Tectonophysics 621 (2014) 101–122

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Tectonophysics

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Rates of geodetic deformation across active faults in southern Italy

Luigi Ferranti a,⁎, Mimmo Palano b, Flavio Cannavò b, Maria Enrica Mazzella a,1, John S. Oldow c,Erwan Gueguen d, Mario Mattia b, Carmelo Monaco e

a DiSTAR — Dipartimento di Scienze della Terra, delle Risorse e dell'Ambiente, Università Federico II, Largo S. Marcellino 10, 80138 Napoli, Italyb Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Catania, Italyc Department of Geosciences, University of Texas at Dallas, Richardson, TX, USAd Istituto di Metodologie per l'Analisi Ambientale, C.N.R., Tito Scalo, PZ, Italye Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Catania, Italy

⁎ Corresponding author at: Tel.: +39 812538180.E-mail address: lferrant@unina.it (L. Ferranti).

1 Present address: Dipartimento di Scienze della Terra, UItaly.

http://dx.doi.org/10.1016/j.tecto.2014.02.0070040-1951/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 November 2013Received in revised form 10 February 2014Accepted 11 February 2014Available online 20 February 2014

Keywords:GNSS velocityActive faultGeodetic slip rateSouthern Italy

Active deformation in southern Italy is accommodated by a distributed number of faults with low–moderate sliprates. Outcropping extensional faults andmostly blind transcurrent faults aremappedwithin awestern (or axial)and an eastern domain, respectively. We use a combination of continuous (2001.00–2011.84) and episodic(1995.68–2010.79) GNSS observations to firstly estimate the geodetic deformation rate on 32 faults. Geodetic re-sults were successively compared with geological displacement estimates. In agreement with seismological andgeological information, a net spatial segregation emerges between the extensional axial belt, and the eastern do-main where strike–slip faults are geodetically active. Although uncertainties are at times large, average displace-ment rates show broadly consistent patterns within both domains. A longitudinal gradient in extension rate isobserved for the axial fault array, with two sectors of higher magnitude (~0.8–1.7 mm/yr for individual faults).This result is consistent with geological observations and supports the notion that extension occurs in discretepatches. Faults of the eastern domain have lower (few 0.1 to ~1.2 mm/yr) strike–slip rates and an eastward-decreasing extensional component, but significant geodetic displacement is detected in areas lacking clear evi-dence of activity. Few faults with 1–2 mm/yr extension rate are locally found in the eastern domain, but, basedon their limited length and on inconsistency with seismology and geology, they are considered as due to deep-seated gravitational spreading. For crustal faults, although geodetic slip andmoment rates are larger than geolog-ical rates, the broad trend of long- to short-term rates is similar, indicating the feasibility of geodetic analysis tocontribute estimating fault slip rate and testing tectonic models in the region. Whereas the western domainextension is thought to be controlled by potential energy related to the Tyrrhenian Moho uplift beneath theApennines, strike–slip in the east is related to shear on inherited faults within the Adriatic crust.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Southern Italy is characterized as one of the highest seismic hazardin the Mediterranean region, as documented by historical and instru-mental catalogs where several large (6.5 ≤ M ≤ 7.5) and destructiveevents have occurred (Fig. 1; Rovida et al., 2011). Despite that the larg-est earthquakes have been associated to a causative fault, large uncer-tainties still exist regarding definition of source geometric, kinematicand energetic parameters (e.g. slip rates); this represents a severe draw-back for the growing practice of seismic hazard estimation based on tec-tonic information for individual seismogenic sources (e.g. Peruzza et al.,2010).

niversità degli Studi di Perugia,

In southern Italy, active faults and focal mechanism solutions ofcrustal earthquakes define two laterally juxtaposed belts of contempo-rary deformation, with strike–slip faulting beneath the Apulia plainsand plateau to the east, and extension along the axis of the SouthernApennines chain in the west (Figs. 2, 3). The current deformationregime in the Apennines started to form between Late Pliocene–EarlyPleistocene, when it replaced contractional tectonics responsible forNeogene accretion of the Southern Apennines fold-and-thrust belt(Fig. 2, inset; Hyppolite et al., 1994; Patacca and Scandone, 2007).

Because of this relatively recent change in tectonic regime, the cur-rent deformation pattern in the Apennines is diffuse and is character-ized by low geodetic (~3–5 mm/yr) and geologic (1–2 mm/yr) rates(Faure Walker et al., 2012; Ferranti et al., 2008; Giuliani et al., 2009;Palano et al., 2011; Papanikolaou and Roberts, 2007; Serpelloni et al.,2005), and the landscape still bears the dominant signature of contrac-tional tectonics. Consequently, detection and characterization of activeand seismogenic faults present significant challenges in the region,

Fig. 1. Instrumental seismicity (M ≥ 2.2) in southern Italy 1983–2012 (http://iside.rm.ingv.it). Blue squares are historical earthquakes from CPTI catalog (Rovida et al., 2011); Imax,maximummacroseismic intensity.

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and to this end the GNSS (Global Navigation Satellite System; formerlyknown as GPS) observations can offer a vital contribution.

In the last decade, GNSS data analysis has contributed to theunderstanding of the pattern of active deformation in southern Italy(D'Agostino et al., 2008; Devoti et al., 2011; Ferranti et al., 2008;Hunstad et al., 2003; Oldow et al., 2002; Palano et al., 2011; Serpelloniet al., 2005). Although published geodetic studies have singled out theregional scale pattern of crustal deformation, the relation between sur-face displacement and inter-seismic strain accumulation at the scale ofindividual faults or fault arrays has been established only locally andwith a significant uncertainty (Ferranti et al., 2008; Giuliani et al.,2009). In addition, a comparison between geodetic and geologic ratesof fault slip has not been attempted yet. The geodetic displacement re-solved onto the fault may embed components of seismic and aseismicslip, and of strain accumulation because of block coupling on the faultslip surface; whether this displacement pattern is however representa-tive of the long-term behavior of crustal faults requires comparison ofdifferent datasets.

In this paper, we use a dense combination of: i) continuous GNSSdata collected between 2001.00 and 2011.84 on geodetic networksinstalled in southern Italy, and ii) episodic GNSS measurements collect-ed between 1995.68 and 2010.79, in order to provide the first estima-tion of geodetic horizontal slip rate on a significant number of active

faults. Later, we compare the geodetic resultswith information obtainedfrom geological studies of active faults with the aim of reconciling con-temporary and geologic displacements and provide insights into thepattern of moment accumulation and release over different time spans.

Although our estimation of geodetic displacement is on occasion bi-ased by a considerable uncertainty, results of this study help to refineand implement the poorly constrained magnitude of slip rate on activefaults, which is a timely contribution for fault-based seismic hazard as-sessment in this highly-populated region (e.g. Peruzza et al., 2010,2011; Roberts et al., 2004). Combination of site velocities with localseismicity and structural studies as discussed in this paper indicatesthat, although broadly driven by convergence between Europe andNubia (e.g. the African plate west of the East African Rift), the patternof crustal displacement in southern Italy is strongly influenced by acombination of processes not immediately related to plate boundaryforces, such as crustal delamination and mantle upwelling, and exploi-tation of intra-plate anisotropy.

2. Regional setting

Southern Italy is floored by the Southern Apennines fold-and-thrustbelt in the western and central part, and by the Apulia foreland in theeastern part (Fig. 2, inset). The Southern Apennines accreted from

Fig. 2. Active faults in southern Italy. Compiled from the Database of Italian Seismogenic Sources (DISS; http://diss.rm.ingv.it/diss/), the GNDT (Gruppo Nazionale Difesa dai Terremoti)database (Galadini et al., 2000), the Neotectonic map of Italy (Ambrosetti et al., 1987) and from recent literature as quoted in the text. The dashed white line in figure and in the insetrepresents the boundary between the Adriatic and Tyrrhenian Moho (slightly modified from Di Stefano et al., 2011). Localities abbreviations are as follows: Ci, Cilento; CP, CampaniaPlain; CV, Crati Valley; Da, Daunia Mountains; Ga, Gargano; Ir, Irpinia; Mu, Murge; MM, Matese Mountains; PhF, Phlegrean Fields; PM, Picentini Mountains; Pn, Pollino; Po, Potenza;RV; Si, Sila; Sn, Sannio; SP, Sele Plain; Ta, Tavoliere; Tr, Tremiti Islands; VdA, Val d'Agri; VdD, Vallo di Diano; VV, Vesuvius Volcano. Inset shows the main tectonic provinces in southernItaly, and the front of the fold and thrust belt.

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Miocene to Early Pleistocene during the westward-directed subductionand delamination of the Adriatic slab, a small lithospheric fragmenttrapped within the articulated collision boundary between Africa andEurope (Faccenna et al., 2001; Malinverno and Ryan, 1986; Pataccaet al., 1990). To the east, the Apulian sector of the Adriatic continentalmargin served as the foreland to the eastward-migrating thrust belt.The Apulia foreland has an average ~30 km crustal thickness(Chiarabba and Amato, 1996) typical for continental crust, and is char-acterized by a ~6 km thick Mesozoic–Cenozoic carbonate platform sed-imentary cover (Mostardini andMerlini, 1986). To thewest, contractionwas followed by extension, which since the Early Miocene resulted inhinterland stretching of continental crust and local emplacement ofoceanic crust beneath the southern Tyrrhenian Sea (Fig. 1, inset;Malinverno and Ryan, 1986).

From Late Pliocene to Early Pleistocene, east-directed displacementof the contractional front in the Southern Apennines comes to an end(Fig. 2, inset; Patacca and Scandone, 2007). During this time, involve-ment of thick Apulian continental crust in the collision system causedthe (re-)activation of faults in the foreland plate with strike–slip and lo-cally transpressional motion, that in the west broke through the

previously emplaced thin-skinned allochthon (Ferranti et al., 2009;Monaco et al., 1998). These faults are at times seismically active in theeastern part of southern Italy, in the Apulia foreland and beneath thefrontal part of the belt (Figs. 2, 3).

Since Late Pliocene, extension related to opening of the TyrrhenianSea and crustal delamination of the Adriatic slab affected the westernsector of the Apennines, forming coastal and intermontane basins con-trolled by high-angle normal faults which crosscut the pre-existingthrust structures (Ferranti and Oldow, 2005; Hyppolite et al., 1994). Ex-tension progressed from west to east, and starting from Early Pleisto-cene an array of active normal faults was nested along the axial part ofthe belt and includes major recognized seismogenic structures (Fig. 2).Inwestern Campania region, extensionwas locally accompanied by vol-canism, whose emplacement was controlled by the intersecting faultnetworks (e.g. Milano et al., 2004).

The regional gravity anomaly map and Deep Seismic Soundingsstudies outline the existence of a doubling of the Moho beneath theSouthern Apennines (Fig. 2, inset; Cassinis et al., 2003; Di Stefanoet al., 2011; Morelli, 2000; Nicolich, 1989; Tiberti et al., 2005). This pat-tern is related to crustal delamination (Channell and Mareschal, 1989)

Fig. 3. Lower hemisphere, equal area projection of focal mechanisms compiled from recent literature and on-line catalogs for earthquakes with M ≥ 3.5; focal mechanisms are coloredaccording to rake: red, thrust faulting, blue, normal faulting, yellow, strike–slip faulting. Localities are as in Fig. 2. Dashed line is the same as in Fig. 2.

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and impinging of the Tyrrhenian Moho, formed since the Miocene be-neath the stretched (thickness down to ~10 km) back-arc basin, overthe sinking, old Moho of the Adriatic plate (Cassinis et al., 2003).

