Journal of Volcanology and Geothermal Research 251 (2013) 27–40

Contents lists available at SciVerse ScienceDirect

Journal of Volcanology and Geothermal Research

j ourna l homepage: www.e lsev ie r .com/ locate / jvo lgeores

Soil gases and SAR measurements reveal hidden faults on the sliding flankof Mt. Etna (Italy)

Alessandro Bonforte a,⁎, Cinzia Federico b, Salvatore Giammanco a, Francesco Guglielmino a,Marco Liuzzo b, Marco Neri a

a Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Osservatorio Etneo, Italyb Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy

⁎ Corresponding author. Tel.: +39 095 7165809; fax:E-mail address: bonforte@ct.ingv.it (A. Bonforte).

0377-0273/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.jvolgeores.2012.08.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 November 2011Accepted 19 August 2012Available online 30 August 2012

Keywords:CO2

RadonInSARFaultsEtnaVolcano-tectonics

From October 2008 to November 2009, soil CO2, radon and structural field surveys were performed onMt. Etna,in order to acquire insights into active tectonic structures in a densely populated sector of the south-easternflankof the volcano, which is involved in the flank dynamics, as highlighted by satellite data (InSAR). The studied areaextends about 150 km2, in a sector of the volcanowhere InSAR results detected several lineaments that were notwell-defined from previous geological surveys. In order to validate and better constrain these features withground data evidences, soil CO2 and soil radon measurements were performed along transects roughly orthog-onal to the newly detected faults, with measurement points spaced about 100 m. In each transect, the highestCO2 values were found very close to the lineaments evidenced by InSAR observations. Anomalous soil CO2 andradon values were also measured at old eruptive fractures. In some portions of the investigated area soil gasanomalies were rather broad over transects, probably suggesting a complex structural framework consisting ofseveral parallel volcano-tectonic structures, instead of a single one. Soil gas measurements proved particularlyuseful in areas at higher altitude on Mt. Etna (i.e. above 900 m asl), where InSAR results are not very informative/are fairly limited, and allowed recognizing the prolongation of some tectonic lineaments towards the summit ofthe volcano. At a lower altitude on the volcanic edifice, soil gas anomalies define the active structures indicatedby InSAR results prominently, down to almost the coastline and through the northern periphery of the city ofCatania. Coupling InSARwith soil gas prospectingmethodshas thus proved to be a powerful tool in detecting hiddenactive structures that do not show significant field evidences.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Mt. Etna volcano is located in eastern Sicily (Fig. 1), and it is thehighest active volcano in Europe (3329 m above sea level in summer2007; Neri et al., 2008). The volcano is ~500 ka old and rises at thefront of the Apennine–Maghrebian Chain (AMC in Fig. 1b), borderingthe northern portion of Malta Escarpment (ME in Fig. 1b) and lying onclayish–sandy Pliocene–Pleistocene Catania–Gela foredeep deposits(CGF in Fig. 1b) (Lentini, 1982; Lanzafame et al., 1997, and referencestherein). Its near-continuous activity is characterized by eruptions atfour summit craters and by flank eruptions that are mainly groupedalong the NE, S and W rift zones (Fig. 1a) (Acocella and Neri, 2003).

The eastern to southern flanks of Mt. Etna are involved in seawarddisplacement (Neri et al., 2009; Ruch et al., 2010; Solaro et al., 2010,and references therein), affecting an on-shore area of >700 km2

(Neri et al., 2004). The margins of this unstable area comprise thePernicana Fault System (PFS; Fig. 1a; Acocella and Neri, 2005) to thenorth and the Ragalna Fault System (RFS; Fig. 1a; Neri et al., 2007)

+39 095 7165826.

rights reserved.

to the south-west. The possible relationship between flank deforma-tion and eruptive activity has recently been highlighted by several au-thors (Acocella et al., 2003; Burton et al., 2005; Walter et al., 2005;Bonaccorso et al., 2006; Neri and Acocella, 2006; Bonforte et al.,2009; Neri et al., 2011a).

Analysis of long-periods of InSAR (Froger et al., 2001; Neri et al.,2009; Solaro et al., 2010) and PSInSAR results (Bonforte et al., 2011)has added some details to the sliding motion, especially on thelower south-eastern side of the volcano. Field geological and instru-mental data revealed the slip activity of the Trecastagni (TF),Tremestieri-Mascalucia (TMF) and Aci Trezza (ATF) faults (Fig. 1;Acocella et al., 2003; Neri et al., 2009; Gambino et al., 2011;Bonforte et al., 2013–this issue), but some of the features detectedby InSAR are not visible in the field and were never detected beforeby classical geological surveys (Fig. 2). For this reason, a ground vali-dation was necessary to verify the existence of such discontinuitiesand to better constrain their position and geometry at the surface.

In volcanic settings, CO2 is normally the most abundant volatilespecies and it drives natural soil degassing in areas other than fuma-rolic fields, where, conversely, water vapor prevails. There is an abun-dance of literature on soil degassing studies in active volcanic and

Fig. 1. Structural map of Mount Etna (modified from Solaro et al., 2010, and references therein) (a), showing the distribution of Etnean volcanics, eruptive fissures and pyroclasticcones grouped in rift zones (NE, S and W), main faults and the sector of the volcano that is affected by flank instability (highlighted in light gray). CC, central craters; PFS, Pernicanafault system; RFS, Ragalna fault system; RNF, Ripe della Naca faults; TFS, Timpe fault system; ARF, Acireale fault; ACF, Aci Catena fault; FIF, Fiandaca fault; ATF, Aci Trezza fault; TF,Trecastagni fault; TMF, Tremestieri-Mascalucia fault; and BOL, Belpasso–Ognina lineament. Features investigated in this work are colored in purple. Large black arrows indicatedirections of movement in different portions of the mobile sectors. Regional tectonic context is shown in inset (b); AMC, Appennine–Maghrebian chain; CGF, Gela–Cataniaforedeep; HF, Hyblean foreland; ME, Malta Escarpment; CoF, compressional front; and EF, extensional front. The pre-Quaternary sedimentary rocks include the AMC and CGFdeposits.

28 A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

geothermal areas (Allard et al., 1991; Farrar et al., 1995; Chiodini etal., 1998; Giammanco et al., 1998; Sorey et al., 1998; Williams-Joneset al., 2000; Baubron et al., 2002; Cartagena et al., 2004; Lewickiet al., 2005; Neri et al., 2006; Notsu et al., 2006; Alparone et al.,2005; Chiodini et al., 2008), mostly devoted to identifying hiddenfault systems, including on Etna volcano (D'Alessandro et al., 1992;Giammanco et al., 1998, 1999; De Gregorio et al., 2002).

Recent studies have demonstrated that soil radon emissions arealso useful to detect hidden faults at Etna (Burton et al., 2004;Giammanco et al., 2009; Siniscalchi et al., 2010; Neri et al., 2011b).Radon is a noble gas and the only naturally occurring radioactivegas. It has three isotopes: 222Rn (radon), 220Rn (thoron), and 219Rn(actinon). Radon is a short-lived decay product from the 238U decayseries, with a half-life of 3.8 days. Thoron is a decay product fromthe 232Th decay series and has a relatively short half-life (55 s),which makes it useful in discriminating areas with very fast soil gastransport and/or Th-rich mineral outcrops. Actinon is part of the

decay series of 235U and its half-life is so short (4 s) that it is ignoredin geochemical exploration. Elevated 222Rn emissions are closely cor-related with high CO2 emissions (e.g., Giammanco et al., 2007, 2009),thus 222Rn provides an additional means to map active, high porosityregions.

This work is aimed at defining a detailed structural outline of thesouthern boundary of the unstable flank of Mt. Etna. The studiedarea is highlighted in Fig. 1. A large portion of this area is involvedin the flank dynamics, as shown by satellite results (SAR), whereasonly few segments of the Trecastagni fault crop out (Branca et al.,2011). SAR data revealed/a wider extension of some faults with re-spect to their field evidence and highlighted some new discontinu-ities in the ground deformation field, but these features have to bevalidated by field data. The target is obtained applying field geochem-ical (CO2 and Rn) investigations, coupled with classical geo-structuralfield survey and comparing the results with the discontinuitiesrevealed by satellite data. The hypothesis is that if any physical

Fig. 2. PS InSAR LOS ground velocity obtained from ascending radar images and GPS station velocities for the 1995–2000 period, with linear features (PSF) as mapped in Bonforte etal. (2011). Blue diamonds indicate the soil CO2 measurement sites, red stars indicate the soil Rn measurement sites. Dashed lines indicate the profiles shown in Fig. 6.

29A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

discontinuity exists in the ground, it could be used by soil gas as apreferential way to reach the surface.

