Damage mechanics preceding the September–October 1989 flank eruption at Mount Etna volcano inferred by seismic scaling exponents

Seismicity preceding the September–October 1989 flank eruption at Mt. Etna volcano has been analysed by means of the fractal two-point correlation dimension Dc relative to epicentres distribution and the scaling exponent D inferred from the seismic b-value. Time evolution of Dc and D preceding the eruption onset revealed: a general decrease of Dc and increase of D (9–4 months prior the eruption onset), intermittent rates of Dc increase and D decrease (4 to 1 month), steady high values of Dc and low values of D (1 month) and sharp decreases of Dc and D marking the eruption onset. Results allowed to constrain the increment of seismic damage by recognising negative (mechanical hardening) and positive (mechanical softening) feedback prior the eruption onset.