High Precision Locations of Seismic Events at SE Flank of Mount Etna: Characteristics of the Seismogenetic Structures

On 9 of January 2001 the INGV seismic network of Catania (Italy) recorded a seismic swarm of about 60 earthquakes with 1.0>Md>3.6, and nearly identical seismograms. The main shock caused damage to the town of Zafferana Etnea 2.5 km distant from the epicenter located on SE flank of the volcano. Some other events with similar waveform forecasted (up to 2 months before) and followed (for about one month) this swarm; moreover a check of Mt. Etna SE flank seismicity, starting from 1994, has permitted to identify a cluster of 6 multiplets recorded on August 1995 whose seismograms are strongly coherent with the January 9, 2001 ones. Thus we recognize for the first time the occurrence of seismic events on Mount Etna with nearly identical sources separated by years. We applied a cross-spectral technique that allows to obtain the relative relocation of events within a multiplet to at least an order of magnitude higher precision than is typically possible with traditional techniques. The results describe clearly the geometry of the seismogenetic structure; the events lie on planes oriented ENE-WSW that are coincident with one of the planes of the focal mechanism obtained by first-arrivals polarities. This alignment is coherent with one of the main regional tectonic trends that characterize the Mt. Etna area The multiplets analysis has allowed to recognize, on SE flank, a right-lateral strike seismic source along ENE-WSW fault plane, 4 km deep, time repeated and able to produce strong releases.