Fault-plane solutions and hypocentral distribution of some 1977 Friuli aftershocks

Abstract

Summary. The polarity of first-arrivals of P-waves recorded by the vertical-component seismometers of the Friuli telemetric network of the Osservatorio Geofisico Sperimentale, Trieste have been used to fmd a number of cumulative fault-plane solutions of aftershocks of the large 1976 May Friuli earthquake. In spite of its poor quality, the cumulative solution for the whole period 1977 May—September is similar to the greatest majority of partial cumulative solutions which exhibit much better quality. The polarities of opposite sign, characterizing the first-arrivals of seismic rays which have travelled along subparallel paths and which are responsible for reducing the quality of the solutions, seem to be due mainly to variations in the direction of slip on faults with constant orientation. The inversion of P- and T-axes revealed by the solutions, however, occurred in certain periods at particular hypocentral depths or in specific epicentral zones and has been responsible for the reversal of some polarities. The almost constant orientation of the principal stresses, revealed by most fault-plane solutions, suggests that they represent the orientation of the regional tectonic stresses related to this phase of the earthquake. Although this orientation is similar to that of the 1976 May shock, an inversion of the P- and T-axes suggests that normal faulting has taken over from reverse faulting observed for the main shock. The reversal of the P- and T-axes may be due to the partial release of the stresses related to the earthquake or to the shift of location of the hypocentres, possibly associated with a different limb of a major thrust-fold.