Modelling long-term seismic noise in various environments

The strongest seismic noise, called secondary microseisms, is generated by ocean wave interactions and we model this noise using the theory of Longuet-Higgins generalized to random ocean gravity waves. Noise sources are computed with an ocean wave model that takes into account coastal reflections. Variations of the source locations are consistent with seasonal variations of seismic noise spectra. Noise spectra are modelled over many years for stations representative of various environments such as continent, island and polar area to constrain, for each environment, the parameters involved in the modelling. For each station, we quantify the trade-off between ocean wave coastal reflection and seismic wave attenuation that both affect the amplitude of the seismic spectrum. We show their adjustment and the need, at some stations, for an extra parameter representing the three-dimensional (3-D) seismic wave propagation effects. The long-term analysis demonstrates the stability of the fitted parameters which can be used in future noise studies. The modelling enables to reproduce the frequency content and amplitude of the different noise peaks of seismic spectra. The strongest peaks are generated by deep ocean sources whereas coastal reflections generate numerous smaller sources that contribute to the background noise level. Coastal reflection effects can be neglected only for the Pacific island station PPT. The modelling also reproduces the peculiar noise spectrum variation in Antarctica (station DRV) which is related to the presence of sea ice around the stations.