Ocean-bottom seismic measurements off the California coast

Abstract

As a part of the Vela Uniform program seismograms were recorded between December 19, 1961, and April 1, 1962, from instruments placed on the ocean bottom off the California coast in water depths up to 1.2 km. Three components of velocity and water pressure were recorded digitally. During the test period a total of five underground nuclear blasts originating from the Nevada test site and several small unidentified seismic events were recorded by the ocean-bottom seismometer. Recordings of pressure and vertical velocity in the frequency range of 0.5 to 5 cps for three nuclear blasts were of sufficient quality for detailed analysis. Spectral analyses were performed on several of these nuclear blast seismograms and the noise data immediately preceding them. These were compared with similar analyses of the same events obtained from a land station in the southern California area. The results indicated that signal-to-noise ratio is slightly poorer on the ocean bottom in 1.2 km of water than on a land station at the same range. The absolute noise level appears to be higher at the deep water sites relative to the land station; however, the ocean-bottom noise level is near the continental ‘average’ of Brune and Oliver for the samples investigated. Finally, the simultaneous recording of pressure and velocity appears to afford a distinct advantage in the identification and classification of arrivals recorded on the ocean bottom because of the P-V relationship that exists for the various propagation modes.