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Sharp and seismically transparent inner core boundary region revealed by an entire network observation of near-vertical PKiKP

Abstract

We present an entire network observation of near-vertical PKiKP from the Japanese seismic network, Hi-net. The record section of an intermediate depth (298 km) earthquake in Mariana (2001/07/03 13:10:43, Mw 6.5) shows remarkably clear arrivals of PKiKP in an epicentral distance range of 14–24 degrees. From more than 170 individual picks of PKiKP, we performed array analyses of PKiKP-related seismic phases to infer the structure near the inner core boundary (ICB) with unprecedented high quality. Both PKiKP and PcP showed strong similarity in waveforms up to a frequency of 2 Hz. Both amplitude and waveform analyses indicate that the ICB may be as sharp as the CMB and no thicker than 2 km. The slant stack for a 1–2 Hz frequency band, where PKiKP and PcP show energy peaks and where the background noise level is 3% of PKiKP, indicated no conspicuous phase other than SKiKP. We also noted that no gradual energy build-up of the inner core scattering signal was observed. Thus, this part of the inner core appears highly transparent seismically in this frequency range. In the 0.5–1.0 Hz frequency band (10–15% noise level), we observed one possible reflection phase from a slightly dipping (5°) reflector/discontinuity inside of the inner core around a depth of 470 km below the ICB (3% reflection). However, this phase may be due to pPKiKP water/crustal layer reverberations. Other than this, there appears to be no sharp (5 km thick) horizontal discontinuity in the top 400 km part of the inner core whose reflection amplitude is larger than 2%.

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Correspondence to Hitoshi Kawakatsu.

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Kawakatsu, H. Sharp and seismically transparent inner core boundary region revealed by an entire network observation of near-vertical PKiKP. Earth Planet Sp 58, 855–863 (2006). https://doi.org/10.1186/BF03351990

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Key words

  • Inner Core
  • PKiKP
  • ICB
  • stacking
  • array analysis