Analysis of a ten-day wave record obtained near Middleton Island in the Gulf of Alaska

Abstract

A bottom mounted surface wave gauge was operated in 70 m of water near Middleton Island in the Gulf of Alaska for 10 days in October and November 1973. Standard fast-Fourier transform techniques have been applied to the data, and a second-order lowpass Butterworth filter has been designed to examine low-frequency components in the record. During the time the wave gauge was in operation, two earthquakes were reported with epicenters near the middle of the Aleutian Islands. The first had a surface wave magnitude of 6.4 on the Richter scale; the second, which occurred about 9 hours later, had a surface wave magnitude of 6.3. Spectra for data taken after the occurrence of these earthquakes have shown that generation of ocean waves by these quakes is questionable. Hourly spectra from the first part of the record reveal a peak around 0.065 Hz which moves toward higher frequencies for about 18 hours. The frequency of the peak then remains constant for about 24 hours, after which it again increases. The changes are well correlated with a large storm which remained stationary in the North Pacific, then moved rapidly into the Gulf of Alaska and subsided. Wave group velocities are used to estimate possible distances of the wave source from the gauge. The actual distances of the storm from the gauge show a close correlation with wave-derived distances. Comparison with changes in wave spectra for a storm in the North Atlantic in March 1968 indicates the same time rate of change in the spectral peak as was found in the North Pacific for time periods when the storms are subsiding.