Introduction

It is well understood that the background level of seismic noise is affected by fluid covers surrounding the solid Earth, such as the atmosphere and ocean. Microseism due to ocean waves is the most significant seismic noise at periods between 2 to 25 sec. For very long periods ($>$ 1000 s), variations in atmospheric pressure affect more significantly the level of seismic noise with increasing period. Between the two period ranges, from 150 to 500 s, weak but dominant background noise is the continuous free oscillations of the Earth. [e.g. Nawa, et al., 1998; Suda et al., 1998]. The amplitude level of this excitation is very low and it can be only detected by stations with a quiet seismic environment or by stacking of several seismic records. Rhie and Romanowicz [2004] showed that the geographic locations of the sources generating ubiquitously propagating Rayleigh waves are mostly in the oceans and that infragravity waves may play an important role in the transfer of energy from the atmosphere to the solid Earth. The theoretical work explaining the oceanic origin of the continuous free oscillations followed, and demonstrated that infragravity waves have enough energy to generate continuous free oscillations. [Tanimoto, 2005]. Significant wave height measurements at buoys on the Pacific show significant correlations with the variations of seismic amplitude in the microseism band (2-25 s). An interesting observation is that the ocean measurements show a good correlation with the variation in the seismic amplitude in the hum band (150-500 s) as well. This observation may indicate that seismic excitations in both frequency bands, 2-25 s and 150-500 s, have the same source or similar generating mechanism, i.e. ocean waves propagating to the shore. However, long period energy propagates long distance more efficiently and is able to excite observable continuous free oscillations of the Earth.

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