Sharp and seismically transparent inner core boundary region
revealed by an entire network observation of near-vertical PKiKP
Kawakatsu, H.
Earth Planets Space, , submitted, 2005.
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 a epicentral distance range of 14-24 degrees.
From more than 170 individual picks of PKiKP, we perform
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 show remarkable 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 $\sim$3\% of PKiKP
indicates no conspicuous phase other than SKiKP.
We also note that no gradual energy build up
of the inner core scattering signal is observed.
Thus this part of
the inner core appears highly transparent seismically in this frequency range.
In the 0.5-1.0 Hz frequency band (10-15 \% noise level),
we observe one possible reflection phase from a slightly dipping ($\sim 5^o$)
reflector/discontinuity inside of the
inner core around a depth of 470 km below the ICB (3 \% reflection).
However, this phase may be due to pPKiKP water/crustal layer reverberations.
Other than this,
there appears no sharp ($\sim$5km thick) horizontal discontinuity
in the top 400km part of the inner core whose reflection
amplitude is larger than $\sim$2\%.