On the excitation, detection and damping of core modes

Abstract

We continue work on the possibility that internal oscillations in the Earth's core might be detectable by a gravity meter at the surface. As a possible excitation mechanism we use earthquakes and for the damping we consider the contribution of anelasticity in both solid and liquid parts of the Earth. The calculation of the eigenfunctions is restricted in this paper to a non-rotating Earth model. We first review the spectrum of modes for degrees l up to 20 using a stably stratified fluid core based on the Preliminary Reference Earth Model (PREM) and compute the earthquake excitation and Q values using standard seismic theory. We then show that the effects of boundary rigidity and both Boussinesq and subseismic approximations in the core have little effect on the eigenfrequencies of the core modes. The rigid boundary core solutions are then extended into both the mantle and inner core by making use of a generalised internal load Love number approach. This method yields good approximations to the elastic eigenfunctions throughout the Earth, the agreement with full theory improving as the degree l increases. Thus we have confidence that this method can be extended successfully to models with rotation.