2. Methods and techniques

2.1. PSInSAR

The PS technique represents a substantial improvement to classicdifferential interferometry approaches. This approach (Ferretti et al.,2001) allows the main limitations of traditional SAR interferometrictechniques, such as temporal and geometrical decorrelation prob-lems, topographic effects, orbit errors, as well as atmospheric artifacts(Massonnet and Feigl, 1995; Bonforte et al., 2001) to be overcome. In-deed, PS analysis is able to separate the interferometric phase contri-butions on single selected targets, isolating the ground deformationcomponent. Phase stable targets can be detected on the basis of a sta-tistical analysis on the amplitude of electromagnetic echoes. So, PSconsist in natural or artificial targets detected by the satellite sensor;they are usually artifacts like buildings or antennas or natural ele-ments like rocky outcrops, capable of maintaining high stability intheir reflectivity in time. In this way, each target is comparable to anatural geodetic benchmark on which its average LOS (along theLine Of Sight) deformation rate and the time series of LOS displace-ments can be measured. Bonforte et al. (2011) exploited all availableInSAR images acquired on Mt. Etna by ERS1 and ERS2 spacecraftsfrom 1995 to 2000 in order to depict the ground deformation patternof the volcano (Fig. 2). From the combination of ascending and de-scending orbit views, the vertical and horizontal (E–W) componentsof ground motion were discriminated and, taking advantage of thevery dense distribution of pixels above the area, the main discontinu-ities in the ground velocity filed were tracked (Fig. 2). These disconti-nuities consist of sudden jumps in the ground velocity pattern andcould represent faults that allow different blocks to move at differentvelocity. Here, we investigate the discontinuities tracked by Bonforte

et al. (2011) in a densely urbanized area on south-eastern flank of Mt.Etna in order to validate PS results and eventually relate these linearfeatures to probable hidden faults and/or fractures draining soil gases.

2.2. Soil CO2 degassing

Between October 2008 and November 2009, an intensive survey ofsoil CO2 degassing was carried out in a wide sector of thesouth-eastern flank of Mt Etna. The studied area extended about150 km2 (box in Fig. 1), and 345 measurements of soil CO2 degassingwere performed. This area corresponds to that where InSARmeasure-ments evidenced differential ground movement along lineamentstrending NW–SE, as above mentioned (Fig. 2). Gas measurementswere performed along transects of sampling points roughly orthogo-nal both to the main fault planes suggested by InSAR and to outcrop-ping fault segments (Solaro et al., 2010; Bonforte et al., 2011), withmeasurement points spaced about 100 m. Each profile was complet-ed in a few hours, in order to carry out measurements under suffi-ciently stable atmospheric conditions and/or volcanic activity.

The method adopted to perform the 345 soil CO2 measurements isthe “dynamic concentration” method (Gurrieri and Valenza, 1988;Camarda et al., 2006), which provides a proxy for soil CO2 fluxes.The method is based on an empirical relationship between soil CO2

flux and CO2 concentration in a gas mixture obtained by dilutingsoil gas with air (dynamic concentration), by means of a specific50 cm probe inserted into the soil. The details of the method are de-scribed elsewhere (Gurrieri and Valenza, 1988; Camarda et al.,2006; Gurrieri et al., 2008). The relationship between CO2 flux andthe dynamic concentration of CO2 was experimentally obtained fromseveral laboratory tests (Gurrieri and Valenza, 1988; Camarda et al.,2006). Initially, the method was calibrated for CO2 fluxes ranging be-tween 0.44 and 9.2 kg m−2 d−1 in soils characterized by a perme-ability of 24 μm2 (Gurrieri and Valenza, 1988). Thereafter, the rangewas extended to the range 0.1–22 kg m−2 d−1 and the equation

30 A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

used to calculate the CO2 flux was modified in order to introduce theeffects of soil permeability (whose range of applicability was 0.36–123 μm2) and pumping flux (in the range 0.4–4.0 l min−1) (Camardaet al., 2006). The formula used in this work is:

ϕCO2¼ 32−5:8⋅k0:24

� �⋅Cd þ 6:3⋅k0:6⋅C3

d ð1Þ

whereϕCO2is the soil CO2 flux in kg m−2 d−1, k is the gas permeability

(μm2) and Cd is the CO2 dynamic concentration (molar fraction).Soil CO2 concentration measurements were performed with an IR

spectrophotometer with an accuracy of ±2% of the full scale (0–10 vol.% or 0–100 vol.% depending on the site) and ±0.01 vol.% reso-lution. While considered unnecessary, the tuning of the spectropho-tometer was checked every day with different standard gas mixtures.

2.3. Soil radon (222Rn) and thoron (220Rn) degassing

Soil 222Rn and 220Rn measurements were carried out at about thesame sampling points surveyed for CO2 and along two different pro-files (stars in Fig. 2), using a portable electronic detector (mod.RAD7, Durridge Company Inc., USA). This detector measures concen-trations of both 222Rn and 220Rn in the bulk sampled gas phase, bycounting the alpha particles emitted by the decay of their respectivedaughter nuclides 218Po (t1/2=3.04 min) and 216Po (t1/2=0.145 s).The soil gas is extracted from the ground at 40 cm depth by an inter-nal pump and the measurements require approximately 15 min, inorder to reach equilibriumwith the measured daughter nuclide 218Po.

The whole investigation covered the period from November 2008to September 2009. The profiles of sampling points were orientedSW–NE, that is orthogonal to the main fault planes suggested byInSAR survey. The transect surveyed at the highest altitude was locat-ed at ~800 m asl between Nicolosi town and Tarderia zone and it was~8 km long. A second transect length of ~1 km passed through thesouthern portion of Pedara town at ~600 m asl. A third profile(~2 km long) was located along the southern periphery ofTrecastagni town, at 550–500 m asl. The last two profiles (~1.2 and~1.6 km long, respectively) were located in a zone at the NE periph-ery of Catania named Ognina, close to the coastline. A total of 78222Rn and 220Rn measurements were performed, and each transectconsisted of several measurement points, spaced some tens of meters.

3. Data presentation

3.1. PSInSAR ground velocities

The ground deformation maps resulting from PS analysis(Bonforte et al., 2011; Fig. 2) highlighted how the entire southernand south-eastern flanks of the volcano are characterized by a generaleastward and downward motion, as already measured by GPS sur-veys (Bonaccorso et al., 2006; Bonforte and Puglisi, 2006). This do-main is divided into several blocks affected by different velocities, asearlier suggested by the analysis of the dense GPS network installedhere (Fig. 2; Bonforte and Puglisi, 2006) and by long-period InSARdata analysis (Solaro et al., 2010). In addition, the PS velocity mapsallow imaging the discontinuities in the ground deformation field,where the relative motion of the blocks occurs, giving the opportunityto define the complex structural framework affecting the southernand south-eastern lower flanks of the volcano. Some discontinuitiesidentified by Bonforte et al. (2011) correspond to known faults,while in other cases the analysis of velocity maps allowed eitherextending the information to known faults or identifying new struc-tures. In a few cases, indeed, the discontinuities are more extendedthan the fault segments identified by classical geological surveys(Monaco et al., 2008; Branca et al., 2011), while some features onthe southern flank were only supposed or partially described by

previous field surveys (Acocella et al., 2003; Neri et al., 2004, 2005;Walter et al., 2005). Only some long-period DInSAR interferograms(Froger et al., 2001; Neri et al., 2009; Solaro et al., 2010) and studieson the propagation of seismic tremor (Falsaperla et al., 2010) havepreviously suggested some features conditioning the ground dis-placement pattern on this flank of the volcano.

On the southern flank, PS data detected some interesting linearfeatures affecting the ground velocity distribution; these abrupt in-terruptions and changes in the ground deformation field could repre-sent the main structures decoupling the eastern seaward movingsector from the southern more stable flank of the volcano. The south-ernmost feature, running roughly NNW–SSE, does not correspond toany known geological structure or fault visible on the field. This fea-ture is marked by an abrupt increase of about 2–3 mm/yr of the east-wards velocity and of about 3–4 mm/yr of the downwards one, thislast one generally decreasing from the west (3 mm/yr) to the east(0 mm/yr). The main evidence of the abrupt velocity variationalong this discontinuity is on the easternmost part of this feature(Borrello–Ognina lineament; BOL in Fig. 1), where it reaches thecoast. Here, it cuts and closes the growing Misterbianco ridgewhich upraises the northern part of Catania at a rate of about5 mm/yr; vertical velocities, in fact, dramatically drop to zero northof this structure. Even if the Borrello–Ognina represents the south-ernmost border of the mobile flank, the main decay of deformationoccurs slightly northwards along another discontinuity that runsroughly parallel to the first one. This discontinuity, hereafter named“Tremestieri-Mascalucia fault” (TMF in Fig. 1), shows both horizontaland vertical intense movements. This feature partly corresponds tosome short fault segments known from geological surveys, but it ex-tends beyond the known faults from upper altitude to the coast.With respect to the previous one, this feature shows an increasingeastward movement jump, from a few mm/yr on its westernmostpart to 15 mm/yr near the coastline. The vertical movement progres-sively decreases from the west (4 mm/yr) to the east (0 mm/yr).Both BOL and TMF are then characterized by a mainly transcurrentkinematics, from transtensive to purely right lateral on movingfrom the west to the east; this is probably the reason why they donot produce any morphological anomaly and, considering also thehighly urbanized areas they cross, they have never been detectedby classical geological surveys. The last discontinuity investigatedon the southern flank follows the well-known NNW–SSE Trecastagnifault (TF in Fig. 1) but extending, again, beyond the mapped fault;southwards, it joins the TMF feature (to form a Y-arranged structure)and northwards it joins the area where NNW–SSE dislocations weremodeled by Bonforte et al. (2004, 2009) during the 2001 eruption,and where also a dry fracture opened during the 1989 eruption(Falsaperla et al., 2010). This structure, differently from the previousones, shows a main vertical kinematics, with a mean subsidence rateof 4 mm/yr of the eastern downthrown block as also confirmed by arecently installed leveling network (Bonforte et al., 2013–this issue),accompanied by a minor eastward motion of this block, also con-firmed by data recorded at two extensometers installed across thefault (Gambino et al., 2011).

