Aftershock sequence analysis using a static fatigue approach

Four aftershock sequences that occurred in the area between the Friuli-Venezia Giulia region and Slovenia, from 1988 to 1998, are analyzed. The spatial patterns of the aftershocks are related to the main shock focal mechanisms. The computed focal mechanisms of major aftershocks evidence a complex slip faulting, in most cases with fault plane solution of similar type to that of the main shock, but with different plane orientation. The sequences are modelled with a log-linear relation between the released cumulative seismic moment and the time elapsed from the mainshock. The results support the hypothesis that the aftershocks energy decay is caused by a static fatigue process. Omori modelling of the sequences shows that the aftershock decay doesn’ t follow a smooth relaxation process. The occurrence rate of the aftershocks is characterized by sudden increases that are probably related to local stress concentration at some asperities in proximity of the main shock fault. The value of the exponent p is related to the heat-flow of the area.