The September--October 1989 flank eruption at Mt. Etna marked a change in the recent eruptive style of the volcano. Seismicity, at mid (order of months) and short (order of days) time, preceding the eruption onset has been analyzed by means of D (scaling exponent), inferred from the seismic b value, that represents the scaling exponent of fracture length distribution. Time evolution of D preceding the eruption onset revealed: a general increase of D (9 to 4 months prior the eruption onset), intermittent rates of D decreases and increases (4 to 1 month), steady values of D (1 month) and sharp decreases of D marking the eruption onset. Integration of these results with ground deformation data reveals that at Mt. Etna stress corrosion crack growth, enhanced by fresh magma arrival, is a plausible mechanism for weakening the country rock, by promoting crack growth and the coalescence of major faults. ¿ 1999 American Geophysical Union