The 2002 Mount Sernio and the 2004 Kobarid sequences: static stress changes and seismic moment release

We analyze the aftershock sequences caused by the 2002 MD 4.9 and the 2004 MD 5.1 mainshocks, occurred in the Friuli area and western Slovenia, with two different approaches. The Coulomb stress changes induced by the mainshocks were calculated to explore the interaction with the aftershock patterns. The aftershock decay is investigated analyzing the temporal behaviour of cumulative seismic moment released, derived from a static fatigue model. The Coulomb stress changes, calculated on the receiver fault of the largest aftershock, show that the aftershock sequences are mostly located in a stress shadow zone. The modelling of Coulomb stress variations, that incorporates the regional stress field, fits better the aftershock pattern. The lack of correlation between the Coulomb stress perturbations and part of the aftershocks is attributed to stress heterogeneities not accounted in the model. The decay rate of the seimic moment release is different in the 2002 and 2004 sequences. According to this model, the distribution of stress in the aftershock volume appears more uniform in the 2004 sequence. Stress fluctuations induced by the mechanical heterogeneities of the medium is suggested by the static fatigue modelling, particularly for the 2002 sequence.