Discrimination of Secondary Microseism Origins Using Ocean Tide Modulation

Abstract

The ocean activity produces continuous and ubiquitous seismic energy mostly in the 2-20 s period band, also known as microseismic noise. The secondary microseisms (2-10 s period) 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. Since both type of secondary microseisms (coastal or deep-ocean) can occur simultaneously at different places and are continuously evolving in terms of frequency, it is very difficult to discriminate them usgin seismic stations 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 secondary microseismic peak. Comparison with swell spectra at surrounding buoys suggests that the largest part of this extra energy comes from deep-ocean-generated microseisms. Focusing on two different storms which occurred in the North Atlantic Ocean, we show that both deep-ocean and coastal microseisms coexist.