ABSTRACT

Simple models of fault displacement yield acceleration and displacement spectra from which seismic moment and corner frequency can be determined. The classic Brune model is used here to predict the shape of the source spectrum and provide scaling relationships between spectral and source parameters. In order to obtain reliable estimates of the source spectrum, the effects of attenuation need to be estimated and corrected. In this preliminary study, QP = 25 ± 7 and QS = 22.5 ± 10 are obtained by applying the spectral-ratio method to a selected set of microseismic events with high signal-to-noise ratio. The Brune model is then fit to several observed S waveforms, yielding source parameters that depend strongly on the geophone sensitivity, instrument gain and Q. More work is needed to reduce uncertainty in these estimates, but these initial results show promise for spectral characterization of microseismic events. The attenuation considerations give potentially useful constraints for predicting magnitude detection distance relations and suggest that, even neglecting near surface attenuation, only low-frequency (~ 50 Hz) P waves could be observed at propagation distances of ~ 3 km required for surface monitoring.

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