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Attenuation of earthquake generated P waves along the western flank of the Andes

CARNEGIE INSTITUTION OF WASHINGTON DEPARTMENT OF TERRESTRIAL MAGNETISM, WASHINGTON, D. C.

Abstract

The attenuation of longitudinal body waves generated by local shocks in southern Peru was investigated. Thirty-four shocks, with focal depths between 20 and 180 km, were used in the study of attenuation as a function of depth and as a function of frequency (1-30 cps). Shocks were recorded on magnetic tape at three stations along the western flank of the Andes. The dimensionless specific attenuation factor Q was computed from measurements of the relative change in shape of P-particle velocity spectra with distance from the source. Near surface effects on amplitudes were taken into account. It was found that: Q_p(1.5-15 cps) > 1 x 10³; the attenuation of P waves increases markedly above 15 cps, probably due to scattering losses; and although absolute Q_p values as a function of depth could not be resolved, an increase in P wave attenuation at a depth of about 80 km is suggested. V_p/V_s, determined from S-P and P readings at seven stations for 150 shocks, is significantly higher for oceanic shocks than inland shocks.

Footnotes

^* Present address: Gulf Research and Development Company, Pittsburgh, Pennsylvania.

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