrees-162A degrees). The former is most sensitive to inner core velocities in the depth range of 350-590 km and the latter from 0 to 100 km below the inner core boundary. The differential travel-time residuals exhibit the roughly "east-west" hemispheric pattern especially beneath the Pacific Ocean. Here, the separation between the two quasi-hemispheres is near the 150A degrees W meridian (from PKP-Bdiff observations), but possibly rotated toward a NW-SE orientation, from PKP-Cdiff. Alternately, a velocity heterogeneity could be present here, in agreement with the previously reported PKIKP vs. PKPBC observations. Assuming the conventional definition for the IC quasi-hemispheres (i.e., the QEH between 0A degrees and 180A degrees E, and the QWH between 0A degrees and 180A degrees W), the quasi-Eastern hemisphere exhibits perturbations of (+1.17 +/- 0.04) (from PKIKP-PKP-Bdiff) and (+0.19 +/- 0.01) (from PKIKP-PKP-Cdiff). The corresponding perturbations in the quasi-Western hemisphere are of (-0.06 +/- 0.06) and (-0.19 +/- 0.02) , respectively. An anomalous IC zone beneath the Indian Ocean is not conspicuous in our PKIKP-PKP-Cdiff observations. Beneath the North Atlantic, the separation between the IC quasi-hemispheres appears to be located between 60A degrees W and 30A degrees W from the PKP-Bdiff observations, but it remains unresolved from the PKP-Cdiff data, due to scarce observations here.