EarthRef.org Reference Database (ERR) -- Schulte-Pelkum et al. 2004

and 2) of ocean swells and concurrent microseisms. Our results have a number of implications for different fields of research. A useful by-product of our finding that microseisms are a strongly directional noise source is the possibility of using automated processing of the continuous noise as a near real-time check on station polarity and calibration problems, which would be a simply implemented indicator for the state of health of a seismic network. Consistent monodirectional noise may have an influence on seismic azimuthal measurements such as shear wave splitting. Most importantly, our findings should be taken into account in proposed studies which will use seismic noise as a proxy for ocean wave height in investigations of interdecadal climate change.

Animation 1
Regional Movie. Array beam directions (red arrow-single frequency, green arrow-double frequency microseisms), station direction and amplitude of double frequency microseisms based on particle motion (magenta arrows), buoy swell directions and energy (grey arrows; swell energy scales with double frequency microseism amplitude by theory), and CDIP model wave heights (color scale for significant wave height Hs, the height of the maximum 1/3rd waves) and directional spectrum. The wave heights are calculated from buoy data at the Harvest Platform using refraction and diffraction modeling <O'Reilly and Guza, 1993>. Since time delays due to the propagation velocity of swells are not taken into account, the wave height is a short-range forecast for NW swells and a hindcast for S swells. A typical time lag for a WNW swell is 6 hours between the buoy and the SE corner of the image. Frames are in hourly intervals (day and hour indicated). The animation has some discontinuities where CDIP images were not available. See Figure 5 for color scale.
We measure direction and amplitude of ocean-generated continuous seismic noise in the western United States. Slowness direction of the noise is determined using array beamforming, and particle motion direction from individual three-component stations. We find two surprising results. First, the noise is highly monodirectional at all sites, regardless of coastal distance. A single narrow generation area dominates for most of the time period, interrupted by a second well defined direction during ocean swell events. Second, we find that a storm off the Labrador coast with not unusual wave heights generates coherent noise across the entire continent. We show the causal relationship between swells arriving at different North American coastal areas and the triggered microseisms in time-lapse movies (Animations 1