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The continuous volcanic and seismic activity at Mount Etna makes this volcano an important laboratory for seismological and geophysical studies. We used repeated three-dimensional tomography to detect variations in elastic parameters during different volcanic cycles, before and during the October 2002-January 2003 flank eruption. Well-defined anomalous low P- to S-wave velocity ratio volumes were revealed. Absent during the pre-eruptive period, the anomalies trace the intrusion of volatile-rich (>/=4 weight percent) basaltic magma, most of which rose up only a few months before the onset of eruption. The observed time changes of velocity anomalies suggest that four-dimensional tomography provides a basis for more efficient volcano monitoring and short- and midterm eruption forecasting of explosive activity.

On July, 12, 2001, Mt. Etna experienced a sudden increase of seismic activity heralding one of the most intense eruptions of the past 30 years. Between July 12 and July 18, when the eruption started, thousands of small magnitude earthquakes occurred and were recorded by a dense seismic network run by the Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania (INGV‐CT). Hypocentral depths of earthquakes were very shallow, mostly located above 3 km b.s.l. and clustered near the summit area. The high quality seismic dataset gives us the unique opportunity to study the process of magma migration before the eruption. In this study we present the three‐dimensional earthquake locations and the velocity structure obtained by a tomographic inversion. The shallowness of seismicity allowed us to enhance the details of the structure beneath the summit craters, in a volume poorly defined by previous tomographic studies. The presence of a high Vp‐body previously observed at Mt. Etna is confirmed at shallow depth beneath the southeastern part of the summit area. The earthquakes preceding the eruption onset concentrated at its western border. A low Vp/Vs anomaly is found at 0–1 km depth, just at the top of the volume where the magma intruded before the eruption. This anomalous zone can be considered as molten material wealthy in gas. The relocated seismicity occurs in a cylinder below the vents activated along the fracture system and exhibited an upward migration until the eruption. All these results show evidence for the emplacement of a near‐vertical dyke striking about N‐S and a few kilometres south of the summit craters.

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