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Looking at seismic noise around the Earth there is commonly a peak in the seismic noise around a frequency of 200 mHz. This peak is typically referred to as the microseism (an example is shown below). What is the source of these low frequency oscillations.

From experience I know that in southern Louisiana the motion at this peak is generally stronger throughout the winter than in the summer. I have also been told that it is associated with water waves in the gulf, but it is unclear how those waves get converted to ground motion tens of miles inland.

Seismic noise taken from www.ligo.caltech.edu/~jharms/data/Plots/Histograms/GR_MOX_BHZ_specvar.png

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The microseism is ocean waves that couple to the ground to make Rayleigh waves. The ocean waves are gravity waves which are dispersive (different frequencies travel at different speeds), and 0.2 Hz (not 20 mHz) is the frequency of the waves that travel at the same speed as storm winds to which they couple.

The ground motion doesn't die off, even thousands of kilometers inland, because Rayleigh waves are bound to the surface (so waves from a point source would spread circularly instead of spherically). The waves are produced all along the coast, which negates the circular spreading.

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    $\begingroup$ Oops, I forgot a zero in the frequency. So, 200 mHz waves couple to storm winds because of velocity matching between the winds and the waves? Does that mean that if a Hurricane is sitting off of the coast the microseism frequency should shift up? $\endgroup$ – Chris Mueller Apr 19 '14 at 15:07

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