Robust Normal Mode Constraints on Inner-Core Anisotropy from Model Space Search

Abstract

A technique for searching full model space that was applied to measurements of anomalously split normal modes showed a robust pattern of P -wave and S -wave anisotropy in the inner core. The parameter describing P -wave anisotropy changes sign around a radius of 400 kilometers, whereas S -wave anisotropy is small in the upper two-thirds of the inner core and becomes negative at greater depths. Our results agree with observed travel-time anomalies of rays traveling at epicentral distances varying from 150° to 180°. The models may be explained by progressively tilted hexagonal close-packed iron in the upper half of the inner core and could suggest a different iron phase in the center.