Observably split multiplets—data analysis and interpretation in terms of large‐scale aspherical structure

Abstract

We study aspherical structure of the deep Earth using normal mode splitting. Analysing 10 previously unresolved high‐Q multiplets in the frequency band 5‐10 mHz allows us to double the number of modal constraints on the aspherical structure of the core. We find that all the multiplets are anomalously split and exhibit predominantly parabolic splitting though some of the low‐m singlets are not well‐resolved. The parabolic splitting suggests that coupling to nearby modes is unimportant and that the structure sensed by these multiplets is dominantly axisymmetric and of harmonic degree 2. Four of the newly observed modes have less than 3 per cent of their potential energy in the inner core. Taken together with published observations of anomalously split multiplets, the following statements can be made. Anomalously split modes have typically 45 per cent of their potential energy below the core‐mantle boundary (CMB) whereas modes with little energy in the core (less than 30 per cent) are not anomalously split. Some of the anomalously split modes have less than 3 per cent potential energy below the inner core boundary (ICB). These observations make it extremely difficult to invoke inner core structure of any kind as the source of anomalous splitting and have led to a search for alternative causes. Several studies have shown that the magnetic field is too small to cause anomalous splitting and a steady fluid flow in the outer core would cause splitting that is symmetrical in frequency and can therefore be ruled out. Anomalous structure of harmonic degree s= 2 in the outer core, with the density constrained to be in hydrostatic equilibrium, can explain a large amount of the signal but leads to an extremely unusual Vp/ρ relation. We therefore conclude that the cause of anomalous splitting is currently unknown. Copyright © 1992, Wiley Blackwell. All rights reserved