Tectonophysics 471 (2009) 78–86

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Tectonophysics

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Intrusion of eccentric dikes: The case of the 2001 eruption and its role in thedynamics of Mt. Etna volcano

Alessandro Bonforte ⁎, Salvatore Gambino, Marco NeriIstituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma n. 2, 95123 — Catania, Italy

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

0040-1951/$ – see front matter © 2008 Elsevier B.V. Adoi:10.1016/j.tecto.2008.09.028

a b s t r a c t

a r t i c l e i n f o

Article history:

The 2001 eruption represen Received 19 November 2007Received in revised form 3 September 2008Accepted 10 September 2008Available online 27 September 2008

Keywords:Stress releaseDikeVolcano-tectonicsFlank instabilityMt. EtnaInstrumental monitoring

ts one of the most studied events both from volcanological and geophysical pointof view on Mt. Etna. This eruption was a crucial event in the recent dynamics of the volcano, marking thepassage from a period (March 1993–June 2001) of moderate stability with slow, continuous flank sliding andcontemporaneous summit eruptions, to a period (July 2001 to present) of dramatically increased flankdeformations and flank eruptions. We show new GPS data and high precision relocation of seismicity inorder to demonstrate the role of the 2001 intrusive phase in this change of the dynamic regime of thevolcano. GPS data consist of two kinematic surveys carried out on 12 July, a few hours before the beginning ofthe seismic swarm, and on 17 July, just after the onset of eruptive activity. A picture of the spatial distributionof the sin-eruptive seismicity has been obtained using the HypoDD relocation algorithm based on thedouble-difference (DD) technique. Modeling of GPS measurements reveals a southward motion of the uppersouthern part of the volcano, driven by a NNW–SSE structure showing mainly left-lateral kinematics. Precisehypocenter location evidences an aseismic zone at about sea level, where the magma upraise wascharacterized by a much higher velocity and an abrupt westward shift, revealing the existence of a weakenedor ductile zone.These results reveal how an intrusion of a dike can severely modify the shallow stress field, triggeringsignificant flank failure. In 2001, the intrusion was driven by a weakened surface, which might correspond toa decollement plane of the portion of the volcano affected by flank instability, inducing an additional stresstestified 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, whichcan culminate in catastrophic failure and debris avalanches, orproceed as slow gravitational spreading of the instable flanks (Voightet al., 1981; Van Wyk de Vries et al., 2001).

Mt. Etna volcano (Italy) was affected several thousands of yearsago by a giant lateral collapse (Calvari et al., 1998), which caused amassive tsunami along the coasts of the eastern Mediterranean Sea(Pareschi et al., 2006). At present, this volcano is characterized by slowand continuous displacement of its eastern to southern flanks (Borgiaet al., 1992), involving an on-shore area of N700 km2 (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.

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Recent works (Acocella and Neri, 2003; Acocella et al., 2003;Walter et al., 2005) have highlighted that there are feedback processesat Etna between flank deformation and eruptive activity. Flankinstability produces extension in the upper part of the volcano (i.e.the summit crater zone) facilitating shallow intrusions (Neri andAcocella, 2006). In turn, flank instability is accelerated by magmaintrusions 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 remarkableeruptive and deformative events, giving us the opportunity to improveour 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 itwas characterized by a double magmatic plumbing systems: central-lateral and eccentric (Behncke and Neri, 2003; Neri et al., 2005); thismeans that magma did not upraise only from the main conduit tointrude towards the volcano’s flank, but it upraised also directly from adeeper reservoir to the surface, forcefully opening a new path. Clearly,such an event induces an exceptional stress on a volcanic edifice, withconsequent ground deformation and seismic energy release that affectthe volcano for several years, heavily conditioning its future activity.

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Understanding the dynamics of an eccentric intrusion can beimportant to reveal the main regional stress tensor controlling theearly formation of the dike, the main crustal discontinuities perturb-ing the dike propagation, geometry and kinematics and the laterresponse of the volcano to the additional stress induced by the forcefulintrusion. Here we discuss in detail the events encompassing a majorand complex flank eruption in 2001 trough by seismic and GlobalPositioning System (GPS) data. Kinematic GPS data were collectedtwice per day during the development of the pre- and syn-eruptivefracture field, giving us a unique opportunity to measure the velocitiesof its opening and propagation. The aim of this work is to resolve, ifpossible, some open questions about (I) the path used by the magmaduring its migration across the shallow crust; (II) the meaning of thedistribution of seismicity in space and time; (III) the related grounddeformation, and finally (IV) the role of this eruption in the recentdynamics of Mt. Etna.

