Tectonic features of the Lipari–Vulcano complex (Aeolianarchipelago, Italy) from 10 years (1996–2006) of GPS data

Mario Mattia,1 Mimmo Palano,1 Valentina Bruno,1,2 Flavio Cannavo,1 Alessandro Bonaccorso1 andStefano Gresta2

1Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy; 2Dipartimento di Scienze

Geologiche, Universita Degli Studi di Catania, Corso Italia 57, 95129 Catania, Italy

Introduction

The Aeolian islands are a volcanicarchipelago located in the southernTyrrhenian Sea, off the north-eastcoast of Sicily (Fig. 1a). The present-day tectonic framework of this area isthe result of geodynamic processescaused by the complex N–S toNNW–SSE trending convergence be-tween the African and the Eurasianplates (Barberi et al., 1973; Faccennaet al., 2001, 2004; Goes et al., 2004).The convergence process, spanningthe entire Mediterranean area, is char-acterized by several geodynamic fea-tures such as trench roll-back, back-arcextension and rotating blocks (Dogli-oni et al., 1999). Several hypotheses,often contrasting, have been proposedin the past to explain the currentgeodynamics of this area. However,now it seems widely accepted that,starting in the Neogene–Quaternary,the roll-back of the Ionian plateunderneath Calabria accompaniedthe clockwise rotation of southernItaly and the opening of the Tyrrhe-nian back-arc basin (Malinverno andRyan, 1986; Gueguen et al., 1998;Faccenna et al., 2001, 2004; Pondrelli

et al., 2004; Rosenbaum and Lister,2004; Jenny et al., 2006). NorthernSicily and the southern Aeolian is-lands are a key area for understandingthis complex tectonic framework.With this aim, many authors haverecently investigated this area throughgeodetic and seismological data (An-zidei et al., 2001; Bonaccorso, 2002;Hollenstein et al., 2003; Neri et al.,2003, 2005; D�Agostino and Selvaggi,2004; Argnani et al., 2007). Theseauthors evidenced that, in the north-western part of Sicily, the present-daycrustal motion is dominated by a N–Scompression consistent with geologi-cal and seismological data. On thenorth-eastern part of Sicily and south-ern Calabria, velocity vectors show aclockwise rotation with respect toAfrica, indicating extension of thisarea. All these features clearly evi-dence the presence of two differenttectonic regimes: a compressional do-main in western Sicily and an exten-sional one in eastern Sicily andCalabria. The transition between com-pressional and extensional domainsseems to occur in the proximity ofthe southernmost part of the Aeolianarchipelago, where geological, geo-detic and seismological data suggestthat the dominant deformation isright-lateral shear (Mazzuoli et al.,1995; Doglioni et al., 2001; Bonac-corso, 2002; D�Agostino and Selvaggi,2004; Pondrelli et al., 2004) on a

NNW–SSE tectonic lineament namedthe Aeolian–Tindari–Letojanni faultsystem (hereinafter ATL; Fig. 1a).In this work, we will analyse peri-

odical and continuous GPS (CGPS)data collected between 1996 and 2006on a dense geodetic network installedon the Lipari–Vulcano complex. Wewill present and discuss the mainresults, with respect to the (1) velocityfield (computed in Eurasian andAfrican reference frames) and (2)strain-rate maps of this area. Inparticular, our data show a contrac-tion acting between the two islandsof Lipari and Vulcano and a clearsubsidence of both the islands.

Background setting

The Aeolian archipelago representsthe emerged portion of an extendedring-shaped submarine volcanic com-plex rising from 1–1.5 km b.s.l. andemplaced on a 15- to 20-km-thickcontinental crust. From the struc-tural ⁄volcanological point of view,the archipelago can be divided intothree main sectors: (1) a western sector(Alicudi and Filicudi islands) domi-nated by NW–SE oriented tectoniclineaments; (2) a central sector (Sali-na, Lipari and Vulcano islands)aligned along the NNW–SSE tectonictrend; and (3) an eastern sector (Pan-area and Stromboli islands) charac-terized by prevailing NE–SW oriented

