Observations of infragravity waves at the ocean-bottom broadband seismic stations Endeavour (KEBB) and Explorer (KXBB)

We report observations of infragravity waves in the period band 30-400 s at two buried broadband ocean-bottom seismic stations, KEBB and KXBB, located off the coast of the Pacific Northwest, at a water depth of similar to 2370 m. When compared to the energy of short-period ocean waves recorded at local buoys, the low-frequency seismic noise is found to be mainly generated when the short-period ocean waves reach the coast, and not when the storm passes directly above the station. Two types of modulation of the infragravity signal are observed. First, a longer-period modulation is observed and is best correlated with the energy of the 14-16 s period ocean waves. Second, the entire infragravity band signal is modulated in phase with tides. The results suggest that the infragravity waves originate from the nearshore region east of buoy 46041, which lies off the coast of Washington State, and not from the nearshore regions farther to the north that are closer to KEBB. Strong polarization of the KEBB horizontal motions in the NW-SE direction also suggests that the infragravity waves arrive at KEBB from the SE direction. Since the infragravity waves are not generated uniformly along the shore, modeling of their spatial variability prior to the selection of new sites could help improve the quality of future ocean-bottom seismic deployments and may also help understand processes at the origin of the Earth's low-frequency hum.