Inner core anisotropy in three dimensions

Abstract

The purpose of this paper is the investigation of cylindrical anisotropy in the inner core based on the travel time anomalies of the PKIKP phase. We use the arrival times reported in the International Seismological Centre Bulletins for years 1964–1990. We select only earthquakes which have a good azimuthal coverage and a sufficiently large number of reporting stations. The earthquakes are relocated using corrections for lateral heterogeneity computed for our most recent three‐dimensional mantle model. We use a total of 313,422 observations of the DF branch of PKP travel time anomalies within the epicentral distance ranges of 120°–140° and 150°–180° reported by 2335 stations for 26,377 earthquakes. We process the data using an averaging procedure, which we call cylindrical anisotropy stacking, that enhances the effects of anisotropy, but is expected to suppress those due to lateral heterogeneity and random errors. The processed residuals show a remarkably consistent pattern. This confirms the dominance of the cylindrical elastic anisotropy in the inner core with, approximately, a constant axis of symmetry. This axis of symmetry is found to be tilted 10.5°±1° from the Earth's rotation axis in the direction 160°E±5° in the northern hemisphere. In this new coordinate system we determine a four‐layer axisymmetric model of transverse anisotropy with each layer approximately 300 km thick. The model shows that the anisotropy is strongest (>3%) within the innermost part of the core. The travel time anomalies show significant (±1.5 s) longitudinal variations, even when the tilt of the axis of symmetry is considered. There is a substantial increase in the amplitude of the longitudinal variations, which are dominated by the second and fourth harmonics, for rays with bottoming depths exceeding 400 km below the inner core boundary. The measurable tilt of the axis of symmetry and the presence of significant nonaxisymmetric signal may provide important clues with respect to the possible causes of anisotropy. The increase in the anisotropy in the innermost part of the core departs from earlier inferences that anisotropy may be limited to the outermost 200–300 km of the Earth's inner core.