2. The recent eruptions

After the large 1991–1993 flank eruption, Etna was quiescent untilthe second half of 1995, when summit eruptive activity resumed. Frommid-1995 to mid-2001 a progressive increasing in the eruptive

Fig.1.Map showing the areas affected by the eruptive fractures of the 2001 eruption (a) and thin the southern to eastern sectors of the volcano and the main faults confining three slide bloand its possible off-shore continuation, 3) Etneanvolcanic rocks, 4)Undifferentiated sedimentunstable sector, PFS=Pernicana Fault System, TFS=Timpe Fault system, RFS=Ragalna Faultstations too. Inset c) shows the relationship between central–lateral and eccentric eruptive cconsecutive eruptive activity. Central–lateral activity occurred at vents fed from the centraVOR=Voragine, BN=Bocca Nuova, SEC=Southeast Crater), whereas the eccentric vents w

activity occurred, manifested through N120 lava fountaining episodesand numerous lava overflows from the summit craters (Harris andNeri, 2002; Calvari et al., 2003; Behncke et al., 2003, 2005, 2006). Inthe same period, a continuous inflation of the volcano, with an almostconstant rate, was recorded by the GPS, Tilt and Electronic DistanceMeasurements (EDM) networks managed by the Catania Section ofthe Istituto Nazionale di Geofisica e Vulcanologia (INGV) (Bonforteand Puglisi, 2003; Puglisi and Bonforte, 2004; Bonaccorso et al., 2004;Neri et al., 2005). Moreover, the entire eastern flank of the volcanowasaffected by a continuous and fairly constant seaward motion, detectedalso by InSAR (Froger et al., 2001) and GPS data at a rate of 2–3 cm/yand modeled as a large-scale sliding involving the volcanic piletogether with the upper part of the sedimentary substratum, down toa depth of about 4 km (Bonforte and Puglisi, 2003).

The highly explosive 2001 eruption was the first flank eruptionafter that of 1991–1993 (Behncke and Neri, 2003a,b). It was followedin 2002–2003 by a second, even more explosive flank eruption(Andronico et al., 2005; Neri et al., 2005), and in 2004–2005 by a thirdflank eruption, this time mainly effusive (Burton et al., 2005; Neri andAcocella, 2006). In 2006–2007, summit activity resumed withintermittent explosive–effusive eruptions at the South-East Crater(Neri et al., 2006; Behncke et al., 2008).

e position of theGPSN–S profile (black triangles). Inset b) indicates the area of instabilitycks; 1) unstable sliding blocks, 2) Front of the anticline that delimits the unstable sectorary rocks, 5) Faults, 6) Directionofmovement of theunstable blocks, 7) Boundaries of theSystem (simplified fromNeri et al., 2004). Themap shows the locations of seismic and tiltentres (from Neri et al., 2005). The vents (black bold lines) are numbered according tol conduit system, which corresponds to the Summit Craters (NEC=Northeast Crater,ere not related to the central conduits. Thin lines indicate dry fractures and faults.

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2.1. The 2001 eruption: a dual feeding system

The 2001 eruption lasted from17 July until 9 August 2001 (Behnckeet al., 2005) and occurred from a total of seven eruptive fissures (Fig.1,Billi et al., 2003; Lanzafame et al., 2003). This eruption showed highdegrees of explosivity, producing 5–10×106 m3 of ash and~25×106m3 of lava (Behncke andNeri, 2003a). This event representedboth types of Etnean flank eruptions as defined by Rittmann (1973):“lateral” and “eccentric”. In the first case, the eruptions originate fromthe lateral draining of the central conduit system (the case of vents 1, 2,3, 6 and 7 in Fig. 1c), while the “eccentric” eruptions originate withmagmaascending throughdistinct, independent conduits (vents 4 and5 in Fig. 1c) with respect to the central conduit system (Acocella andNeri, 2003; Behncke and Neri, 2003b; Allard et al., 2006). The twogroups of vents simultaneously delivered two distinct magma types(Clocchiatti et al., 2004; Métrich et al., 2004): degassed trachybasalticmagma was drained from the central conduits, while a volatile-richprimitive basaltic–trachybasaltic magma was erupted from the twoeccentric vents.