ABSTRACT

The tectonic deformation of the Lipari–Vulcano complex, one ofthe most important active volcanic areas of the Mediterraneanregion, is studied here through the analysis of 10 years (1996–2006) of GPS data from both three permanent and 13 non-permanent stations. This area can be considered crucial for theunderstanding of the interaction between the Eurasian andAfrican plates in the Mediterranean area, and, in general, thiswork emphasizes a methodological approach, already appliedin other areas worldwide (J. Geophys. Res., 1996, 101, 27 957;J. Geodyn., 1999, 27, 213) where geodetic data and strainparameters maps of critical areas can help to improve ourunderstanding of their geodynamical aspects. In this frame-

work, this study is aimed at providing a kinematic deformationmodel on the basis of the dense geodetically estimatedvelocities of the Lipari–Vulcano complex. In particular, theobserved deformation pattern can be described by a combi-nation of (1) the main N–S regional compression and (2) aNNE–SSW compression with a small right-lateral strike slipcomponent acting along a tectonic structure trending N�40Wbetween the two islands. This pattern was inspected through asimplified synthetic model.

Terra Nova, 20, 370–377, 2008

Correspondence: Mattia Mario, Istituto

Nazionale di Geofisica e Vulcanologia,

Sezione di Catania, Piazza Roma 2, 95123

Catania, Italy. Tel.: +39 95 7165805; fax:

+39 95 435801; e-mail: mattia@ct.ingv.it

370 � 2008 Blackwell Publishing Ltd

doi: 10.1111/j.1365-3121.2008.00830.x

tectonic lineaments (De Astis et al.,2003). In this study, we focus on thecentral sector, where the volcanismstarted at 0.4 Ma and is still active atLipari (580 AD) and Vulcano (1888–1890 AD). Detailed structural analyses

have shown that this sector has devel-oped along a graben-like structure,oriented orthogonally to the arc(Ventura, 1994). The most developedsystem of discontinuity affecting thegraben-like basin consists of NW–SE-

to NNW–SSE-trending faults (Ven-tura, 1994). Most of these faults arearranged with an �en echelon� config-uration along two main branchesbordering the western and easternflanks of Vulcano and cutting thesouth-western flank of Lipari(Fig. 1). The area between the twomain right-hand-stepping, involvingboth the southern part of Lipari andthe northern sector of Vulcano,defines a pull-apart basin boundedby N–S to NNE–SSW trending faults.All these features are related to theATL tectonic lineament (Ghisetti andVezzani, 1982; Barberi et al., 1994;Ventura et al., 1999) representing asystem of lithospheric faults, withright-lateral strike-slip movements.Since 1983, more than 800 earth-

quakes (M > 2.5) have occurred inthe southern portion of the Aeolianarchipelago. Most seismicity occurredinside the �Gulf of Patti� area, showinga mix of dextral strike-slip and exten-sional fault-plane solutions, in agree-ment with the tectonic pattern of thenorthern shore of Sicily at this longi-tude (Billi et al., 2006). The remainingseismicity is clustered around theSalina, Lipari and Vulcano islands,in particular on their western side,showing reverse focal mechanismswith NNW–SSE and NW–SE com-pressive axes (Fig. 1b).

Geodetic network and dataprocessing

Continuous GPS monitoring of theground deformations of Lipari andVulcano islands started in April 1995,when a four-site permanent GPS net-work (VVLC, VCSP, LOSV andVGPL; Fig. 2a) was installed by theInternational Institute of Volcanolo-gy, now Istituto Nazionale di Geofisi-ca e Vulcanologia (INGV-CT). VGPLstation has been excluded from thisanalysis because of the anomalousmovements observed, caused by set-tlement of the volcanic pile beneaththe geodetic pillar. In addition, an oldgeodetic network (Bonaccorso, 2002)was surveyed yearly by GPS tech-niques, from 1996 (Fig. 2a and Ta-ble 1). All non-permanent and CGPSdata collected between 1996 and 2006have been processed with the GAMIT

and GLOBK software packages(Herring et al., 2006a,b) with IGS(International GNNS Service) precise

km

0 5 10

N

S

GULF OF PATTI

Lipari

Vu lcano

Salina

N

S

20 km

Filicudi

Salin a Panarea

Strombol i

Lipar i

Vu lcano

Alicudi

S O U T H E R N

T Y R R H E N IA N S E A

AT

L

A E O L I A N A R C H I P E L A G O

SICI L Y

VF CM

N

S

km

0 1. 5 3

Lipari

Vulcano

LC

V

LFC

VP

(a)

FF

(c)

(b)

T

MEDI GRAS

GRAZ

AL yrrhenian

Sea

NOTO

CAGL

NOT1

LAMP

MATE

AJAC

IT Y

Fig. 1 (a) Structural sketch map of the Aeolian archipelago, redrawn from Baroneet al. (1982) and Argnani et al. (2007). ATL, Aeolian–Tindari–Letojanni fault system;VF, Vulcano Fold; CM, Capo Milazzo. (b) Focal mechanisms of larger earthquakesbetween 1976 and 2005 (http://seismology.harvard.edu; Neri et al., 2005). Inset showsthe location of the IGS stations included in the GPS processing. (c) Structural sketchmap of the Lipari–Vulcano complex redrawn from Ventura (1994) and Mazzuoliet al. (1995). V, Vulcanello; LC, Lentia Complex; LFC, La Fossa Caldera; VP,Vulcano Piano; FF, fumaroles field.

Terra Nova, Vol 20, No. 5, 370–377 M. Mattia et al. • Tectonic features of Aeolian area from GPS

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ephemerides and Earth orientationparameters from the InternationalEarth Rotation Service. Data fromnine continuously operating IGS sta-tions (AJAC, CAGL, GRAS, GRAZ,LAMP, MATE, MEDI, NOT1 andNOTO; Fig. 1b) were also included inthe processing (Table 1). The basicproducts of the processing are GAMIT

�h-files�, loosely constrained solutionscontaining a set of 1-day site estimatepositions, Earth orientation para-meters and associated error covari-ance matrices. Next, the individual�h-files� are combined, on a daily basis,by using the GLOBK Kalman filter withregional (IGS1, IGS2, IGS3, IGS4and EURA) solutions provided bythe SOPAC (Scripps Orbit and Per-manent Array Center), to create adaily unconstrained combined net-work solution. By using the GLORG

module of GLOBK (Herring et al.,2006b), the loosely constrained dailysolutions were transformed into theITRF2005 reference frame (Altamimiet al., 2007) and then rotated into afixed Eurasian frame and a fixedAfrican frame (Fig. 2). In this laststep, the Eurasia rotation pole wastaken from Altamimi et al. (2007),while the Africa rotation pole wasestimated with a weighted least squareinversion using the ITRF2005 sitevelocities and associated errors re-

ported in Altamimi et al. (2007) withthe exception of NOTO and NOT1sites, because they are located north-ward of the Sicily channel character-ized by extensional deformation (e.g.Hollenstein et al., 2003) (Table 2).

Velocity and strain-rate fields

The GPS velocity field in the Eurasianand African reference frames for theVulcano-Lipari complex is reported inFig. 2, while some examples of thecollected coordinate time series andthe corresponding normalized rootmean square (NRMS) values of thelinear trend for North, East and Upcomponents are reported in Fig. 3 andTable 3. The Eurasian velocity fieldframe (Fig. 2b) indicates that the vol-canic complex is moving N7–9�W-ward (for details see Table 1). Asclearly observable in Fig. 2b, there aresignificant differences in the velocitiesat the scale of the Lipari–Vulcanocomplex with a remarkable negativevelocity gradient moving from S to N:GPS stations lying on Vulcano Piano(Fig. 1c) show velocity values between8.8 and 9.9 mm yr)1 with an N7�Wtrend; GPS stations lying on LentiaComplex (Fig. 1c) show velocityvalues between 8.2 and 8.7 mm yr)1

