Ground deformation and seismicity related to the propagation and drainage of the dyke feeding system during the 2007 effusive eruption at Stromboli volcano (Italy) Emanuele Marchetti, Riccardo Genco, Maurizio Ripepe ⁎ Dipartimento di Scienze della Terra, Università di Firenze, via G. La Pira, 4, 50121, Firenze, Italy abstract article info Article history: Received 14 May 2008 Accepted 7 November 2008 Available online 24 November 2008 Keywords: tiltmeters ground deformation seismicity slope instability feeding dyke Stromboli The 2007 effusive eruption of Stromboli volcano produced a ground deformation detected by two high- resolution borehole tiltmeters installed at an elevation N 500 masl and at a distance of b 800 m from the active vents. Ground inflation was recorded about 12 h before the eruption onset as a consequence of magma ascent within the shallow feeding conduit. Tilt vectors converging towards the summit area indicate a confined or vertically elongated source. The increased magma input rate led to a well alimented lava flow, which was detected as a major tilt deflation during the 6 days following the eruption onset. Unlike during the inflation, the ground deformation that accompanied the lava effusion, is not consistent with a confined source close to the summit area, but rather suggests a process of deflation of the volcanic structure along a dyke-shaped structure striking 40°N. The tilt data presented in this paper represent a great opportunity to investigate the mechanism of magma effusion and the response of the feeding system to the increased magma flux. High value of seismic tremor amplitude and surface fracturing is the evidence of the increased magma input rate, which led to the SW-NE propagation of the dike, acting as a linear source of deflation when the system was rapidly drained by the sustained lava emission. A secondary inflation process, acting during the tilt decay, was clearly recorded in the afternoon on February 27 2007, representing the evidence of the magma intrusion, that led to the opening of an effusive vent at 400 m elevation. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Stromboli volcano is an open-conduit basaltic system, character- ized by mild explosive activity from three summit craters with emissions of gas and scoria every 5–15 min, typically reaching heights of 50–200 m above the craters. The explosive activity is consistently located within a 150×300 m crater terrace elongated along the main NE-SW structural trend, affecting the entire structure of the volcanic edifice, as marked by the Vallonazzo tectonic lineament, and by the position of the ancient volcanic center of Strombolicchio (e.g. Rosi, 1980). Since 2002, activity at Stromboli has been characterized by two effusive eruptions, in December 2002–July 2003 (e.g. Bonaccorso et al., 2003) and in February–April 2007. In both cases at the eruption onset the summit crater terrace was deeply fractured along the main NE-SW tectonic lineament, leading to lava effusion initially located at the base of the NE crater before it moved towards lateral vents at lower elevation (Calvari et al., 2005; Ripepe et al., 2005, 2009-this issue). An important new feature of the 2007 eruption is the evidence of clear ground deformation (Bonaccorso et al., 2008) associated with the main phases of the eruption, and providing important information on the eruptive source mechanism. Despite its sustained and permanent explosive activity, a clear ground deformation has been recorded only few times at Stromboli. Examples include deformation associated with deep dyke emplacement in 1994–1995 (Bonaccorso, 1998) and the April 5, 2003, paroxysm (Mattia et al., 2004). In this paper we present ground deformation of the 2007 effusive eruption by high-resolution and fast-sampling tilt sensors deployed at short distance (b 800 m) from the craters. Tilt measurements com- bined with seismic data allow us to infer the mechanism of magma migration and the evolution of the feeding system in response to the increased magma input rate during the first hours of the eruption. We suggest a process of dyke emplacement, as possible mechanism to explain the migration of the effusive vents. 2. The tiltmeters network Tiltmeters provide valuable information on ground deformation and are employed worldwide in different research fields for scientific and monitoring purposes. The importance of ground deformation monitoring on active volcanoes in modeling and tracking magma transport (e.g. Cervelli and Miklius, 2003), dyke emplacement (e.g. Battaglia and Bachèlery, 2003) and shallower processes related to the plumbing system (e.g. Voight et al., 1998; Iguchi et al., 2008), has been proved during the past decades. However, for a long time the low Journal of Volcanology and Geothermal Research 182 (2009) 155–161 ⁎ Corresponding author. Dipartimento di Scienze della Terra, Università di Firenze, via G. La Pira, 4, 50121, Firenze, Italy. E-mail address: maurizio.ripepe@geo.unifi.it (M. Ripepe). 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.11.016 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores