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Journal of Geodynamics 71 (2013) 1– 13

Contents lists available at ScienceDirect

Journal of Geodynamics

j ourna l h om epage: ht tp : / /www.e lsev ier .com/ locate / jog

inematic interpretation of present-day crustal deformation inentral Greece from continuous GPS measurements

onstantinos Chousianitisa,∗, Athanassios Ganasa, Michail Giannioub

National Observatory of Athens, Institute of Geodynamics, Lofos Nymfon, Athens 11810, GreeceKtimatologio S.A., 288 Messogion Avenue, Athens 15562, Greece

r t i c l e i n f o

rticle history:eceived 11 April 2013eceived in revised form 17 June 2013ccepted 18 June 2013vailable online 2 July 2013

eywords:PSermanent networkime series analysis

a b s t r a c t

We processed 30-s GPS data from continuous GPS stations in central Greece using the Kalman filteringapproach and accounting for time-correlated noise content obtaining a velocity field in the ITRF2008 andthe Eurasian-fixed reference frame. The station distribution allowed us to compute 1D strain throughrates of baseline length changes as well as to construct the image of the 2D strain and rotation ratefields. The obtained baselines range in length from 11 to 132 km and show rates from −1.95 mm/yr up to14.14 mm/yr (estimated uncertainties from 0.3 to 0.8 mm/yr), while the calculated 1D strain rate rangesfrom −27 ns/yr up to 226 ns/yr (average uncertainty ∼15 ns/yr). Largest extension (192–226 ns/yr) isobserved in the western and central part of the Corinth rift while similar extension rates (80–120 ns/yr)are obtained for the eastern part of the Corinth rift and its continuation in the south Viotia–south of

rustal extensionreece

Evia region and across the Sperchios–Kammena Vourla rift. The coherent picture of the velocity pat-tern for Attica and north-eastern Peloponnese (Corinth) stations indicates that these areas belong tothe same crustal block, separating by the Viotia region by a nearly E–W crustal discontinuity along theKaparelli–Asopos valley faults. However, some internal strain is present within Attica’s crust as wellas across the Saronic Gulf resulting in extension rates of the order of 25 ns/yr. We also find extension(54–71 ns/yr) across “rigid” Peloponnese taken by normal faults in the greater Kalavryta region.

. Introduction

Greece is located at a complex plate boundary region wherewo tectonic plates (Africa-Nubia and Eurasia) converge and theountry has very high risk of major earthquakes along the Hel-enic Arc (Makropoulos and Burton, 1981; Pirazzoli et al., 1982;apazachos, 1990; Shaw et al., 2008). Relative motion of theselates accumulates stress in the lithosphere, causing observablerustal deformation (e.g. McClusky et al., 2000; Ganas and Parsons,009; Floyd et al., 2010). Earthquake rupture occurs along bothrustal faults and Nubia-Eurasia plate interface to release tectonictress, a fact that makes the study of crustal deformation an essen-ial issue for studying earthquakes. In southern Greece, earthquakesre caused primarily by interaction between the relatively smallegean Sea and the larger Africa (Nubia) plates. In northern Greece

he main seismic hazard comes from the two branches of the Northnatolian Fault that terminate inside the Aegean Sea. In western

reece there are two large offshore faults, the Cephalonia Trans-

orm Fault and the Apulian Thrust, further northwest. Moreover,ther strong, shallow earthquakes frequently occur away from

∗ Corresponding author. Tel.: +30 2103490184.E-mail address: chousianitis@noa.gr (K. Chousianitis).

264-3707/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.jog.2013.06.004

© 2013 Elsevier Ltd. All rights reserved.

the major fault systems mentioned above, toward the interior of“rigid” crustal blocks (e.g. Grevena–Kozani earthquake in 1995,Mountrakis et al., 1998; Resor et al., 2005; Athens earthquakein 1999, Papadopoulos et al., 2002; Pavlides et al., 2002). Spacegeodesy technologies, such as GPS, represent a powerful tool incrustal deformation monitoring, especially when high precision isrequired and geodetic measurements are increasingly more widelyapplied in geodynamics and earthquake studies (e.g. Subarya et al.,2006; Reilinger et al., 2010; Floyd et al., 2010; Ganas et al., 2013). Inaddition, GPS measurements are able to quantify interseismic andtransient postseismic processes (e.g. Henry et al., 2001), as wellas coseismic deformation (e.g. Anzidei et al., 2009; Ganas et al.,2009; Pollitz et al., 2011) and are able to correlate the rate of strainaccumulation with earthquake occurrence (e.g. Fialko, 2006).

