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Attenuation of refracted seismic waves in young oceanic crust

SCHOOL OF OCEANOGRAPHY UNIVERSITY OF WASHINGTON WB-10, SEATTLE, WASHINGTON 98195

Abstract

It is commonly observed in seismic refraction experiments over young oceanic crust that refracted energy is concentrated in the 10 to 15 Hz band, even though the source may have far greater bandwidth. We apply spectral ratio methods to data from a midwater hydrophone array towed 10 km from the Juan de Fuca ridge and confirm that the impulse response of the crust to refracted energy has a peak near 10 Hz. This peak appears to result from a combination of two factors that produce attenuation. One is related to the large velocity gradient in the upper crust caused by crack closure with increasing depth. The other is related to anelastic attenuation, which is probably also related to cracks and voids in the upper crust. At low frequencies (wavelengths greater than or equal to the thickness of the high gradient zone), it is shown that attenuation is dominated by the reflectivity of the upper crust. At high frequencies attenuation is dominated by anelastic effects associated with upper crust Q's of between 10 and 50. The wave propagation is dispersive, causing observable phase shifts in the response function, and Q may be frequency dependent. This model of the propagation explains why refraction data over young oceanic crust have energy concentrated near 10 Hz.