Abstract

A comprehensive spectral analysis of Love and Rayleigh waves was made on the Parkfield earthquake of June 28, 1966. Using tentative Q values obtained by the usual two-station method for continental and oceanic paths, the observed spectra were corrected for dissipation, and used for obtaining a tentative source model. The resultant model, having the fault length 37 km, the strike direction N43°W and the rupture velocity 2.2 km/sec, is consistent with various near field measurements.

Based on this tentative source model, a least-squares method is applied to separate source, dissipation and interference effects on the spectra. The logarithm of the ratio of the observed to the theoretical spectra was plotted against the epicentral distance for various frequencies. The slope and zero-crossing value of the fitted straight line were used for revising the attenuation coefficient and the seismic moment respectively. The variability of individual station spectra was attributed to the interference effect. The revised value of the seismic moment of the Parkfield earthquake is e(0.33±0.40)×1025dyne-cm. The revised attenuation coefficients indicate that Love waves have consistently higher dissipation than Rayleigh waves at periods 15 to 40 seconds. Both show marked minimum of dissipation (Q over 800 for Love and over 1000 for Rayleigh waves) at periods between 20 and 25 seconds.

We found that our Q data are consistent with a model which has a high-Q zone in the lower crust and uppermost mantle, overlying a low-Q zone which correlates well with the Gutenberg low-velocity zone. There is an indication that the intrinsic Q is dependent on period at depths shallower than 90 kilometers in such a way that it increases with period up to 20 seconds. For periods beyond 20 seconds, the intrinsic Q may be constant for all periods and all depths above 90 kilometers.

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