3. Active deformation

Active tectonics in southern Italy appears segregated in two discretedomains, which are characterized by different deformation mecha-nisms. The two domains are separated by a localized zone which runsalong the axis of the mountain chain, and strikingly coincides with thedeep boundary between the Tyrrhenian andAdriaticMoho. In thewest-ern domain, active normal faults (Fig. 2) and seismicity (Figs. 1, 3) areconcentrated within an ~50 km wide belt running along the mountainridge (Papanikolaou and Roberts, 2007; Valensise and Pantosti, 2001).Within this belt, earthquakes have relatively shallow focal depths(~10–15 km), and exhibit focal mechanisms with primarily normalfaulting features and tensile axis roughly orthogonal to the Tyrrhenianmargin (Fig. 3). The belt of extensional earthquakes and active normalfaults follows to the south the curvature of the orogenic arc. Seismolog-ical observations are generally consistent with borehole breakouts(Montone et al., 2012) and fault-slip data from Quaternary rocks(Faure Walker et al., 2012; Hyppolite et al., 1994; Maschio et al., 2005;Papanikolaou and Roberts, 2007).

The swarm of active normal faults along the axis of the mountainbelt has been associated to most of the large historical earthquakes,which concentrate in the extensional domain (Figs. 1 to 3; DISSWorking Group, 2010). These faults are regarded as having an EarlyPleistocene and in some cases an even younger age (Improta et al.,2010; Maschio et al., 2005; Pantosti et al., 1993). Although the locationof the active normal faults is relatively well constrained, significantuncertainties exist on the age initiation, geological displacements, aswell as on the rate of slip for individual structures.

Geodetic data so far available supply the rate of integrated extensionin the Apennines but not the strain accumulation across individualnormal faults. The geodetically-estimated rate of regional extensionvaries from ~2–3.5 (Ferranti et al., 2008; Serpelloni et al., 2005) to ~4–6 mm/yr (Giuliani et al., 2009; Hunstad et al., 2003; Ward, 1994).

On the opposite, the eastern domain encompassing the foothills ofthe Apennines and the Apulia foreland locally show strike–slip andsubordinately thrust earthquakes (Fig. 3) with focal depths deeper(~15–35 km) than the extensional earthquakes in the west (Boncioet al., 2007; Pondrelli et al., 2006). Within the Apulia foreland, whereasthe Gargano block and its offshore extension forms an active seismicbelt with moderate-magnitude thrust and transpressional earthquakes,the southern part including the Murge and Salento blocks lacks signifi-cant seismicity (Figs. 1, 3). Notwithstanding, NE–SW convergence

a

b

Fig. 4.GNSS velocities and 95% confidence ellipses referred to a fixed: (a) Central Europe frame; and (b) Apulian frame (see text and Table 1 for additional details). Dashed line is the sameas in Fig. 2.

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Table 1Relative GPS–Euler vector and associated errors (95% of confidence) for the fixed CentralEurope (Eu) and the Apulia (Ap) reference frame. The Euler vector parameters (latitudeand longitude of pole, rotation rate) were estimated by minimizing, with a weighted leastsquares inversion (Palano et al., 2010), the adjustments to horizontal GNSS velocities alignedto a fixed Central Europe reference frame (Nocquet and Calais, 2003). Sites used for the poleparameters computations are reported on Table S1 of the Supplementary section.

Plate pair Latitude N° Longitude E° Ω (°/Myr)

Eu–Ap −31.362 ± 1.468 −145.253 ± 2.251 0.137 ± 0.044

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between southern Apulia and the seismologically active front of thecontractional orogen in Albania and the Dinarids across the southernAdriatic Sea (Fig. 3) occurs at ~5 mm/yr (Hollenstein et al., 2003;Jouanne et al., 2012; Serpelloni et al., 2005). South of Apulia, seismicityin the eastern domain resumes in north-eastern Calabria, where strike–slip and subordinately thrust and extensional earthquakes are recordedalong the continental margin, the shelf and into the Ionian Sea (Fig. 3).

Seismogenic deformation in the eastern domain is characterized bysequences, mainly segregated in the northern part of the region(Pondrelli et al., 2003). These clusters, characterized by a sparse seis-micity and by events with moderate magnitude, show an east–westalignment (Fig. 3) which outlines the surface and subsurface locationof broad systems of right-lateral faults which are thought to connectthe deep part of the Apennines to the Apulia foreland (e.g. Boncioet al., 2007; Di Bucci et al., 2006). A northern array of right-lateral faultsis buried at depth beneath the Daunia foothills, but resurfaces across theGargano upland (Fig. 2). To the south, two additional arrays of E–Wstriking right-lateral faults are thought to join the Apennines with theMurge and Salento blocks of Apulia. Further south, NW–SE strikingtranscurrent faults are mapped in northern Sila and in the Ionian Seaoffshore the Pollino range (Fig. 2).

Unlike the normal faults in the west, the location and geometry ofactive transcurrent faults in the eastern domain is not well understood,because most of the faults are blind and supposedly segmented. Thesefaults are thought to be Mesozoic or older extensional faults affectingthe Apulia platform, and nowadays reactivated in strike–slip motion(Boncio et al., 2007; Di Bucci et al., 2006).

Boncio et al. (2007) evidenced that the 1990–91 sequence thatstroke the frontal belt of the Apennines near Potenza town in the east-ern domain (Fig. 3), concentrated between a 15–23 km depth range,corresponding to the Apulia middle crust underthrust beneath theApennines belt. The apparent anomalous earthquake depths (N15 km)and the confinement within a relatively narrow depth range areexplained by Boncio et al. (2007) according to the crustal rheology,which consists of a strongmid crustal brittle layer sandwiched betweentwo plastic horizons.

The subdue seismogenic role of faults within the eastern domain isreflected in the minor historical seismicity record when compared tothe western normal faults. In the eastern domain, strong historicalevents are only sparsely documented and notably affect the northernpart of the region only (Fig. 2). On the other hand, no major historicalearthquakes are reported during the last 1000 yr in the central-southern Apulian foreland and eastern Lucania region (see 2011, for ad-ditional details).

4. GNSS velocity analysis

4.1. Network and data processing

To improve the detail of the geodetic velocity field in the investigat-ed area, we updated the GNSS results reported in Palano et al. (2011)by extending the data processingup to the 2011.84 epoch and includinginto the processing the periodic measurements carried out on the Peri-Tyrrhenian GPS Array (PTGA; Ferranti et al., 2008). The 15-benchmark

network, surveyed for the first time in 1995, was distributed in the en-tire southern peninsular Italy. Since then, the PTGA network has beensurveyed in 1997, 2000, 2008 and 2010, and it has been expanded toreach the current configuration of 42 benchmarks (Mazzella, 2011).All the benchmarks were installed within hard bedrock not affected byfractures as far as possible. Particular care was devoted to locate obser-vation sites within coherent and rooted blocks with no evidence ofgravitational sliding. Overall, this analysis builds on 95 continuous and31 episodic measurement sites (Fig. 4).

GNSS data were processed by using the GAMIT/GLOBK software(Herring et al., 2010). To tie the regional measurements to theITRF2005 reference frame (Altamimi et al., 2007), data coming from12 continuously operating IGS (International GNSS Service) stations(AJAC, BRUS, CAGL, GRAS, GRAZ, JOZE, LAMP, MATE, MEDI, NOTO,NOT1 and ZIMM) were introduced in the processing. By using theGLOBK software we combined, on a daily basis, our solutions (andtheir covariance matrices) with global solutions (IGS1, IGS2, IGS3,IGS4, IGS5, IGS6 and EURA) provided by the Scripps Orbit and Perma-nent Array Center (SOPAC) to generate site time series. As a final step,by using the GLORG module of GLOBK we combined these solutionsand their full covariance matrices to estimated a consistent set of posi-tions and velocities in the ITRF2005 reference frame by minimizingthe horizontal velocity of the 12 continuously operating IGS stationsmentioned above. To account for correlated noise, we applied the “real-istic sigma” algorithm of Herring (2003) to each of our time series, afterremoving the best-fitting annual and semi-annual signals and includingthe estimated random walk component for every single component ofeach station in our final velocity solution.

Our GNSS network shares some stations with those processed byD'Agostino et al. (2011) allowing for a rigorous integration of the twoestimated velocity fields. To this end, we aligned the published veloci-ties to the ITRF2005 reference frame by applying a seven-parameterHelmert transformation, obtained by finding the transformation thatminimizes the Root Mean Square (RMS) of differences between veloci-ties of common sites. The average discrepancies are small, and the RMSfor the used 15 common stations is less than 0.4 mm/yr. The resultingvelocity field and additional details are reported in the Supplementarymaterial section.

4.2. GNSS velocity fields

In order to analyze the crustal deformation pattern over the investi-gated area, the estimated GNSS velocitieswere aligned to afixed CentralEurasia reference frame (Nocquet and Calais, 2003) and to an additionallocal reference frame (that we label Apulia), whose Euler pole wasestimated by using GNSS sites located in southern Apulia and easternLucania regions (Tables 1; S1).

In a Central Europe reference frame, the geodetic velocity fielddepicts a general NW-directed motion at a rate of 1–3 mm/yr onthe Tyrrhenian sector, and a N-to-NE-directed motion at a rate of3–6 mm/yr on the Adriatic sector (Fig. 4a). The divergent velocityboundary is sharp and is positioned along the crest of the Apennines,where it coincides with the seismological and active faults boundary(Figs. 2, 3). Further south, the geodetic boundary is loosely traced acrosscentral Calabria. Sites located in the Campania coastal plain show apattern that differs from the general trend and underlines local effectsderiving from volcanic centers on the western Campania margin(Vesuvius and Phlegrean Fields, Fig. 2).

Velocities referred to the Apulia frame evidence, on the Tyrrhenianside of Campania and Lucania regions, a pattern of WSW-oriented mo-tion at a rate of 1 to 4 mm/yr (Fig. 4b). The Tyrrhenian sector of Calabriashows a similar pattern but with rates generally b1.5 mm/yr. On theAdriatic side of southern Italy, the velocity field shows a broad ENE-directed pattern with rates of about 1–4 mm/yr in the area betweeneastern Molise and northern Apulia. Sites in central-southern Apulia

a

b

Fig. 5. Geodetic strain-rate parameters: (a) rate of the areal change magnitude (background color code) and greatest extensional and contractional horizontal strain-rates (arrows);(b) rate of maximum shear strain.

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Table 2Modeled active crustal faults of thewestern and eastern domain, and gravitational faults. For each structure, the geological source parameters, the kinematic and theQuaternary slip rate (bold for latest Pleistocene–Holocene slip rate), and the sourcereference are listed. NMI = Neotectonic map of Italy (Ambrosetti et al., 1987); DISS = Database of Italian Seismogenic Sources, Version 3.1.1 (http://diss.rm.ingv.it/diss/); ITIS = Individual Seismogenic Source; ITCS = Composite SeismogenicSource; GNDT = Gruppo Nazionale Difesa Terremoti; CNR MS = Consiglio Nazionale delle Ricerche, Modello Strutturale d'Italia.

