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Abruzzi region (central Italy) producing vast damage in the L'Aquila town and surroundings. In this paper we present the location and geometry of the fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks. The distribution of aftershocks, 712 selected events with M L ! 2.3 and 20 with M L ! 4.0, defines a complex, 40 km long, NW trending extensional structure. The main shock fault segment extends for 15-18 km and dips at 45°to the SW, between 10 and 2 km depth. The extent of aftershocks coincides with the surface trace of the Paganica fault, a poorly known normal fault that, after the event, has been quoted to accommodate the extension of the area. We observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes.

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[1] On 6 April (01:32 UTC) 2009 a M W 6.1 normal faulting earthquake struck the axial area of the Abruzzo region in central Italy. We study the geometry of fault segments using high resolution foreshock and aftershock locations. Two main SW dipping segments, the L'Aquila and Campotosto faults, forming an en echelon system 40 km long (NW trending). The 16 km long L'Aquila fault shows a planar geometry with constant dip (∼48°) through the entire upper crust down to 10 km depth. The Campotosto fault activated by three events with 5.0 ≤ M W ≤ 5.2 shows a striking listric geometry, composed by planar segments with different dips along depth rather than a smoothly curving single fault surface. The investigation of the spatiotemporal evolution of foreshock activity within the crustal volume where the subsequent L'Aquila main shock nucleated allows us to image the progressive activation of the main fault plane. From the beginning of 2009 the foreshocks activated the deepest portion of the fault until a week before the main shock, when the largest foreshock (M W 4.0) triggered a minor antithetic segment. Seismicity jumped back to the main plane a few hours before the main shock. Secondary synthetic and antithetic fault segments are present both on the hanging and footwall of the system. The stress tensor obtained by inverting focal mechanisms of the largest events reveals a NE trending extension and the majority of the aftershocks are kinematically consistent. Deviations from the dominant extensional strain pattern are observed for those earthquakes activating minor structures.

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.

After a period of deflation during the 1991-1993 flank eruption, Mount Etna underwent a rapid inflation. Seismicity and ground deformation show that since 1994, a huge volume of magma intruded beneath the volcano, producing from 1998 onward a series of eruptions at the summit and on the flank of the volcano. The last of these, started on 27 October 2002, is still in progress and can be considered one of the most explosive eruptions of the volcano in recent times. Here we show how geodetic data and seismic deformation, between 1994 and 2001, indicate a radial compression around an axial intrusion, consistent with a repressurization of Mount Etna's plumbing system at a depth of 6 to 15 kilometers, which triggered most of the seismicity and provoked the dilatation of the volcano and the recent explosive eruptive activity.

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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.