Rayleigh Wave Dispersion and Crustal Structure in the Eastern Pacific and Indian Oceans

Abstract

Summary Rayleigh wave dispersion data are presented for a number of earthquakes having their epicentres in the Easter Island area and which were recorded at Pasadena, U. S. A. The dispersion data differ in a small but significant way from those obtained for Pacific Ocean basin paths. When the seismic refraction data of Raitt are used to compute theoretical Rayleigh wave dispersion curves, discrepancies with the observed Easter Island-Pasadena data are found which are resolved by the addition to the assumed theoretical model of a modified low velocity zone in the upper mantle. This modification consists of lowering the mean shear velocity in the upper mantle to 4.5 km/s. It is further demonstrated that it is possible to have a theoretical model compatible with the compressional velocities obtained by seismic refraction measurements and the dispersion data which does not require changing Poisson's ratio from the usual value and consequent reduction of the shear velocities in the upper mantle. This is accomplished by reducing the thickness of the layer with compressional velocity 8.2 km/s to the minimal thickness required to give a refraction arrival. The mean shear velocity of 4.5 km/s implies a “soft” upper mantle along the Easter Island to Pasadena path. Dispersion data also show that the mean thickness of unconsolidated sediments between Easter Island and Peru is 0.57 km. Rayleigh wave dispersion data are also presented for Indian Ocean shocks recorded at Wilkes Station, Antarctica. Within the limits of observational error, the Indian Ocean data are in agreement in the period range from 37 to 25 seconds with the theoretical model assumed for the Easter Island to Pasadena data.