with an N9�E trend; GPS stationslying on Vulcanello (Fig. 1c) and the

southern part of Lipari show velocityvalues between 6.0 and 8.0 mm yr)1

with an N9�W trend; GPS stationslying on the central part of Liparishow velocity values of about4.5 mm yr)1. VCRA station, locatedon the summit of La Fossa Caldera(Fig. 1c), shows a different behaviour(both in magnitude and azimuth)related to volcanic activity and ⁄orsettlement of volcanic rocks. Thesesomewhat �clustered� differences in themagnitudes of the velocities couldindicate the existence of buried tec-tonic lineaments that are not sup-ported by field observations.Concerning the vertical velocity, allGPS stations show negative valuesand a decrease from about )15 to)7 mm yr)1, from the northernmoststations to the southernmost ones.The horizontal and vertical velocitygradients are also visible in the Afri-can reference frame (Fig. 2c). Weconsider the vertical velocities as areliable product of our data process-ing for five reasons: (1) the relativelyhigh number of field surveys between1996 and 2005 (Table 1 and Fig. 3);(2) the benchmarks of the Lipari–Vulcano network are set in concretepillars, reducing the uncertainty re-lated to the use of tripods; (3) thegradient of vertical velocities detectedat the discrete benchmarks is compat-

N

S

km

0 1.5 3

VMOL

VSER

LROS

VROS

VCRA

VVUL

VSAR

VVLC

VPLI

LCAP

VCSP

LOSV

VLEN

LGUA

LF AL

LSAN LMAZ

N

S

km

0 1.5 3

5 mm yr–1

N

S

km

0 1.5 3

5 mm yr–1

2 –2

0

–15

–10 –5

h (mm yr–1

)4

6 8

10

12

14

16

Dis

tanc

e al

ong

prof

ile (

km)

VC

RA

VS

ER

LR

OS

V

PL

I

VL

EN

LG

UA

LF

AL

VGPL

(a) (b) (c) (d)

Nuv

el1A

Fig. 2 (a) Permanent (white diamonds) and non-permanent (light-grey points) GPS benchmarks. (b) GPS velocities and 95%confidence ellipses in Eurasia fixed reference frame; grey arrow shows the convergence vector between Africa and Eurasiaaccording to the Nuvel1A model (from DeMets et al. (1994)). (c) GPS velocities and 95% confidence ellipses in Africa fixedreference frame. (d) Vertical velocity in the N167�E direction as a function of distance along the same direction.

Tectonic features of Aeolian area from GPS • M. Mattia et al. Terra Nova, Vol 20, No. 5, 370–377

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372 � 2008 Blackwell Publishing Ltd

ible with the one observed at theCGPS stations; (4) although this areais a volcanically active area and non-linearity in the displacements can beexpected, during the analysed period,the time series of the vertical compo-nent shows a linear trend (Fig. 3); and(5) the overall vertical deformationpattern is in good agreement with themeasurements carried out on a level-ling route in Vulcano island (Bonac-corso, 2002).By adopting a least square ap-

proach and taking into account (1)the network geometry, (2) the esti-mated velocity at each site and (3)

the associated covariance matrix, C,the horizontal strain-rate field wascalculated (e.g. Shen et al., 1996; El-Fiky and Kato, 1999). In particular,we assumed that the field is isotropicand homogeneous, and we multipliedthe covariance matrix by a weightingfactor expressed as a function of thesite distance according to a distance-decaying constant, a. The weightingfunction for C follows an exponentialdecay allowing us to exclude stationsfarther than a threshold distance, D,usually ‡ 2a. To achieve realisticresults, the strain-rate was computedonly in those points of a grid that

had at least four stations in theirD-distance neighbourhood. On thebasis of the frequency distributionof the lengths of the baselines, valuesof 2.5 and 6 km were chosen for aand D respectively. For the above-mentioned reasons, VCRA stationwas excluded from the strain fieldcomputation. Finally, the strain-ratefield was computed on a regular gridwith 0.3 km stepping. The estimatedstrain-rate distributions are shown inFig. 4a–d as dilatation, principal axesof strain, maximum shear strain androtation. The dilatation strain-ratefield shows a general contraction ofthe investigated area with a maximumof about 2–2.5 ± 0.8 lstrain yr)1 inthe area between the two islands(Fig. 4a). The principal axes of strainhighlight a more complex pattern(Fig. 4b): (1) the northern part ofLipari island exhibits smallshortening (about 0.5 lstrain yr)1)along both NE–SW and NW–SEdirections; (2) the area between thetwo islands shows shortening in the

Table 1 Site code, geodetic coordinates, observation history and East, North and Up velocity components and associated errors

relative to Eurasia for all CGPS and non-permanent benchmarks.

