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.