National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1994

Potential for monitoring low-level seismicity on the Juan de Fuca Ridge using military hydrophone arrays

Fox, C.G., R.P. Dziak, H. Matsumoto, and A.E. Schreiner

Mar. Technol. Soc. J., 27(4), 22–30 (1993)

The Juan de Fuca Ridge is generally considered to be "aseismic," averaging only 1 event/year recorded by the global seismic networks. Analyses of seismic tertiary-phases recorded by hydrophone arrays provide a more sensitive means of detecting low-level seismicity in the ocean. Direct, indirect, and theoretical methods are developed and applied to data from various sources to estimate the detection thresholds of fixed hydrophone systems. These thresholds, when combined with frequency-magnitude scaling relationships derived from the global networks, provide predictions of the mean number of events expected for the Juan de Fuca Ridge system. The results indicate that the global networks are limited to a detection threshold of m = 4.2 and 1 event/year; earlier work with Pacific Missile Range/Missile Impact Location System hydrophones and analog techniques was limited to m = 3.4 and 15 events/year; U.S. Navy SOSUS (Sound Surveillance System) hydrophones combined with digital signal processing techniques can detect a minimum m = 2.5 and 265 events/year, perhaps as low as 2.4 as indicated by events recorded from northern California; and beam forming of the SOSUS arrays can reduce the detection threshold to m = 1.8-2.1 and 1,000-2,000 events/year.

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