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Excitation of microseisms: views from the normal-mode approach

Toshiro Tanimoto

Published September 2013, SCEC Contribution #1821

Non-linear interaction of ocean waves is a widely accepted mechanism of microseism excitation for frequencies approximately between 0.05 and 0.5 Hz.
Longuet-Higgins published the most influential paper on this subject in 1950 and his main contributions can be summarized in two points; the first is on the double-frequency mechanism and the second is on the dependency of excitation on ocean depth. The two results were derived for different media, the first point for an incompressible liquid (ocean) and the second point for a compressible liquid layer over an elastic halfspace. The two features naturally come out in the normal-mode formulas. We note, however, that the use of the Longuet-Higgins formula, that showed efficiency of excitation based on ocean depth, is not suitable for interpreting land-based observations because his formula was for displacement at ocean bottom. Re-evaluation of this dependency for a land-based observation shows similar results for the depth of maximum excitation, but the sharpness of this peak is reduced significantly.
Some recent observational studies tried to identify the source area of excitation by using this ocean-depth dependency, but such a result needs to be revised to a larger source region according to our results. We confirm that the maximum excitation of microseisms exists and occurs when the ocean depth is about 2.7 km. It is thus not surprising to find an efficient pelagic source for microseisms, although existence of the wave-wave interaction in the source region is essential.

Tanimoto, T. (2013). Excitation of microseisms: views from the normal-mode approach. Geophysical Journal International, 194(3), 1755-1759. doi: 10.1093/gji/ggt185.