Intrusion of eccentric dikes: The case of the 2001 eruption and its role in the dynamics of Mt. Etna volcano Alessandro Bonforte ⁎, Salvatore Gambino, Marco Neri Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma n. 2, 95123 — Catania, Italy abstract article info Article history: Received 19 November 2007 Received in revised form 3 September 2008 Accepted 10 September 2008 Available online 27 September 2008 Keywords: Stress release Dike Volcano-tectonics Flank instability Mt. Etna Instrumental monitoring The 2001 eruption represents one of the most studied events both from volcanological and geophysical point of view on Mt. Etna. This eruption was a crucial event in the recent dynamics of the volcano, marking the passage from a period (March 1993–June 2001) of moderate stability with slow, continuous flank sliding and contemporaneous summit eruptions, to a period (July 2001 to present) of dramatically increased flank deformations and flank eruptions. We show new GPS data and high precision relocation of seismicity in order to demonstrate the role of the 2001 intrusive phase in this change of the dynamic regime of the volcano. GPS data consist of two kinematic surveys carried out on 12 July, a few hours before the beginning of the seismic swarm, and on 17 July, just after the onset of eruptive activity. A picture of the spatial distribution of the sin-eruptive seismicity has been obtained using the HypoDD relocation algorithm based on the double-difference (DD) technique. Modeling of GPS measurements reveals a southward motion of the upper southern part of the volcano, driven by a NNW–SSE structure showing mainly left-lateral kinematics. Precise hypocenter location evidences an aseismic zone at about sea level, where the magma upraise was characterized by a much higher velocity and an abrupt westward shift, revealing the existence of a weakened or ductile zone. These results reveal how an intrusion of a dike can severely modify the shallow stress field, triggering significant flank failure. In 2001, the intrusion was driven by a weakened surface, which might correspond to a decollement plane of the portion of the volcano affected by flank instability, inducing an additional stress testified by GPS measurements and seismic data, which led to an acceleration of the sliding flanks. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Flank instability affects numerous volcanoes in the world, which can culminate in catastrophic failure and debris avalanches, or proceed as slow gravitational spreading of the instable flanks (Voight et al., 1981; Van Wyk de Vries et al., 2001). Mt. Etna volcano (Italy) was affected several thousands of years ago by a giant lateral collapse (Calvari et al., 1998), which caused a massive tsunami along the coasts of the eastern Mediterranean Sea (Pareschi et al., 2006). At present, this volcano is characterized by slow and continuous displacement of its eastern to southern flanks (Borgia et al., 1992), involving an on-shore area of N 700 km 2 (Neri et al., 2004), confined to the north by the Pernicana fault system (PFS, Acocella and Neri, 2005 and references therein; Bonforte et al., 2007a) and to the south–west by the Ragalna fault system (RFS, Rust et al., 2005; Neri et al., 2007 , and references therein), as shown in Fig. 1 . Recent works (Acocella and Neri, 2003; Acocella et al., 2003; Walter et al., 2005) have highlighted that there are feedback processes at Etna between flank deformation and eruptive activity. Flank instability produces extension in the upper part of the volcano (i.e. the summit crater zone) facilitating shallow intrusions (Neri and Acocella, 2006). In turn, flank instability is accelerated by magma intrusions in the upper feeding system (Bonforte and Puglisi, 2003; Neri et al., 2004; Puglisi and Bonforte, 2004; Neri et al., 2005). During the last seventeen years, Mt. Etna has produced remarkable eruptive and deformative events, giving us the opportunity to improve our understanding of how the volcano works (for a detailed descrip- tion of these events see Allard et al., 2006; Neri et al., 2008). The 2001 eruption represents a particular and rare event, since it was characterized by a double magmatic plumbing systems: central- lateral and eccentric (Behncke and Neri, 2003; Neri et al., 2005); this means that magma did not upraise only from the main conduit to intrude towards the volcano’s flank, but it upraised also directly from a deeper reservoir to the surface, forcefully opening a new path. Clearly, such an event induces an exceptional stress on a volcanic edifice, with consequent ground deformation and seismic energy release that affect the volcano for several years, heavily conditioning its future activity. Tectonophysics 471 (2009) 78–86 ⁎ Corresponding author. Tel.: +39 0957165809; fax: +39 095 435801. E-mail address: bonforte@ct.ingv.it (A. Bonforte). 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.09.028 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto