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

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: Qp(1.5-15 cps) > 1 × 103; the attenuation of P waves increases markedly above 15 cps, probably due to scattering losses; and although absolute Qp 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. Vp/Vs, determined from S-P and P readings at seven stations for 150 shocks, is significantly higher for oceanic shocks than inland shocks.