Structural and seismological features of the 1989 Syn‐Eruptive NNW‐SSE fracture system at Mt. Etna

During the September–October 1989 eruption at Mt. Etna, structural (faults, folds, fractures) and seismological data (locations, focal mechanisms, shear wave splitting) on the opening of a 6.5km long NNW‐SSE fracture system were collected. The fracture system reflected the surface deformation associated to the emplacement of a blade‐like dyke at shallow depth. Seismic activity occurred only at the southern tip of the fracture system and at about 1–1.5km depth, i.e. at the interface between the less stiff clay basement and the stiffer volcanic pile. Structural data indicate that fractures formed in response to an ENE‐WSW striking σ 3 =σ hmin . In contrast, the NNW‐SSE compressive structures, which developed only at the southern tip of the main fracture system, were related to an ENE‐WSW σ Hmax . The inversion of focal mechanisms shows a thrust‐type deformation related to an ENE‐WSW striking σ 1 =σ Hmax . The association of compressive structures is consistent with experimental models of dykes stopping at a less stiff‐stiffer interface. S‐wave splitting parameters have been measured during the fracture system propagation. No change was observed in the qS1 polarization eigendirections (NE‐SW), while the TD values seemed to vary with time. The results of seismological and structural analyses suggest that the dyke stopping mechanism produces: i) a seismic swarm associated to a local compressive stress field; and (ii) dyke‐induced stress generating shallow aligned secondary fractures (EDA cracks).