A physical model of the long-period precursor to the 1976 Friuli earthquake

We suggest that the regular, long-period perturbations recorded by the Marussi pendulums at the Trieste Earth Tide Station over a period of three years preceding the 1976 Friuli earthquake, were due to silent earthquakes which occurred below the focal zone of the impending 1976 earthquake. The source of silent earthquakes is modelled in terms of slow slippage episodes on a buried fault with viscoelastic properties. The computed radiation emitted by such a source reproduces at least the order of magnitude of the dominant frequency, the mean duration and amplitude of the events observed at Trieste. Owing to the very long period and the low amplitudes involved, the wave emission is found to be detectable in the near field only, with appropriate instrumentation.