Wind-generated ocean gravity waves excite pressure fluctuations that couple energy into seismic waves at the ocean bottom, causing the gravity-wave induced seismic signals called microseisms. Primary or "single-frequency" microseisms, observed at the gravity wave frequencies, are generated only in shallow water by direct pressure coupling of the ocean wave pressure signal with the sloping seafloor and/or the breaking of waves at the shoreline [Hasselmann, 1963]. Microseisms at double the ocean wave frequency result from nonlinear interaction of ocean gravity waves with opposing components, with the amplitude of "double-frequency" (DF) microseisms proportional to the product of the amplitudes of the opposing ocean wave components producing them [Longuet-Higgins, 1950]. Wave-wave interactions that generate DF microseisms occur nearshore [Bromirski et al., 1999; Bromirski, 2001; Bromirski and Duennebier, 2002] as well as over all oceanic regions [Bromirski et al., 2005; Bromirski, 2009].
When ocean waves impact the shore, a portion of the wave energy is reflected and interacts with the incoming ocean wavefield, producing a pressure pulse that propagates to the seafloor where it is transformed into DF microseisms. DF microsiesm levels at near-coastal seismic stations are dominated by wave activity at the nearest coastlines [Bromirski et al., 1999; Bromirski and Duennebier, 2002]. This is evident in the nearly one-to-one correspondence between the ocean wave and microseism spectra (figure). This correspondence allows the reconstruction of the ocean wave climate using archived seismograms, with the long term ocean wave climatology along the central California coast under construction from analog seismograms from UC Berkeley [see SeisDig software].
A portion of ocean swell reaching coasts (wave periods from 30-10s) is transformed by a non-linear process into long period "infragravity" waves, having periods from 350-50s, that are refractively trapped along the coast. The infragravity waves generate a seismic signal called the Earth's "hum", which is dominated by coastal generation and is observed globally [Bromirski and Gerstoft, 2010]. Some of the infra gravity wave energy leaks off the continental shelf along the Pacific coast of North America and propagates to Antarctica, where it may have a significant impact on the stability of the Ross Ice Shelf [Bromirski et al., 2010; Antarctic research page].