3.2. Soil CO2 measurements

The measured values of CO2 dynamic concentration are reportedin Table 2, together with the coordinates of the 345 measurementpoints. Some statistics on the measured values are reported inTable 3. The range of measured values spans from 200 ppmv to24,000 ppmv, with a mean value of 1800 ppmv, median of1100 ppmv and standard deviation of 2350 ppmv. The positive skew-ness computed for this dataset (5.3) is due to a tail of values signifi-cantly higher (up to one order of magnitude) than the median value.

As generally observed in volcanic areas, soil CO2 fluxes aresustained both by organic CO2 production, which represents the

Table 1Radon and thoron measurements. Lat/lon in UTM WGS84.

No. Latitude Longitude Radon Thoron

Nicolosi–Tarderia (profile 1) 1 37.65283 15.06720 2160 45002 37.65283 15.06720 2930 43753 37.65282 15.06468 1690 41654 37.65203 15.06388 3840 65105 37.65145 15.06265 1910 38656 37.65119 15.06146 1790 53657 37.65042 15.05834 2220 21058 37.64937 15.05724 1375 48309 37.65011 15.05626 1021 5040

10 37.64745 15.05553 1185 118911 37.64684 15.05351 2020 191512 37.64567 15.05211 955 121513 37.64341 15.05016 3065 393014 37.64029 15.04862 1895 328515 37.63872 15.04584 3125 299016 37.63852 15.04386 3515 257017 37.63874 15.04146 1129 52818 37.63846 15.03827 376 140019 37.63784 15.03574 1675 153520 37.63687 15.03397 2425 55821 37.63596 15.03227 2935 102422 37.63490 15.03018 2320 112223 37.63406 15.02850 2955 176524 37.63382 15.02763 228 191525 37.63274 15.02488 770 235026 37.63221 15.02395 2185 369527 37.62996 15.02286 1160 68528 37.63079 15.02056 261 68229 37.62990 15.01934 342 48730 37.62963 15.01705 1063 45631 37.62883 15.01403 619 71432 37.62753 15.01332 32 109033 37.62566 15.0195 277 32434 37.62356 15.00950 146 54935 37.62182 15.00665 64 6336 37.62045 15.00279 97 037 37.61855 15.00190 570 220538 37.62025 14.99614 2330 688

Ognina (upper profile 6) 1 37.53376 15.10518 999 31302 37.53459 15.10576 2620 96903 37.53527 15.10630 9745 18,2004 37.53602 15.10952 2775 11,3005 37.53673 15.11062 2675 21,2506 37.53812 15.11209 1730 12,4507 37.53871 15.11353 1940 12,6008 37.53940 15.11756 2760 43609 37.54081 15.12006 2570 12,250

Ognina (lower profile 6) 1 37.53117 15.11312 1510 10,4502 37.53285 15.11151 3215 4023 37.53378 15.11336 3220 18,8004 37.53424 15.11490 4960 22,0505 37.53404 15.11669 3655 10,3506 37.53470 15.11809 2245 99257 37.53493 15.11976 6175 21,3508 37.54576 15.12104 2215 72959 37.54577 15.12230 11,580 2495

10 37.54636 15.12443 1210 5570Trecastagni (profile 3) 1 37.60911 15.07716 5605 13,100

2 37.60899 15.07839 4350 11,4003 37.60895 15.08048 4105 14,2504 37.60919 15.08185 5055 12,4005 37.60986 15.08297 3555 86206 37.61140 15.08388 10,575 77557 37.61230 15.08454 5765 85508 37.61321 15.08386 5760 13,7009 37.61466 15.08452 3900 15,250

Pedara (profile 2) 1 37.62226 15.07198 835 30552 37.62128 15.068270 3750 19,7003 37.62130 15.06721 2095 83954 37.61957 15.06655 3110 12,5505 37.61796 15.06567 482 32056 37.61884 15.063620 3625 92357 37.61740 15.06016 8455 12,8508 37.61698 15.059290 2855 10359 37.61669 15.05701 2120 9235

10 37.61575 15.05224 1129 232

31A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

background degassing, and by volcanic emissions, mostly focusedalong active tectonic structures (Chiodini et al., 1998, 2008;Giammanco et al., 1998). These two different sources of CO2 can berecognized as two statistically distinguishable populations, as pro-posed by Chiodini et al. (1998, 2008). The choice of a threshold to dis-tinguish between the two populations is crucial in order to identifydegassing structures that act as preferential paths for the ascent ofdeep gases. As a first step, the highest CO2 flux sustained by soil res-piration (7.4·10−8 g/cm2/s) proposed by Kanemasu et al. (1974)could be considered as the lowest limit for volcanic CO2 degassing(Giammanco et al., 1998), provided that organic matter is homoge-neously distributed over the studied area. Considering the range ofsoil permeability where Eq. (1) applies (0.36–123 μm2), the highestCO2 flux sustained by soil respiration would correspond to a “dynam-ic” concentration of about 2500 ppmv. This value, though providingan approximate indication, could be inconsistently too high to repre-sent microbial CO2 production in volcanic soils, where the vegetationcover can be inhomogeneous and somewhat scanty. In order to pro-vide more specific clues on the anomaly threshold, we followed theapproach proposed by Aiuppa et al. (2004), based on statistics. Al-though the statistical identification of the anomaly threshold in adataset normally distributed can be straightforward (and usually as-sumed to correspond to the mean value of a dataset plus 1 or 2times its standard deviation), it becomes complicated when dealingwith non-Gaussian populations of data. For this reason, we appliedthe Kolmogorov–Smirnov test to our dataset in order to check if it ap-proaches a normal distribution (computed for the sample mean andstandard deviation). In the case of non-normal distribution, thevalue corresponding to the maximum difference (d) between thesample cumulative distribution and the Gaussian distribution is as-sumed as the threshold (Fig. 3). The choice of a single thresholdvalue, nevertheless, could lead to puzzling results. Actually, as gener-ally observed in volcanic areas, CO2 degassing is unevenly distributedover the surface of a volcanic edifice, and the most intense degassingzones could have higher CO2 background values. Moreover, it is suit-able to compare surveys of soil CO2 performed during a short timespan (few days). This should overcome the eventual influence ofchangeable volcanic activity or different atmospheric conditions,both factors potentially able to affect soil degassing. For these reasons,the statistical analysis of CO2 data has been performed separately onsamples collected along transects surveyed in the same few days. InTable 3, the threshold values computed with the Kolmogorov–Smirnov test are listed for the different groups of samples, givingvalues ranging from 1000 to 5400 ppmv.

The spatial distribution of CO2 concentration values in the studiedarea, as well as that of the anomalous points, is plotted in Fig. 4. In themaps, the main discontinuities identified in this sector of the volcanoare also plotted (Bonforte et al., 2011 and references therein). As ob-served in Fig. 4a, the highest values have been measured in the low-ermost part of the SE flank of the volcano, with dynamicconcentration values as high as 24,400 ppmv. Elsewhere, the CO2

concentration values are generally below 1000 to 2200 ppmv, withpeaks of 6000–8000 ppmv, mostly measured in proximity of the east-ernmost lineament (TF in Fig. 1). The map of Fig. 4b, which shows thespatial distribution of anomalous data, provides a clearer pattern ofdegassing areas. The anomaly thresholds have been definedaccording to the procedure described above. Several anomalies canbe observed along the whole BOL lineament. In the westernmostpart of this lineament, the anomaly threshold is fixed at 1100 ppmv.According to this threshold value, several anomalous measurementsare found, with values as high as 5400 ppmv, which define a widearea of anomalous degassing, rather than a single well-definedfracture. Other CO2 anomalies are recognized down slope along theBOL lineament, with maximum values of 24,000 ppmv measured atlower elevation. For this set of measurements the anomaly thresholdis fixed at 5400 ppmv, due to the high background degassing.

Table 2Carbon dioxide measurements, expressed as ppmv. Geographical coordinates in UTMWGS84.

