Observation of deep water microseisms in the North Atlantic Ocean using tide modulations

Ocean activity produces continuous and ubiquitous seismic energy mostly in the 2-20 s period band, known as microseismic noise. Between 2and 10 s period, secondary microseisms (SM) are generated by swell reflections close to the shores and/or by opposing swells in the deep ocean. However, unique conditions are required in order for surface waves generated by deep-ocean microseisms to be observed on land. By comparing short-duration power spectral densities at both Atlantic shoreline and inland seismic stations, we show that ocean tides strongly modulate the seismic energy in a wide period band except between 2.5 and 5 s. This tidal proxy reveals the existence of an ex situ short-period contribution of the SM peak. Comparison with swell spectra at surrounding buoys suggests that the largest part of this extra energy comes from deep ocean-generated microseisms. The energy modulation might be also used in numerical models of microseismic generation to constrain coastal reflection coefficients.