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, Hinet. 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%.

Deep Earth Structure: The Earth’s Cores

“…Seismic waveforms indicate that the ICB can generally be considered as a first-order discontinuity, < 2 km thick (Cummins and Johnson, 1988a;Kawakatsu, 2006). This is consistent with thermodynamic considerations: although the uppermost inner core may incorporate fluid inclusions when it forms, it is likely to collapse under its own weight, the unconsolidated material having a thickness probably smaller than 1 km (Deguen et al, 2007).…”

Irregular topography at the Earth’s inner core boundary

“…PKiKP phase has been enigmatic in its observability in the precritical distances (16)(17)(18)(19)(20)(21)(22)(26)(27)(28)(29). Although its observability was predicted based on the elastic parameters of Earth's models, it was only occasionally observed when the seismic station coverage was sparse, which can be explained by the characteristics of the ICB topography inferred here.…”

Complex inner core boundary from frequency characteristics of the reflection coefficients of PKiKP waves observed by Hi-net

“…3a, b). This was done for all records except the Mariana event (event 1), which was examined by Kawakatsu (2006) and observed to have large PKiKP signals around 1 Hz. Note that we use band-pass filtering only for initial identification of PKiKP.…”

“…Their analysis did not reveal any differences between the eastern and western hemispheres of the inner core. Using phases from a Mariana event observed in Japan, whose raypaths sampled the ICB beneath the western Pacific, Kawakatsu (2006) found little scattering energy in the PKiKP coda. However, Leyton and Koper (2007b) analyzed the coda of PKiKP and suggested the existence of small-scale heterogeneities in the uppermost inner core.…”

“…For example, Koper et al (2003) analyzed a significant number of PKiKP phases at shorter distances recorded by small aperture arrays of the International Monitoring System. This study was followed by a large number of new studies (Cao and Romanowicz 2004;Koper and Pyle 2004;Poupinet and Kennett 2004;Koper and Dombrovskaya 2005;Krasnoshchekov et al 2005;Leyton et al 2005;Kawakatsu 2006;Leyton and Koper 2007a, b;Peng et al 2008). …”