Seismic measurements on the ocean floor: 1. Bermuda area

Abstract

A seismograph system was placed on the ocean floor 65 km south of Bermuda in May 1964, at a depth of 4.3 km. Instrumentation consisted of three long-period seismometers (natural period = 15 sec) and one short-period vertical-component seismometer (natural period = 1 sec). Data were telemetered acoustically to a shipborne receiver for 8½ days. This experiment represents the first successful atttempt to operate long-period seismographs on the ocean floor. Predominant periods, time variations in average level, and associated energy flux of observed microseisms are approximately the same at Bermuda and on the ocean floor. It is concluded that (1) the energy of the microseisms is coupled into the layered medium by water-wave interaction, (2) the observed microseisms were generated near Bermuda and not directly beneath storms at sea, and (3) these microseisms propagate primarily as Rayleigh waves of the fundamental mode. Phases from nine earthquakes were identified on the records from the ocean-bottom instruments. The general character of the observed phases does not differ substantially from those recorded at the Bermuda standard station except for the presence of greater high-frequency amplitudes on the ocean bottom from a series of Dominican Republic shocks. The signal-to-noise ratio is larger at Bermuda for periods longer than about 1 sec, but is larger at the ocean-bottom site for shorter periods. Energy associated with a short-period Rayleigh wave propagating through the ocean bottom in the fundamental mode is concentrated near the water-sediment interface. In the island structure, the energy is distributed more uniformly with depth. This difference explains the relatively large microseismic amplitudes measured on the ocean floor. On the basis of the model used to represent the structure at the ocean-bottom site, for a given energy flux, removal of the unconsolidated sediment layer reduces the theoretical particle-motion amplitude of the water-solid interface by factors of 8 and 94 for the vertical and horizontal components, respectively. Such a reduction in background level would make the ocean-bottom site an order of magnitude quieter than the station on Bermuda. This result indicates the possible advantages of locating instruments at sediment-free sites on the ocean bottom.