No. CO2 Lat Lon

1 800 37.5950 15.07672 1200 37.5955 15.07733 800 37.5960 15.07794 1400 37.5954 15.07845 1600 37.5949 15.07956 600 37.5941 15.08077 1000 37.5935 15.08148 800 37.5925 15.08199 600 37.5919 15.082910 10,400 37.5918 15.084211 2200 37.5919 15.084412 600 37.5921 15.085013 600 37.5924 15.085714 3000 37.5927 15.086715 800 37.5932 15.087416 4200 37.5936 15.087817 800 37.5945 15.088218 400 37.5952 15.088519 1200 37.6088 15.080520 1400 37.6087 15.079521 1000 37.6089 15.078322 1200 37.6093 15.077223 1000 37.6093 15.075624 400 37.6096 15.073925 1400 37.6092 15.081826 1200 37.6099 15.083127 1600 37.6114 15.083928 7600 37.6113 15.083829 2200 37.6116 15.084030 6000 37.6116 15.084031 1400 37.6123 15.084532 4000 37.6131 15.083933 1600 37.6146 15.084634 1000 37.6156 15.085935 1400 37.6157 15.086836 600 37.6161 15.087737 800 37.6224 15.071938 800 37.6226 15.070939 600 37.6222 15.0769040 1200 37.6213 15.068441 2200 37.6213 15.067242 1400 37.6222 15.066343 1400 37.6197 15.066544 400 37.6180 15.065645 1800 37.6185 15.064646 2200 37.6188 15.063747 1400 37.6189 15.062748 1400 37.6175 15.060249 2000 37.6170 15.059350 7400 37.6167 15.057151 1800 37.6162 15.054552 1800 37.6158 15.052253 800 37.6158 15.050854 1000 37.6084 15.072755 2200 37.6078 15.071356 1800 37.6073 15.070857 600 37.6067 15.070058 1600 37.6061 15.069359 400 37.6054 15.068860 1800 37.6055 15.069061 1000 37.6043 15.066662 1000 37.6039 15.065963 1600 37.6034 15.065264 1400 37.6033 15.064665 2200 37.6038 15.060866 1000 37.6040 15.059267 800 37.6039 15.058668 1000 37.6037 15.058169 600 37.6028 15.058670 2200 37.6024 15.057271 2200 37.6017 15.055272 1200 37.6002 15.052073 1200 37.6000 15.053674 600 37.6007 15.051075 2000 37.5992 15.0477

Table 2 (continued)

No. CO2 Lat Lon

76 800 37.5996 15.045777 200 37.5985 15.044678 1400 37.5975 15.043679 600 37.5967 15.042380 1200 37.5955 15.042381 400 37.5953 15.041382 4200 37.5946 15.041183 1200 37.5940 15.040484 800 37.5919 15.038785 1000 37.5918 15.036586 800 37.6040 15.063087 1800 37.6043 15.061588 800 37.6234 15.084789 1000 37.6232 15.083890 400 37.6232 15.083091 1000 37.6227 15.080392 1000 37.6219 15.079393 600 37.6206 15.085894 1400 37.6205 15.085295 400 37.6200 15.085096 800 37.6178 15.083297 600 37.6169 15.083398 5200 37.6147 15.083499 400 37.6130 15.0807100 600 37.6153 15.0814101 1200 37.6036 15.0837102 1000 37.6042 15.0855103 1000 37.6062 15.0870104 600 37.6066 15.0880105 1800 37.6064 15.0888106 3000 37.6121 15.0925107 800 37.6113 15.0921108 3400 37.6104 15.0928109 600 37.6101 15.0915110 600 37.6102 15.0907111 1200 37.6079 15.0912112 800 37.6100 15.0903113 3400 37.6307 15.0589114 5400 37.6319 15.0587115 2600 37.6326 15.0589116 6400 37.6332 15.0594117 2200 37.6339 15.0600118 2000 37.6353 15.0607119 2000 37.6363 15.0615120 2000 37.6363 15.0634121 800 37.6385 15.0666122 400 37.6366 15.06895123 2800 37.6311 15.0570124 800 37.6299 15.0568125 1800 37.6284 15.0564126 1400 37.6260 15.0563127 1800 37.6247 15.0561128 600 37.6241 15.0532129 800 37.6244 15.0509130 200 37.5908 15.0234131 200 37.5902 15.0223132 2000 37.5895 15.0207133 2400 37.5890 15.0196134 1000 37.5888 15.0189135 400 37.5881 15.0176136 600 37.5875 15.0167137 400 37.5871 15.0144138 3200 37.5874 15.0112139 600 37.5874 15.0103140 200 37.5874 15.0083141 600 37.5875 15.0074142 400 37.5872 15.0062143 400 37.5873 15.0048144 200 37.5878 15.0037145 800 37.5713 15.0281146 600 37.5712 15.0297147 400 37.5690 15.0352148 400 37.5686 15.0365149 800 37.5685 15.0391150 3200 37.5708 15.0420151 1800 37.5710 15.0432152 2400 37.5711 15.0440

32 A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

Table 2 (continued)

No. CO2 Lat Lon

153 800 37.5717 15.0459154 400 37.5715 15.0480155 800 37.5727 15.0491156 2200 37.6088 14.9976157 1400 37.6080 14.9965158 1400 37.6078 14.9960159 1600 37.6073 14.9947160 3000 37.6066 14.9934161 400 37.6068 14.9922162 1600 37.6054 14.9912163 1800 37.6048 14.9905164 1400 37.6039 14.9891165 600 37.6035 14.9873166 1000 37.6037 14.9866167 400 37.6021 14.9853168 1600 37.6016 14.9835169 2600 37.6014 14.9823170 5400 37.6008 14.9816171 800 37.6010 14.9806172 1600 37.6013 14.9783173 1600 37.6008 14.9771174 800 37.6006 14.9762175 800 37.6008 14.9752176 600 37.5608 15.0575177 800 37.5613 15.0564178 1200 37.5615 15.0555179 600 37.5616 15.0541180 2600 37.5608 15.0525181 400 37.5609 15.0513182 1000 37.5588 15.0516183 2600 37.5582 15.0507184 600 37.5575 15.0511185 600 37.5575 15.0490186 400 37.5588 15.0486187 2600 37.5587 15.0469188 800 37.5572 15.0447189 800 37.5516 15.0622190 1000 37.5522 15.0612191 5400 37.5530 15.0613192 8000 37.5564 15.0605193 4000 37.5576 15.0621194 1000 37.5606 15.0609195 800 37.5642 15.0599196 600 37.5528 15.0596197 1200 37.5827 15.0922198 800 37.5813 15.0907199 1600 37.5804 15.0903200 1200 37.5796 15.0901201 800 37.5789 15.0899202 600 37.5780 15.0894203 800 37.5770 15.0890204 2200 37.5760 15.0885205 1600 37.5757 15.0879206 400 37.5767 15.0858207 1200 37.5763 15.0835208 1600 37.5767 15.0812209 600 37.5767 15.0793210 400 37.5763 15.0766211 800 37.5758 15.0744212 1000 37.5743 15.0727213 600 37.5759 15.0712214 800 37.5735 15.0708215 200 37.5751 15.0686216 4000 37.5751 15.0669217 1200 37.5739 15.0657218 1200 37.5740 15.0662219 600 37.5733 15.0640220 1600 37.5732 15.0624221 600 37.5731 15.0609222 1200 37.5727 15.0593223 1800 37.5729 15.0561224 800 37.5729 15.0536225 3000 37.6170 15.0051226 1000 37.6187 15.0018227 600 37.6176 15.0037228 400 37.6170 15.0070229 400 37.6165 15.0085

Table 2 (continued)

No. CO2 Lat Lon

230 400 37.6156 15.0109231 800 37.6154 15.0128232 400 37.6157 15.0144233 2000 37.6156 15.0161234 1200 37.6149 15.0187235 800 37.6157 15.0210236 3200 37.6164 15.0222237 600 37.6170 15.0234238 600 37.6176 15.0246239 800 37.6513 15.0681240 2600 37.6516 15.0662241 800 37.6518 15.0640242 4000 37.6510 15.0626243 400 37.6505 15.0614244 800 37.65499 15.0606245 1800 37.6503 15.0577246 5600 37.6501 15.0564247 800 37.6514 15.0570248 400 37.6522 15.0563249 2200 37.6537 15.0554250 800 37.6541 15.0553251 600 37.6560 15.0510252 2000 37.6558 15.0492253 2400 37.6551 15.0477254 1800 37.6548 15.0465255 1000 37.6546 15.0451256 3000 37.6536 15.0431257 1000 37.6522 15.0422258 1000 37.6519 15.0408259 3800 37.6514 15.0401260 600 37.6520 15.0386261 600 37.6551 15.0376262 2200 37.6573 15.0356263 400 37.6572 15.0337264 400 37.6581 15.0333265 200 37.6534 14.9806266 2200 37.6527 14.9817267 1000 37.6512 14.9830268 600 37.6501 14.9845269 800 37.6492 14.9861270 400 37.6486 14.9886271 1400 37.6484 14.9911272 400 37.6451 14.9951273 1000 37.6463 15.0019274 400 37.6646 15.0037275 1000 37.6326 15.0115276 1000 37.6336 15.0104277 1200 37.6347 15.0091278 0 37.6361 15.0079279 200 37.6284 15.0150280 200 37.6279 15.0141281 200 37.6277 15.0129282 800 37.6278 15.0114283 400 37.6283 15.0088284 400 37.6297 15.0064285 400 37.6231 14.9707286 1400 37.6221 14.9704287 600 37.6211 14.9702288 3400 37.6199 14.9701289 1800 37.6188 14.9699290 2400 37.6195 14.9700291 1000 37.6181 14.9695292 2000 37.6178 14.9686293 600 37.6170 14.9683294 2000 37.6088 14.9660295 1600 37.6097 14.9664296 1000 37.6107 14.9668297 1800 37.6114 14.9670298 400 37.6122 14.9674299 200 37.6135 14.9678300 600 37.6141 14.9672301 3000 37.6152 14.9672302 2400 37.6155 14.9677303 1400 37.6078 14.9664304 2000 37.6067 14.9660305 2000 37.6055 14.9656

(continued on next page)

33A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

Fig. 3. An example of application of the Kolmogorov–Smirnov test. The plot shows thecumulative frequency (black curve) for the population including samples from 1 to129, together with the corresponding theoretical normal distribution (dashed line).The maximum distance d (0.229) between the two curves is taken as anomaly thresh-old (2200 ppmv).

