Sharp and seismically transparent inner core boundary region revealed by an entire network observation of near-vertical PKiKP
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. 2006
Received: 12 August 2005
Accepted: 15 February 2006
Published: 26 July 2006
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%.