- © 2008 by the Seismological Society of America
Microseisms are the most ubiquitous continuous seismic signals on Earth at periods between approximately 5 and 25 s (Peterson 1993; Kedar and Webb 2005). They arise from atmospheric energy converted to (primarily) Rayleigh waves via the intermediary of wind-driven oceanic swell and occupy a period band that is uninfluenced by common anthropogenic and wind-coupled noise processes on land (Wilson et al. 2002; de la Torre et al. 2005). “Primary” microseisms (near 8-s period) are generated in shallow water by breaking waves near the shore and/or the nonlinear interaction of the ocean wave pressure signal with the sloping sea floor (Hasselmann 1963). Secondary microseisms occur at half of the primary period and are especially strongly radiated in source regions where opposing wave components interfere (Longuett-Higgins 1950; Tanimoto 2007), which principally occurs due to the interaction of incident swell and reflected/scattered wave energy from coasts (Bromirski and Duennebier 2002; Bromirski, Duennebier, and Stephen 2005). Coastal regions having a narrow shelf with irregular and rocky coastlines are known to be especially efficient at radiating secondary microseisms (Bromirski, Duennebier, and Stephen 2005; Shulte-Pelkum et al. 2004). The secondary microseism is globally dominant, and its amplitudes proportional to the square of the standing wave height (Longuett-Higgins 1950), which amplifies its sensitivity to large swell events (Astiz and Creager 1994; Webb 2006).
Microseisms have long been recognized as an indicator of large-storm intensity (Gilmore 1946; Gutenberg 1947; Gilmore and Hubert 1948). However, contributions from deep-water open-ocean storms are not generally significant in records from land-based seismometers (Bromirski and Duennebier 2002; Haubrich and McCamy 1969). Rather, microseism levels predominantly measure spatially distributed wave activity associated with wave interactions along coasts. The efficient propagation and ubiquity of short-period microseism Rayleigh waves (Bromirski, Duennebier, and Stephen 2005; Schulte-Pelkum …