Table 2 (continued)

No. CO2 Lat Lon

306 400 37.6058 14.9645307 1400 37.6062 14.9631308 800 37.6064 14.9623309 2000 37.5715 15.1153310 800 37.5710 15.1147311 21,500 37.5696 15.1143312 6300 37.5685 15.1123313 5200 37.5679 15.1115314 1100 37.5668 15.1105315 1100 37.5651 15.1087316 11,500 37.5647 15.1080317 10,300 37.5644 15.1069318 1900 37.5636 15.1054319 1400 37.5620 15.1057320 1200 37.5603 15.1053321 1200 37.5593 15.1052322 2800 37.5700 15.1122323 1000 37.5704 15.1132324 2300 37.5713 15.1118325 3900 37.5665 15.1061326 9400 37.5657 15.1024327 3100 37.5646 15.1023328 4000 37.5640 15.1015329 2400 37.5625 15.1043330 1200 37.5618 15.1035331 4600 37.5634 15.0977332 2100 37.5694 15.0987333 24,400 37.5433 15.0972334 5100 37.5422 15.0959335 2700 37.5416 15.0945336 2400 37.5412 15.0929337 9500 37.5404 15.0923338 7800 37.5398 15.0908339 3000 37.5392 15.0900340 1900 37.5384 15.0879341 2700 37.5470 15.1003342 4700 37.5463 15.0980343 5400 37.5454 15.0997344 4400 37.5468 15.1018345 1500 37.5490 15.1002

34 A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

CO2 anomalies are also identified in correspondence to the TMFlineament, with concentrations as high as 4000 ppmv, measured atthe lowermost portion.

In the easternmost part of the study area, several anomalies areidentified. If compared with the TF lineament, a close correspondenceoccurs in some cases, whereas elsewhere anomalous degassing occurseast of the identified lineament. Furthermore, some portions of the TFlineament are apparently not actively degassing. The measurementsmade at the highest elevation, close to the virtual extension of theTF lineament, show various anomalies, with concentrations in therange 1400–5000 ppmv.

3.3. Soil radon (222Rn) and thoron (220Rn) measurements

All 222Rn and 220Rn values measured in the studied area, togetherwith the coordinates of measurement points, are shown in Table 1.Values for 222Rn span from 32 Bq m−3 to 11,580 Bq m−3, with

Table 3Statistical data on soil CO2 measurements.

Sample Min Max Mean Median Skewness Std dev Threshold

Whole dataset 200 24,400 1800 1100 5.3 2350 22001–129 200 10,400 1600 1200 3 1540 1400130–196 200 8000 1400 800 2.4 1400 1000197–238 200 4000 1100 800 1.8 800 1200239–308 200 5600 1300 1000 1.6 1060 1000309–348 800 24,400 4800 2800 2.5 5200 5400

mean value of 2679 Bq m−3, median of 2218 Bq m−3 and standarddeviation of 2305 Bq m−3. Values for 220Rn range from 0 Bq m−3 to22,050 Bq m−3, with mean value of 6317 Bq m−3, median of4048 Bq m−3 and standard deviation of 6011 Bq m−3.

All profiles were NE–SW-oriented, that is orthogonal to the knownfaults affecting the SE flank of Etna between ~800 m asl and the Ioni-an Sea coastline. Significant differences both in 222Rn and 220Rnvalues were detected along the five profiles surveyed. The Nicolosi–Tarderia profile (Fig. 5a, 1 in Fig. 2) shows a progressive increaseboth in 222Rn and 220Rn values on moving from SW to NE. Althoughthe median values of both gases (1525 Bq m−3 for 222Rn and1650 Bq m−3 for 220Rn, respectively) are lower than those in theother profiles, the highest values of 222Rn (close to or higher than3000 Bq m−3) suggest the presence of well-defined fractured zones,highlighted with vertical gray bars in Fig. 5a. The values of 220Rnalong this profile in general show a similar behavior to those of222Rn. However, in the southwest part of the profile the 222Rn/220Rnratio was higher than in its northeast part. In particular, the ratiowas very high in correspondence of the southwesternmost anomalybecause of the very low 220Rn values. This suggests local conditionsof relatively low soil permeability, such that only the long-lived iso-tope 222Rn can reach the surface through the presumed fault.

The Trecastagni profile (Fig. 5b, 3 in Fig. 2) shows median values of5055 Bq m−3 (222Rn) and 12,400 Bq m−3 (220Rn). Only one clearpeak in 222Rn values (10,575 Bq m−3) can be observed and it corre-sponds to the lowest value of 220Rn. This again suggests the presenceboth of a fractured zone and a low soil permeability.

The Pedara profile (Fig. 5c, 2 in Fig. 2) shows median values of2488 Bq m−3 for 222Rn and 8815 Bq m−3 for 220Rn. Also in thiscase only one peak in 222Rn values (8455 Bq m−3) is observed, butdifferently from the previous profile it corresponds to high 220Rnvalues, thus indicating the presence of a fractured zone with highsoil permeability. The upper Ognina profile (Fig. 5d, 6 in Fig. 2), locat-ed at the northeastern periphery of Catania town, shows a soildegassing behavior similar to that of the Pedara profile. Along thisprofile, median values of 222Rn and 220Rn were, respectively,2620 Bq m−3 and 12,250 Bq m−3, and only one clear 222Rn anomalywas detected. This anomaly corresponded to a positive anomaly of220Rn, but other anomalous 220Rn values were observed further tothe NE along this profile, without a corresponding 222Rn anomaly.This suggests the presence of very shallow ground fractures, althoughno surface evidence of them could be found. Lastly, the lower Ognina

Fig. 4. a) Distribution map of CO2 dynamic concentration values; b) distribution map of anomalous points, defined according to the Kolmogorov–Smirnov test (see text for details).

35A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

profile (Fig. 5e, 6 in Fig. 2) showed median values of 3218 Bq m−3

and 10,138 Bq m−3, respectively for 222Rn and 220Rn. Along this pro-file, the highest peak value of 222Rn (11,580 Bq m−3) of all sampledprofiles was measured, corresponding to a low 220Rn value. Therefore,similar to what has been described above, this suggests a fracturedzone with relatively low soil permeability. The two high 220Rn peakvalues along this profile correspond to high 222Rn values, althoughthe latter were not as high as the highest 222Rn value, and hencethey suggest the presence of other fractured zones of relativelyminor importance.

4. Discussion

The southern boundary of the seaward moving Eastern flank ofMt. Etna has shown complex kinematics that we investigated throughthe comprehensive integration of geological, geodetic and soildegassing data. The different records are merged in a series of profiles(Fig. 6), as reported in Fig. 2. Along each profile, ground LOS

(ascending) velocity is plotted along with the dynamic concentrationof soil CO2 and/or 220Rn soil concentrations. We have chosen to plotthe LOS velocities in order to have a denser information (seeBonforte et al., 2011) able to better evidence jumps in the ground de-formation pattern. Intersections with fault segments as defined inBonforte et al. (2011) are also plotted.

The analysis of every profile reveals peculiar kinematics anddegassing patterns. Profile 1 (Nicolosi–Tarderia) is almost 12 kmlong and runs SW–NE at about 600 m of elevation. The LOS velocityprofile shows a smooth and progressive variation in the westernmostpart, in the first 5 km, in a sector comprised between BOL and TMF(Fig. 1). In this area, several anomalies of soil CO2 have been mea-sured. These features would imply a large fractured area, instead ofa single well-defined fault. The possible degassing fractures areevidenced in the graph by shaded gray areas. A CO2 anomaly fairlycorresponds to the Tremestieri–Mascalucia fault (TMF); the groundvelocity appears stable from there towards the east. From 6 to 8 kmalong the profile, where no CO2 data are available, the ground velocity

Fig. 5. Graphs plotting radon (222Rn) and thoron (220Rn) emission, in Bq m−3, from soil along the five profiles. For locations see stars in Fig. 2. Gray vertical bar indicates the zones ofhighest radon values that may be associated with the presence of a fault.

36 A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

appears rather constant, while several peaks of both 222Rn and 220Rnsoil concentrations are measured, which claims for degassing fromshallow ground fractures. A change in ground velocity is related tothe seismogenetic Trecastagni fault (TF) (Acocella et al., 2003, and ref-erences therein) and is paralleled by aminor CO2 anomaly; a sharp peakof CO2 (up to 6200 ppmv) occurs a fewhundredmeters east of the fault,on its hangingwall. The very low 222Rn and 220Rn concentrations mea-sured in correspondence to the CO2 anomaly suggest that the ascendingdeep gas (mainly CO2) dilutes both 222Rn and 220Rn, probably releasedby local volcanic rocks. An analogous effect has recently been observedalong the Pernicana fault (Siniscalchi et al., 2010). The ascent of mag-matic CO2-rich gas is confirmed by helium isotopes, measured in thegas dissolved in a water well, located close to the soil CO2 anomaly

(sample # 31, Allard et al., 1997). The increase of 222Rn and especiallyof the short-living 220Rn northeast of the TF, seems to indicate an im-portant surface fracturing affecting the hangingwall of the fault.