Site Longitude Latitude VE (mm yr)1) VN (mm yr)1) VU (mm yr)1)

AJAC 8.763 41.927 0.41 ± 0.67 0.27 ± 0.62 )0.74 ± 0.8

CAGL 8.973 39.136 0.83 ± 0.36 0.25 ± 0.36 )0.94 ± 0.47

GRAS 6.921 43.755 0.72 ± 0.36 0.48 ± 0.36 0.84 ± 0.47

GRAZ 15.493 47.067 0.87 ± 0.36 0.34 ± 0.36 )0.16 ± 0.48

LAMP 12.606 35.5 )2.38 ± 0.5 3.72 ± 0.47 )1.41 ± 0.58

LCAP 14.954 38.444 )1.43 ± 0.73 5.5 ± 0.7 )10.45 ± 1.08

LFAL 14.945 38.444 )0.63 ± 0.75 6.98 ± 0.71 )11.66 ± 1.07

LGUA 14.946 38.455 )1.88 ± 0.75 5.65 ± 0.71 )18.45 ± 1.14

LMAZ 14.905 38.483 0.87 ± 0.72 4.96 ± 0.69 )14.95 ± 0.93

LOSV 14.948 38.446 )1.59 ± 0.52 8.15 ± 0.5 )18.18 ± 0.61

LROS 14.977 38.483 )2.93 ± 0.72 4.71 ± 0.7 )11.01 ± 1.16

LSAN 14.934 38.485 )0.51 ± 0.72 3.95 ± 0.7 )14.78 ± 1.08

MATE 16.704 40.649 0.94 ± 0.35 4.22 ± 0.35 )0.11 ± 0.47

MEDI 11.647 44.52 2.26 ± 0.38 1.8 ± 0.37 )2.38 ± 0.48

NOT1 14.99 38.876 )1.59 ± 0.52 4.91 ± 0.49 )2.16 ± 0.65

NOTO 14.99 38.876 )1.83 ± 0.58 3.51 ± 0.56 )3.04 ± 0.78

VCRA 14.965 38.403 )3.87 ± 0.71 10.37 ± 0.68 )12.31 ± 0.94

VCSP 14.952 38.409 1.54 ± 0.55 8.02 ± 0.53 )9.43 ± 0.65

VLEN 14.945 38.408 1.24 ± 0.69 8.64 ± 0.67 )8.44 ± 0.87

VMOL 14.986 38.396 )0.18 ± 0.69 8.76 ± 0.67 )7.9 ± 0.91

VPLI 14.956 38.430 )0.44 ± 0.7 7.68 ± 0.68 )10.88 ± 0.95

VROS 14.974 38.395 )1.17 ± 1.31 8.79 ± 1.16 )9.18 ± 1.93

VSAR 14.961 38.390 )0.92 ± 0.71 9.88 ± 0.68 )7.88 ± 0.99

VSER 14.985 38.378 )1.01 ± 0.72 9.44 ± 0.69 )5.46 ± 1.04

VVLC 14.961 38.426 )2.36 ± 0.56 6.87 ± 0.53 )9.77 ± 0.7

VVUL 14.963 38.426 )0.87 ± 0.69 6.45 ± 0.68 )10.59 ± 0.94

Site code, geodetic coordinates, observation history and east, north and up velocity components and associated errors are also reported for IGS stations included in

the processing. For non-permanent benchmarks, the occupation time window ranged from 4 to 6 h for each day of the surveys carried out during 1996–2002, and up

to 24 h for each day of the surveys carried out after 2002. In general, the duration of each survey ranged from 2 to 4 days. For CGPS benchmarks, the observation

time window was scheduled to 3 h during 1996–early 2002. After May 2002, all CGPS stations were scheduled with a 24-h observation time window.

