Relationship of D″ structure with the velocity variations near the inner‐core boundary

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Abstract
Variations in regional differential times between PKiKP (i) and PKIKP (I) have been attributed to hemispheric P‐velocity variations of about 1% in the upper 100 km of the inner core (referred to as HIC). The top of the inner core appears relatively fast beneath Asia where D″ is also fast. An alternative interpretation could be the lateral variation in P velocity at the lowermost outer core (HOC) producing the same differential times. To resolve this issue, we introduce the diffracted PKP phase near the B caustic (Bdiff) in the range of 139–145° epicenter distances, and the corresponding differential times between Bdiff and PKiKP and PKIKP as observed on broadband arrays. Due to the long‐wavelength nature of Bdiff, we scaled the S‐wave tomography model with k values (k ≡ dlnVs/dlnVp) to obtain large‐scale P‐wave velocity structure in the lower mantle as proposed by earlier studies. Waveform synthetics of Bdiff constructed with small k's predict complex waveforms not commonly observed, confirming the validity of large scaling factor k. With P‐velocity in lower mantle constrained at large scale, the extra travel‐time constraint imposed by Bdiff helps to resolve the HOC‐HIC issue. Our preliminary results suggest k > 2 for the lowermost mantle and support HIC hypothesis. An important implication is that there appears to be a relationship of D″ velocity structures with the structures near the inner core boundary via core dynamics.