As in profile 1, also along profile 3 (Trecastagni) a uniform gradi-ent of ground velocity is observed from the origin of the profile toTMF lineament, with only a little step in correspondence of BOL,while a drastic increase marks the TMF and TF lineaments. Thechanges in ground velocity in the first 8 km are also evidenced bythe only two soil CO2 anomalies, measured at the intersections withBOL and TMF lineaments. After soil gas evidences, Belpasso-OgninaLineament does not represent only a ground deformation feature; itcan be hereafter considered as a fault and then named more correctlyas Belpasso-Ognina Fault (BOF). Conversely, the abrupt change in

Fig. 6. Plots of ground velocities (blue circles), CO2 (red line), radon (222Rn, green line) and thoron (220Rn, light green line) fluxes/concentrations projected along the 6cross-sections numbered in Fig. 2. All plots have the same X-axis scale. Dotted vertical lines indicate the intersection of the cross-sections with the discontinuities defined byBonforte et al. (2011). Gray areas indicate the zones where anomalous gas fluxes/concentrations have been measured.

37A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

ground velocity, which marks the TF lineament, is not paralleled byany CO2 anomaly. A sharp CO2, 222Rn and 220Rn anomaly, instead, isobserved a few hundred meters north-eastwards. The peaks of bothCO2 and 222Rn, and the simultaneous negative peak of 220Rn, claim

for a deeper origin of the ascending gas, which dilutes the locallyproduced 220Rn, and convective transport of 222Rn.

A similar abrupt variation of ground velocity is measured along thesame lineament (TF) at higher elevation (profile 2 — Pedara). In this

38 A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

latter case, the hypothesized lineament drains 222Rn and also 220Rn pro-duced in shallow soil levels, while no CO2 anomalies are observed incorrespondence of the change in ground velocity. The CO2 and 220Rnpeak just west of the fault probably corresponds to an eruptive fracture,draining superficial gases (organic CO2?). In any case, the dispersion ofthe 220Rn anomalies and the ground velocity pattern here define an~1 km wide fractured area rather than a unique structure cutting thesurface.

At lower elevation, as evidenced along profile 4, a dramatic changein ground velocity clearly marks the lineament TMF, while only a mod-erate but evident variation in ground velocity corresponds to the linea-ments BOL and TF. Concerning the degassing pattern, several anomaliesin soil CO2 are observed along the whole profile, with the highest con-centrations measured in proximity of ground velocity variations rele-vant to the BOL and TF lineaments. TMF does not present an evidentCO2 anomaly but, as for profile 1, some anomalies are present betweenBOL and TMF. Unlike profile 1, here no gradient affects the ground ve-locity that remains constant between the two lineaments.

In the westernmost portion of the BOL, between 500 and 600 m ofaltitude, anomalous CO2 concentrations were measured also ingroundwaters, coupled with high CH4 and dissolved salts contents(Parello et al., 2001; Aiuppa et al., 2002). According to the availableliterature, in this area shallow meteoric waters mix with saline andCO2-rich fluids that ascend from an underlying hydrothermal system(Chiodini et al., 1996). In the SW part of Etna these fluids rise to thesurface without being modified and form the “Salinelle” mud volca-noes (Brusca et al., 2001). Together with CO2-rich magmatic gases,thermogenic CH4 rises from hydrocarbon reservoirs hosted in thesedimentary basement (Aiuppa et al., 2003 and references therein).

At lower elevation, in the area affected both by the BOF and by theTMF–TF lineaments, the low CO2 contents in local groundwaters seemto exclude significant CO2 input to water from below (D'Alessandro etal., 1999; Parello et al., 2001). Therefore, extensive drainage of deepgases coming from the underlying hydrothermal aquifer appears un-likely in this sector of the studied area. Conversely, the soil gas

Fig. 7. Schematic map summarizing the structural framework defined for the southern flanklines (1) indicate the position of the main faults detected by geological and/or geodetic survindicate all open fractures/faults, including some newly detected lineaments, producing ga

anomalies detected in correspondence of the identified tectonic line-aments, characterized by a non-volcanic isotopic signature, wouldsuggest that they are probably related to the release of organic gasfrom small pockets trapped within the clayey sediments that underliethe volcanic deposits and crop out locally.

At lowest elevation, along the profiles 5 and 6, marked changes inground velocity highlighted the uplift of an active anticline (positiveLOS velocities, see Fig. 2) on the southernmost part of the volcano. Parallelto the gradient of ground velocity, some well defined soil gas anomalieshave been detected, in either CO2 or 222Rn. The isotopic marker of CO2,measured in the points of highest CO2 soil concentration in profile 5, in-dicates an organic origin for the soil gas (δ13C=−22‰). This would sug-gest that a series of fractures drain crustal gases. In profile 6 (Ognina), twoRn measurements transects (one – lower – along the coastline, Fig. 5d,and one – upper – about 1 km inland, Fig. 5e) are projected. The 222Rnanomaly measured on the upper profile (plotted with dashed lines) cor-responds to the position of the BOL as tracked in Bonforte et al. (2011),while the lower profile (plottedwith solid lines) shows an evident anom-aly about 1 kmnorth-eastwards. As shown in Fig. 2, the BOF should inter-sect the lower profile in its central part, while the anomaly affects thenorthernmost measurement points; this aspect suggests a more pro-nounced curvature of the BOL close to the coastline.

The merging of all available data, including field evidence, groundvelocities, soil gas concentrations and fluxes, is represented in the in-terpretative map shown in Fig. 7. The southernmost lineament (BOF),that runs roughly NNW–SSE, has no surface evidence, but it is in-ferred both from ground deformation data and from soil gas anoma-lies, confirming the existence of a fault. This tectonic line marks achange in the kinematic behavior of the two bounded sectors of Mt.Etna. As observed along the profiles of Fig. 6, CO2 anomalies have al-ways been detected along this lineament, and several others havebeen detected also close to it, claiming for a wide fractured area rath-er than a unique and well defined linear structure.

The lineament named TMF is marked by a drastic downward in-crease of ground velocity, all along its length; some segments of this

of Mt. Etna from ground deformation, geological and geochemical investigation. Greeneying (Solaro et al., 2010 and references therein; Bonforte et al., 2011). Shadowed areass conveying and/or significant ground displacement.

39A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

fault are partially visible in the field because of ground fracturing anddislocations. This lineament roughly corresponds also to the southernhydrogeological divide identified at depth (Ogniben, 1966).

The easternmost lineament in the studied area (TF) shows a rathercomplex pattern. Between 700 and 900 m of altitude, the change inground velocity corresponds (or are very close) to clear anomalouspeaks in soil degassing. In particular, degassing of deep CO2 of proba-ble volcanic origin prevails on shallow-sourced radon. At lower eleva-tion, CO2 degassing occurs rather sporadically, thus suggesting theexistence of different non-contiguous fault segments. Ground velocityvariations are not sharp but they occur gradually over a few hundredmeters, thus confirming the presence of a wide fractured area ratherthan a single well defined fault. Conversely, as observed along profiles2 and 3 in Fig. 7, 220Rn and 222Rn anomalies, coupled or not with CO2

anomalies, help identifying actively degassing structures. In truth, wesuspect that either CO2 or Rn are drained by fractures from smallpockets of crustal (i.e., non-volcanic) gas. This is particularly evidentat about 300 m of altitude along the TF lineament, where a markedCO2 anomaly occurs in correspondence of a well-known, monitored(Gambino et al., 2011; Bonforte et al., 2013–this issue) segment ofthe fault. In this area Pleistocene clays crop out, thus the origin ofsoil CO2 is probably organic.

5. Conclusions

InSAR results, recently published by Bonforte et al. (2011), havebeen re-analyzed in detail for the southern flank of Mt. Etna andre-interpreted on the grounds of new soil degassing data. Thematching between geological, structural, geodetic and soil gas dataallowed shedding more light on the complex dynamics of the slidingeastern flank of Mt. Etna, gathering a wealth of information that thesingle disciplines would not have obtained alone. In particular,InSAR results, have proved very useful in detecting buried segmentsof active structures without surface evidence in this sector of the vol-cano (Bonforte et al., 2011) but, especially at higher elevation, envi-ronmental conditions make this method sometimes less precise. Themeasurements of soil degassing have provided further insights intohidden faults, although local geological conditions (i.e. existence offields of compact lavas over buried faults) could prevent the detectionof gas anomalies or, as in the case of degassing eruptive fractures,could bring about erroneous interpretations. The multidisciplinaryapproach adopted here allowed overcoming the drawbacks of the sin-gle methodologies, and a better outlining of actively degassing hiddenfaults (Fig. 7). The shear zones revealed by this study are linked to theactive SE rift of Mt. Etna and probably accommodate both the differ-ent velocities of ground deformation along the faults that bound theunstable blocks of the volcano (Solaro et al., 2010; Bonforte et al.,2011), and the E–W extensional strain associated with magma em-placement within the volcano.

Furthermore, the information acquired improved our understand-ing of the nature of diffuse soil degassing on Mt. Etna in relation bothto faults and to local geology. This approachmay significantly contrib-ute to improve the structural hazard assessment and geodynamicmodeling of Mt. Etna. From a wider point of view, the proposedmeth-odology can be successfully applied to any volcanic or tectonically ac-tive area, where ground displacement and soil degassing are bothrelated to fault movements.

