Soon after Lehmann discovered the earth's inner core in 1936 through the analysis of travel times of
teleseismic body waves, Birch (1940) suggested that the inner
core should be solid as a result of freezing of liquid iron in the outer
core. Thirty years later, the first indirect evidence of the solidity of the
inner core was documented by means of seismic normal mode eigenfrequency measurements(Dziewonski
and Gilbert, 1971). However, the
observation of the phase PKJKP, which traverses the
inner core as a shear wave, is still a controversial issue, until now attempted by only few
Julian et al. (1972) and Okal and Cansi (1998) each suggested the detection
of PKJKP based on data from short-period seismic arrays in the Hz and
0.1-0.5 Hz frequency ranges, respectively.
Deuss at al. (2000) argued that these two claims were misidentifications,
and instead, proposed an observation of pPKJKP+SKJKP, based on stacking data from the global
broadband network, in the frequency range 0.01-0.1 Hz. On the other hand,
the existing estimates in the inner core are based solely on normal
mode observations (Dziewonski and Anderson, 1981; Widmer et al., 1991).
There has been no attempt at estimating
the shear wave attenuation in the inner core using body wave data.
PKIKP, which traverses the inner core as a compressional wave, is now a
routinely observed phase. It should be observed simultaneously with PKJKP
in the epicentral distance range to , according to the seismic reference model
PREM. The relative amplitude of PKJKP varies strongly with frequency. Although
we cannot rule out the possibility of observing PKJKP in the frequency range 0.1 to 0.5 Hz, it is more
likely to be found
at lower frequencies.
Evidence for the observation of PKJKP. (a) Observed vespagram
in the slowness and travel time domain.
(b) Stacked waveform corresponding to the energy maximum in (a).
(c) Observed vespagram in the back-azimuth and travel time domain.
back-azimuth is and
the expected back-azimuth of PKJKP is .
(d) Synthetic vespagram for the pseudo-liquid inner core model.
Berkeley Seismological Laboratory
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