Observation of high-frequency PKiKP in Japan: Insight into fine structure of inner core boundary

We observed clear PKiKP phases steeply reflected at the inner core boundary (ICB) in seismograms recorded by the dense short-period seismic network on the Japan Islands, and used 119 high-quality waveforms of the PKiKP phases to study the detailed structure of the ICB. The PKiKP phases are observed in seismograms band-pass filtered at 2–3 Hz from three deep earthquakes at epicentral distances of 7–20° at 105 seismic stations of High-Sensitivity Seismic Network (Hi-net) on the Japan Islands. After phase weighted stacking, the PKiKP waveforms exhibit some weak but clear phases that appear both before and after the PKiKP arrival, which could be explained by a transition zone with a few thin layers existing in the vicinity of the ICB. We used the reflectivity method to simulate the PKiKP waveforms and adopted a forward modeling approach to determine the optimal multi-layer model. Our results show that 8–17 km thick layers with higher density (0.1–1.0%) and higher P-velocity (0.1–1.5%) than the PREM model exist above a sharp ICB, while 10–32 km thick layers with lower density (0.4–2.2%) and lower P-velocity (0.4–2.5%) than PREM exist below the ICB. The big jump of density and velocity at ICB in our layered model (0.35–0.43 g/cm3, 0.33–0.47 km/s) indicates that the ICB itself is still a sharp discontinuity. Our observations of the complex high-frequency PKiKP waveforms suggest that the sharp transition from the outer to inner core may be accompanied by laterally varying layering. The ICB stratification may reflect the gradual solidification process during the growth of the inner core.

►Clear PKiKP phases reflected at the inner core boundary are observed in Japan. ► Pre- and post-PKiKP phases are identified after phase-weighted stacking. ► The PKiKP waveforms are simulated to study the subtle structure of ICB. ► The ICB is not a simple reflector but exhibits a stratification structure.