The eruption was heralded by a seismic swarm, which startedduring the night of 12–13 July (Patanè et al., 2003b). Epicentres werelocated south of the summit craters. At the same time, numerous dryfractures began to dissect the ground surface in the Belvedere zone(Fig. 1). Ground uplift of up to 0.2 m in the Montagnola area becameevident on 13 July and continued until 16 July (Bonaccorso et al.,2002). On 16–17 July a 1400 m long, 500 m wide, and NNE–SSWtrending graben formed in the Cisternazza area (Fig. 1c; Billi et al.,2003). Total extension measured across the fractures was over 2 m,with vertical displacements of up to 0.5 m (Lanzafame et al., 2003).

Fig. 2. Epicenter map, W–E and N–S cross sections of re-located seismicity. M cluster is compbetween deep and shallow intrusions (see Fig. 3). S cluster is characterized by left-lateral f

In the following chapters, we will discuss a high precisionrelocation of seismicity and new data obtained by a kinematic surveyground deformation measured by GPS close to the summit area.

3. Instrumental data

3.1. Seismicity

In 2001, the permanent seismic network of INGV comprised 35seismic stations (Fig.1b)mainly equippedwithmono-component shortperiod sensors. Good coverage of the area and the recent definition ofrobust 3D velocity models, which adequately represent the internalstructure of the volcano, allow accurate locations of the seismicity.

The intrusion of the 2001 eccentric dike and the opening of theeruptive fractures were accompanied by a strong seismic activity(Patanè et al., 2003b) and significant ground deformation (Bonac-corso et al., 2002). INGV local permanent seismic network detected atotal of 2645 MdN1.0 earthquakes, 62 with Md=3.0, Mmax=3.9starting from 12 July (at 22:00 GMT) to the end of the eruption. Forour analysis, we considered a best subset of 350 earthquakes withmagnitude MdN2.0 previously located with Hypoellypse (Patanè etal., 2003b) and characterized by a location accuracy range between 0.2and 1 km (average 0.4) for the epicentral coordinates and between 0.3and 1.4 km (average 0.6) for focal depth.

In order to obtain a better picture of the spatial distribution of theearthquakes, we relocated earthquakes using the HypoDD relocationalgorithm that is based on the double-difference (DD) techniquedescribed in Waldhauser and Ellsworth (2000) and Waldhauser(2001). This method allows obtaining a spatial distribution of the

osed of earthquakes clustered in correspondence to the dike. 15/07 h. 10:00 is the limitocal mechanism (Patanè et al., 2003b; Gambino, 2004).

Fig. 3. Time migration of re-located earthquake foci related with evolution of intrusive phases, surface fracturing and eruptive phenomena. M cluster (black dots) and peripheralseismicity (grey dots) are defined in Fig. 2. Times of kinematic GPS surveys are also reported.

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earthquakes with a higher precision with respect to the traditionallocalization techniques, highlighting several clusters of events. Themethod takes advantage of the fact that, if the hypocentral separationbetween two earthquakes is small compared to the event-stationdistance and the scale of velocity heterogeneity, then the ray paths canbe considered identical along their entire length. Under theseassumptions, the differences in the travel times can be attributed todifferences in their hypocentral spatial separation.

Moreover, before HypoDD processing we improved the absolutelocation using the SIMULPS14 code (from Thurber, 1993) and anaccurate 3D velocity model developed by Patanè et al. (2003a) withthe same data. Using 3D starting hypocentral parameters in double-difference routine strongly reduces differences between absolute andrelative location producing much more accurate relative eventlocations (Gambino et al., 2004).

The final HypoDD dataset consists of 341 earthquakes character-ized by a mean relative location accuracy of 0.15 km for the epicentralcoordinates and 0.17 km for focal depth. Epicentral maps and N–S andE–Wcross sections of the re-located seismicity are shown in Fig. 2; thetime-depth evolution of the earthquakes is presented in Fig. 3.

Table 1Kinematic stations measured during the July 17th survey.

Stationname

1stoccup.

2ndoccup.

Eastdisplac.(m)

Northdisplac.(m)

Heightdisplac.(m)

Easterror(m)

Northerror(m)

NS07 11.48NS08 11.41 12.09 −0.004 −0.023 0.105 0.006403 0.007071NS09 11.34 12.16 0.007 −0.02 0.116 0.006403 0.007071NS10 11.27 12.22 0 −0.005 0.114 0.005657 0.007071NS11 11.19 12.30 0.008 −0.005 0.12 0.005657 0.00640NS16 10.57 12.52 −0.023 −0.008 0.118 0.006403 0.007071NS17 10.50 12.59 −0.016 −0.013 0.119 0.007071 0.007071NS18 10.40 13.06 −0.004 −0.01 0.124 0.006403 0.00640NS20 10.23 13.15 −0.015 −0.007 0.122 0.006403 0.00640NS21 10.09 13.27 −0.036 −0.027 0.112 0.006403 0.00640NS22 09.12 13.37 0.001 −0.034 0.114 0.007211 0.00781

Times (GMT) of first and second occupations are listed. Displacements measured along e(2 sigma). Displacements are positive towards East, North and Up.

