Determination of S-wave energy release of earthquakes in the region of Friuli, Italy

The S-wave energy released by the 1976 Friuli earthquake (M(L) = 6.6) and nine aftershocks with local magnitudes ranging between 4.7 and 6.4 was estimated using strong-motion records near the source and at regional distances. To determine the seismic energy radiated from the source, we corrected the spectral records for attenuation along the path using the Q-frequency relation Q = 20.4f(1.01), and for the effect of nearsurface attenuation using specific values of kappa(0), reported by Castro et al. (1996) for the recording sites of the region. The remaining path effects were separated from the source functions using a non-parametric inversion. The displacement source functions show an intermediate omega(-1) fall-off before a high-frequency fall-off of omega(-2), which provides more energy compared with a single omega(-2) decay. When we-compare the seismic energy (E(s)) evaluated by integrating the square of the velocity source spectra with the energy estimated from the local magnitudes via Gutenberg & Richter's (1956) relation, we find that E(s) radiated from the source is, on average, a factor of 6 higher than that obtained from M(L). In addition, comparing the seismic moments (M(0)) reported in previous studies (Cipar 1981; Cocco & Rovelli 1989) with E(s), we observed that E(s) increases proportionally to M(0)(1.24) This rate of increase of the seismic energy with moment indicates that the Orowan stress drop also increases with M(0). The apparent stress calculated with the ratio E(s)/M(0) is also greater than half the Brune stress drop (sigma(a) > Delta sigma/2), suggesting that, for the thrust faulting earthquakes of Friuli, the final stress was greater than the frictional stress during the 1976 sequence.