Bulletin of the Seismological Society of America
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Bulletin of the Seismological Society of America; December 1999; v. 89; no. 6; p. 1543-1557
© 1999 Seismological Society of America
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The effect of local wind on seismic noise near 1 Hz at the MELT site and in iceland

William S. D. Wilcock, Spahr C. Webb and Ingi Th. Bjarnason

School of Oceanography University of Washington, Box 357940, Seattle, WA 98195
Marine Physics Laboratory, 0205 Scripps Institution of Oceanography University of California, LaJolla, CA 92093
Science Institute University of Iceland, Reykjavík, Iceland

Abstract

The mantle electromagnetic and tomography (MELT) experiment on the east Pacific rise near 17°S was the first large teleseismic experiment on a midocean ridge. During the six-month deployment, no compressional arrivals were well recorded above 0.5 Hz. In comparison, the ICEMELT experiment in Iceland recorded compressional arrivals at 1-2 Hz from about 2 earthquakes per month. We compare noise spectra from the two experiments and show that this difference in detection is at least in part a result of noise. Near 1 Hz, seismic noise in the oceans is produced locally by wind-generated waves. At both experiment sites, 1-Hz noise levels are well correlated with local sea-surface-wind speeds derived from satellite observations. For a given wind speed, 1-Hz noise levels are about 10-20 dB lower in Iceland. At the MELT site, cross-correlations of wind speed with the logarithm of noise in a narrow-frequency band yield correlation coefficients exceeding 0.7 at frequencies between 0.4 Hz and 2 Hz. Noise levels at 1 Hz increase with wind by 1.3-1.4 dB per m/sec for wind speeds less than 10 m/sec. For the ICEMELT experiment, high correlation coefficients extend to markedly higher frequencies for coastal stations, and there is a 10-dB drop in 1-Hz noise levels 100-km inland. Noise levels increase by about 0.8 dB per m/sec. The strong correlation between wind speed and 1-Hz seismic noise provides justification for using satellite wind speed data to search for locations on the global spreading system where there is a better probability of recording high-frequency arrivals. The calmest sites are found on the northern east Pacific rise, near the equator in all oceans, and near 34° N and 22° S on the mid-Atlantic ridge.




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