The 12–19 July earthquakes affected the southern sector of thevolcano in a main (M) cluster, related to the dike intrusion, and inperipheral areas (Figs. 2 and 3)with aminor cluster (“Peripheral seism.”in Fig. 2) on the upper S-SE sector of the volcano (Gambino, 2004).Precise locations have allowed evidencing several aspects of thisseismicity:

– a fast upward migration of the seismicity starting from 15 July (at10:00 GMT). In particular, earthquakes of the M cluster from 13July to 14 July occurred at a depth between 2.5 and 1.2 km b.s.l.(bsl); from 15 July (at 10:00 GMT) the depth of this seismicity wasconfined at ~0.5 km a.s.l. (asl);

– a spatial gap in the seismicity appeared at ~0–0.5 km bsl (Fig. 2);– a time interval (several hours) characterized by very low seismic

activity, lasted from late 14 July to themorningof thenext day (Fig. 3);– a clear shift of epicentral locations of M earthquakes toward W–

SW occurred during the uprise of magma (grey events in Fig. 2),just below the eccentric eruptive fractures (Fig. 1c).

After the dike reached the surface, seismicity remained at very lowlevels for the whole period of the eruption.

Heighterror(m)

Eastvelocity(cm/h)

Northvelocity(cm/h)

Heightvelocity(cm/h)

Easterror(cm/h)

Northerror(cm/h)

Heighterror(cm/h)

0.016279 −0.857 −4.929 22.500 1.372 1.515 3.4880.016279 1.000 −2.857 16.571 0.915 1.010 2.3260.016279 0.000 −0.545 12.436 0.617 0.771 1.776

3 0.016279 0.676 −0.423 10.141 0.478 0.541 1.3760.016279 −1.200 −0.417 6.157 0.334 0.369 0.8490.016279 −0.744 −0.605 5.535 0.329 0.329 0.757

3 0.016279 −0.164 −0.411 5.096 0.263 0.263 0.6693 0.016971 −0.523 −0.244 4.256 0.223 0.223 0.5923 0.016971 −1.091 −0.818 3.394 0.194 0.194 0.514

0.017692 0.023 −0.770 2.581 0.163 0.177 0.401

ach component and relative computed velocities are reported with associated errors

Fig. 4. Ground deformation observed by GPS measurements (a) from July 12 to 17 and (b) on July 17 AM to PM. Arrows indicate the horizontal displacements measured while thecolour thematic map has been obtained by interpolating the station velocities. It is interesting how the maximum velocity zone expands towards SSE during the last hours of theintrusion (b).

Table 2Parameters of the model for the July 12th–17th time interval from GPS data.

Parameter Starting values(Bonaccorso et al., 2002)

Dike

X UTM zone 33N (km) 501.06 500.16Y UTM zone 33N (km) 4176.1 4175.42Azimuth 7° W 7.8° WZ (km) 0.5 0.2Length (km) 2.2 3.8Width (km) 2.3 1.1Dip angle 90° 82.5° EStrike slip (cm) 0 103 Right lateralDip slip (cm) 0 155 Normal (east down)Opening (cm) 350 208

Starting values, taken from Bonaccorso et al. (2002) are also reported. The modelindicates a roughly N–S dike, slightly East dipping, with a main tensile kinematics andsignificant normal and right lateral components of motion.

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3.2. The Global Positioning System (GPS) survey

A dense GPS network on Etna has been surveyed periodically (atleast once per year) since 1988. During the past two decades, itsconfiguration has been continuously improved so that it now coversthe entire volcanic area (Bonforte and Puglisi, 2003); in particular, in1995, another 22 benchmarks were installed along an approximately10 km N–S profile, starting and ending at about 2000 m asl andcrossing the summit crater area (Puglisi and Bonforte, 2004).

Two GPS surveys (reported in Puglisi et al., 2008) were carried outon 12 July, a few hours before the beginning of the seismic swarm, andon 17 July, just after the onset of eruptive activity from thefirst eruptivefractures at 2900 m asl (Fig. 3). These surveys were conducted on thesouthern flank of the volcano,measuring only the upper stations of theMt. Etna GPS network (from 2000 to 3100 m asl; Fig. 1) and along partof the N–S profile. The profile was measured forward and backward insemi-kinematic mode (Puglisi and Bonforte, 2004).

