Insights Into the Origin of Earth's hum and Microseisms

We have recently documented that the sources of Earth's background free oscillations in the 2-7 mHz frequency range, are primarily in the oceans. We used array data from two regional networks of VBB seismometers, in California (Berkeley Digital Seismic Network) and in Japan (F-NET). We proposed a mechanism for their generation, involving the coupling of infragravity waves with the ocean floor, where the infragravity waves would be produced through non-linear interactions between storm generated ocean waves. The precise mechanism of this atmosphere-ocean-seafloor coupling is however not yet well understood, as well as their relation to microseisms, another type of seismic noise originating from ocean waves, which dominates seismic records in the period range 0.1-1 Hz. In order to gain further insight into these intriguing phenomena, we have focused on the North Pacific region, where the dominant sources of hum appear to be located in the northern hemisphere winter. We have assembled a dataset of significant wave height recordings at buoys deployed by the National Oceanographic Atmospheric Administration (NOAA) and Japan Meteorological Agency (JMA) for a five day period in 2000, during which two large storms develop and propagate across the Pacific, there are no significant earthquakes, and the background "hum" is maximum. From the temporal correlations between the signature of the storms on the buoy data and the seismic low frequency Rayleigh wave background noise, we infer that the coupling with the seafloor most likely occurs near the coast, around the rim of the north Pacific ocean. We also show that we can track the generated Rayleigh waves as they propagate from West to East across the continental United States. We have also assembled and processed 3 years of microseism data at stations of the BDSN, F-NET arrays, and show a strong correlation of amplitude fluctations in the microseismic band with that in the "hum" band during the northern hemispheric winter, but not during summer months, suggesting that in the winter, the microseisms and hum have a common "regional" or "local" origin, whereas in the summer, the origin of the hum is indeed distant (southern hemisphere) while the microseisms remain local.