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Spatio-temporal characterisation of microseism sources in the north east Atlantic region

2016, Craig, David, Bean, Christopher J., Lokmer, Ivan

Oceans generate persistent low frequency background seismic signals known as microseisms through a mechanical coupling with the Earths crust. Microseism energy originates as regions of low barometric pressure (depressions) over the oceans where it is transmitted to the sea-floor and propagates as elastic energy in the Earths crust. Consequently, microseisms carry important meteorological information relating to both the atmosphere and the hydrosphere. The relationship between the two, leads to the possibility of obtaining information on the ocean wave-field from near coastal seismic records by developing a transfer function between an ocean buoy and a near coastal seismic receiver. However, this assumes that the seismic record is dominated by a source relatively close to the buoy.Microseisms are also used in many passive seismological methods, including noise tomography and cross-correlation methods. In these methods it is assumed that when averaged over a sufficiently long time period the seismic wavefield is random. This places importance on understanding the degree of non-uniformity within the seismic source region. Both these applications highlight the importance of understanding how the microseism distributions vary both spatially and temporally. Previous studies have identified the North East Atlantic region near Ireland as one of the global hotspots for producing microseisms.The main aim of this thesis is to investigate the spatial and temporal variability in the near coastal microseism spectrum recorded in Ireland. Three broadband seismic arrays are used which are located to provide maximum spatial coverage of the continental shelf in the North East Atlantic near Ireland. The arrays are used to identify the dominant source locations in this region. The spectra for microseisms generated in this region are dominated by R g waves. Contributions from sources beyond the continental shelf are also identified. These primarily consisted of P-waves. However, in one instance R g waves from a source beyond the continental shelf are clearly identified. However, these do not appear to form a significant portion of the microseism spectrum.The relationship between microseisms recorded in the region and the ocean wave field is also highlighted through a correlation of microseism amplitudes and ocean wave heights. The ocean wave heights were obtained from a hindcast dataset for global ocean wave parameters. Wavefield separation is also considered and a frequency wavenumber filter designed and applied to allowed propagation directions prior to correlation with the wavefield. This allows the identification of secondary generation areas that are otherwise masked.The microseisms spectra recorded by seismometers in Ireland are closely related to the ocean wavefield in the North East Atlantic. However, several generation areas exist on the continental shelf near Ireland. Each area relates to different parts of the ocean wavefield. Wavefield separation is thought to be a necessary step before attempting to develop a transfer function between microseism amplitudes and ocean wave heights.