ID Fault name Length(km)

Coordinate Down-dipwidth (km)

Min depth(km)

Max depth(km)

Strike(°)

Dip(°)

Kinematicgeology

Dipdirection

Rake (°) Slip-(Throw, T)-rate(mm/yr)

Horizontal slip-rate(mm/yr)

Source

West tip East tip

Eastern domain1 Biferno 30 41°43′12″

14°42′36″41°58′48″15°1′12″

18.3 2 16 55 50 n/a SE n/a n/a n/a NMI

2 Frosolone 36 41°40′30″14°20′31″

41°40′30″14°46′42″

14.9 11 25 269 70 RN N 230 0.1–1.0 0.1–1.0 DISS (ITIS095)

3 Ripabottoni–S. Severo 64 41°41′7″14°47′51″

41°43′23″15°33′47″

19.1 6 25 270 85 R N 180–220 0.1–0.5 0.1–0.5 DISS (ITCS053)

4 S. Marco in Lamis–Mattinata 45 41°43′28″15°34′31″

41°42′52″16°5′39″

25.1 0 25 270 85 RN N 200–230 0.1–10.8

0.1–10.8

DISS (ITCS058),(Tondi et al., 2005)

5 Castelluccio dei Sauri–Trani 93 41°16′49″15°24′58″

41°16′28″16°31′30″

12.2 11 23 270 80 R N 170–190 0.1–0.5 0.1–0.5 DISS (ITCS004)

6 Mirabella Eclano–Monteverde 60 41°2′22″14°56′29″

40°58′21″15°38′43″

16.6 1 16 280 65 RN N 230–250 0.1–1.0 0.1–1.0 DISS (ITCS084)

7 Potenza 15 40°39′51″15°47′23″

40°38′8″15°58′19″

6 15 21 95 88 R S 175 0.1–0.5 0.1–0.5 DISS (ITIS084)

8 Palagianello 65 40°39′31″16°19′51″

40°37′31″17°5′35″

9 13 22 90 85 R S 170–190 0.1–0.5 0.1–0.5 DISS (ITCS005)

9 Amendolara–Sibari 68 39°47′38″16°29′10″

39°33′43″17°13′56″

26.9 3 22 286 45 TL N 235 0.7–1.0 0.7–1.0 Ferranti et al. (2009)

10 Rossano 40 39°36′37″16°22′28″

39°33′52″16°49′50″

28.3 2 22 280 45 LT N 220 1.0–5.0 (T) 1.0–2.0⁎ GNDT (99),(Van Dijk et al., 2000)

11 Cecita 20 39°29′52″16°29′52″

39°19′28″16°34′8″

25.4 0 22 162 60 NL SW 320 n/a n/a n/a

12 Lakes 30 39°19′50″16°32′59″

39°6′57″16°46′01″

25.4 0 22 142 60 NL SW 320 1.2 0.6 Galli and Bosi (2003)

13 Lamezia–Catanzaro 21 38°55′34″16°12′38″

38°54′41″16°27′3″

n/a n/a n/a 95 70 L S n/a n/a n/a GNDT (104)

Eastern domain, shallow-crustal creeping faults14 Varano 35 41°54′20″

15°41′53″41°45′46″16°5′23″

5.8 0 5 315 60 N NE 270 n/a n/a Ithaca, (Bertottiet al., 1999)

15 Castel del Monte 44 41°7′4″16°13′19″

40°54′43″16°40′34″

20.8 0 18 300 60 N NE 270 n/a n/a CNR MS, (Del Gaudioet al., 2007)

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16 Avena–Lauropoli 28 39°45′43″16°23′17″

39°56′45″16°36′16″

4.6 0 4 222 60 N E 270 x 0.3 Ferranti et al. (2009)

Western domain17 Acquae Iuliae 22 41°32′2″

14°2′31″41°25′9″14°15′30″

13.2 1 13 125 65 N SW 270 0.2–0.40.45–1.9

0.1–0.20.2–0.8

GNDT (52) (Galli andNaso, 2009)

18 Matese Lake 36 41°30′22″14°8′47″

41°21′33″14°32′19″

14.3 0 13 120 65 N SW 270 0.5–1.0 0.2–0.4 GNDT (53), [this paper]

19 Boiano Basin 24 41°31′22″14°19′5″

41°24′5″14°33′56″

13.8 1 12.3 55 55 N NE 270 0.1–1.0 (T)0.5–0.9

0.1–0.60.3–0.5

DISS (ITIS004), (Galliand Galadini, 2003;Porfido et al., 2002)

20 Volturno Plain N 15 41°15′60″14°3′49″

41°11′32″14°13′18″

13.9 1 13 120 65 N SW 270 0.5–1.5 (T) 0.2–0.5 (T) 0.2–0.60.1–0.2

GNDT (65)

21 Ariano Irpino 25 41°25′40″14°45′8″

41°13′23″14°52′16″

14.9 11 25 277 70 RN N 230 0.1–1.0 0.1–0.3 DISS (ITIS092)

22 Lauro 11 40°54′55″14°32′25″

40°51′32″14°39′11″

13.9 1 13 125 60 N SW 270 0.2–0.5 (T) 0.1–0.2 GNDT (76) (Ambrosettiet al., 1987)

23 Volturara 30 40°54′5″14°50′54″

40°46′31″15°9′48″

13.9 1 13 310 60 N NE 270 0.3 (T) 0.2 Papanikolaou andRoberts (2007)

24 Irpinia Fault (Colliano segment) 28 40°51′12″15°8′4″

40°41′28″15°23′17″

15 1 14 310 60 N NE 270 0.4–0.6 0.2–0.3 DISS (ITIS077)(Papanikolaou andRoberts, 2007)

25 Irpinia Fault (S. Gregorio segment) 9 40°40′15″15°24′38″

40°37′49″15°30′11″

15 1 14 300 60 N NE 270 0.4–0.6 0.2–0.3 DISS (ITIS078)

26 Melandro–Pergola 18 40°33′51″15°30′4″

40°26′46″15°38′44″

11.3 1 10.8 317 60 N NE 270 0.1–1.0 0.1–0.5 DISS (ITIS010)

27 East Agri 20 40°24′55″15°42′40″

40°19′29″15°55′6″

13.1 5 15 120 50 N SW 270 0.7–1.30.83 (T)

0.60.5

GNDT (90), (Papanikolaouand Roberts, 2007)

28 M. Maddalena 23 40°23′53″15°39′18″

40°14′58″15°50′37″

13.5 1 12.7 316 60 N NE 270 0.1–1.00.2

0.1–0.50.2

DISS (ITIS008),(Maschio et al., 2005)

29 South Diano 15 40°19′54″15°38′56″

40°12′35″15°43′40″

14.4 1 12 140 50 N SW 270 0.5–0.7 (T) 0.3–0.4 GNDT (86), (Villani andPierdominici, 2010)

30 Mercure Basin 32 40°5′52″15°52′30″

39°55′40″16°10′42″

15.0 0 13 120 60 N SW 270 0.2–0.4 (T) 0.1–0.2 Papanikolaou andRoberts (2007)

31 Pollino 32 39°59′15″15°59′3″

39°49′23″16°17′32″

17.3 0 15 125 60 N SW 270 0.2–0.5 (T)0.2–0.4 (T)

0.1–0.20.1–0.2

GNDT (97) (Papanikolaouand Roberts, 2007)

32 Crati Valley 51 39°36′15″16°15′55″

39°8′46″16°18′43″

10.4 1 10 175 60 N W 270 0.1–1.0 0.1–0.50.8

DISS (ITCS015),(Galli et al., 2007)

⁎ Adopted value.

109L.Ferrantietal./Tectonophysics

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101–122

110 L. Ferranti et al. / Tectonophysics 621 (2014) 101–122

have an E-directed motion at a rate b1 mm/yr, and the Ionian side ofnorthern Calabria displays a SE-directed velocity pattern at b1.5 mm/yr.

4.3. Geodetic strain-rate field

To compute the 2D strain-rate tensor over the studied area, startingfrom the observed horizontal velocity field and associated covarianceinformation, we derived a continuous velocity gradient tensor on a reg-ular 0.2° × 0.2° grid using a “spline in tension” technique (Wessel andBercovici, 1998). The tension is controlled by a factor T, where T = 0leads to a minimum curvature (i.e. natural bicubic spline), while T =1 leads to a maximum curvature, allowing for maxima and minimaonly at observation points. In the computations we set T = 0.4, slightlydifferent from the value (T = 0.3) suggested by Wessel and Bercovici(1998) for topographic interpolation. As a final step, we computed theaverage 2D strain-rate tensor as derivative of the velocities at thenodes of each grid cell. The estimated strain-rates are shown in Fig. 5aas greatest extensional (εHmax ) and contractional (εhmin ) horizontalstrain-rates and rate of the areal change, and in Fig. 5b as rate of themaximum shear strain.

We present here a refined version of the results shown by Palanoet al. (2011). A continuous belt of positive area change with highstrain-rate values (up to ~110 nanostrains/yr) and NE–SW-orientedεHmax is recognized along the crest of the Apennines (Fig. 5a), in agree-ment with active crustal stretching documented by geological and seis-mological data. However, stretching does not appear quantitativelyuniform, with areas of relative maxima and minima alternatingalong the belt of extensional strain and positive area change rate.In detail, from north to south, the border between central and south-ern Apennines (at the boundaries of Molise, Lazio and Campaniaregions, Fig. 5a) is characterized by a rate of area change of up to~80 nanostrains/yr with a NE–SW trendingεHmax and a locally positiveεhmin indicating a bi-axial extensional pattern. Southward, the belt ofhigh-extension rate follows the curvature of the chain, with principalaxis having a constant NE–SW trend, and theεhmin swinging from nega-tive to positive. The rate of area change, which is slightly reduced in theborder sector between Campania and Apulia, increases again at theboundary between Campania and Lucania and has two peaks up to~110 nanostrains/yr. Southward, the belt of rapid area dilation andhigh extension rate sharply disappears at the Calabria–Lucania border(Fig. 5a).

A second belt of area dilatation, albeit of minor magnitude(~30 nanostrains/yr, with local peaks up to ~60 nanostrains/yr) andwith E- to SE trendingεHmax, characterizes the eastern coast of northernCalabria, and pushes northward into central Apulia where the maxi-mum principal axis progressively turns to a NE trend (Fig. 5a). Furthernorth, this positive areal change belt is offset to the left and crossesthe Gargano promontory, withεHmax axis keeping to a NE trend.

Areas of negative area change rate are limited. Apart from the coastalsector of the Campania Plain, where alternating patches of negative andpositive areal changes may testify local volcanic processes, in theApennines they are restricted to an arcuate belt that follows the frontof the chain between Molise and Lucania, with prevailing NE- to E-trendingεhmin axis and up to−80 nanostrains/yr (Fig. 5a). A marginallysignificant contraction was already evidenced in this area by Giulianiet al. (2009).

The maximum shear strain rate map (Fig. 5b) shows highest valuesalong the crest of the Apennines, which spatially coincide with thoseareas that in the last 103 yr were struck by strong earthquakes and cur-rently experience a diffuse low-to-moderate seismic release (Figs. 1, 3).Particularly, the Campania–Molise border in the north and the Campa-nia–Lucania border in the south are the sectorswith highest shear strainof up to 80 nanostrains/yr. High maximum shear strain-rate up to 60nanostrains/yr can be locally recognized at the volcanic districts of theTyrrhenian margin of Campania as well. On the opposite, the shearstrain-rate in northern Calabria, Lucania and Apulia is low, even in the

Adriatic and Ionian coastal region that shows locally moderate dilata-tion rates (Fig. 5a).

