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The seismic motion of the deep ocean floor

LAMONT GEOLOGICAL OBSERVATORY COLUMBIA UNIVERSITY, PALISADES, NEW YORK

Abstract

Results of the first measurements of ambient seismic noise of the deep ocean floor below 2000 fathoms are presented. The measurements were made with vertical component seismometers at nine locations. At five stations in the Atlantic Ocean, amplitudes observed at 1 cps were 100 millimicrons at one site, 35 millimicrons at two sites, and less than 1 millimicron at the other two sites. At a station in the Gulf of Mexico, amplitudes of 35 millimicrons at 1 cps were observed. Amplitudes of about 1 millimicron at 1 cps were observed at three sites in the Arctic Ocean.

Amplitudes and periods remained essentially constant during the two days that data were recorded in the Gulf of Mexico and showed no correlation with nearby local meteorological storm activity. During the three days that data were recorded at the noisiest Atlantic site, rising and falling winds and seas over a broad area were weakly reflected as small changes in the amplitude of the ocean bottom noise. Similar small amplitude variations were recorded at Bermuda, 150 nautical miles to the northeast, but the absolute Bermuda amplitudes were an order of magnitude less than those at the noisiest Atlantic site.

Measurements in samples of up to one hour in duration were made in the Arctic Ocean at three stations. The measurements were made at intervals during a 16-day period, and a good correlation was found in both time and amplitude between seismic noise on the ice surface and on the ocean bottom in the band from 1 to 10 cps.

A P-wave train from a distant earthquake was recorded at the noisiest Atlantic site with a signal-to-noise ratio of about 3:1; this was the same signal-to-noise ratio seen nearby at Bermuda with a seismograph of similar response. In the Arctic, P waves from an Alaskan shock and a possible PKP phase from a New Zealand shock were recorded. The maximum P-wave signal-to-noise ratio recorded in the Arctic for the Alaskan shock was 250:1. After a correction is made for the difference in epicentral distance, the signal-to-noise ratio at Ogdensburg, New Jersey, for this shock is only 20:1. Ogdensburg is quieter than an average station.

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