Shear response to an intrusive episode at Mt. Etna volcano (January 1998) inferred through seismic and tilt data

Abstract

The space–time evolution of a seismic swarm (MDmax=3.7) occurring on 9–14 January 1998 at Mt. Etna volcano is analysed. The hypocentral pattern and most reliable focal mechanisms indicate that seismic activity during 10–14 January mainly occurred within a fault-zone located in the volcano's western flank. The fault showed an approximately NE–SW overall trend, and shallow rupture mode of right-lateral strike-slip type. It is noteworthy that earthquakes occurring during the initial phase (9 January) of the swarm showed prevalent dip–slip rupture mechanisms, which are consistent with tensile opening of an approximately NNW–SSE-trending fault-zone. This NNW–SSE structural lineament belongs to a regional lithospheric structure, which has also been identified as a main volcano-genic structural trend of Mt. Etna. The observed continuous tilt data are not explainable by the seismic slip associated with strain released along the NE–SW structure. However, an intrusive source would be compatible with the rupture mechanisms observed during the initial phase of the swarm and could explain the observed tilt. Therefore, we evidence that an intrusive episode occurred along the NNW–SSE structure starting 9 January and during the following days it forced the NE–SW fault system to work in a right-lateral mode. These mechanisms, leading to the February–November 1999 summit eruption, are analogous to the ones observed before the 1991–93 eruption.