Table 2 ITRF2005 absolute rotation poles.

Plate Latitude N� Longitude E� x (� Myr)1)

Eurasia 56.330 ± 0.549 )95.979 ± 0.969 0.261 ± 0.003

Africa 49.270 ± 0.456 )83.466 ± 1.139 0.270 ± 0.002

Parameter x is the rotation rate. The Eurasia rotation pole is taken from Altamimi et al. (2007). The Africa

rotation pole was estimated with a weighted least square inversion by using the ITRF2005 site velocities and

associated errors reported in Altamimi et al. (2007) with the exception of NOTO and NOT1 sites.

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Terra Nova, Vol 20, No. 5, 370–377 M. Mattia et al. • Tectonic features of Aeolian area from GPS

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order of 1.5–2 lstrain yr)1 along theNE–SWdirection, coupledwith a smalllengthening (about 0.3 lstrain yr)1)along the NW–SE direction; (3) theLentia area shows a shortening inthe order of 2–2.5 lstrain yr)1 alongthe E–W direction and a minorshortening (about 0.5 lstrain yr)1)along the N–S direction; and (4)the Vulcano Piano area is affectedby a shortening of about 0.5 lstrai-n yr)1 along the N–S direction.Regarding the maximum shear strainrate, the greatest values (about2.5 lstrain yr)1 along a NW–SE direc-tion) (Fig. 4c) are detected in thenorthern part of Vulcano and in the

southern part of Lipari. The rotationrate, defined as the antisymmetricpart of the velocity gradient tensor,recasted in terms of an axial vectorwhose magnitude is equivalent to themagnitude of the vertical axis rota-tion rate (the rotations are withrespect to a downward-positive ver-tical axis with clockwise positive andcounterclockwise negative) is shownin Fig. 4d. A large area of counter-clockwise rotation in the northernpart of Vulcano, with an amplitudeof about 0.2–0.3 lrad yr)1, is clearlyvisible, while clockwise rotationoccurs in the remaining part of theLipari–Vulcano complex, with great-

er values by about 0.5 lrad yr)1 inthe area between the two islands.

Discussion

Velocity and strain-rate fields of theLipari–Vulcano complex have beeninvestigated through non-permanentand CGPS measurements collectedbetween 1996 and 2006. The mainresults, described in the previous par-agraph, highlight some key questionsposed by this important seismic andvolcanic area.A significant velocity gradient be-

tween the southernmost stationsand the northernmost ones has been

1996 2000 2004 200620021998

020

40

–20–40–60

–80

LSA

NLO

SV

North East

LFA

L 020

40

–20–40–60

–80

–20

020

40

–20–40–60

–80

020

40

–20–40–60

–80

020

40

–20–40–60

–80

Up

020

40

–20–40–60

–80

80

120

40

0

–40

80

120

40

0

–40

80

120

40

0

–40

1996 2000 2004 200620021998 1996 2000 2004 200620021998

80

120

40

0

–40

020

40

–20–40–60

–80

VLE

N

020

40

–20–40–60

–80

020

40

–20–40–60

–80

VC

SP

020

40

–20–40–60

–80

VV

LC

80

120

40

0

–40

80

120

40

0

–40

020

40

–20–40–60

–80

020

40

–20–40–60

–80

Fig. 3 Examples of North, East and Up coordinates time series of benchmarks relative to Eurasia fixed reference frame. The errorbars show the 95% confidence interval. The dotted black lines represent the weighted linear regression of the solutions. Ordinateunits are millimetres.

