Speaker: Sharon Kedar
“Correlation of ambient seismic noise is now widely used to determine earth structure. The reason for this is the relative ease of determining the Green’s functions along station to station paths compared to the complexity that an earthquake source introduces, and because of the ability to do this in absence of earthquake sources. The technique has been applied to both short and long-range surveys in many parts of the world.
Recently, correlation methods are being applied to the problem of monitoring temporal variations in the subsurface. For this application the technique would appear almost ideal because the source is omnipresent compared to earthquakes. However, the need to look at correlations for relatively short time windows can lead to a violation of the underlying assumption of the technique. That is that the sources need to be distributed randomly off (either) end of the station-station path. If this assumption is not met, the technique estimate can be biased by a favored projection of the Green’s function. This will lead to an incorrect travel time estimate and consequently an incorrect velocity estimate.
To enable reliable 4D noise tomography (dominated by microseisms), the source must be characterized in space and time. Most microseisms source studies to date are based on correlations between seismic data, buoys, barometer measurements and wave height models. While these statistical associations do reflect a causal relationship, they do not unambiguously highlight the physical process at work nor the exact generation areas or depths. To do that, physical modeling of wave-wave interactions in the open ocean as well as in coastal regions is required. Moreover, the complete theory of microseisms generation in a compressible ocean must be used. While this can be accomplished in a straightforward manner in the open ocean, it is far more challenging in coastal regions, where the wave reflection characteristics is likely to depend on a number of variables, such as the beach slope, the wave height and tidal conditions. In this presentation we report explicit calculations of microseisms amplitudes making use of hindcast ocean wave spectra from the North Atlantic Ocean, and comparison of the calculations to seismic data collected at stations in North America, Greenland, and Iceland. We find that a particularly energetic source area stretches from the Labrador Sea to a region south of Iceland, where climatological conditions are conducive to generating oppositely traveling waves of same period, and where the ocean depth corresponds to an “”organ-pipe”” resonance of the compression waves generated by the opposing wave-wave interaction, as predicted by the theory. It is demonstrated that deep ocean nonlinear wave-wave interactions are sufficiently energetic to account for the observed seismic amplitudes in North America, Greenland and Iceland. Differences between Atlantic and Pacific microseisms are highlighted.”