Cao_et_al_2005 - REPORTS 31 B C Faust et al Geochim...

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25. E. Apra et al. , NWChem, version 4.7 (Pacific Northwest National Laboratory, Richland, WA, 2005). 26. D. R. Hamann, Phys. Rev. B 40 , 2980 (1989). 27. L. Kleinman, D. M. Bylander, Phys. Rev. Lett. 48 , 1425 (1982). 28. J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 , 3865 (1996). 29. Materials and methods are available as supporting material on Science Online. 30. M. C. Cruickshank, L. S. Dent Glasser, S. A. I. Barri, I. J. F. Poplett, J. Chem. Soc. Chem. Commun. 23 , 23 (1986). 31. B. C. Faust et al. , Geochim. Cosmochim. Acta 59 , 2651 (1995). 32. L.-O. O ¨ hman, Geochim. Cosmochim. Acta 61 , 3257 (1997). 33. D. A. Palmer, D. J. Wesolowski, Geochim. Cosmochim. Acta 57 , 2929 (1993). 34. We thank J. Weare for helping to develop the simu- lations. The research was supported by separate grants from the Office of Basic Energy Science of the U.S. De- partment of Energy to W.H.C. and E.B., from the U.S. NSF to B.L.P. and W.H.C., and from the Natural Sciences and Engineering Research Council of Canada to T.W.S. Supporting Online Material www.sciencemag.org/cgi/content/full/1110231/DC1 Materials and Methods Fig. S1 Tables S1 and S2 References 25 January 2005; accepted 18 April 2005 Published online 28 April 2005; 10.1126/science.1110231 Include this information when citing this paper. An Observation of PKJKP: Inferences on Inner Core Shear Properties Aimin Cao, 1 * Barbara Romanowicz, 1 Nozomu Takeuchi 2 The seismic phase PKJKP, which traverses the inner core as a shear wave and would provide direct evidence for its solidity, has been difficult to detect. Using stacked broadband records from the Gra ¨fenberg array in Germany, we documented a high signal-to-noise phase, the arrival time and slowness of which agree with theoretical predictions for PKJKP. The back azimuth of this arrival is also consistent with predictions for PKJKP, as is the comparison with a pseudoliquid inner core model. Envelope modeling of the PKJKP waveform implies a shear velocity gradient with depth in the inner core that is slightly larger than that in the preliminary reference Earth model. Soon after Lehmann ( 1 ) discovered Earth _ s inner core in 1936 through the analysis of travel times of teleseismic body waves, Birch ( 2 ) suggested that the inner core should be solid as a result of freezing of liquid iron. Thirty years later, indirect evidence of the solidity of the inner core was documented by means of seis- mic normal mode eigenfrequency measure- ments ( 3 ). However, the observation of the phase PKJKP, which traverses the inner core as a shear wave (Fig. 1A), has been a con- troversial issue. Julian et al . ( 4 ) and Okal and Cansi ( 5 ) each suggested the detection of PKJKP on the basis of data from short-period seismic arrays at frequencies of È 1.0 Hz and 0.1 to 0.5 Hz, respectively. Deuss et al . ( 6 ) argued that these two claims were misidentifi- cations, and instead proposed an observation of pPKJKP þ SKJKP between 0.01 and 0.1 Hz. PKIKP, which traverses the inner core as a compressional wave (Fig. 1A), is routinely ob- served. It should be observed simultaneously with PKJKP in the epicentral distance range 116 - to 180 - , according to the seismic prelim- inary reference Earth model (PREM) ( 7 ). The relative amplitude of PKJKP varies strongly with frequency (Fig. 1B). Although we cannot
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