Cause of continuous oscillations of the Earth

Spheroidal fundamental mode oscillations of the Earth for frequencies betwe en 2 and 7 mHz (millihertz) are observed even on seismically quiet days. Tw o hypotheses of the cause of these oscillations are investigated: the cumul ative effect of small earthquakes and atmospheric pressure variations. The cumulative effect of earthquakes, assuming that earthquakes follow the Gute nberg-Richter law, is shown to be 1-2 orders of magnitude too small. The ob served amplitudes of modes require an equivalent earthquake of magnitude 6. 0 everyday, which cannot be achieved by summing up contributions from small earthquakes. The hypothesis of atmospheric excitation is favored because o f the discovery of seasonal variations in stacked modal amplitudes for sphe roidal modes between S-0(20) and S-0(40) It is also evaluated by comparing observed modal amplitudes with theoretical amplitudes, derived from a stoch astic normal mode theory. The source of excitation is atmospheric pressure variations, which indicate turbulent motion of the atmosphere for the frequ ency range of interest and are estimated by barometer data. The observed mo dal amplitudes can be matched by the stochastic normal. mode theory, indica ting that atmospheric pressure variation is large enough to excite solid Ea rth normal modes up to the observed amplitudes. Therefore two lines of evid ence, detection of seasonal variations and approximate match of overall mod al amplitudes, support the hypothesis that the continuous background oscill ations are excited by atmospheric pressure variations.