Array Analysis of Microseism Sources in the NE Atlantic

Abstract:

The World’s oceans generate persistent low frequency background signals or ‘microseisms’ through a mechanical coupling with the crust. This can occur through a direct interaction between ocean gravity waves and the crust resulting in microseisms at ocean wave frequencies, known as primary frequency microseisms (PFM). Microseism generation also occurs beneath standing waves generated by the interaction of opposing ocean wave fields, these have twice the frequency of the causative ocean waves and are known as double frequency microseisms (DFM). Microseism theory limits primary microseisms to shallow water regions but secondary microseisms can occur in deep water. For near coastal stations it is possible to establish a relationship between microseism amplitudes and ocean wave heights under the premise that microseisms recorded on near coastal stations are dominated by local sources. The continental shelf in the NE Atlantic is a known source area for ocean generated microseisms. To develop an understanding of the ocean generated microseism spectrum recorded in Ireland we use array analysis to locate primary and secondary microseism sources. The local microseism wave-field can consist of contributions from several regions within the continental shelf near Ireland and Briton. Comparison to ocean wave model data indicates PFM and DFM sources for a particular region tend to be adjacent and in regions with similar ocean wave characteristics. Typical seismic velocities are ~3km/s which is consistent with Rayleigh wave velocities for the region. However, occasionally higher velocities dominate and appear to be related to storms off the continental shelf. A possible explanation for this is the reflection of Rayleigh waves at the continental margin, whilst body waves prevail and monopolize the microseism wave-field for short periods.