5. Geodetic slip rate across active faults

5.1. Geometric and kinematic parameters

In order to estimate the geodetic slip rate and later themoment ratefor active faults, in a first step we derived the fault geometric and kine-matic information from published and unpublished studies and data-bases (Table 2). In particular, based on the location of GNSS stations,we have singled out 32 structures equally distributed within the west-ern and eastern domain. For each structure we stipulated length (L),strike (θ), dip (φ), minimum (Dmin) and maximum (Dmax) depth,down-dip width (H; defined as H = d / sinφ where d is the differencebetween Dmax and Dmin values), rake and slip rate. Parameters ofmany structures (particularly for those of the eastern domain)were ex-tracted from theDatabase of Italian Seismogenic Sources (DISS) Version3.1.1 (http://diss.rm.ingv.it/diss/). Two types of seismogenic sourceswere drawn from the DISS database, namely Composite and Individualsources (Table 2). Unlike Individual sources, which are relatively wellparameterized faults which can produce “characteristic” earthquakeruptures, Composite sources are viewed as seismogenic volumeswhich incorporate an unspecified number of Individual sources, andthus their “characteristic” rupture length is poorly defined or unknown.Although this may imply a smaller accuracy of source description, theyare nonetheless used to achieve completeness of the record of potentialearthquake sources, and thus contribute to the development of regionalprobabilistic seismic hazard assessment (Basili et al., 2008). In thepresent study, because of the limited number of stations used toconstrain the geodetic slip rate, both Composite and Individual sourceswere treated as single structures. Parameters of some faults wereextracted from the GNDT (Gruppo Nazionale Difesa dai Terremoti)database (Galadini et al., 2000), or from individual papers and maps(Table 2). For these structures, source information is less completerelative to the DISS database, particularly as regards the fault depth.For few faults, the parameters were firstly defined based on our detailedor reconnaissance investigation.

The modeled deformation sources have length typically in the ~10–35 km range, with only some of them, and namely the Compositesources or some other regional fault array, exceeding40 km.Dmin rangestypically from null (outcropping fault) to 1 km for the major andminorfaults of the extensional domain, respectively. Faults of the foreland partof the transcurrent domain have the top at shallow depth (1 km) or areexposed, as occurring in the Gargano promontory. On the opposite,faults of the eastern domain which underlay at depth the foothills ofthe Apennines and are structurally covered by the thrust belt haveDmin at 6–11 km, rarely less (Table 2). In northern Calabria, strike–slipfaults outcrop or have tips at shallow depths below the coastal plainor the continental shelf and slope.

The Dmax values stipulated in the sourced datasets, which is relatedto the crustal seismogenic thickness (hereinafter Hs), was calibratedthrough the analysis of the depth distribution of seismicity. To thisaim, we defined four different sectors (Fig. 6a), one in the western do-main and three in the eastern domain. The three sectorswithin the east-ern domain are: the foothills belt in the Apennines, the foreland in theApulia region and the eastern part of Calabria. By taking into accountall earthquakes with M ≥ 2.2 occurred in the investigated area since1983 (http://iside.rm.ingv.it), we computed a cumulative frequencyplot of the depth-distribution of instrumental seismicity for each sector.The value of 2.2, usually defined as magnitude of completeness, repre-sents the lowest magnitude for which the catalog can be consideredcomplete (see Palano et al., 2011 for additional details). The plotshows that 90% of earthquakes (e.g., Marone and Scholz, 1988) occurin the upper crust within a depth that increases from the western tothe eastern domain (Fig. 6b). The limiting depth for 90% of M ≥ 2.2

111L. Ferranti et al. / Tectonophysics 621 (2014) 101–122

earthquakes is 13, 17 and 19 km in the western, frontal and forelandpart of the Apennines; the limiting depth in the Calabria frontal belt is22 km. These results are in good agreement with conclusions achievedin the study of local seismic sequences beneath the frontal part of theApennines (Boncio et al., 2007; Di Luccio et al., 2005; Latorre et al.,2010). The Dmax listed in the DISS database is slightly deeper than ourestimate of the limiting depth for 90% of M ≥ 2.2 earthquakes, becausethe former is based on thedepth of themost energetic shockswithin thesequence associated to the seismogenic source. We mostly adopted themaximum depth value reported in the DISS where available, or alterna-tively, used our estimate.

When not listed, strike and dip of faults were estimated based onpublished and unpublished geological information. For the purpose ofcomputation, strikes were averaged between the mapped surface tipsof faults or fault systems (compare Figs. 2 and 6). Although this simpli-fication may introduce errors for low-angle dipping faults, the modeledstructures have all moderate- to high-angle dips and thus errors relatedto wrong strikes are negligible. Faults of the eastern domain haveassigned dips between 70° and 90°, a range of value consistent forstrike–slip and oblique-reverse faults. Fault dips for thewestern domainare in the range of 55°–65° typical for normal faults.

The kinematics of modeled faults rely on structural analysis carriedout in recent deposits and on stress tensor data derived from availablefocal mechanisms. We list in Table 2 the general kinematic role ofeach structure as reported in the consulted databases and studies, andthe rake stipulated in published accounts or based on our knowledge.Finally, because we are mostly interested at scalar rates, we drawfrompublished sources the geologic slip rate of faults,which are typical-ly averaged on a Pleistocene or on a Holocene time interval (the twodifferent time spans are reported in Table 2). It must be noted that thegeological estimates are in some cases loosely constrained particularlyfor blind faults of the eastern domain. For sake of comparison with thegeodetic rates, the slip rate reported in consulted repositories wasconverted to the horizontal slip rate by using the dip angle. For thestrike–slip faults, the indicated value represents the horizontal compo-nent of slip.

5.2. Computation of geodetic slip rates

By selecting the closest GPS stations straddled by each fault, we com-puted the parallel (strike) and perpendicular (tensile or shortening)horizontal components of the geodetic slip rate (Table 3) by using ageometrical approach that projects the vector decomposition of GNSSstations velocities onto the fault trace. Only in ten cases we used twostations for each pair of blocks across the fault in order to compute thehorizontal slip rate. For the remaining faults, selected stations werethree or more (see Table 3 for details).

For each fault, the kinematic estimation was carried out by using aMonte Carlo simulation (Metropolis andUlam, 1949) to a set of selectedstations surrounding the fault. During the iterations ofMonte Carlo sim-ulation, values of station velocities were sampled at random from theirprobability distributions. The geometrical approach was used to calcu-late the kinematic parameters at each iteration. After 10,000 iterationsthe result was a probability distribution of possible parameters. Thekinematic parameter estimations were the expected values for theobtained probability distributions and the associated errors were therelated standard deviations (Table 3).

Within the time period considered in this study, no significant earth-quake has occurred in southern Italy nor in the immediate surround-ings, apart for the 2002 Molise sequence in the Daunia sector of theeastern domain (Fig. 3), which had a cumulative co-seismic offset of120 mm for the two main shocks (both with Mw = 5.7) as recordedby GNSS data (Giuliani et al., 2007). Within the western domain, no es-timates are available for the offset associated to the 1998 Mw = 5.6Mercure earthquake. The permanent GNSS stations considered herewere installed after both earthquakes, and only some episodic PTGA

stations have an observational record that partly span the 2002 seismicevents. Thus, large co-seismic offset related to these earthquakes do notsignificantly affect our geodetic velocity field.

The geodetic displacement resolved onto the faults may embedcomponents of both seismic and aseismic slip, and of strain accumula-tion because of friction on the fault plane (e.g. Savage, 1983). Sitesused here to compute slip rates for individual faults are reasonably farfrom any of the faults (typically N1 km), and thus we assume that ourslip estimate reflects the steady surface deformation. Based on thisreasoning, we compared the observed interseismic displacement tothe geologically recorded slip rate. We assume the long-term or geologic(overmany earthquake cycles) fault displacementmeans that the slip hasoccurred on frictionless fault planes.

6. Scalar geodetic and geologic moment rate

To better understand the relation between long-term and contem-porary deformation on the faults for which geodetic slip rate were com-puted, we estimated in a second step the scalar geological and geodeticmoment rates on these faults (Table 3).

The geological moment rate was computed using the Brune (1968)formulation:

Mgeol ¼ μHLu ð1Þ

where μ is the shear modulus of the rocks involved in faulting withassigned value of 30 GPa, L and H are length and down-dip width, re-spectively, of the dislocation along which faulting occurred and u isthe average displacement on the dislocation, which corresponds to thestipulated geologic slip rate (Table 2). We estimated both the Pleisto-cene and Holocene moment rate depending on the availableinformation.

The geodetic moment rate was estimated using the Savage andSimpson (1997) formulation:

Mgeod ¼ 2μHsA Max εHmax

�� ��; εhmin

�� ��; εHmax þεhmin

�� ��� �� � ð2Þ

where μ is the shear modulus of rocks within the focal volume, withassigned value of 30 GPa, Hs is the seismogenic thickness (Table 2), Ais the surface area of the investigated region and εHmax and εhmin arethe principal horizontal strain-rates previously described. For eachfault listed in Table 2 we defined a rectangular-hull with surface areadefined as A = (4α)(L + 2α) where 4α and L + 2α are the short andthe long side of the rectangular-hull, L is the length of the fault and α,with arbitrary assigned value of 8 km, represents theminimumdistancebetween the fault trace and the side of the rectangular-hull (Fig. 7). Bytaking into account the observed horizontal velocity field and associatedcovariance information, in a first step we computed the expected veloc-ity values at the nodes of each rectangular-hull and as a second step wederived the average 2D strain-rate tensor at the center of eachrectangular-hull. Finally, by adopting the formulation described in (2)and parameters reported in Table 2, we computed the geodeticmomentrate (Table 3).

We did not incorporate in this study an estimation of the seismicmoment rates because of the limited time-span (~30 yr) covered bythe instrumental seismicity catalog, which is too short to cover therecurrence time of most of the large earthquakes and consequently isnot an adequate measure of the seismically-released strain.

7. Results

7.1. Strike–slip faults of the eastern domain

In the eastern part of southern Italy, GNSS velocities depict substan-tial displacement across a deformation belt spanning between theApennines and northern Apulia (Fig. 8). Moving from west to east, the

a

b

Fig. 6. a) Sketchmap of the four seismogenic domains identified in southern Italy; b) Cumulative frequency plot of the depth-distribution of instrumental seismicity with M≥ 2.2 for thefour seismogenic domains. Earthquakes having depth fixed at the default values of 5 and 10 km (usually adopted for mid-continental areas when the depth of the earthquake is poorlyconstrained by available seismic data) were also included in the computation.

112 L. Ferranti et al. / Tectonophysics 621 (2014) 101–122

first significant velocity gradient is encountered across an ~30 km NE–SW-trending boundary spatially coinciding with the Biferno RiverValley (n. 1, Fig. 8) in the Daunia sector. Here, no known seismogenicsource is listed in available databases, but our analysis reveals 0.9 ±0.5 mm/yr right-lateral slip with a minor contraction component(Fig. 9a) across a structure inferred to run beneath the valley. It is possi-ble that this structure, reported in generalized structural maps(Ambrosetti et al., 1987), is part of a broader deformation belt runningoffshore the Adriatic Sea north of the Gargano upland and characterizedby thrust and strike–slip earthquakes (Fig. 3; Del Gaudio et al., 2007).Along the offshore extension of the Biferno structure, oblique thrustfaults accommodate uplift of the Tremiti Island high (Fig. 2) and foldthe sea-floor (Ridente and Trincardi, 2006). Thus, geodetic result on-land is consistent with marine geophysical data.

