Studying Earth's Wave Climate Using the Global Microseism

Studying Earth's Wave Climate Using the Global Microseism Excursions of the number of hours per winter wave year (November - March in the Northern Hemisphere; May - September in the Southern Hemisphere) relative to mean values in hours of microseism excitation exceeding the 95th percentile in the 14-20 s period band for six or more contiguous hours observed at GSN and co-sited precursor stations. Note global oscillations of this microseism index and strong positive excursions during El Nino periods (e.g., 1978, 1983, 1998). After Aster et al. (2010).
Globally ubiquitous seismic background noise peaks, visible as broad peaks in seismic spectra with modes near 7 and 14 s period, are generated via two distinct mechanisms that transfer storm-generated gravity wave energy to the seismic wave field. Continuous digital ground motion data recorded by the Global Seismographic Network and precursor networks provide a unique integrative record of ocean wave to seismic coupling at regional to global scales [Aster et al., 2008] that can be applied to chronicle microseism power extreme events over hourly to decadal time periods. Because most land-observed microseism surface wave energy is generated at or near coasts [e.g. Bromirski et al., 1999], microseism metrics are particularly relevant to assessing changes in coastal ocean wave energy. For example, extreme microseism winter storm season event counts quantify the widespread wave influence of the El Nino Southern Oscillation (ENSO). The double-frequency (near 7 s) microseism is generated by interacting (standing wave) components of the ocean wave field, and is observed to be particularly volatile, likely both because of its quadratic dependence on wave height and because of its sensitivity to both incident wave angle and to coastal conditions that control ocean swell reflectivity. This suggests that the weaker single-frequency (near 14 s) microseism directly generated by ocean swell at coasts is a more representative seismic proxy for broad-scale ocean wave energy estimation. Metrics of extreme microseism events since the 1970s suggest slight positive trends in the northern hemisphere, and slightly declining trends in the Southern Hemisphere [Aster et al., 2010]. Microseism metrics are extendable to the pre-digital era through the digitization of analogue seismograms from long-running observatories, and thus offer the opportunity to quantify and characterize the wave influence of ENSO and other climate variations through to the early 20th century.
</p><p>Aster, R., D.E. McNamara, and P. Bromirski (2008), Multi-decadal climate-induced variability in microseisms, Seismol. Res. Lett., 79, 194-202, doi:10.1785/gssrl.79.2.194.
</p><p>Aster, R., D.E. McNamara, and P. Bromirski (2010), Global Trends in Extremal Microseism Intensity, Geophys. Res. Lett. in press.
</p><p>Bromirski, P. D., R. E. Flick, and N. Graham (1999), Ocean wave height determined from inland seismometer data: Implications for investigating wave climate changes in the Northeast Pacific, J. Geophys. Res., 104, 20,753-20,766.
</p><p>Acknowledgements: The Global Seismographic Network is a cooperative scientific facility operated jointly by the Incorporated Research Institutions for Seismology, the United States Geological Survey (USGS), and the National Science Foundation. P. Bromirski has been supported in this research by the California Department of Boating and Waterways. Richard Boaz contributed significantly to database programming.</p>


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