Tectonic features of Aeolian area from GPS • M. Mattia et al. Terra Nova, Vol 20, No. 5, 370–377

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374 � 2008 Blackwell Publishing Ltd

measured, indicating that this area issubject to the effects of Africa–Eurasiaconvergence. In addition, the largegeodetic dataset used in this studyallows us to improve the acting strainanalysis in the Lipari–Vulcano com-plex and to compare it with previouspublished results (e.g. Hollensteinet al., 2003; D�Agostino and Selvaggi,2004; Argnani et al., 2007) where the

available data were limited to only onestation each for Lipari and Vulcano.This last aspect was investigatedthrough the use of some simple syn-thetic models (Fig. 5). In a first model,we simulated a N–S convergence pro-cess and calculated the expected prin-cipal axes of strain (Model A;Fig. 5a). The resulting strain fieldpattern is very close to the one pro-posed in recent literature (e.g. Hollen-stein et al., 2003; D�Agostino andSelvaggi, 2004; Argnani et al., 2007);however, it strongly disagrees with theone resulting from our analyses. Thisdiscrepancy clearly indicates that inthis area the deformation pattern ismore complex than that predicted bya simple N–S convergence process. Inlight of this, in a second model, wesimulated a near-vertical right-lateralstrike-slip fault having a ratio betweenthe closure component and the dextralmovements of 3 : 1 (Model B;Fig. 5b). The fault is N40�W oriented(azimuth of the ATL fault, Fig. 1a).Although the expected strain fieldpattern shows several shortening axesNE–SW oriented, coupled withlengthening axes NW–SE oriented,this is unable to explain our overallobserved strain pattern. In a lastmodel, we combined model A andmodel B: the expected strain fieldpattern is very close to the one shownin this study (Fig. 4). In light of this,the deformation pattern detected in

between the two islands can be de-scribed by a combination of (1) themain N–S regional compression and(2) a NNE–SSW compression with asmall right-lateral strike-slip compo-nent acting along a tectonic structurehaving the same trend as the onedetected between Capo Milazzo andVulcano by Argnani et al. (2007).As above mentioned, the ATL fault

system has played a central role in thedevelopment of a tensional transferzone with the formation of a pull-apartbasin, where the volcanism of Lipariand Vulcano started (Ventura, 1994).Our findings (on Lipari–Vulcano com-plex), together with the results of Arg-nani et al., 2007 (about the submarinearea between Capo Milazzo and Vul-cano), indicate that the ATL faultsystem is actually working in trans-pression and ⁄or is part of a maintranspressive belt extending fromCapoMilazzo to Salina, overprinting theformer extensional regime active dur-ing the Pliocene (Ventura, 1994).Some hypotheses can be made

about the subsidence gradient fromthe southernmost stations to thenorthernmost ones. This pattern couldbe traced to the opening of the Tyr-rhenian Sea, although the subsidence,in this case, should be accompanied byextension. It is more likely that there isa direct relationship with a local thrustloading, but this question is an openpoint to be better investigated at a

0 1.5 3

0.0

(a)

–2.6

–1.3

µstr

ain

yr–1

0 1.5 3

km

N

S

N

S

0 1.5 3

2.5

0.5

1.5

µstr

ain

yr–1

0 1.5 3km

N

S

N

S

0 1.5 3

km

1.5µstrain yr–1

lengtheningshortening

(b) (c) (d)

0 1.5 3km

0.8

–0.2

0

µrad

yr–1

Fig. 4 Strain-rate parameters: (a) dilatation strain-rate; (b) principal axes of strain-rate. Blue (thicker lines) is for contraction, red(thinner lines) for extension; (c) maximum shear strain-rate (earthquakes are also reported; black and white dots for hypocentersshallower and deeper than 4.5 km respectively); (d) rotation rate (positive for clockwise and negative for counterclockwise, withrespect to a vertical axis).

Table 3 Normalized root mean square

(NRMS) of the linear trend for North,

East and Up coordinates time series

relative to Eurasia for all the

non-permanent and permanent

(the last three sites) GPS stations on

the Lipari–Vulcano complex.

