An investigation into ocean wave sources of ambient seismic noise

• Main Author: Neale, Jennifer F. Ward
• Published: University of Southampton 2017
• Subjects: 551.46
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.720195

Periodic pressure fluctuations beneath ocean waves deform the seabed beneath them and can generate seismic waves that propagate around the globe. These pressure fluctuations are recorded on seafloor pressure gauges and the associated ground displacements on seismometers, where they contribute to ambient seismic noise. The signals offer an opportunity to study or monitor ocean waves that are traditionally difficult to measure because of their low wave heights (deep water infragravity waves) or their remote locations (deep water swell waves). However, the link between ocean waves and the oscillations of the pressure and seismic wavefields has remained unclear. The aim of this thesis was to increase our understanding of ocean sources of ambient seismic noise, including their location, how well they can be located, and their relationship with ocean wave parameters. Using cross-correlated pairs of seafloor pressure records, infragravity waves offshore California/Oregon were found to originate mostly from local coastlines during northern-hemisphere winter and from the south during summer. A first attempt to estimate the coastal reflection coefficient of remote arrivals was made and found to be 0.49-0.74, which has implications for infragravity energy in the deep ocean and may be important for models in which infragravity waves are propagated across ocean basins. P-wave sources in the North Pacific during winter were located using seismometers in California and found to be concentrated around 40-50◦N 160-180◦E. Observed source locations were within 10◦ of the modelled source locations. Significant wave height in the deep ocean was estimated from P-waves and correlated with modelled wave height with a correlation coefficient of 0.63. Previous work only attempted to estimate coastal wave heights. Combining additional records from Japan and Europe improved source location, including imaging of multiple sources. Accuracy in source location and amplitude estimation are essential if microseisms are to be used to monitor wave activity in the deep ocean.