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 investigators. 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 $\sim 1.0$ 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 $Q_\beta$ 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 $116^o$ to $180^o$, 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.

Figure 13.43: Evidence for the observation of PKJKP. (a) Observed vespagram for PKJKP 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. The estimated back-azimuth is $\sim 223^o$ and the expected back-azimuth of PKJKP is $218.^o$. (d) Synthetic vespagram for the pseudo-liquid inner core model.
\epsfig{file=aimin05_1_1.eps, width=8cm}\end{center}\end{figure}

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