DEPTH DEPENDENCE OF ANISOTROPY OF EARTHS INNER-CORE

Both body wave (PKP) travel times (Creager, 1992; Song and Helmberger, 1993a; McSweeney and Creager, 1993; Shearer, 1994) and fits to the sp litting of core modes (Tromp, 1993) show general agreement that the to p 300 km of inner core is very anisotropic. The anisotropy displays ax ial symmetry around the Earth's spin axis, with the polar direction 3% faster than the equatorial direction. One key problem now is the dept h dependence of the inner core anisotropy. Here we extend our polar pa th studies to include both long-period and short-period modeling for t he PKP phases at ranges 120 degrees to 173 degrees. Arrivals from the top of the inner core (PKIKP) and reflections from the inner core boun dary (PKiKP) can be observed distinctly in short-period records at ran ges 130 degrees to 140 degrees and as waveform distortions in the long -period records at ranges 130 degrees to 146 degrees. These waveforms provide a new set of data for examining the topmost 150 km of the inne r core, which is not well sampled by the previous body wave travel tim es. Record sections of waves traversing the inner core nearly parallel to the Earth's spin axis (polar paths) from three events, two beneath the South Sandwich Islands and one along the Macquarie Ridge, recorde d at World Wide Standardized Seismograph Network, Canadian Network, an d Long Range Seismic Measurements stations are analyzed. Our results s uggest that the top 150 km of the inner core is only weakly anisotropi c (less than 1%), with strong evidence indicating that the top 60 km i s not anisotropic at all.