abstract

Thirty-five shallow, distant earthquakes located in the Pacific and recorded at the Ocean Bottom Geophysical station OBS III by a long-period vertical seismograph and a long-period crystal hydrophone were selected for analysis of the dispersion of Rayleigh waves. The sensors are part of the Lamont Geological Observatory's instrument package implanted in May 1966 approximately 200 km west of San Francisco at a water depth of 3.9 km. The location of the station and that of the epicenters, all in the ocean, give us for the first time the oppurtunity of studying purely oceanic paths. The group velocity dispersion curves in the period range 12-40 sec show minor regional differences in the oceanic crustal structure. For the comparison, dispersion curves were obtained for 24 of these earthquakes from the seismographs recorded at the Berkeley seismograph station, BKS. Most of the pairs of dispersion curves show no significant differences due to crossing the continental margin. However, the group velocities of Rayleigh waves from southern Alaska and Easter Island are higher at OBS III than at Berkeley by as much as 0.1 km/sec. Realizing that theoretical models based exclusively on surface-wave data are not unique, and taking Dorman's oceanic model 8099 as our starting point, we were able to fit our experimental dispersion curves using models characterized by a low-velocity zone starting at a depth of about 60 km. According to this type of solution the crust is thicker along the paths from south Alaska and Easter Island, parallel to the coast, than along the other paths examined. The pressure-to-displacement ratio (P/D) is not very sensitive to changes in the models for periods greater than 12 sec. It is, however, useful in determining the local sedimentary structure from short-period waves.

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