The transpressional structure underlying the Biferno Valley mergesto the southwest with the ~150 km long E–W striking array of right-lateral faults connecting the Apennines to the Apulia foreland (Fig. 2).The location of these structures has been proposed in the aftermath ofthe 2002 seismic sequence, caused by ruptures along right-lateralblind fault segments (Fig. 3; Di Luccio et al., 2005). At the western partof the array, the geodetic signal is however inadequate to precisely esti-mate the horizontal slip rate across the Frosolone (n. 2, Fig. 8) andRipabottoni–S. Severo sources (n. 3) of the DISS database (Fig. 9a).

The geodetic signal, on the other hand, is better in the eastern part ofthe array, within the Gargano highland, where it suggests right-obliquetranstensional displacement at ~0.5 mm/yr across the exposedMattinata–S. Marco in Lamis Fault (n. 4). The fault spatially coincideswith a seismically active belt characterized by a mixture of thrust andstrike–slip earthquakes (Fig. 3). The structure consists of three seg-ments, all of them straddled by GNSS sites. Across the eastern segment,Holocene dextral slip at ~0.8 mm/yr is recorded by paleoseismologicalstudies (Piccardi, 1998; Tondi et al., 2005), and, together with the

Pleistocene strike–slip rate estimate range, is consistent with the geo-detic result for the whole fault system (Fig. 9a). The eastern extensionof the deformation belt is tracked by high-resolution seismic reflectionprofiles offshore Gargano, where it deforms the sea-bed with right-lateral displacement which decreases to ~0.2 mm/yr (Di Bucci et al.,2009; Ridente and Trincardi, 2006).

To the south, residual GNSS velocities across the E–W striking faultsystem separating Murge and the lowland south of Gargano(Castelluccio dei Sauri–Trani, n. 5; Fig. 8) indicate 1.2 ± 0.3 mm/yrright-lateral slip. The fault system is buried under the lowland anddoes not show seismogenic activity (Figs. 2, 3), but the geodetic dis-placement is over twofold the estimated maximum geologic slip rate(Tables 2, 3; Fig. 9a).

An ~E–W trending fault system further south is located beneath thefoothills of the Apennines (Mirabella Eclano–Monteverde, n. 6, Fig. 8),and is characterized by 1.1 ± 0.5 mm/yr right-lateral slip with a slightextensional component. The geodetic signal is in broad agreementwith the inferred geological slip rate and kinematics (Fig. 9a), andwith the proposition of normal-right oblique slip during the MS = 6.6,1930 earthquake (Pino et al., 2008) on a fault spatially coinciding withthe location of source n. 6. GNSS network configuration and observedvelocities are not adequate to estimate a possible eastern extension ofthis fault system in the Murge plateau, however.

Further south, an ~80 km long systemof deeply buried (top at 13–15depth) E–W striking faults has been proposed (DISS Working Group,2010) to stretch from the Lucania Apennines to the Apulia foreland atthe border between the Murge and Salento blocks (sources n. 7 and 8,Fig. 8). Geodetic estimate for the western part of the system (PotenzaFault), are of a small right-normalmotion (Fig. 9a). This result is consis-tentwith the seismological characterization of the structure responsiblefor several seismic sequences in the last two decades, with the largershocks occurring on E–W striking, right-lateral fault planes (Boncio

Table 3Modeling results for the active faults (same as in Table 2), listing for each fault the GNSS sites assigned to individual blocks straddled by the fault, the geodetic slip rate and kinematics(positive for right-lateral and extensionalmotion, and negative for left-lateral and contractionalmotion), and the geodetic and geological moment rate (bold formoment-rate estimationsderived from latest Pleistocene-Holocene geologic slip rate). Question mark on kinematics indicates a more uncertain estimate.

ID Fault name GPS sites Geodetic slip-rate (mm/yr) Kinematic geodesy Moment rate (Nm/yr)

Block 1 Block 2 Strike–slip Opening Geodetic Geologic

Eastern domain1 Biferno TRIV, MAUR, FRES, PETC LARI, MELA 0.92 ± 0.50 −0.29 ± 0.47 R 5.64E + 15 n/a2 Frosolone BSSO, CABA TRIV, MAUR 0.07 ± 0.96 0.43 ± 0.35 RN? 2.36E + 16 1.61E + 15–1.61E + 163 Ripabottoni–S. Severo FOGG, MOCO, SVIT MELA, LARI, SPCI 0.13 ± 0.98 1.12 ± 0.92 RN? 2.81E + 16 3.66E + 15–1.83E + 164 S. Marco in Lamis–Mattinata ACQ1, MSAG LASE, SGRT 0.45 ± 0.57 0.53 ± 0.29 RN 7.02E + 15 3.39E + 15–3.39E + 16

2.71E + 165 Castelluccio dei Sauri–Trani FOGG, MARG CADM, MRVN 1.16 ± 0.28 0.23 ± 0.39 R 2.49E + 16 3.40E + 15–1.70E + 166 Mirabella Eclano–Monteverde ANDR, CAFE, GROT ACCA, SERB, SGTA 1.05 ± 0.54 0.70 ± 0.58 RN 1.49E + 16 2.98E + 15–2.98E + 167 Potenza TITO CAMA 0.28 ± 0.28 0.26 ± 0.24 RN 3.72E + 15 2.70E + 14–1.35E + 158 Palagianello SVTO, MATE, GINO NOCI, FASA 1.03 ± 0.66 −0.10 ± 0.29 R 1.52E + 16 1.76E + 15–8.81E + 159 Amendolara–Sibari TREB ROSS −1.01 ± 1.12 0.76 ± 0.60 LN? 1.31E + 16 3.84E + 16–5.48E + 1610 Rossano ROSS PIPA 1.22 ± 0.37 −0.78 ± 0.38 RT? 9.00E + 15 3.39E + 1611 Cecita LUZX, CECI, CAMO PIPA 0.31 ± 0.22 0.81 ± 0.25 RN 5.12E + 15 n/a12 Lakes CRLM, SERS CCRI, STSV 1.06 ± 0.78 1.05 ± 0.29 RN 1.55E + 16 2.74E + 16

1.37E + 1613 Lamezia–Catanzaro SERS, AMAN LAME 0.14 ± 0.32 0.02 ± 0.18 RN? 2.79E + 15 n/a

Eastern domain, shallow-crustal creeping faults14 Varano ISCH LASE, MSAG 0.16 ± 0.46 2.12 ± 0.32 N 3.05E + 15 n/a15 Castel del Monte MRVN, CADM, AMUR VALZ 0.98 ± 0.33 1.51 ± 0.31 NR 1.26E + 16 n/a16 Avena–Lauropoli TIMP TREB −0.07 ± 0.44 1.09 ± 0.44 N 3.26E + 15 1.16E + 15

Western domain17 Acquae Iuliae VAGA LONG 0.37 ± 0.23 1.21 ± 0.23 N 8.10E + 15 1.75E + 15–3.50E + 15

3.93E + 15–1.66 + 1618 Matese Lake LONG, CAMP VAGA, PTRJ −0.65 ± 0.37 0.76 ± 0.52 NL 8.99E + 15 7.75E + 15–1.55E + 1619 Boiano Basin LONG, CAMP BSSO, CABA 1.53 ± 0.90 1.74 ± 0.54 NR 6.87E + 15 1.21E + 15–1.21E + 16

4.97E + 15–8.94E + 1520 Volturno Plain N VAGA MODR 0.80 ± 0.21 0.27 ± 0.21 N 4.57e + 015 3.29E + 15–9.86E + 15

1.31E + 15–4.60E + 1521 Ariano Irpino PSB1, SACR SVIT, MOCO, CICL 0.15 ± 0.54 2.06 ± 0.81 N 1.31E + 16 1.34E + 15–1.34E + 1622 Lauro AVEL PACA −0.59 ± 0.37 1.61 ± 0.30 N 2.20E + 15 1.06E + 15–2.64E + 1523 Volturara BAGN, MCRV TOBE 1.86 ± 0.38 1.30 ± 0.87 NR 1.11E + 16 4.32E + 1524 Irpinia Fault

(Colliano segment)ANDR, SNAL, MRLC BAGN, MCRV 0.51 ± 0.35 1.52 ± 1.06 NR 1.08E + 16 5.04E + 15–7.57E + 15

25 Irpinia Fault(S. Gregorio segment)

GIUL DONG, CDRU −0.08 ± 0.32 1.0 ± 0.27 N 6.72E + 15 1.62E + 15–2.43E + 15

26 Melandro–Pergola CDRU, DONG TITO 0.45 ± 0.37 2.83 ± 0.22 N 7.47E + 15 6.11E + 14–6.11E + 1527 East Agri VOLX GRUM, MCEL 1.63 ± 0.52 −0.66 ± 0.29 RT 5.67E + 15 5.48E + 15–1.02E + 16

8.49E + 1528 M. Maddalena MTSN, SLCN MCEL, GRUM −1.23 ± 0.64 1.69 ± 1.43 NL 6.08E + 15 9.32E + 14–9.32E15

2.80E + 1529 South Diano MTSN CARM 0.43 ± 0.30 1.05 ± 0.30 N 4.13E + 15 4.22E + 15–5.90E + 1530 Mercure Basin TIMP CUCC 0.47 ± 0.27 1.54 ± 0.27 N 4.48E + 15 3.33E + 15–6.66E + 1531 Pollino SERR, CASS ACRI 0.32 ± 0.34 0.09 ± 0.57 RN? 5.71E + 15 3.84E + 15–9.60E + 15

3.84E + 15–7.68E + 1532 Crati Valley BISI, LUZZ, LUZX CETR, CARO 0.35 ± 0.45 0.16 ± 0.45 RN? 3.20E + 15 1.59E + 15–1.59E + 16

2.54E + 16

113L. Ferranti et al. / Tectonophysics 621 (2014) 101–122

et al., 2007; Di Luccio et al., 2005). To the east, a parallel structure(Palagianello source) has been proposed to connect the mountain beltand the foreland plateau (Fig. 8); geodetic data indicate possibly sub-stantial right-lateral slip at 1.0 ± 0.7 mm/yr (Fig. 9a), in agreementwith the inferred seismogenic source kinematics albeit at slightly largerrate.

On the Ionian margin of southern Italy, significant differential mo-tion exists between GNSS sites in eastern Lucania and northern Calabriaand those further south (Fig. 8). Sites ROSS and PIPA in northern Silahave left-directedmotionwith respect to sites in and around the PollinoRange. The differential motion occurs across a deformation belt whichborders the southern side of Pollino and continues offshore along thesouthern flank of the Amendolara Ridge (Fig. 8; n. 9). Morphotectonicand marine geophysical studies (Ferranti et al., 2009, under revision)documents that this ~70-km long fault system (Amendolara–Sibari) isactive and involves SW-oriented, left-oblique transpressional blind seg-ments extending down to ~10 km depth. Thanks to the irregular coast-line and the geodetic network configuration, the GNSS data are able totrack displacement on the offshore fault. The broad left slip estimated

by geodetic data, although biased by uncertainty (Fig. 9a), is in agree-ment with seismic profile analysis and focal mechanisms of middle-upper crustal earthquakes, that along the Amendolara Ridge are charac-terized by left-slip on NW-striking nodal planes (Ferranti et al., 2009,under revision; Fig. 3).

