abstract

The seismic activity of the May 6, 1976 Friuli earthquake has been investigated. It provides clear evidence of internal clustering of shocks, with the largest aftershocks being followed by their own series of aftershocks. Late large aftershocks with their own aftershock series occurred 4 months after the main shock, when aftershocks had subsided. Thus, in the entire series of aftershocks, six phases of strain release are found, and part of the aftershock region is not included in the aftershock volume of the main shock. All this indicates that a few aftershocks are at least partially independent from the main shock.

The value of b is estimated for the entire sequence and for the separate phases; during the activity, b shows an increase after the main shock, a decline immediately before the largest aftershock, and a second increase immediately afterward. This can be explained in terms of stress changes, and is consistent with laboratory studies of rock deformation.

The compressive stress is perpendicular to the Eastern Alps, and may be considered as the principal cause of the earthquake sequence. The solution of the main shock of the sequence is a reversed fault movement, unlike most of the mechanisms in the focus of the earlier Friuli earthquakes which are of the transcurrent type.

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