Abstract

A program of seismic noise measurements (frequency range 0.2-5.0 cps) in deep boreholes has been under way for the past two years. Preliminary results are consistent with the hypothesis that below about 1.0 cps the noise consists of an energy-equipartition mixture of Rayleigh and Love modes and a P-wave component with vertical motion energy at the surface almost equal to that of the sum of the trapped modes. Observed ratios of surface horizontal-to-vertical component motions are also compatible with this hypothesis, as are the amplitude-depth relationships. However, nearly equipartition mixtures of the theoretically computed trapped modes can be found which simulate many of the time averaged effects of P-waves remarkably well, so that unequivocal conclusions are difficult to draw.

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