Further south, on the northern border of the Sila massif, GNSS veloc-ities document that the NW–SE striking Rossano–S. Nicola Fault array(n. 10), has a right-lateral motion at 1.2 ± 0.4 mm/yr, coupled to ashortening component at 0.8 ± 0.4 mm/yr (Fig. 9a), which may resultfrom sites ROSS and PIPA being located at a restraining bend of thefault system (Figs. 2, 8). Thisfinding is consistentwith studies proposinga transpressive character of the fault system (Ferranti et al., 2009; Tansiet al., 2007; Van Dijk et al., 2000), and with focal solutions of the mostenergetic earthquakes (Fig. 3), although the right lateral geodetic mo-tion is not consistent with geology and seismology. The magnitude ofthe geodetic estimation is at the lower boundary of the stipulated geo-logic rate (1–5 mm/yr; Galadini et al., 2000). However, we believethat the geological slip rate, when compared to all other faults of the re-gion, is probably overestimated. We have arbitrarily reduced the upper

Fig. 7. Draft of the rectangular hull used for the computation of the geodetic moment-rates. The upper surface area (A) of the rectangular-hull is defined as A = (4α)(L + 2α)where 4α and L+2α are the short and the long side of the rectangular-hull, L is the lengthof the fault and α, with arbitrary assigned value of 8 km, represents theminimumdistancebetween the fault trace and the side of the rectangular-hull. H, defined as H = d / sinφ(where d is the difference betweenDmax andDmin values; seemain text and Table 2 for de-tails) represents the down-dip width of the fault; Hs is the crustal seismogenic thickness.

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bound to 2.0 mm/yr (Table 2) to maintain coherency with the regionaldisplacement field.

In the interior of the Silamassif, the NNW–SSE striking Sila Fault sys-tem (Fig. 8) has been proposed, based on trench investigations, to be re-sponsible for large (M 6.2–6.7) historical earthquakes (Galli and Bosi,2003; Galli and Scionti, 2006). According to Galli and Scionti (2006),the system is composed of two segments, the Cecita and Lakes Faultsto the NW and SE, respectively, and of minor segments further SE, thathave normal-left oblique kinematics. In the last four centuries, this sys-tem ruptured almost entirely from NW toward SE (1638–1744–1832),with the exception of the north-western segment, the ‘silent’ CecitaFault. Geodetic observations, on the other hand, indicate right-obliquetranstensional motion for both faults (n. 11, 12, Figs. 8, 9a). The geodeticstrike–slip rate for the Lakes Fault is consistent with the reportedHolocene rate, but no geological information available for the CecitaFault.

In the central part of Calabria, a main E–Wmorphological boundarycorresponds to the trace of the Lamezia–Catanzaro Fault (n. 13; Fig. 8),which is thought to limit northward the active subduction zone beneaththe Calabrian Arc (Faccenna et al., 2011). Disagreement exists on the re-cent activity of the fault, which is alternatively considered to have a left-lateral (Tansi et al., 2007) or a normal (Faccenna et al., 2011) kinematics.GNSS sites show only a very small differential motion across the fault(Fig. 9a), however.

7.2. Shallow-crustal creeping faults of the eastern domain

Within the eastern domain, few faults characterized by high-rategeodetic extension are locally encountered within the transcurrentfault arrays. These faults have also an inferred or proved geologic exten-sion, but they are not typically associated to extensional earthquakes orto large seismic events.

In northern Apulia, on the northeastern tip of Gargano, a well-constrained diverging velocity gradient documents horizontal exten-sion of up to 2.1 ± 0.3 mm/yr across the NW–SE striking Varano Faultwhich dissects the Gargano promontory (n. 14; Fig. 8). The contradicto-ry pattern of slip when compared to the extent of strike–slip faulting inthe region is reflected in the sharp variation in area change pattern atGargano from negative in the west to positive in the east (Fig. 5a), andclearly calls for different processes at hand. Although geologic sliprates are not available for structure n. 14, the high geodetic extensionis not consistent with the limited length of the fault to be a crustalseismogenic structure. According to Bertotti et al. (1999), the Garganopromontory was structured by Neogene thrusts and transpressionalfaults rooted above a shallow-crustal NNE-dipping detachment. Basedon the above arguments, we regard the Varano Fault as an ancillary

structure, possibly reactivating the old thrust surface, which accommo-dates gravity-controlled, seawardmotion of the eastern part of Gargano.Accordingly, we assign to the fault a shallow rooting depth of 5 km(Table 2).

To the south, on approaching the Adriatic coast of Murge, NE diver-gence occurs between sites straddled by a NW–SE striking discontinuitywhich cuts through the plateau (here baptized Castel del Monte Fault,n. 15; Fig. 8). This structure may coincide with the location of thesmall-energy 1991 Castel del Monte seismic sequence (Mmax = 3.1)which was characterized by prevailing extensional focal mechanisms(Del Gaudio et al., 2005). Like for the Varano Fault, the significanthorizontal extension (1.5 ± 0.3 mm/yr) across the Castel del MonteFault is suggestive of a contribution from gravity-controlled surfacemotion toward the bathymetric low.

On the Ionian side of northern Calabria, significant divergence is ob-served between sites TREB on the coast and TIMP in the Pollino Moun-tain range, and indicates horizontal extension at 1.4±0.5 mm/yr acrossa NE–SW striking fault system (Avena–Lauropoli, n. 16; Fig. 8). The geo-detic results are consistent with geological analysis, which documentedthat the Avena–Lauropoli Fault is composed of discontinuous and small-length fault segments rather than a through-going lineament, and isaccompanied by locally dramatic stratal titling indicative of rooting atshallow depths (Ferranti et al., 2009). The fault was interpreted byFerranti et al. (2009) as part of a superficial extensional detachmentrelated to deep uplift of the Pollino range above transpressional faults.

7.3. Normal faults of the western domain

Moving from the northwest along the axis of the Apennines, the firstmajor extensional structures are encountered at the Matese massif, lo-cated at the Campania–Molise boundary. On the southwest side of themassif, sites LONG and VAGA support a geodetic extension of 1.2 ±0.2 mm/yr on theAcquae Iuliae Fault (n. 17, Figs. 8, 9b),which is viewedas the source of large historical and Holocene earthquakes, and was al-ready identified as the location of a significant geodetic velocity gradient(Giuliani et al., 2009). Our geodetic extension estimate is by far lesserthan the value (2.5–3 mm/yr) shown by Giuliani et al. (2009), but it isbroadly consistent with the upper bound (0.8 mm/yr) Late Holocenerange proposed on the base of trench data by Galli and Naso (2009).

Within theMatesemassif, geodetic data indicate divergence across aNW–SE striking and SW-dipping fault system (Matese Lake Fault, n. 18)which cuts along the central part of the range. The fault is thought tohave Holocene activity (Bousquet, 1973; Galadini et al., 2000), but it isnot well parameterized nor included in the DISS database. Geodetic ve-locities of sites straddled by the fault, however, are consistent with anormal-left oblique horizontal motion at ~0.7 mm/yr. This estimate ismoderately larger than the geologic extension computed from parame-ters drawn from a published cross-section (Calabrò et al., 2003), whichyield component rate of ~0.2 to ~0.4 mm/yr, assuming an Early(1.8 Myr) to Middle (0.8 Myr) Pleistocene inception of faulting, respec-tively (Fig. 9b).

On the contrary, agreement exists on the seismogenic role of theNE-dipping Boiano normal fault, located at the northern border ofthe Matese massif (n. 19, Fig. 8), which is considered responsiblefor the Me = 6.6, 1805 earthquake (Fig. 1; DISS Working Group,2010). Geodetic analysis indicates significant extension (1.7 ±0.5 mm/yr) and a component of right-lateral slip (1.5 ± 0.9) forthe fault (Fig. 9b). Our extension slip estimation is moderately largerthan the inferred Pleistocene and Holocene (Galli and Galadini,2003) rate and broadly consistent with the estimate of Giulianiet al. (2009).

Between theMatesemassif and the Tyrrhenian coastline of northernCampania region, the horizontal GNSS velocities indicate an additionalalbeit minor right-lateral slip and extension (Figs. 8; 10). However,the stations distribution is not adequate to assess where the gradientis precisely occurring, and thus to constrain the location of the active

Fig. 8.Modeled active faults and seismogenic sources (numbers according to Tables 2, 3), and labeled GNSS sites and related velocity in the Apulian reference frame. Localities as in Fig. 2.

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fault(s). These are commonly thought to lie at the inner border of thenorthern part of the Campania plain (Volturno Plain Fault, n. 20,Galadini et al., 2000) where they form a NW–SE trending, ~80 kmlong system of right-hand en-echelon strands of limited length(Fig. 8). If the Volturno Plain Fault is the active faults, then the strain ac-cumulation on it is consistent with the Pleistocene and Holocene rates.

In the Sannio area, geodetic extension at 2.1 ± 0.8 mm/yr is ob-served across the Ariano Irpino Fault (n. 21; Figs. 8, 10). This sourcelies at the boundary between the western and eastern domain, and isthought to be a blind normal-oblique structure (Table 2) associated todestructive historical earthquakes (Fig. 1). Note that, because velocitiesof sites SACR and PSB1 are similar and are both different from SVIT,MOCO and CICL, we slightly turned counter-clockwise the reportedstrike of the fault to let SACR and PSB1 in the western or footwallblock and the remaining sites in the hanging-wall block. Although the

fault parameters proposed in the DISS database make this fault moreakin to the eastern domain (Table 1), our results are more consistentwith it being part of the western, extensional domain.

Within the Picentini Mountains, albeit constrained by few GNSSsites, geodetic velocities suggest motion across some faults whichhowever lack evidence of Holocene activity. Both the Lauro (n. 22)and the Volturara (n. 23) Faults (Fig. 8) have a geodetic extensionof ~1.6 mm/yr, the latter associated to a significant (2.2 ± 0.2 mm/yr)right-lateral slip. For both faults, the geodetic estimate is larger than thegeological slip rate (Fig. 9b).

East of the Picentini Mountains, the Irpinia Fault is a well character-ized structure because it slipped during theMw=6.9, 1980 earthquake(Pantosti et al., 1993). Co-seismic rupture observations showed that thefault is segmented, and paleo-seismological investigations have sug-gested a 0.4–0.6 mm/yr extension rate (DISS Working Group, 2010).

a) b)

Fig. 9.Horizontal fault-parallel and fault-normal components of the geodetic and geologic slip rate for individual faults in the: a) eastern domain; and b)western domain of southern Italy.Geologic and geodetic rates drawn from Tables 2 and 3, respectively. The geologic rate for source n. 28 (thicker dashed line) includes the estimate for theMelandro–Pergola (DISSWorkingGroup, 2010, Table 2) Fault, and an additional ~0.4 mm/yr including the summed contribution from the Vallo di Diano (VD) and Alburni (A) Faults (Papanikolaou and Roberts, 2007),assuming a ~60° dip.

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GNSS data indicate 1.5 ± 1.1 and 1.0 ± 0.3 mm/yr extension for theColliano (n. 24) and S. Gregorio (n. 25) segments of the fault, respective-ly, which are larger than the trench data (Tables 2, 3, Fig. 10). Probably,the estimate for the S. Gregorio segment include contributions fromother faults southwest of the Irpinia Fault and north of sites DONGand CDRU.

The Cilento promontory, which faces the Tyrrhenian Sea in southernCampania, appears to behave as a geodetically and geologically coher-ent block. The boundary sector between the Cilento promontory andthe Apennines is marked by Quaternary extensional basins (Vallo diDiano and Val d'Agri, Fig. 2). Different views exist about the locationof the master fault controlling the destructive seismogenic activity inthe area (Cello et al., 2003; DISS Working Group, 2010; Maschio et al.,2005), specifically whether it is NE- or SW-dipping structure.