Acknowledgments

We thank the anonymous reviewers for their very punctual and con-structive comments. We wish to thank also S. Conway for the funda-mental improvement in the English language. This work was fundedby the DPC-INGV project “Flank”.

References

Acocella, V., Neri, M., 2003. What makes flank eruptions? The 2001 Etna eruption andthe possible triggering mechanisms. Bulletin of Volcanology 65, 517–529. http://dx.doi.org/10.1007/s00445-003-0280-3.

Acocella, V., Neri, M., 2005. Structural features of an active strike-slip fault on the slid-ing flank of Mt. Etna (Italy). Journal of Structural Geology 27/2, 343–355. http://dx.doi.org/10.1016/j.jsg.2004.07.006.

Acocella, V., Behncke, B., Neri, M., D'Amico, S., 2003. Link between major flank slip anderuptions at Mt. Etna (Italy). Geophysical Research Letters 30, 2286. http://dx.doi.org/10.1029/2003GL018642.

Aiuppa, A., Brusca, L., D'Alessandro, W., Giammanco, S., Parello, F., 2002. A case study ofgas–water–rock interaction in a volcanic aquifer: the south-western flank of Mt.Etna (Sicily). In: Stober, I., Bucher, K. (Eds.), Water–Rock Interaction. Kluwer Aca-demic Publishers, Dordrecht, pp. 125–145.

Aiuppa, A., Bellomo, S., Brusca, L., D'Alessandro, W., Federico, C., 2003. Natural and an-thropogenic factors affecting water quality of the aquifers hosted in an active vol-cano (Mt. Etna, Italy). Applied Geochemistry 18, 863–882.

Aiuppa, A., Caleca, A., Federico, C., Gurrieri, S., Valenza, M., 2004. Diffuse degassing of carbondioxide at Somma Vesuvius volcanic complex (Southern Italy) and its relation withregional tectonics. Journal of Volcanology and Geothermal Research 133, 55–79.

Allard, P., Carbonelle, J., Dajlevic, D., Le Bronec, J., Morel, P., Robe, M.C., Maurenas, J.M.,Faivre-Pierret, R., Martin, D., Sabroux, J.C., Zettwoog, P., 1991. Eruptive and diffuseemissions of CO2 from Mount Etna. Nature 351, 387–391.

Allard, P., Jean-Baptiste, P., D'Alessandro, W., Parello, F., Parisi, B., Flehoc, C., 1997. Man-tle-derived helium and carbon in groundwaters and gases of Mount Etna, Italy.Earth and Planetary Science Letters 148, 501–516.

Alparone, S., Behncke, B., Giammanco, S., Neri, M., Privitera, E., 2005. Paroxysmal summitactivity at Mt. Etna monitored through continuous soil radon measurements.Geophysical Research Letters 32, L16307. http://dx.doi.org/10.1029/2005GL023352.

Baubron, J.-C., Rigo, A., Toutain, J.P., 2002. Soil gas profiles as a tool to characterise ac-tive tectonics areas: the Jaut Pass example (Pyrénées, France). Earth and PlanetaryScience Letters 196, 69–81.

Bonaccorso, A., Bonforte, A., Guglielmino, F., Palano, M., Puglisi, G., 2006. Compositeground deformation pattern forerunning the 2004–2005 Mount Etna eruption. Jour-nal of Geophysical Research 111, B12207. http://dx.doi.org/10.1029/2005JB004206.

Bonforte, A., Puglisi, G., 2006. Dynamics of the eastern flank of Mt. Etna volcano (Italy)investigated by a dense GPS network. Journal of Volcanology and Geothermal Re-search 153, 357–369. http://dx.doi.org/10.1016/j.jvolgeores.2005.12.005.

Bonforte, A., Ferretti, A., Prati, C., Puglisi, G., Rocca, F., 2001. Calibration of atmosphericeffects on SAR interferograms by GPS and local atmosphere models: first results.Journal of Atmospheric and Solar–Terrestrial Physics 63, 1343–1357.

Bonforte, A., Guglielmino, F., Palano, M., Puglisi, G., 2004. A syn-eruptive ground defor-mation episode measured by GPS, during the 2001 eruption on the upper southernflank of Mt Etna. Bulletin of Volcanology 66, 336–341. http://dx.doi.org/10.1007/s00445-003-0314-x.

Bonforte, A., Gambino, S., Neri, M., 2009. Intrusion of eccentric dikes: the case of the2001 eruption and its role in the dynamics of Mt. Etna volcano. Tectonophysics471, 78–86. http://dx.doi.org/10.1016/j.tecto.2008.09.028.

Bonforte, A., Guglielmino, F., Coltelli, M., Ferretti, A., Puglisi, G., 2011. Structural assess-ment of Mt. Etna volcano from Permanent Scatterers analysis. Geochemistry, Geo-physics, Geosystems 12, Q02002. http://dx.doi.org/10.1029/2010GC003213.

Bonforte, A., Carnazzo, A., Gambino, S., Guglielmino, F., Obrizzo, F., Puglisi, G., 2013. Amultidisciplinary study of an active fault crossing urban areas: the TrecastagniFault at Mt. Etna (Italy). Journal of Volcanology and Geothermal Research 251,41–49 (this issue).

Branca S., Coltelli M., Groppelli G., Lentini F., 2011. Geological map of Etna volcano,1:50,000 scale. Italian Journal of Geosciences, Vol. 130, No. 3, 265–291, doi:10.3301/IJG.2011.15

Brusca, L., Aiuppa, A., D'Alessandro, W., Parello, F., Allard, P., Michel, A., 2001. Geochem-ical mapping of magmatic gaswater–rock interactions in the aquifer of Mount Etnavolcano. Journal of Volcanology and Geothermal Research 108, 201–220.

Burton, M., Neri, M., Condarelli, D., 2004. High spatial resolution radon measurementsreveal hidden active faults on Mt. Etna. Geophysical Research Letters 31 (7),L07618. http://dx.doi.org/10.1029/2003GL019181.

Burton, M., Neri, M., Andronico, D., Branca, S., Caltabiano, T., Calvari, S., Corsaro, R.A., DelCarlo, P., Lanzafame, G., Lodato, L., Miraglia, L., Muré, F., Salerno, G., Spampinato, L.,2005. Etna 2004–05: an archetype for geodynamically-controlled effusive eruptions.Geophysical Research Letters 32. http://dx.doi.org/10.1029/2005GL022527.

Camarda, M., Gurrieri, S., Valenza, M., 2006. CO2 flux measurements in volcanic areasusing the dynamic concentration method: influence of soil permeability. Journalof Geophysical Research 111, B05202. http://dx.doi.org/10.1029/2005JB003898.

Cartagena, R., Olmos, R., Lopez, D.L., Soriano, T., Barahona, F., Hernandez, P.A., Perez,N.M., 2004. Diffuse soil degassing of carbon dioxide, radon, and mercury at SanMiguel volcano, El Salvador. In: Rose, W.I., et al. (Ed.), Natural Hazards in El Salva-dor: Geol. Soc. Am. Special Papers, 375, pp. 203–212.

Chiodini, G., Frondini, F., Raco, B., 1996. Diffuse emissions of CO2 from the Fossa crater,Vulcano Island (Italy). Bulletin of Volcanology 58, 41–50.

Chiodini, G., Cioni, R., Guidi, M., Marini, L., Raco, B., 1998. Soil CO2 flux measurements involcanic and geothermal areas. Applied Geochemistry 13, 534–552.

Chiodini, G., Caliro, S., Cardellini, C., Avino, R., Granieri, D., Schmidt, A., 2008. Carbonisotopic composition of soil CO2 efflux, a powerful method to discriminate differ-ent sources feeding soil CO2 degassing in volcanic–hydrothermal areas. Earth andPlanetary Science Letters 274, 372–379.

D'Alessandro, W., De Domenico, R., Parello, F., Valenza, M., 1992. Soil degassing in tec-tonically active areas of Mt Etna. Acta Vulcanologica 2, 175–183.

40 A. Bonforte et al. / Journal of Volcanology and Geothermal Research 251 (2013) 27–40

D'Alessandro, W., Inguaggiato, S., Federico, C., Parello, F., 1999. Chemical compositionof dissolved gases in groundwaters from Mt. Etna, Eastern Sicily. In: Armannsson,H. (Ed.), Geochemistry of the Earth's Surface. Balkema, Rotterdam, pp. 491–494.

De Gregorio, S., Diliberto, I.S., Giammanco, S., Gurrieri, S., Valenza, M., 2002. Tectoniccontrol over large-scale diffuse degassing in eastern Sicily (Italy). Geofluids 2,273–284.

Falsaperla, S., Cara, F., Rovelli, A., Neri, M., Behncke, B., Acocella, V., 2010. Effects of the1989 fracture system in the dynamics of the upper SE flank of Etna revealed byvolcanic tremor data: the missing link? Journal of Geophysical Research 115,B11306. http://dx.doi.org/10.1029/2010JB007529.

Farrar, C.D., Sorey, M.L., Evans, W.C., Howle, J.F., Kerr, B.D., Mack Kennedy, B., King, C.-Y.,Southon, J.R., 1995. Forest-killing diffuse CO2 emission at Mammoth Mountain as asign of magmatic unrest. Nature 376, 675–678.

