Anisotropy in central part of inner core

The anisotropy in the central part of the inner core has been difficult to study because of the paucity of PKP data at antipodal distances, especially for those paths running nearly north-south (polar paths). In this paper, we systematically search through the International Seismological Center (ISC) picks of PKIKP travel times for polar paths at near-antipodal distances reported recently by Su and Dziewonski [1995]. Corresponding seismograms were obtained from the Worldwide Standardized Seismographs Network (WWSSN) film chip library at Lamont-Doherty Earth Observatory. By using seismograms, we can impose quality control on the data, measure differential times (AB-DF in this case) by cross correlation and use long-period waveforms if available. Nonpolar paths at near-antipodal distances are assembled as well, to address biases from lateral variation in the lowermost mantle on AB-DF times. Our results 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 polar paths exhibit fast anomalies of the same magnitude relative to nonpolar paths, while AB times show no obvious difference between polar and nonpolar paths. These observations are consistent with an axisymmetric anisotropy of ∼3% around the spin axis throughout the bulk of the inner core, with the possibility of even stronger anisotropy near Earth's center. This degree of anisotropy is still greater than what the most recent models based on normal modes would indicate.