In the northern part of the boundary sector between Cilento and theApennines, substantial divergent motion is recorded in a sector span-ning sites CDRU and DONG in the west and TITO in the east (Fig. 8). Ex-tension is here modeled as largely accommodated by the NE-dippingMelandro–Pergola Fault (n. 26), although motion could be also occur-ring across the northern strand of the 45-km long Vallo di Diano Fault,and on the Alburni Fault (Fig. 2). The geodetic extension (2.8 ±0.2 mm/yr) is larger than the geological estimate (Fig. 9b), even whenthe additional contribution of the Vallo di Diano and Alburni Faults(Papanikolaou and Roberts, 2007) are included, and suggests currentslip on all the faults plus additional internal strain in the surroundingcrust.

PTGA sites CARM and MTSN straddling the southern strand of theVallo di Diano Fault (n. 29, Fig. 8), which limit the eastern side of theVallo di Diano basin, have a velocity residual indicative of 1.1 ±0.3 mm/yr extension. This result appears consistent with structuraland paleoseismological investigations (Spina et al., 2008; Villani andPierdominici, 2010) although at a slightly higher geodetic value(Fig. 9b). Unfortunately, we have no sites straddling the central part ofthe Vallo di Diano Fault to assess its geodetic activity.

Regarding the Val d'Agri basin, uncertainty exists on the geodetichorizontal slip rate for the NE-dipping Monti della MaddalenaFault (n. 28; Fig. 8), where field studies and detailed seismological ob-servations (Improta et al., 2010; Maschio et al., 2005; Valoroso et al.,2009) record active extension controlling the recent evolution of thebasin. Geodetic observations broadly document an extension (1.7 ±1.4 mm/yr) and a left-lateral slip component (1.2 ± 0.6 mm/yr) acrossthe M. Maddalena Fault, at the upper bound of the Pleistocene andHolocene estimate (Fig. 9b).

To the eastern border of the Val d'Agri basin, the SW-dipping EastAgri Fault (n. 27) has been proposed to be an active normal to left-oblique fault (Benedetti et al., 1998; Cello et al., 2003). Our analysis isnot consistent with this view, but rather indicates right-lateral slip at1.6 ± 0.5 mm/yr and a shortening component for the fault (Fig. 9b).

The southward continuation of the active extensional belt in south-ern Lucania is not well constrained either by focal mechanisms or by ac-tive fault studies. Diverging GNSS velocities between sites of the axialzone (TIMP) and of the Tyrrhenian margin (CUCC) suggests current

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extension (1.5±0.3 mm/yr) accommodated across inferred active nor-mal faults of theMercure basin (n. 30; Fig. 8). The basin has been struckby the 1998 Mw 5.6 Castelluccio Earthquake and in the last few years ithas been experiencing an intense, low-energy seismic sequence (Arrigoet al., 2006; Totaro et al., 2013). The geodetic estimate is substantiallylarger than theHolocene estimates (Fig. 9b), but the significant distancebetween the GNSS sites (Fig. 8) could hinder the existence of additionalsources and/or diffuse strain.

To the southeast of theMercure Basin, along the south-western bor-der of the Pollino range, a number of PTGA sites constrainmotion on theWNW-striking Pollino Fault (n. 31, Fig. 8),which is regarded as themainNeogene transcurrent shear zone of northern Calabria (Ghisetti andVezzani, 1982; Van Dijk et al., 2000). Investigators proposed a mainleft-lateral kinematics (Bousquet and Gueremy, 1969; Catalano et al.,1993; Ghisetti and Vezzani, 1982), and some suggested it wasreactivated as normal since the Middle Pleistocene (Schiattarella,1988). Based on paleoseismological investigations, it was argued thatthe central part of the fault is active in extension together with theNNW–SSE striking Castrovillari Fault to the south, but the whole faultsystem is currently characterized by an important seismic gap (Cintiet al., 1997, 2002; Michetti et al., 1997). Although uncertainty is signifi-cant, our geodetic observations are not supportive of the left-lateral slip,rather they possibly indicate minor right-lateral slip and extension(Fig. 9b), in agreement with the reactivation model. Geodetic data, onthe other hand, are consistent with the geological slip rate estimatefor the fault (Fig. 9b).

South of the Pollino Fault, the Crati Valley is bordered on both sidesby normal faults (Monaco and Tortorici, 2000; Tortorici et al., 1995), co-incident with strong historical and moderate instrumental earthquakes(Figs. 2, 3). GNSS station distribution allows to estimate extension onthe eastern border fault (n. 32, Fig. 8), although the activity of the north-ern part of the western fault system, suggested by geologic studies(Monaco and Tortorici, 2000), cannot be neglected. A small right-oblique extension displacement is detected (n. 30), in agreement withfault-kinematic studies (Faccenna et al., 2011) and Pleistocene rateestimate.

a)

Fig. 10. Geodetic and geologic moment rates computed for individual faults in the: a) westernand 3, respectively.

8. Discussion

8.1. Lateral variability of geodetic deformation in southern Italy

It is a common believe that the current tectonic regime in southernItaly is dominated by extension along the axis of the Apennines(e.g. Montone et al., 2012), with localized strike–slip faulting innorthern Apulia. Our analysis, supported by a detailed inspection ofgeological and seismological data, is rather consistent with a sharpand continuous spatial segregation between the western and easternparts of the region, which are characterized by extension and strike–slip deformation, respectively (Figs. 2 to 4). In the following, wediscuss the broad pattern of geodetic horizontal slip rates for faultsystems within both domains, and specifically how do they varyacross and along the orogen. For sake of simplicity, we refer to theaverage geodetic estimate and neglect uncertainties.

Within the western sector, we find that the extension rate on indi-vidual normal faults is broadly within the ~0.8–1.7 mm/yr range(Fig. 9b). Three faults, located at the tip (n. 31, 32) or off (n. 20) themain axial fault array, have lower extension rates of ~0.1–0.3 mm/yr.Only two sources (n. 21 and 26) have larger rates, but the geodetic esti-mate probably includes contributions frommore than a single fault and/or significant internal block strain. A mostly right-lateral strike–slipcomponent is found for faults of the western domain, but it is limitedat ~0.4–0.6 mm/yr, and, given uncertainty, is of low statistical signifi-cance. Only few faults have a significant (up to ~1.5 mm/yr) componentof strike–slip motion (Fig. 9b).

Although we acknowledge that extension in the western sector ofsouthern Italy is focused within the high mountains, we refine thisview and surmise that it is not concentrated on a single, through-going fault. Rather, it appears that at least two parallel fault swarmsalong the axis of the Apennines (e.g. Matese, Irpinia, Val d'Agri sectors,Fig. 8) are needed to accommodatemost (~2/3 to ~9/10) of the regionalgeodetic strain gradient. Other faults to the west, located at the bor-der of the coastal plains (e.g. n. 20, 22), accommodate the remainingstrain.

b)

and, b) eastern domain of southern Italy. Geologic and geodetic rates drawn from Tables 2

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More significantly, the magnitude of geodetic extension and strainappears to vary systematically moving along the axis of the chain paral-lel to the main faults arrays (Figs. 8, 10). Within the Campania–Molisesector in the north, the aggregated extension accommodated by faultsat theMatesemassif (faults n. 17 to 19) is ~3.6 mm/yr, and an addition-al 0.3 mm/yr extension may occur west of this location (n. 20). Movingto the southeast, extension decreases to ~2 mm/yr in Sannio (n. 21),and then to ~2.3 mm/yr in the Picentini Mountains (n. 22–23 at~1.5 mm/yr, with an additional ~0.8 mm/yr extension accommodatedby obliquemotion on fault n. 6 to the east) and ~1–1.5 mm/yr in Irpinia(n. 24–25). Thus, a 1.5–2 mm/yr decrease in extension rate from~3.5 mm/yr to ~1.5–2 mm/yr characterizes the transition fromnorthern (Matese) to central (Sannio–Picentini–Irpinia) Campania(Fig. 9b).

A second sector of relatively greater extension (~2.8 mm/yr) isfound in the Vallo di Diano–Val d'Agri area at the Campania–Lucaniaborder (aggregated contribution of n. 28–29, and n. 26 with additionalstrain admittedly taken by faults west of it). South of here, in southernLucania, the magnitude of extension decreases progressively, butrapidly to ~1.5 mm/yr (n. 30) and down to near null in northernCalabria (n. 31–32). In southern Lucania and northern Calabria, itappears that only a single fault swarm is capable of accommodatingthe extension (Fig. 8). The variation in extension magnitude alongthe axis of the chain, with two patches of greatest values at theMolise–Campania and Campania–Lucania borders, is reflected inthe pattern of area and shear strain change (Fig. 5a, b).

The observed trend in geodetic data is consistentwith the long-termslip rate pattern (Fig. 9b), and appears a persistent feature of crustalfaults in the Southern Apennines (Papanikolaou and Roberts, 2007).The latter authors studied the active faults between Irpinia andnorthernCalabria (roughly between our sources n. 23 and 31, encompassing thesouthern sector of greater extension) and found individual and aggre-gated slip rates which are greater at the center and diminish towardthe tips of the individual faults and regional fault array.We found a sim-ilar pattern in geodetic extension not only for the southern fault array,but also for the northern one fromMatese to Irpinia (Fig. 9b). Althoughwhen compared to geologic rates, the geodetic rates are one to twofoldhigher (Fig. 9b), our findings lend support to the conclusion that region-al extension in the Apennines occurs in discrete patches, and crustalfaults interact to maintain the established geometry and slip ratepattern of the array (Papanikolaou and Roberts, 2007). The possible rea-sons for the magnitude discrepancy between geologic and geodeticrates are discussed in the next section.

When compared to normal faults of the western domain, faults ofthe eastern sector have lower strike–slip rates distributed within twoaverage clusters of ~0.1–0.3 mm/yr and ~1–1.2 mm/yr (Fig. 9a). Unlikethe western domain, the lower slip rates and the higher uncertainty forfaults of the eastern domain hinders the detection of possible longitudi-nal gradients in magnitude. The associated fault-normal component ofthe horizontal slip rate appears larger in northern Calabria, and smallerin the Apulian sector, where it mostly characterizes faults closer to thewestern domain. The fault-normal component typically changes fromextensional close to the western boundary (n. 2, 3 and 6, but also n.21 at the border with the western domain), to null or slightly contrac-tional in the Apulia foreland (n. 1, 2, 8). A similar pattern was alreadynoted in seismological data (Boncio et al., 2007) for the Apulian faults,and supports the correlation between surface geodetic observationsand deep seismogenic processes.

Finally, extension at ~1–2 mm/yr is extracted for some faults withinthe eastern domain (n. 14 to 16, Fig. 8) which are: a) located near thecoast; b) of limited length; c) surrounded by seismologically activestrike–slip and reverse faults. These faults do not form a through-going belt, but rather they are isolated segments. The observedextension magnitude is clearly off-scale when compared to theirlength according to well-established empirical relations (Wells andCoppersmith, 1994), and firmly sustain the interpretation, supported by

field study (e.g. Ferranti et al., 2009, for source n. 16), of their gravitationalnature.