Ferretti, A., Prati, C., Rocca, F., 2001. Permanent scatterers in SAR interferometry. IEEETransactions on Geoscience and Remote Sensing 39, 8–20.

Froger, J.L., Merle, O., Briole, P., 2001. Active spreading and regional extension at MountEtna imaged by SAR interferometry. Earth and Planetary Science Letters 187,245–258.

Gambino, S., Bonforte, A., Carnazzo, A., Falzone, G., Ferrari, F., Ferro, A., Guglielmino, F.,Laudani, G., Maiolino, V., Puglisi, G., 2011. Displacement across the TrecastagniFault (Mt. Etna) and induced seismicity: the October 2009–January 2010 episode.Annals of Geophysics 54, 414–423. http://dx.doi.org/10.4401/ag-4841.

Giammanco, S., Gurrieri, S., Valenza, M., 1998. Anomalous soil CO2 degassing in relationto faults and eruptive fissures on Mount Etna (Sicily, Italy). Bulletin of Volcanology60, 252–259.

Giammanco, S., Gurrieri, S., Valenza, M., 1999. Geochemical investigations applied toactive fault detection in a volcanic area: the North East Rift on Mt. Etna (Sicily,Italy). Geophysical Research Letters 26, 2005–2008.

Giammanco, S., Sims, K.W.W., Neri, M., 2007. Measurements of 220Rn and 222Rn andCO2 emissions in soil and fumarole gases on Mt. Etna volcano (Italy): implicationsfor gas transport and shallow ground fracture. Geochemistry, Geophysics,Geosystems 8, Q10001. http://dx.doi.org/10.1029/2007GC001644.

Giammanco, S., Immè, G., Mangano, G., Morelli, D., Neri, M., 2009. Comparison betweendifferent methodologies for detecting radon in soil along an active fault: the case ofthe Pernicana fault system, Mt. Etna (Italy). Applied Radiation and Isotopes. http://dx.doi.org/10.1016/j.apradiso.2008.09.007.

Gurrieri, S., Valenza, M., 1988. Gas transport in natural porous mediums: a method formeasuring CO2 flows from the ground in volcanic and geothermal areas.Rendiconti della Societa Italiana di Mineralogia e Petrologia 43, 1151–1158.

Gurrieri, S., Liuzzo, M., Giudice, G., 2008. Continuous monitoring of soil CO2 flux on Mt.Etna: the 2004–2005 eruption and the role of regional tectonics and volcano tec-tonics. Journal of Geophysical Research 113, B09206. http://dx.doi.org/10.1029/2007JB005003.

Kanemasu, E.T., Powers, W.L., Sij, J.W., 1974. Field chamber measurements of CO2 fluxfrom soil surface. Soil Science 118, 233–237.

Lanzafame, G., Leonardi, A., Neri, M., Rust, D., 1997. Late overthrust of the Appenine–Maghrebian Chain at the NE periphery of Mt. Etna, Italy. Comptes Rendus de l'Academie des Sciences, Paris 324, 325–332.

Lentini, F., 1982. The geology of the Mt. Etna basement. Memorie della SocietaGeologica Italiana 23, 7–25.

Lewicki, J.L., Bergfeld, D., Cardellini, C., Chiodini, G., Granieri, D., Varley, N., Werner, C.,2005. Comparative soil CO2 flux measurements and geostatistical estimationmethods on Masaya volcano. Bulletin of Volcanology 68, 76–90.

Massonnet, D., Feigl, K.L., 1995. Discrimination of geophysical phenomena in satelliteradar interferograms. Geophysical Research Letters 22, 1537–1540.

Monaco, C., De Guidi, G., Catalano, S., Ferlito, C., Tortorici, G., Tortorici, L., 2008. CartaMorfotettonica del Monte Etna. Litografia Artistica Cartografica, Firenze.

Neri, M., Acocella, V., 2006. The 2004–05 Etna eruption: implications for flankdeformation and structural behaviour of the volcano. Journal of Volcanology

and Geothermal Research 158, 195–206. http://dx.doi.org/10.1016/j.jvolgeores.2006.04.022.

Neri, M., Acocella, V., Behncke, B., 2004. The role of the Pernicana Fault System in thespreading of Mt. Etna (Italy) during the 2002–2003 eruption. Bulletin of Volcanol-ogy 66, 417–430.

Neri, M., Acocella, V., Behncke, B., Maiolino, V., Ursino, A., Velardita, R., 2005. Contrast-ing triggering mechanisms of the 2001 and 2002–2003 eruptions of Mount Etna(Italy). Journal of Volcanology and Geothermal Research 144, 235–255. http://dx.doi.org/10.1016/j.jvolgeores.2004.11.025.

Neri, M., Behncke, B., Burton, M., Galli, G., Giammanco, S., Pecora, E., Privitera, E., Reitano,D., 2006. Continuous soil radon monitoring during the July 2006 Etna eruption. Geo-physical Research Letters 33, L24316. http://dx.doi.org/10.1029/2006GL028394.

Neri, M., Guglielmino, F., Rust, D., 2007. Flank instability on Mount Etna: radon, radarinterferometry and geodetic data from the southern boundary of the unstable sector.Journal of Geophysical Research 112, 1–15. http://dx.doi.org/10.1029/2006JB004756.

Neri, M., Mazzarini, F., Tarquini, S., Bisson, M., Isola, I., Behncke, B., 2008. The changingface of Mount Etna's summit area documented with Lidar technology. GeophysicalResearch Letters 35. http://dx.doi.org/10.1029/2008GL033740.

Neri, M., Casu, F., Acocella, V., Solaro, G., Pepe, S., Berardino, P., Sansosti, E., Caltabiano,T., Lundgren, P., Lanari, R., 2009. Deformation and eruptions at Mt. Etna (Italy): alesson from 15 years of observations. Geophysical Research Letters 36, L02309.http://dx.doi.org/10.1029/2008GL036151.

Neri, M., Acocella, V., Behncke, B., Giammanco, S., Mazzarini, F., Rust, D., 2011a. Structuralanalysis of the eruptive fissures at Mount Etna (Italy). Annales Geophysicae 54, 5.http://dx.doi.org/10.4401/ag-5332.

Neri, M., Giammanco, S., Ferrera, E., Patanè, G., Zanon, V., 2011b. Spatial distribution ofsoil radon as a tool to recognize active faulting on an active volcano: the exampleof Mt. Etna (Italy). Journal of Environmental Radioactivity 102, 863–870. http://dx.doi.org//10.1016/j.jenvrad.2011.05.002.

Notsu, K., Mori, T., Chanchah Do Vale, S., Kagi, H., Ito, T., 2006. Monitoring quiescentvolcanoes by diffuse CO2 degassing: case study of Mt. Fuji, Japan. Pure and AppliedGeophysics 163, 825–835.

Ogniben, L., 1966. Lineamenti idrogeologici dell'Etna. Rivista Mineraria Siciliana100–102 (XVII, n.).

Parello, F., D'Alessandro, W., Aiuppa, A., Federico, C., 2001. Cartografia geochimica degliacquiferi etnei. Gruppo Naz. per la Difesa dalle Catastrofi Idrogeologiche, C.N.R. — n.Officine Grafiche Riunite, Palermo, p. 2190.

Ruch, J., Acocella, V., Storti, F., Neri, M., Pepe, S., Solaro, G., Sansosti, E., 2010. Detachmentdepth revealed by rollover deformation: an integrated approach at Mount Etna. Geo-physical Research Letters 37, L16304. http://dx.doi.org/10.1029/2010GL044131.

Siniscalchi, A., Tripaldi, S., Neri, M., Giammanco, S., Piscitelli, S., Balasco, M., Behncke, B.,Magrì, C., Naudet, V., Rizzo, E., 2010. Insights into fluid circulation across thePernicana Fault (Mt. Etna, Italy) and implications for flank instability. Journal ofVolcanology and Geothermal Research 193, 137–142. http://dx.doi.org/10.1016/j.jvolgeores.2010.03.013.

Solaro, G., Acocella, V., Pepe, S., Ruch, J., Neri, M., Sansosti, E., 2010. Anatomy of an un-stable volcano from InSAR: multiple processes affecting flank instability at Mt.Etna, 1994–2008. Journal of Geophysical Research 115, B10405. http://dx.doi.org/10.1029/2009JB000820.

Sorey, M., Evans, B., Kennedy, M., Farrar, C.D., Hainsworth, L.J., Hausback, B., 1998. Carbondioxide and helium emissions from a reservoir of magmatic gas beneath MammothMountain, California. Journal of Geophysical Research 103 (B7), 15303–15323.

Walter, T.R., Acocella, V., Neri, M., Amelung, F., 2005. Feedback processes betweenmagmatism and E-flank movement at Mt. Etna (Italy) during the 2002–2003 erup-tion. Journal of Geophysical Research 110, B10205. http://dx.doi.org/10.1029/2005JB003688.

Williams-Jones, G., Stix, J., Heiligmann, M., Charland, A., Sherwood Lollar, B., Arner, N.,Garzon, G., Barquero, V.J., Fernandez, E., 2000. A model of diffuse degassing at threesubduction-related volcanoes. Bulletin of Volcanology 62, 130–142.