The kinematic measurements started from the southernmoststation of the profile (NS22, Fig. 1a) with a first session of 30 min,and then the receiver moved northward to the NS07 station, surveyingeach station of the profile with 3 min sessions with an acquisition rateof 5 s (Table 1). At the NS07 station, a session of about 15 min wascarried out, before beginning to return to re-surveying all the stationsand closing the survey with a final 20 min session at NS22.

Considering the rapidly evolving eruptive and deformativedynamics accompanying the 17 July measurements, the day-longkinematic survey will be considered here as two sub-surveys, splittingthe whole data set in order to process independently the forward andthe backward kinematic measurements along the N–S profile (herenamed NS-fore and NS-back sub-surveys). This was possible thanks tothe longer session carried out at 12:00 GMT on the NS07 station, thatcould be used as a new static initialization of the backward path. Alsothe static data were split in two sub-sets, before and after 12:00 GMT.Precise ephemeredes produced by International GPS Service (IGS)were used in the processing. Static data acquired by the Mt. Etna GPSpermanent network during the kinematic measurements are alsoconsidered. This particular processing gave quite larger errors (up to7 mm for the horizontal components and 15 mm for the vertical onesfor kinematic measurements), but still acceptable to study theongoing dynamics, due to the very strong ground deformationrecorded (up to several tens of centimeters).

4. Results

By comparing the 17 July N–S-fore sub-survey to the one carriedout on 12 July, a few hours before the beginning of the seismic swarm(Fig. 3), it is possible to analyze the general surface deformationproduced by the upraise of the dike, before the fractures opening atthe surface. Strong E–W extension of the upper part of the volcano isvisible, (arrows in Fig. 4a), near the La Montagnola area (Fig. 1). From12 to 17 July, important horizontal displacements, up to ~90 cm, affectthe stations very close to the fracturing area (Belvedere-Cisternazza inFig. 1) in the upper southern part of the volcano, where a large grabenformed on 16 July (Fig. 3), confirming also the pattern described byPuglisi et al. (2008). We also normalized the measured horizontaldisplacements to velocities (centimeters/hour, cm/h) interpolatingthem above the investigated area, in order to give more reliableinformation for this comparison (colours in Fig. 4a) in space and time.

The comparison between the forward and the backward pathsmeasured on 17 July (Table 1), allowed us to analyze the rapid grounddeformation produced by the very last hours of the intrusion process,when magma was reaching the surface. In this case it is necessary tonormalize the measured displacements to velocities (Fig. 4b), due tothe increasing time interval between the two occupations fromnorthern to southern kinematic station (Table 1). The horizontalvelocity value for the northernmost station of the kinematic profile

Fig. 5. Comparison between observed (black arrows) and expected (grey arrows)horizontal displacements for the July 12–17 time interval. The modelled dike is shownin light grey. See text for details of the model.

Table 3Parameters of the model for the July 17th AM–PM time interval from GPS data.

Parameter Dike Fault

X UTM zone 33N (km) 500.27 501.63Y UTM zone 33N (km) 4175.137 4172.9Azimuth 2.9° W 24.8° WZ (km) 0.4 0.4Length (km) 3.2 4.6Width (km) 0.4 2.2Dip angle 89° W 66° EStrike slip rate (cm/h) 20.5 Right lateral 13.6 Left lateralDip slip rate (cm/h) 6.9 Inverse (East down) 2.4 Inverse (West down)Opening rate (cm/h) 40.9 7.8

The model indicates a N–S striking vertical dike with a main tensile kinematics and asignificant right lateral component of motion, coupled with a NNW-SSE East-dippingfault with a main left lateral kinematics. The combined strike-slip and dip-slipcomponents of the two sources define a southwards moving and down throwing slicebetween them.

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realized on 17 July is not available because of the unique sessioncarried out there. Furthermore, the high velocities resulting for theadjacent two northern stations of the profile are affected by largeerrors (see Table 1) due to the short time interval elapsed between theforward and backward sessions.

The northern stations of the network, which are far from thefractured area and then are affected by elastic deformation, show aslight horizontal southward motion (Fig. 4) that is in good agreementwith the intrusion of a ~N–S dike controlled by the regional tectonicstress field (Cocina et al., 1997), with a horizontal N–S compressionaxis. On the contrary, a significant horizontal southward motionalways affects the southern flank, involving more stations at loweraltitude on 17 July (Fig. 4b, South of the La Montagnola area up to~1500 m asl) with respect to the 12–17 July comparison (Fig. 4a, onlystations around and North of La Montagnola area).

