( 2005) compared the power spectra of ocean-bottom seismic records at a station located offshore of the Monterey Bay in California with the wave spectral densities measured by the nearby National Oceanic and Atmospheric Administration (NOAA) buoy. IG waves also are considered to excite the Earth's hum (Rhie and Romanowicz 2004, 2006 Tanimoto 2005 Nishida et al.

1995) and may be enhanced by tidal modulation in coastal oceans (Guza and Thornton 1982 Okihiro and Guza 1995 Tomson et al. These waves are considered to be excited by non-linear interactions between oceanic swells (Longuet-Higgins and Stewart 1962 Herbers et al. The phase velocity of IG waves observed in deep ocean environments is accurately explained by the gravity wave theory (e.g., Webb et al. Ocean infragravity (IG) waves are sea surface gravity waves with periods of several minutes and wavelengths of tens of kilometers. Such waves are greatly amplified when IG waves from a distant typhoon are incoming to the trough, suggesting the secondary origin of IG waves that can emit even more energetic waves than the originally incoming waves. Although less energetic, IG waves propagating parallel to the trough and coast are observed. Both sources show a seasonal variation, weak in summer and strong in winter. The sources of the most energetic waves are narrowly and stably localized into two closely adjacent azimuthal windows with mutually different wave spectral characteristics. Application of a slant-stacking technique revealed that the most energetic IG waves are incoming across the Nankai Trough from the Philippine Sea with limited energy of reflected waves back from the nearest coast. Continuous 3-year records of broadband ocean-bottom seismometers and pressure gauges of the seafloor network (Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET)) in the Nankai Trough region made it possible to monitor incoming ocean infragravity (IG) waves.

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