Infragravity (IG) waves are long-period (20-500 s) ocean surface gravity waves. The pressure fluctuations that they create at the ocean bottom result in seafloor deformation, which is the main source of the long-period noise observed at the ocean-bottom seismic stations. Understanding the properties of the IG waves as well as their coupling to the solid earth is important for the study of the earth's hum and structure using non-seismic sources. Also, modeling of their spatial variability could help with the selection of the most quiet locations for future ocean-bottom seismic deployments.
In our previous work, we found that IG waves are generated in the nearshore region from the shorter-period ocean waves and are observed at the ocean-bottom station KEBB only after they propagate from the shelf into the deeper water and pass over the station. In our recent work (Dolenc et al., 2008), we further studied the IG waves at KEBB to identify the nearshore region where the IG waves are generated.
Station KEBB was installed 247 km offshore Vancouver Island at a water depth of 2376 m in August 2003. It comprised a 3-component broadband seismometer Guralp CMG-1T, a recording, and a battery package. The seismometer was completely buried in the ocean floor sediments and the station was recording the data continuously.
The analysis of the horizontal velocity ground motions at KEBB throughout the deployment shows that motions are strongly polarized in NW-SE direction (Figure 2.23). Assuming that the IG waves propagated to KEBB as freely propagating waves, we can approximate them as plane waves. In this case we expect the horizontal motions at KEBB to be primarily in the direction of the IG wave propagation. This suggests that the IG waves arrived from the SE direction.
To identify the origin of the IG waves independently of the above observation, we took advantage of their tidal modulation. Tidal modulation of the IG waves has previously been observed at the ocean-bottom seismic stations and can be explained by a mechanism proposed by Thomson et al. (2006), in which it results from changes in the beach profile from convex at low-tide to concave at high-tide. To identify the origin of the IG waves we used two parameters. First, the tidal phase changes along the shore. Second, the distance from the nearshore region to KEBB changes as we move along the shore. Since the freely traveling surface waves are dispersed, the difference between the arrivals of the longer- and shorter-period IG waves at KEBB is a function of distance that IG waves have to travel. We first calculated the predicted traveltime for the IG waves from every buoy to KEBB (Figure 2.24b, solid lines). We then measured the phase delay between the tidal modulation observed at KEBB and tidal phase at individual buoys within different period bins. The comparison with the modeled traveltimes suggests that the IG waves observed at KEBB originated from the nearshore region close to buoy 46041.
The strong polarization of the horizontal motions at KEBB as well as the analysis of the phase of the tidal modulation observed at KEBB both suggest that IG waves originate from the nearshore region in southern Washington and not from the nearshore regions further to the north that are closer to KEBB. This suggests that long sandy beaches in southern Washington, and not the rocky and rugged coast to the north, play an important role in the IG wave generation.
This work was partially supported by NSF (grant OCE-0648302) as well as BSL funds. The KEBB was deployed as part of the 3-year multidisciplinary prototype NEPTUNE experiment supported by a grant from the W. M. Keck Foundation to the University of Washington. The seismic component of the project was a collaboration between the University of Washington, the University of Oregon, and the Monterey Bay Aquarium Research Institute.
Dolenc, D., B. Romanowicz, P. McGill, and W. Wilcock, Observations of infragravity waves at the ocean-bottom broadband seismic stations Endeavour (KEBB) and Explorer (KXBB), Geochem., Geophys., Geosys., 9, Q05007, doi:10.1029/2008GC001942, 2008.
Thomson, J., S. Elgar, B. Raubenheimer, T. H. C. Herbers, and R. T. Guza, Tidal modulation of infragravity waves via nonlinear energy losses in the surfzone, Geophys. Res. Lett., 33, L05601, doi:10.1029/2005GL025514, 2006.
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