Fig. 6. Comparison between observed (black arrows) and expected (grey arrows) horizontalalso the NNW–SSE fault. The improvement of the expected deformation on the southern fl

continuation of the faults delimiting the movement of the block 3 highlighted in Fig. 1b. Se

4.1. GPS data inversions and deformation models

Inversions of the GPS datasets were performed by using the Okada(1985) dislocation model and a Least Square Algorithm (LQA)approach. For the comparison between the 12–17 July N–S-foredata, the dike modeled by Bonaccorso et al. (2002) was used asstarting point, leaving all geometric and kinematic parameters free tobe searched for. The inversions converged to a single dike showing anazimuth similar to the dike modeled by Bonaccorso et al. (2002), butshallower, eastward dipping and displaced westward (Table 2 andFig. 5). Furthermore, it showed an opening of about 2 m, with asignificant normal component of motion (about 1.5 m) and a minorright lateral behaviour (1 m), downthrowing its eastern side south-eastwards. This pattern was confirmed by the fracture field developedin the upper southern flank of Etna on 13–17 July (Fig. 1; Billi et al.,2003; Lanzafame et al., 2003). The fit between measured andexpected displacements is quite good, but a stronger measured south-ward component of motion (51 cm) in one point (point 18, at about2600 m asl) remains unexplained by the simple elastic model (Fig. 5).

In the case of the forward and backward paths carried out on 17July, the comparison between calculated and measured data showsstrong differences of motion in numerous southern stations, whichcannot be explained by the intrusion of the dike alone (see thedifferences between observed and expected velocities in Fig. 6a), even

velocities for the July 17 AM–PM time interval, by considering (a) only the dike and (b)ank when considering also the fault is evident. Grey dotted lines indicate the possiblee text for details of the model.

Fig.7. 3-D model (looking North) illustrating the magma uprise (red arrow) during 13–17 July 2001, and related deformations. Blue squares indicate the seismicity (see Fig. 2 fordetails). A shift in the magma uprise is clearly visible in correspondence of the aseismic zone at ~0 km above sea level, separating the shallow (a) and deep (b) zones of intrusion.Yellow arrows show the direction of movement of the unstable blocks (in dots the arrows not involved directly during the event here described). 1) and 2) indicate the position of thesliding surface modeled by Bonaccorso et al. (2006) and by Bonforte and Puglisi (2006) respectively. The line inwhite dots (c) indicates the position of the fault modeled in this work,moved during the final stage of the intrusion process. See text for details.

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considering the strong limitations in the modeling due to the nearfield conditions and brittle response of the medium affected by hugefracturing.

A second dislocation source has thus been added, using as startingpoint the NNW–SSE source modeled by GPS data collected during theeruption by Bonforte et al., 2004. The final model shows a doubledeformation source, composed by a dike and a left-lateral transcurrentfault (Table 3 and Fig. 6b). The addition of the NNW–SSE faultimproves the fit on the southern part of the network, but generates anorthward motion on the northern summit stations due to thesymmetric left-lateral kinematics expected for an elastic medium (seethe differences between observed and expected velocities in Fig. 6b).This misfit confirms the actual non-elastic behavior of the volcano,with a southward motion of the western side of the modeled NNW–

SSE fault, not accompanied by a northward motion of its eastern side.The kinematics of the fault, together with the right lateral

component of motion of the modeled dike, define a subsidingtriangular “slice” on the southern flank of the volcano, moving itsouthward (see block 3 in Fig.1b and Fig. 7). Themotion of this portionof the volcano is consistentwith the complex kinematic of the fracturesopened in the same area during the eruption (Lanzafame et al., 2003).

This picture is in agreement with the volcano-tectonic modelswhich describe Mount Etna as affected by flank instability (Neri et al.,2007, and reference therein). This involves a large eastern-to-southernsector of the volcano, divided into three different blocks characterizedby different directions and rates of sliding (Figs. 1b and 7; Neri et al.,2004). Block 3 ismoving southwards, confined toward east by a NNW–

SSE left transtensive fault which could correspond to the aforemen-tioned NNW–SSE structure modeled here. Moreover, this faultevidently was able to change its kinematics (from left to dextraltranstension) when block 2 moved more rapidly southeastward withrespect to block 3. This case occurred a few days (25–27 July 2001)after the 2001 dike intrusion, when the sliding affecting the flanks ofEtna accelerated expanding eastward (Bonforte et al., 2004).