8.2. Relation between geodetic and geologic deformation

Geodetic horizontal slip rates and moment rates computed in thiswork are generally larger than the reported geological rates, and onlyin some cases are at the upper bound of their uncertainty range, partic-ularly for extensional faults of the western domain (Figs. 9 to 10). As al-ready pointed out elsewhere (e.g. Friedrich et al., 2003), there may bevarious reasons for the observed discrepancy between geodetic andgeological observations. On the one hand, there could be missing faults,or significant internal strain between the GNSS stations and themodeled fault trace. This non-recoverable deformation may resultfrom slip on multiple small faults within the blocks that are too smalland have too low slip rates (≪1 mm/yr) to be detected by GNSS obser-vations. Based on the amount of research carried so far, it is unlikely thatimportant active faults have gone unnoticed in the Apennines. For ex-ample, the Volturara Fault (n. 23) has a higher geodetic than geologicrate with no additional large fault intervening between the GNSS sites,and this is possibly indicative of diffuse internal strain.

Another reason for the discrepancy could be the knownpossibility oftime fluctuations in the geodetic record, which in several cases turns tobe unrepresentative of the longer-termdeformation (e.g. Friedrich et al.,2003). These fluctuations may record mechanical transients during theseismic cycle (interseismic recoverable strain or post-seismic reboundeffects) which are smoothed out in the permanent strain recorded bythe geological offsets. Perhaps, post-seismic effects associated to the M6.9, 1980 Irpinia earthquake, which may derive by processes occurringat the ductile layer depth (e.g. Della Via et al., 2003), conspire withother uncertainty sources to augment the discrepancy between the geo-detic and geological record in the Irpinia sector of central Campania(faults n. 24, 25).

Finally, it could be argued that part of the discrepancymay be causedby sliding processes at some of the GNSS sites which led to an overesti-mation of extension magnitude. However, because almost all measure-ment sites are installed in hard bedrock (see Section 4.1), we excludethat gravitational sliding is present in our estimates.

Clearly, our geodetic estimates have a yet large range, because alarge number of GNSS velocity estimation are characterized by uncer-tainties of ~0.3 mm/yr, which is in the same range of the geologicalslip-rate of most of the faults studied here. Because uncertainties areinversely proportional to the observation time span, the currentor planned increment of the observation time interval at thesesites will lead to a reduction of uncertainty values at 0.1 mm/yr level(e.g. Herring, 2003).

Notwithstanding geodetic uncertainty, the geodetic and geologicalestimate still will not match. We are aware that a sound interpretationof the relation between the geodetic and geologic deformation relieson a knowledge of the seismic versus aseismic behavior of the fault,whichmay be provided by a quantitative appraisal of the historical seis-micity associated to the fault, far beyond the scope of the paper. We canonly make a qualitative estimation of the seismic behavior of faults byinspection of their spatial relation with the historical seismicity releasepattern (Fig. 1).

Within the axial part of the chain,where destructive earthquakes arereasonably attributed to extensional faults (Figs. 1, 2), and seismic effi-ciency on the 600 yr time-window is up to 100% (Palano et al., 2011), adiscrepancy between geodetic and geologic rates has different implica-tions depending on the timing of the last earthquake rupture.

Geodetically active faults at the Campania–Molise boundary(Acquae Iuliae, Matese Lake, Boiano n. 17 to 19), have geodetic ratesonly slightly larger than, or comparable to geologic rates. Because theBoiano Fault lastly ruptured in the Me = 6.6, 1805 earthquake (DISSWorkingGroup, 2010), this result would suggest transient effects or dif-fuse strain. On the opposite, the Acquae Iuliae Fault, which is viewed as

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the source of theMe=6.3, 1349 event (Galli andNaso, 2009),may be ofsome concern given the substantial time elapsed since. TheMatese LakeFault, for which large historical events are not reported, might be effi-ciently releasing by means of creep or low seismicity (note however,that the location on a high mountain terrain renders the historical seis-micity record for the fault more uncertain). While this paper was in thereview process, a seismic sequence (Mmax= 4.95) occurred in the area,whose epicentral distribution (http://iside.rm.ingv.it/) coincides withthe Matese Lake Fault. This occurrence allows to argue that if the faultis (at least partially) locked, the significant geodetic strain accumulationon the fault may be of concern, in light of its recent seismic activation.

Faults in central and southern Campania, on the contrary, have geo-detic moments by far larger than geological moment rates (n. 21 to 25,Fig. 10b). For some faults, post-seismic effects (Irpinia Fault segments)or diffuse strain may be postulated. Because no historical earthquakesare associated to the Lauro (n. 22) and Volturara (n. 23) Faults, signifi-cant seismic loading can be excluded, and the discrepancy may resultfrom transient creep. On the other hand, the high geodetic rates forthe Lucania faults may be of concern considering the location in a de-structive seismic area (Maschio et al., 2005; Villani and Pierdominici,2010).

On the opposite, the geodetic and geologic rates are broadly compa-rable in Calabria (Fig. 10b). This result may be insightful for the PollinoFault (n. 31), which is thought to be coincidingwith a seismic gap basedon trench investigations (Cinti et al., 1997, 2002; Michetti et al., 1997).Although the geodetic and geologic rates are both low, their similaritymight be of concern.

Conversely, in regions of low to null seismicity, if the geodetic andgeologicmoment rates are equal, creep or very long recurrence intervalsfor seismic events may be postulated. This is the case of the extensionalarray located at the border of the coastal plains (n. 20, 22), which show aminor albeit appreciable geodetic signal. We do not know whether thismotion is accommodated aseismically (e.g. Ferranti et al., 2008), as sug-gested by the lack of significant seismic events (Figs. 1, 3), or faults ofthe Tyrrhenianmargin have recurrence times longer than the complete-ness interval of the historical catalogs. Certainly, these faults have ac-commodated the most severe displacement of the Apennines uppercrust down to the Tyrrhenian Sea during the last 2–3 Myr, and thus along-recurrence activation cannot be neglected.

Similarly, the eastern domain, which is characterized by diffusestrike–slip deformation and low tomoderate seismicity, may be charac-terized by faults with efficient seismic release or by long-recurrenceearthquakes. For instance, significant geodetic motion, comparablewith geologic rates, is found for sources in northern Apulia and in theApennines foothills (n. 2 to 6, Fig. 10a). On the one hand, it is possiblethat significant creep is occurring on these faults. However, because itwas proposed that large past earthquakes may have activated somefault segments (e.g. Fracassi andValensise, 2007), our observation raisesthe possibility of future ruptures.

8.3. Different engines for the active deformation domains

Appraisal of independent geodetic, geological and seismologicaldatasets shows a remarkable consistency within both the western andeastern tectonic domains. The boundary between the two domains issharp and runs along the high crest of the Apennines, where active nor-mal faults accommodate large part of the geodetic extension (Figs. 2, 3,4). Noteworthy, this boundary spatially coincides with the deep contactbetween the young and uplifted Tyrrhenian Moho to the west, and thesubjacent, older Adriatic Moho to the east (Fig. 2, inset). The eastwardencroachment of the Tyrrhenian Moho is thought to be a result of theprogressive roll-back and delamination of the Adriatic slab and accom-panyingmigration of the locus of stretching and asthenospheric upwell-ing in the Tyrrhenian back-arc basin (Cassinis et al., 2003; Di Stefanoet al., 2011; Malinverno and Ryan, 1986).

The direction of geodetic extension (Fig. 5a) is ~NE–SW along theSouthern Apennines, but turns to ~E–W in Calabria following the oro-graphic bend, the rotation of seismological axes (Fig. 3) and the curva-ture of the deep Moho boundary (Fig. 2). Although numerical modelsare needed to quantitatively appraise the relation between processesacting at different depths, these observations are taken to imply that ex-tension in the high Apennines is a direct result of the impingement ofthe Tyrrhenian Moho and delamination of the Adriatic crust, and isthus controlled by an excess potential energy. Geodetic observationsare consistentwith seismological data in supporting the notion that sur-face and deep crustal processes are intimately twinned along the axis ofthe Apennines.

What is remarkable is the clear spatial segregation, within the axialpart of thewestern domain, of two discrete fault arrays with the highestslip and strain rates localized at the Campania–Molise and Campania–Lucania boundary, respectively (Figs. 5, 10). As already noted, the geo-detic pattern is persistent in the geologic record (Papanikolaou andRoberts, 2007). The two fault arrays and shear strain regions in theApennines are mirrored to the east by the physiographic differencebetween Gargano and Murge, with an intervening lowland. Wespeculate that a deep segmentation between the two foreland blocks(e.g. Doglioni et al., 1994; Ferranti and Oldow, 2005) has caused aspatially shifted encroachment of the Tyrrhenian Moho beneath theApennines, which in turn has controlled the position and evolutionof the two sectors of localized extension in the upper crust of theApennines.

On the other hand, different causesmust be invoked for the geodeticactivity of strike–slip faults in the eastern domain. The concealed anddiffuse pattern of low slip rate faults are consistent with a model of re-activation ofMesozoic or older extensional faults affecting the continen-tal interior of the Adriaticmicro-plate (Boncio et al., 2007; Di Bucci et al.,2006). Although seismological datamostly indicate active displacementonly in Gargano and northern Calabria (Figs. 1, 3), geodetic observationsindicate a more diffuse pattern of strain accumulation and release.Overall, the transcurrent and locally oblique motion currently accom-modated on these faults may be seen as a far-field response to the colli-sion between the southern part of Adria, and the Dinarids and Albaniansector of Europe (Hollenstein et al., 2003; Jouanne et al., 2012; Oldowand Ferranti, 2005; Oldow et al., 2002).

9. Concluding remarks

Despite the limited time-span covered by the GNSS data (about 85%of the permanent sites have an observation interval shorter than 5 yr,and episodic sites have observation interval of ~15 yr), we observe forseveral faults a good agreement between the pattern of geodetic andgeological deformation, albeit the geodetic rates are largely or at theupper bound of geological rates.

Notwithstanding the locally significant uncertainty and the likelyexistence of transient fluctuations in geodetic slip rate estimation, ourresults supply vital information for refining the understanding of activefaults in the region. For most of these faults, the geological slip rate is todate poorly constrained (Table 2). The results presented here are partic-ularly encouraging for the study of the eastern domain strike–slip faults,where the geological slip rate is difficult to assess due to their deep blindnature and the low deformation rates.

Of importance, we detect significant right-lateral geodetic displace-ment in areas of the eastern sector where no active structures are re-ported in available datasets (e.g. n. 1), or where the inferred sourcesshow a weak evidence of activity (e.g. n. 5). Although we are not ableat this stage to discern whether it could represent a seismic hazardthreat or not, this outcome is relevant for near future active tectonicanalysis.

The geodetically-based refinement of slip rate for active faults hasimportant implications for both fault-based and time-dependent seis-mic hazard assessment models. At present, the national seismic hazard

120 L. Ferranti et al. / Tectonophysics 621 (2014) 101–122

map (Meletti et al., 2008) does not include individual seismogenicsources, mostly for the difficulty of characterizing their seismicity be-cause of the large uncertainty on the deformation rate (see Table 2).Thus, incorporation of geodetic slip rates may effectively contribute to-ward a fault-based seismic hazard assessment (Peruzza et al., 2010,2011; Roberts et al., 2004), even in this region of low and diffusedeformation.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.tecto.2014.02.007.

Acknowledgments

This research was conducted under the auspices of a collaborativeresearch initiative funded by various sources in Italy and in the UnitedStates. The GNSS campaigns could not have been carried out withoutthe diligent work of many undergraduate and graduate students fromItaly and the United States. The careful reviews of B. Pace and I.Papanikolaou were instrumental in clarifying several aspects of thework.

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