Vol51,1,2008_DelNegro 16-02-2009 21:27 Pagina 25 ANNALS OF GEOPHYSICS, VOL. 51, N. 1, February 2008 Looking inside Panarea Island (Aeolian Archipelago, Italy) by gravity and magnetic data Luca Cocchi (1), Fabio Caratori Tontini (1), Cosmo Carmisciano (1), Paolo Stefanelli (1), Marco Anzidei (2), Alessandra Esposito (2), Ciro Del Negro (3), Filippo Greco (3) and Rosalba Napoli (3) (1) Istituto Nazionale di Geofisica e Vulcanologia, Fezzano (SP), Italy (2) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy (3) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Italy Abstract This paper shows and discusses the results of gravity and magnetic surveys of Panarea Island and its archipela- go. The most recent volcanic manifestation occurred in November 2002 with a shallow submarine gas eruption between the islets of Dattilo, Panarelli, Lisca Bianca, Bottaro and Lisca Nera. Currently, the activity of Panarea is monitored through a multidisciplinary study under the umbrella of the Italian Department of Civil Protection with the goal of defining the hazard of this area. With this aim, in May 2006 the first gravity and magnetic sur- veys of Panarea Island and its archipelago were performed. The offshore magnetic data were obtained using a marine magnetometer, a Geometrics G880, from the Istituto Idrografico dell Marina (IIM). Onshore and offshore magnetic data were integrated into an unique dataset for complete magnetic coverage of the study area. By us- ing two micro-gravimeters (LaCoste & Romberg), gravity data were collected along tracks every 250 m. The gravity dataset was processed using the standard method. A Bouguer reduction was applied to the free-air grav- ity dataset using a detailed digital elevation model of the island and the neighbouring sea after evaluation of the optimal Bouguer density to reduce the topographic effect. The result is a Bouguer anomaly map that shows lat- eral variations in density distribution and the relationships between the shallow volcanic/crustal features and tec- tonic lineaments. This evidence is also highlighted by the magnetic pattern, which suggests the importance of the youngest volcanic deposits with respect to the magnetic features of the island. Key words magnetism – gravity – structural setting seamounts located in the Southern Tyrrhenian – Panarea Island – hydrothermal systems Sea. This volcanic island arc is currently char- acterized by active volcanism that recently af- fected Stromboli and Panarea Islands with lava 1. Introduction eruptions and gas exhalation, respectively. The geodynamic evolution of the Aeolian arc is Panarea Island is part of the Aeolian Arc, mainly connected to a NW-striking subduction which consists of seven islands and several system in which a portion of the Ionian plate is deep under the Calabrian Arc (Barberi et al., 1973; Beccaluva et al., 1985). The late phase of the Aeolian volcanism is related to the rifting- type process more than the subduction process Mailing address: Dr. Luca Cocchi, Istituto Nazionale di (De Astis et al., 2003). This area is affected by Geofisica e Vulcanologia, U.P. Geofisica e Tecnologie Mari- ne, Via Pezzino Basso 2, 19025 Fezzano (SP), Italy; e-mail: shallow, intermediate and deep seismicity [email protected] (Chiarabba et al., 2005) and active crustal de- 25 Vol51,1,2008_DelNegro 16-02-2009 21:27 Pagina 26 L. Cocchi, F. Caratori Tontini, C. Carmisciano, P. Stefanelli, M. Anzidei, A. Esposito, C. Del Negro, F. Greco and R. Napoli Fig. 1a,b. a) Aeolian Archipelago; b) Morphological and tectonic features of Panarea Island and its archipela- go. The geographic positions of the exhalative centres are from Anzidei et al. (2005). formation (Bonaccorso, 2002; Hollestein et al., effusive and explosive eruptions during its for- 2003; D’Agostino and Selvaggi, 2004; Pondrel- mation. In particular, the activity of Panarea Is- li et al., 2004; Serpelloni et al., 2005). land and its archipelago is characterized by off- The arc is divided into two main parts by a shore and onshore exhalative activities. NNW-SSE-striking fault called the «Tindari- Panarea Island, located between Stromboli Letoianni» fault system (Barberi et al., 1994; and Lipari Islands, is the smallest of the Aeo- Mazzuoli et al., 1995; De Astis et al., 2003: Bil- lian archipelago. The emerging part of the li et al., 2006), where Salina, Lipari and Vulcano Panarea volcano covers about 3.5 km2, with a Islands are located (fig. 1a). The western sector NE-SW orientation (fig. 1b). It represents the of the Aeolian arc is formed by Glauco sub-aerial portion of a submarine stratovolcano seamount, Alicudi and Filicudi Islands, where with a maximum depth of ~2000 m and a length volcanism developed between 1.3 to 0.05 Ma of ~20 km (Gabbianelli et al., 1993; Gamberi et (Tranne et al., 2002a,b). In the eastern sector al., 1997; Favalli et al., 2005). The volcanism that extends from Panarea and Stromboli islands started about 150 ka (Gabbianelli et al., 1990; to Alcione and Palinuro seamounts, the volcan- Calanchi et al., 1999; Lucchi et al., 2003) with ism is still active. At Stromboli the activity start- the deposition of lava domes and lava flows, ed about 200 ka (Gillot and Keller, 1993) with mostly made of andesite to rhyolite, interbed- 26 Vol51,1,2008_DelNegro 16-02-2009 21:27 Pagina 27 Looking inside Panarea Island (Aeolian Archipelago, Italy) by gravity and magnetic data ded with pyroclastic deposits (Calanchi et al., emerged portion of the island. The gravity data 1999, 2002). Between 130±9 and 54±8 ka were sampled using a pair of LaCoste & (Gabianelli et al., 1990), the volcanic activity Romberg micro-gravimeters (Aliod model) formed the islets of Lisca Bianca, Lisca Nera, equipped with a digital data acquisition system, Dattilo, Panarelli and Basiluzzo. Pyroclastic de- GPS tracking and automatic tide corrections, posits of external provenance cover the vol- with a nominal resolution of 1 µgal. The offset canic products of Panarea and its islets. This between the two instruments was analyzed at unit is dated between 70 and 8 ka (Lucchi et al., the beginning of the survey by recording the 2007), clipping the older deposits and repre- gravity data at the same set of stations. This senting the last volcanic/eruptive event in the procedure was designed to reduce the misfit area. At present, volcanic activity produces values between the two instruments and to ob- submerged gas exhalation among the islets and tain a uniform gravity dataset. A reference sta- several fumaroles located in the Calcara zone, tion was fixed with known absolute gravity and north Panarea Island (see fig. 1b). placed at the minimum elevation to check the The last exhalative crisis of November 2002 state of the apparatus and the quality of the occurred between Lisca Bianca, Bottaro and measurements, and to estimate the instrumental Dattilo islets in a shallow bathymetric area (30 drift. Since the gravity survey was performed m depth), affecting ~2.3 km2 with about 109 following accessible paths, the measurement l/day of erupted gas (Caracausi et al., 2005; stations were not distributed according to a reg- Caliro et al., 2004; Anzidei et al., 2005). ular grid, but were scattered. Along the acquisi- Panarea is currently the object of multipara- tion tracks, the gravity field was recorded using metric studies concerning geophysical and geo- a spatial interval of 250 m. The entire cone of chemical observations. We present the results Panarea Island was sampled through 92 stations of gravity and magnetic surveys. (fig. 2). The geographic positions of the gravity The magnetic anomaly field is connected to stations were defined using the integrated posi- lateral and depth variations in the magnetic contri- tion system of the gravimeter and a second butions of magnetized rocks in the shallow crust. portable GPS receiver. The gravity field was The magnetic anomaly field of Panarea Island and recorded with a couple of measurement ses- its archipelago were analyzed to describe the sions with a time-interval of 5 min; for each sta- highly magnetized zones and to determine the re- tion, station-ID, date, time, topography altitude lationship between the distribution of exhalative (by GPS and altimeter) and GPS-position were centres and magnetization trends. The offshore collected. In order to check for instrumental and onshore data were analyzed separately and in- drift, a set of stations was re-acquired daily. The tegrated in a common dataset. The gravity analy- full gravity dataset for Panarea was composed sis was limited to Panarea Island with a set of of two subsets connected to the two different in- measurements covering the volcanic apparatus. A struments. This distinction was useful in the complete Bouguer reduction was computed fol- subsequent data processing, in particular for re- lowing discretization of the bathymetry using a ducing instrumental drift. Each gravimeter is set of prismatic elements after a cross-plot analy- characterized by a different drift effect, which sis of the optimal Bouguer density. consists of a shift from the zero level during the The goal was to provide an interpretation of recording time, and which is strictly connected the Panarea volcanic system based on a new po- to the mechanism and the electronics of the sen- tential field dataset merged to the tectonic and sor. structural features. For each gravimeter, the related dataset was divided by the time of acquisition. The LaCoste & Romberg Aliod model sensor records tidal 2. Gravity survey and processing variation and automatically provides tide-cor- rected gravity data. The recorded gravity data Microgravity survey of Panarea Island was were corrected using the daily drift value. This performed in May 2006 covering the entire value was added to or subtracted from the data 27 Vol51,1,2008_DelNegro 16-02-2009 21:27 Pagina 28 L. Cocchi, F. Caratori Tontini, C.