4.2. Matching GPS data and seismicity

Ground deformation and seismic data allow the dynamics of the2001 eccentric dike intrusion and the response of thevolcano's edifice to

be analyzed in great detail both in space and time. The precisehypocenter locations evidenced an aseismic zone at about sea level,where the magma uprise was characterized by a much higher velocityand by the abrupt westward shift. This peculiarity reveals the existenceof a weakened or ductile zone, which might correspond to a basaldecollement plane of the portion of the volcano affected by flankinstability, and thatwas intercepted by the uprise of the dike. In fact, thiszone well corresponds to the westwards prolongation of the slidingsurface (“1” in Fig. 7) modeled by Bonaccorso et al. (2006). Grounddeformationdatamodelingpresentedhere (Figs. 5 and6, Tables2 and3)confirms the westwards shift of the upper part of the intrusion, as alsorevealed by the distribution of the seismicity (grey M cluster in Fig. 2).

Furthermore, the GPS measurements on the kinematic profileevidence a southwards motion of the upper southern part of thevolcano, bounded by a NNW–SSE structure showing a mainly left-lateral kinematics, in good agreementwith the focal mechanism of the“Peripheral seismicity” (Fig. 2) on the upper S-SE part of the volcano(Patanè et al., 2003b; Gambino, 2004).

Almost all the peripheral seismicity on the eastern side of thevolcano (i.e. not directly related to the eccentric dike uprise),including also the “Peripheral seismicity” cluster, is confined abovethe deeper decollement plane (“2” in Fig. 7) modeled by Bonforte andPuglisi (2006) using GPS data.

4.3. The role of the 2001 eruption in the recent dynamics of Mount Etna

The 2001 eruptionmarks a dramatic change in the eruptive activityof Mt Etna of the last decade. The activity of Mount Etna betweenearly-1993 and 2008 represents an example of an eruptive cycle, froman initial phase of eruptive quiescence (1993–1995) over a phase ofsummit eruptions (1995–2001) to a phase of frequent flank eruptions(2001–present; Behncke and Neri, 2003a; Allard et al., 2006). The firstphase of this cycle is characterized by a temporary quiescence of thevolcano, after the voluminous 1991–1993 eruption. During and for aperiod following that eruption, ground deformation data indicatedthat the volcanic edifice deflated (Puglisi et al., 2001; Bonaccorso et al.,2004).

The second phase marks the recharging of the plumbing system. Itwas characterized by a fairly continuous inflation (from late 1994 to

85A. Bonforte et al. / Tectonophysics 471 (2009) 78–86

mid-2001; Houlié et al., 2006) and the resumption of summit eruptiveactivity (since mid-1995 to mid-2001). These events were accom-panied (since 1996) by a slow~aseismic displacement of the easternto southeastern flanks (Froger et al., 2001; Bonforte and Puglisi, 2003;Lundgren et al., 2004; Puglisi and Bonforte 2004; Bonforte and Puglisi,2006).

The third phase, beginning with the 2001 flank eruption andcontinuing to the present, marked a crucial change in Etna's dynamicregime: the shift from almost continuous summit activity following andaccompanying a fairly continuous inflation, to a series of flank eruptions(2001, 2002–2003, 2004–2005, 2008), accompanied by marked infla-tion/deflation phases and strong increase in flank displacement (Allardet al., 2006 and reference therein; Bonforte et al., 2008) and by theresumption of violent summit activity in the secondhalf of the 2006 andin 2007–2008 (Neri et al., 2006; Behncke et al., 2008). It is possible thatthis shift was triggered by the forceful uprise of magma along an ec-centric path, which was not only manifested in the 2001 flank eruptionbut also caused an accelerated displacement of the unstable flank sector(Neri et al., 2005; Bonforte et al., 2007c; Puglisi et al., 2008).

According to Neri et al. (2005), the 2001 eruptionwas triggered bymagmatic overpressure and intrusion of volatile-rich primitivemagma across the S flank. Refilling of the plumbing system beganseveral months after the end of the 2001 eruption, as evidenced by tilt,GPS and seismic data (Patanè et al., 2003a) and by the resumption ofmild summit activity in the summer of 2002 (Allard et al., 2006). TheSeptember 2002 seismicity on the Pernicana fault was induced by theincreased flank slip (Bonforte et al., 2007c) and was interpreted byAcocella et al. (2003) and Neri et al. (2005) as the main trigger of thefollowing 2002–2003 flank eruption. However, while this phenom-enonmay reasonably have determined the sudden onset and timing ofthe eruption (allowing a partially passive intrusion along the NE rift,as inferred by Bonforte et al., 2007b), it is most likely that the maintrigger of the 2002–2003 eruption was the magma recharging startedin the previous months (Gambino et al., 2004; Bonforte et al., 2007c).

