HR: 08:30h
AN: S31B-03    [PDF]
TI: Entire Array Observation of Near-vertical PKiKP: Observation of Sharp ICB, Sharp Inner Core Reflector/Discontinuity, and no ICS Signal
AU: * Kawakatsu, H
EM: hitosi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Clear observation of near-vertical PKiKP is known for its rareness, preventing detailed seismological investigation of the top part of the inner core. We present an entire array observation of such near-vertical PKiKP from a recently deployed Japanese seismic network (Hi-net). The record section of an intermediate depth earthquake in Mariana (2001/01/07, Mw6.5) shows remarkably clear arrivals of PKiKP in an epicentral distance range of 14-24 degrees. From more than 170 individual picks of PKiKP, we could further perform array analysis of PKiKP related seismic phases with unprecedented high quality. Both PKiKP and PcP show strong similarity in waveform up to a frequency of 2 Hz, which suggests that ICB may be as sharp as CMB and no thicker than 2 km. Slant stacks for a 1-2 Hz frequency band where PKiKP and PcP show the peaked energy, SKiKP can be identified with the appropriate slowness. In this band where the background noise level is 2-3 % of PKiKP, however, no other conspicuous phase is identified except a slight indication of pPKiKP; thus the inner core appears highly transparent in this frequency range. In the lower frequency band of 0.5-1.0 Hz, on the other hand, the slant stack for the PKiKP slowness contains two clear signals (above four standard deviations) at about 75 and 85 seconds after PKiKP. The former phase is likely to be pPKiKP. The later phase, which is about 20 % larger in amplitude, could be identified as a pPKiKP crustal reverberation phase, but the amplitude appears too large; further, no corresponding phase is seen in PcP stack while pPcP is seen at 75 sec after PcP. It is then possible to interpret this phase as a reflection from a slightly dipping discontinuity inside of the inner core at a depth of 470 km below ICB. The reflection coefficient (half of the impedance contrast) should be about $ 3 \pm 1 % $, and the transition thickness is about 5 km. The reflection points are located in the so-called ``eastern hemisphere" of the inner core where a relatively thick ($\sim $400 km) outer isotropic layer have been observed. Geophysical consequence of such sharp ICB and a sharp inner core discontinuity, as well as the apparent lack of ICS (inner core scattering), will be further discussed.
DE: 1015 Composition of the core
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 8115 Core processes (1507)
SC: Seismology [S]
MN: 2003 Fall Meeting