Infragravity Wave Generation and Reflection Off the Coast at Oregon, USA

Extremely low frequency seismic noise, known as earth’s “seismic hum”, is generated by the very long wavelength, low frequency infragravity waves of the ocean which transfer their energy to the solid earth. Infragravity waves are generated along coastlines by the non-linear interaction of incoming ocean swells and can travel long distances across ocean basins with relatively little attenuation. They have been implicated in the break-up of ice shelves (Bromirski et al, Geophys. Res. Lett. 2010) and are important for coupling processes between the ocean, atmosphere, and earth (Rhie and Romanowicz, Nature 2004), but their spatial and temporal variably remains poorly studied.

We investigate the characteristics of the infragravity wave band in the north-east Pacific using co-located ocean bottom seismometers and differential pressure gauges deployed in deep water as part of the Cascadia Initiative array. Using cross correlation techniques and backprojection of the noise correlation function (Harmon et al, Geophys. Res. Lett. 2012) we have found that infragravity wave energy propagates offshore for the majority of the year, indicating that the Oregon coastline is a net producer of infragravity waves. On rare occasions, infragravity wave energy is observed to propagate onshore. In January 2013, a particularly strong event occurred where energy propagated onshore with normal incidence to the coastline. We use this event to investigate the feasibility of estimating coastline reflection coefficients from cross correlation of differential pressure gauge records.