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ISTITUTO NAZIONALE DI GEOFISICA, VIA DI VILLA RICOTTI, 42, 00161 ROME, Italy
Abstract
An aftershock sequence was recorded by an SMA-1 instrument installed at Gemona del Friuli on a massive limestone site during the period from May to October 1976. The sequence consisted of small to moderately large (1021 < M0 
1025 dyne · cm) dominant thrust earthquakes that occurred within 15 km of the station. The instrument was triggered by P waves for 24 events; the resulting records are particularly suitable to the investigation of source properties for the Friuli earthquakes. Acceleration, velocity, and displacement waveforms have been analyzed in order to investigate source properties. Durations (T) and areas (
0) of the shear-wave pulses have been measured from the displacement time histories and compared with the values of corner frequency fc and seismic moment M0 estimated in the frequency domain from the acceleration spectra of the same events. The comparison between these two set of data reveals that attenuation correction is important also for correct estimations of source parameters from ground motions at close focal distance and moderate magnitudes. After correcting for attenuation, stress drops are approximately constant, 
= 300 bars, on the average in the investigated seismic moment range.
For this sequence, the behavior of peak ground motions versus earthquakes size is consistent with the trend obtained from stochastic simulations assuming an omega-squared spectral model with a constant stress drop of 300 bars. The good results obtained by applying to Friuli earthquakes an omega-squared model with a constant stress drop of 300 bars are further confirmed by the analysis of spectral ratios for different size earthquakes of the sequence. This analysis demonstrates that the finding of this spectral-scaling model is not biased, to first order, by whole-path or near-site propagation properties whose effect can be significant when data from a single station are investigated.
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