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Keywords:

  • background free oscillations;
  • love waves;
  • microseisms

[1] Earth's background free oscillations known as Earth's hum have been interpreted as the Earth response to vertical pressure loads due to atmospheric and/or oceanic disturbances. Such excitation mechanisms, however, can hardly excite Love waves. Here we show clear evidence of background Love waves from 0.01 to 0.1 Hz, based on the array analysis of tiltmeters in the Japanese islands. The observed kinetic energy of Love waves is as large as that of Rayleigh waves through the whole period of analysis. The predominant incident azimuths are common to the Love and Rayleigh waves, the strongest in directions along ocean-continent borders, next from deep ocean floors and the weakest from continents. These observations indicate that background Love and Rayleigh waves are largely generated by the same mechanisms other than vertical pressure loading. We suggest that the most likely excitation source is shear traction acting on a sea-bottom horizon due to linear topographic coupling of infragravity waves.