Ornella Cocina

Mt. Etna Volcano: A Seismological Framework

Mount Etna is one of the most active and powerful basaltic volcanoes in the world, with an historical record of documented eruptions going back over 2000 years. It is located in eastern Sicily in a complex geodynamic framework, where major regional structural lineaments play a key role in the dynamic processes of the volcano [e.g., Bonaccorso et al., 1996]. Several years of structural and geophysical observations have revealed how the majority of the eruptive fracture systems activated in the last 30 years correspond to those of historical eruptions [e.g., Azzaro and Neri, 1992]. The orientation of the fractures coincides mostly with two structural trends, NNW-SSE and NE-SW, observed both in the volcanic area and in the regional context. These alignments are hypothesized to be the main volcano-genetic structures [e.g., Bonaccorso et al., 1996; Gresta et al., 1998] controlling the evolution of Mt. Etna, as their interference establishes a weak zone along which magma can rise from depth [Rasa et al., 1995]. In the second half of the last century, after nearly 20 years without any major flank eruption, a series of effusive eruptions started in 1971. In the following 15 years, fourteen main sub-terminal and/or flank eruptions affected different eruptive systems [e.g., Azzaro and Neri, 1992]. Afterwards, about two years withouth eruptive activity separated the October 1986-March 1987 eruption from that of 1989, which was one of the most important in terms of effusion rate. This eruption was probably preceded by a major intrusive episode [Ferrucci et al., 1993; Rymer et al., 1993] which also fed the 1991-1993 flank eruption. The latter was the most important lateral eruption at Mt. Etna in the last three centuries, both in terms of duration (476 days) and volume of lava erupted (ca. 250 x 106 m3). After the end of this eruption, volcanic activity was confined to the summit area until the July-August 2001 flank eruption. Seismological observations have provided information on both the dynamics and structure of the volcano, in addition to their interaction with the regional tectonic structures. Today it appears clear that volcanism and tectonics in the Etnean area interact closely [e.g., Bonaccorso et al., 1996; Cocina et al., 1998; Bonaccorso, 2001; Bonaccorso and Patane, 2001; Patane and Privitera, 2001], although the problem of the driving mechanisms of magma upwelling remains an open question. Unfortunately, the local, permanent seismic network had a low density of stations prior to 1990 which severely affected hypocentral location constraints and our knowledge on magma dynamics in the shallower crust. Only from the early 1990\s has seismic data been available in digital format for a significant number of stations as well as for three-component sensors [e.g., Patane et al., 1999; Barberi et al., 2000; Patane and Privitera, 2001; Patane et al., 2003]. These improvements have allowed to put high-quality constraints on seismic activity occurring at almost all depths in recent years, and to perform studies which tackle the link between seismicity and eruptive activity. In this paper we present an overview of seismic activity affecting the volcano in the period 1978-2001. In particular, we focus our attention on the years between 1988 and 2001. We discuss the interaction between the regional and local stress field in this time span, and define seismic constraints on the magma source which yielded eruptive activity.

Seismogenic stress field beneath Mt. Etna (South Italy) and possible relationships with volcano-tectonic features

Abstract Shallow shear-type seismic activity occurring beneath the Etna volcano during 1990–1995 has been analysed for hypocenter locations, focal mechanisms and stress tensor inversion. The results have been examined jointly with Electronic Distance Measurements and tiltmeter data collected in the same period and reported in the literature. Significant seismicity located in the upper 10 km was found to be confined to the time intervals in which ground deformation data indicated inflation of the volcano edifice (e.g., the periods preceding the December 1991–March 1993 and August 1995–March 1996 eruptive phases). The shocks mostly occurred in a sector approximately centered on the crater area and elongated in the East–West direction. The causative seismogenic stress shows a low-dip East–West orientation of σ 1 . In agreement with existing knowledge on relationships between local fault systems and magma uprise processes, the shallow seismicity in question is tentatively explained as being due to lateral compression by magma inside a nearly North–South system. The volcano deflation phase revealed by Electronic Distance Measurements and tilt data during the 1991–1993 major eruption was not accompanied by any significant shear-type shallow event. Below the depth of 10 km, the North–South prevailing orientation of σ 1 reflects the dominant role of the regional stress.

Seismic Tomography Experiment at Italy's Stromboli Volcano

Stromboli Island, located in the southern Tyrrhenian Sea, is the emerged part (about 900 meters above sea level) of an approximately 3-kilometer-high stratovolcano. Its persistent Strombolian activity, documented for more than 2000 years, is sometimes interrupted by lava effusions or major explosions. Despite the number of recently published geophysical studies aimed at clarifying the volcanos eruption dynamics, the spatial extent and geometrical characteristics of its plumbing system remain poorly understood. In fact, knowledge of the inner structure and the zones of magma storage is limited to the upper few hundred meters of the volcanic edifice [Chouet et al., 2003; Mattia et al., 2004], and P and S wave velocity models are available only in restricted areas [Petrosino et al., 2002].