abstract

Seismograms from Berkeley and from a site on the deep ocean bottom off California were compared to ascertain the effect on Rayleigh waves of propagation across the continental margin. Amplitude ratios as a function of period were formed by dividing the spectral component at one recording site by that at the other recording site. In the period range from 14 to 25 sec and for near-normal incidence, the amplitudes of vertical displacement on the ocean bottom were two-thirds to one-third as large as those at Berkeley. Amplitude ratios for waves propagating in either direction across the margin are consistent within experimental error.

Comparison of the experimental amplitude ratios with ratios that were calculated by using a variational technique recently advanced by McGarr and Alsop suggests that there is a striking contrast in structure between Berkeley and the ocean-bottom site, which is about 235 km WNW of Berkeley. The velocities and densities of the lower crust and upper mantle beneath the oceanic site are much higher than those beneath Berkeley; this result is in qualitative agreement with the results of geophysical studies in this region using other techniques.

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