Lombardo, G., G. Coco, M. Corrao, and S. Gresta. “Features of Seismic Events and Volcanic Tremor During the Preliminary Stages of the 1991-1993 Eruption of Mt. Etna”. Annals of Geophysics, vol. 39, no. 2, Nov. 1996, doi:10.4401/ag-3974.