ANISOTROPY OF THE EARTHS INNER-CORE

The inner core has the intriguing property that seismic waves travelin g parallel to the Earth's spin axis arrive earlier than those travelin g parallel to the equatorial plane. The travel time is some 6 s faster from pole to pole than across the equator. This difference is not the result of different polar and equatorial radii of the inner core, for it would require the inner core to be elongated at the poles by more than one third of its radius. Rather, this 6-s travel time difference is the result of the inner core's being anisotropic; wave speeds diffe r for different directions of wave propagation. This anisotropy, disco vered only in the last decade, is characterized by cylindrical symmetr y about an axis approximately aligned with the Earth's spin axis. The anisotropy is believed to be due to a preferred orientation of anisotr opic iron crystals composing the inner core, but the mechanisms respon sible for creating such a preferred orientation remain uncertain. With the anisotropy now well established as a basic property of the inner core, its quantification and that temporal variations of its orientati on have become a vital tool for probing the structure, composition, an d dynamics of the Earth's deep interior.