The 2004–2005 flank eruption was instead characterized by theabsence of short-term precursory phenomena, a slower propagationof fractures, lack of explosive activity, degassing and any eruption-related seismicity, and a significant oblique shear along the fractures.These observations suggest that there was no direct connectionbetweenmagma refilling and the eruption (Burton et al., 2005). This isalso testified by the composition of magma erupted during this event(Corsaro and Miraglia, 2005) which drained the conduit of theSoutheast Crater, where it had remained since its latest previousactivity in 2001. Therefore, a mechanical trigger, instead of a magmaticoverpressure, can be invoked for this passive lava effusion (Burtonet al., 2005; Neri and Acocella, 2006; Bonaccorso et al., 2006).

Thus, the sequence of flank eruptions since 2001 shows a singularprogression from an event entirely triggered by magmatic over-pressure (2001), over one resulting from renewed magma uprise butfacilitated, in time and space, by the displacement of the unstableflanks (2002–2003), to one that was essentially triggered by thecontinued flank displacement and emitted only evolved residualmagma (2004–2005). Also the summit activity, resumed in thesecond half of 2006, was characterized by complex and intensedynamics, with the opening of numerous fractures affecting the flanksof the Southeast Crater cone. In this light, the 2001 eruptionrepresents a pivotal event in the recent dynamic of Etna, markingthe passage from a period (March 1993–June 2001) of moderatestability with slow, continuous flank sliding and contemporaneoussummit eruptions, to a period (July 2001 to present) of dramaticallyincreased flank deformations and flank eruptions.

5. Conclusions

Instrumental and field data collected on the upper part of Mt. Etnavolcano just before and during the intrusion of the eccentric dike in

July 2001 allowed themagma uprise to be clearly defined in space andtime. The earliest phase of the dike intrusion is characterized by analmost vertical uprise, marked by the distribution of the seismicity,between 2.5 and 0.5 km bsl.

Subsequently, there was a phase when the dike ascended virtuallyaseismically but much more quickly over a distance of 500 m. Seismicactivity then reappeared between 0 and 1 km asl, but was displacedwestward with respect to the earlier phase of dike ascent.

This sequence of events indicates the presence of a zone of ductilebehaviour or intense fracturation, located immediately above the sealevel, where the dike could propagate rapidly and aseismically, along asubhorizontal orweakly inclined plane. This suggests the existence of adecoupling structure, which might correspond to the basal decolle-ment plane of the portion of the volcano affected by flank instability,intercepted by the uprise of the dike. This ductile zone can correspondto the prolongation of a decollement plane modelled by Bonaccorsoet al. (2006), while the deeper plane modelled by Bonforte and Puglisi(2006) seems to underlie the whole seismogenetic volume.

The deformations measured by the GPS network can be explainedby modeling two different sources: one linked to the intrusion of thedike, and a second one located in correspondence with a transtensivesinistral tectonic dislocation, whose movement was triggered by thepush of the intrusion of the same dike. On the other hand, themovement and position of that fault are consistent with thedeformations affecting the eastern and southern flanks of the volcano,which from 2001 onward have been going on at a rate and extentnever recorded by the instrumental monitoring network.

In the light of these observations, the 2001 eruption plays a crucialrole for the understanding of the recent dynamics of Mount Etna, andmorewidely, for the understanding of the short and long-terms effectsof the emplacement of an eccentric dyke. It is possible that the forcefuluprise of an eccentric magmatic volume served to accelerate the flankdeformations, which continued for several years at higher rates thanthose recorded before 2001 (Bonaccorso et al., 2006; Bonforte et al.,2008).

Acknowledgments

This work was funded by the Istituto Nazionale di Geofisica eVulcanologia and by the Dipartimento per la Protezione Civile (Italy).The authors wish to thank A. Gudmundsson, V. Acocella and S.Vinciguerra who solicited our contribute at EGU 2007-Volcano-Tectonics session. Thanks are due to F. Guglielmino, M. Palano, B.Puglisi , M. Cantarero, F. Calvagna and O. Consoli for their fundamentalrole in field data collection, and to the INGV permanent GPS networkstaff for data availability. Authors are indebted to B. Behncke for helpfuldiscussion and to S. Conway for improvements in the English. Thanksare also due to the reviewer C. S. Clemente for his comments thatimproved the early manuscript.

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