Collaborative Research: Characterizing sources of infragravity waves and the earth's hum using data from the Cascadia Amphibious Array

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Seismometers are sensitive to tiny ground motions generated by distant earthquakes and other phenomena. As has long been known, in the absence of earthquakes, these instruments also record a background 'hum', a low level seismic signal originating in the earth that has energy cycling every 6-300 seconds. Broad waves called "infragravity waves" moving through the ocean are the source of most of the 'hum', imparting cyclic pressure on the seafloor as they travel through the seawater. This general relation between ocean 'forcing' on the seafloor and Earth's seismic 'hum' is understood, but how much is generated along the shallow margins of the ocean versus within the deep ocean basins, and why? This project investigates the relationship between the source region(s) of the 'hum' and the passage of major storms. It explores whether the shape of the coastal slopes influences how much hum energy is produced. In order to accomplish the work, seismometers both on the seafloor and on the continent will be used to combine information from the signals detected across the array of instruments, and point toward the location where the seafloor forcing occurred.Through a collaboration between seismologists and oceanographers, the spatial variability of infragravity wave (IG) energy levels, the influence of ocean storms on that pattern, and the generation of microseisms and Earth's long period "hum" will all be documented. Broadband seismic and pressure spectra from the Cascadia amphibious array will be analyzed. Numerical models will be employed to explore the range of possible wave behaviors across the continental shelf. The coupling between wind-driven ocean waves, IG waves and the seafloor will be studied in a near-coastal region of the Pacific Northwest. The results of this study will contribute to the characterization of IG noise levels on the ocean floor, link this variability to the near shore IG wave climate, and provide guidance for future off-shore deployments of seismic and pressure arrays. Improved understanding of the sources of Earth?s hum may help design approaches to using long period seismic noise data for the study of deep earth structure. The west coast of North America is uniquely suited for this purpose as it is has been documented as a strong source region for the "hum". A graduate student will play a significant role in this cross-disciplinary research, obtaining experience in array processing techniques such as stacking, beamforming, and back projection.