Site NRMS (N) NRMS (E) NRMS (U)

LCAP 0.81 1.12 1.09

LFAL 0.89 0.82 0.86

LGUA 1.02 0.92 3.54

LMAZ 0.63 1.13 2.28

LROS 0.62 1.08 1.08

LSAN 0.74 0.96 0.51

VCRA 0.83 0.77 2.02

VLEN 1.05 0.65 0.84

VMOL 0.77 0.93 0.99

VPLI 0.72 1.03 1.25

VROS 0.84 1.12 1.68

VSAR 1.01 0.8 1.66

VSER 0.86 0.65 0.66

VVUL 0.75 1.09 1.29

LOSV 1.00 0.97 1.17

VCSP 1.11 0.83 1.00

VVLC 0.97 1.13 1.00

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broader scale. The almost linearbehaviour of the subsidence goingfrom south to north should excludeany effect of volcanic activity, be-cause, as suggested in many studies(e.g. tomographic studies in Chiarab-ba et al., 2004), there is no continuityof the plumbing system related to thevolcanism observed in Lipari andVulcano islands.Another interesting result of our

analysis is the relatively good agree-ment between the area of maximum

shear and the cluster of shallow (depthrange 0–4.5 km b.s.l.) earthquakeslocalized by the INGV-CT networksince 1999 (Fig. 4c). This is a goodindicator of a seismic response tostrain accumulation resulting fromtectonic activity in the area betweenLipari and Vulcano. Moreover, con-sidering the location of the mainfumaroles of Vulcano island (Fig. 1c),we observe that they are clustered inthe northern sector of the island, closeto the measured maximum shear area.

The relationships between these fuma-roles and the fracturing of the sealedcap above the main hydrothermalsystem of the island (Harris and Ma-ciejewski, 2000) can be inferred byincreased values of shear strain, but,for the moment, this can be consid-ered as a starting hypothesis for futureinvestigations. Volcanic activity in acontractional setting (Tibaldi, 2005) isfavoured by small-scale pull-apartsand such a feature is present in thenorthern part of Vulcano (Ventura,1994). This fact explains local-scalephenomena of shallow fracturing in ageneral compressive context.

Conclusions

The analyses performed in this workthroughadenseGPSnetworkhighlightimportant geodynamic features of theLipari–Vulcano complex. The velocitygradientbetween the southernmost andthe northernmost stations is consistentwith the fact that this area experiencesthe effects of the Africa–Europe colli-sion. The dilatation strain-rate fieldshows a general contraction of theoverall investigated area, evidencingthat the ATL fault system extensionalfeatures are now superseded by a com-pressive tectonic pattern and the faultsystem seems to be part of a maintranspressive belt extending fromCapoMilazzo to Salina, according to theresult of Argnani et al. (2007). More-over, the subsidence gradient observedat the scale of the volcano–tectoniccomplex suggests that the verticaldeformation pattern could be influ-enced by local thrust loading.

Acknowledgements

The authors thank Luigi Ferranti and twoanonymous reviewers for their constructivesuggestions, and the Scientific Editor, CarloDoglioni, for his helpful comments. Thanksare also due to Giuseppe Puglisi and Aless-andro Bonforte for the data of the Lipari–VulcanoGPSnetwork.A research grant hasbeen provided by project FIRB �SviluppoNuoveTecnologie per la Protezione eDifesadel Territorio dai Rischi Naturali� UR b3�SistemiOsservativi Geodetici: LaReteGPSNazionale� to one of the authors (V. Bruno).This research has benefited from fundingprovided by the Italian Presidenza delConsiglio dei Ministri – Dipartimento dellaProtezione Civile (DPC). Scientific papersfunded by DPC do not represent its officialopinion and policies.

Model A Strain axes

m = o + s= +

Model B Strain axes

Model A+B Strain axes

(a)

(b)

(c)

Fig. 5 (a) Simplified model of a N–S convergence process and expected principal axes ofstrain (blue–thick lines–for shortening axes and red–thin lines–for lengthening); (b) sim-plified model of a near-vertical right-lateral strike-slip fault having a negative opening (i.e.convergence) three times greater than the right-lateral strike-slip component. The fault isN40�W oriented, according the main tectonic features of the investigated area;(c) combined model fromModel A and Model B and expected principal axes of strain.

Tectonic features of Aeolian area from GPS • M. Mattia et al. Terra Nova, Vol 20, No. 5, 370–377

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Received 31 January 2008; revised versionaccepted 2 July 2008

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