The horizontal hum of the Earth: A global background of spheroidal and toroidal modes

Abstract

It is well known that the fundamental spheroidal modes of the Earth are continuously excited. The vertical motions of this 'hum of the Earth' can be observed at many stations all over the world. However, there exists no hum observation based on horizontal seismic data yet. In this article, we present observations of continuous horizontal background oscillations of the Earth at four exceptionally quiet seismic stations and show the existence of a so far unknown global background of long-period Love waves and, equivalently, fundamental toroidal modes. Spheroidal and toroidal modes exhibit similar horizontal amplitudes which, for the spheroidal modes, agree with those expected from the vertical component data. Neither earthquakes nor spheroidal-toroidal mode coupling can explain a permanent excitation of toroidal modes at this level. Copyright 2008 by the American Geophysical Union.