Normal modes of the Earth

Summary

The normal modes, or free oscillations, of the Earth are the standing elastic-gravitational vibrations of the whole planet. After a great earthquake the Earth rings like a bell for days, at a discrete set of frequencies. The modes come in two families, spheroidal and toroidal, labelled by an overtone number and a spherical harmonic degree. Their frequencies constrain the density and elastic structure of the whole Earth, including the inner core.