ANISOTROPY IN CENTRAL PART OF INNER-CORE

The anisotropy in the central part of the inner core has been difficul t to study because of the paucity of PKP data at antipodal distances, especially for those paths running nearly north-south (polar paths). I n this paper, we systematically search through the International Seism ological Center (ISC) picks of PKIKP travel times for polar paths at n ear-antipodal distances reported recently by Su and Dziewonski [1995]. Corresponding seismograms were obtained from the Worldwide Standardiz ed Seismographs Network (WWSSN) film chip library at Lamont-Doherty Ea rth Observatory. By using seismograms, we can impose quality control o n the data, measure differential times (AB-DF in this case) by cross c orrelation and use long-period waveforms if available. Nonpolar paths at near-antipodal distances are assembled as well, to address biases f rom lateral variation in the lowermost mantle on AB-DF times. Our resu lts show large AB-DF residuals of 2 to 8 s for the polar paths at near -antipodal distances. Moreover, the absolute DF travel times for the p olar paths exhibit fast anomalies of the same magnitude relative to no npolar paths, while AB times show no obvious difference between polar and nonpolar paths. These observations are consistent with an axisymme tric anisotropy of similar to 3% around the spin axis throughout the b ulk of the inner core, with the possibility of even stronger anisotrop y near Earth's center. This degree of anisotropy is still greater than what the most recent models based on normal modes would indicate.