abstract

Seismograms from island stations within or near the edge of the Pacific plate were used to obtain surface-wave attenuation coefficients in the period range 18 to 110 sec for Rayleigh waves and 20 to 110 sec for Love waves. The average Rayleigh-wave attenuation coefficient values range from a maximum of 1.64 × 10−4 km−1 at short periods to a minimum of 0.72 × 10−4 km−1 at longer periods. Corresponding extreme values for Love waves are 3.30 × 10−4 km−1 and 0.60 × 10−4 km−1. The data are characterized by relatively large standard deviations which reflect departures from an ideal medium having laterally homogeneous elastic and anelastic properties.

The possibility of regional variations in anelastic properties was examined by dividing the Pacific into three regions according to age (0 to 50 m.y., 50 to 100 m.y., >100 m.y.). A systematic decrease in attenuation coefficient values over most of the period range is readily apparent from the data. Qβ−1 models obtained by stochastic inversion of the attenuation data suggest that the observed differences are produced by increasing values of Qβ in the low Q zone (at depths of 40 to 200 km), and possibly also by increasing values of Qβ in the lithosphere, as the age of the Pacific sea floor increases.

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