Geodetic investigation of the kinematics in Greece via campaignGPS observations started in the late 1980s–early 1990s followingthe documentation of large-scale continental extension across theGulf of Corinth (∼1 cm/yr; Billiris et al., 1991). It was soon realizedthat the kinematics of deformation involved a combination of west-erly motion of Anatolia and south-westerly motion of the central

and south Aegean, relative to Eurasia, in tandem with N–S conver-gence across the Hellenic Arc (Le Pichon et al., 1995; McClusky et al.,2000; Kahle et al., 2000). It is also evident that there is a progressiveincrease in GPS velocities southward in northern Greece toward

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K. Chousianitis et al. / Jour

he North Aegean Trough, across which the velocities increase andhange direction dramatically. During the last decade, a number ofroups conducted research on the fragmentation of the upper crustn microplates or continental blocks (e.g. Avallone et al., 2004; Nystnd Thatcher, 2004; Reilinger et al., 2006; Floyd et al., 2010) andn mapping crustal motions and interseismic velocities in the cen-ral and western part of the country (e.g. Cocard et al., 1999; Briolet al., 2000; Bernard et al., 2006; Lagios et al., 2007; Hollensteint al., 2008). Most of the above studies, along with the vast major-ty of the geodetic research in Greece have focused on the use ofampaign-style GPS measurements or on the joint use with contin-ous measurements (CGPS), but very small number of them dealsith continuous measurements only (Peter et al., 1998; Hollenstein

t al., 2006; Ganas et al., 2013).Central Greece is a well-known extensional province where

rustal deformation is associated with rifting since Miocene timesOri, 1989; Doutsos and Poulimenos, 1992; Leeder and Jackson,993; Papanikolaou and Royden, 2007). Rifting is currently orga-ized into three marine basins (Corinth, Evia, Pagasitikos) whileoung land basins (Sperchios, Asopos, South Thessaly) are alsoresent (Fig. 1). Active faults are segmented, with lengths between0 and 20 km, and strike approximately from E–W to N120◦ E. Thective faults are moderately to highly inclined (40–60◦) and showrimarily normal slip vectors with a systematic variation alongtrike, pointing toward the fault centers (Roberts, 1996; Robertsnd Ganas, 2000; Morewood and Roberts, 2001). Earthquakes gen-rally do not exceed M = 7 (Ambraseys and Jackson, 1990) and occurt depths between 5 and 15 km.

The purpose of this paper is to present an updated velocity fieldn central Greece based on data from 33 continuously recording GPStations and use it to map the contemporary distribution of activeeformation in that area. The sufficient observation time span ofhe used stations, the outlier editing and the modeling of the first-rder features of the derived time series ensure reliable velocitystimates. We refine the understanding of the strain distributionnd accumulation within the study area using one-dimensionaltrain rates derived from rates of baseline length changes and two-imensional strain and rotation rate tensors (i.e. the symmetric andntisymmetric part of the velocity gradient tensor) by discretiz-ng the study area using Delaunay triangulation. We then compare

ith geological data to constrain the present-day kinematics ofhe major structural units of central Greece. Our overall geodeticesults clearly reveal different kinematic regimes present in thisegion. The already known or supposed tectonic features of centralreece, such as the extension rates across the northern Gulf of Eviand the major faults of Kammena Vourla and Arkitsa, have been fur-her quantified, while unknown features, such as the extensionalegime across north Peloponnese, have been revealed, providingew insights in the tectonic processes of the studied area (Fig. 1).

. GPS data processing

We process GPS data from 33 continuously recording sta-ions (cGPS) in central Greece, which map the study area with

mean inter-site distance of about 50 km in order to derive aense high-quality velocity field. Most GPS stations are locatedlose to major seismogenic structures, a prerequisite to opti-ally measure tectonic motions. The analyzed stations belong

t different research institutions, one governmental office andne private company (Fig. 2). Three of them belong to theOANET network, a real-time, high-rate GNSS network with

tations with sampling rates up to 5 Hz, installed by the Insti-

ute of Geodynamics of the National Observatory of Athenshttp://www.gein.noa.gr/gps.html). This network has been operat-ng since February 2006 following the EUREF (Regional Referencerame Sub-Commission for Europe) Permanent Network standards.

Geodynamics 71 (2013) 1– 13

Five stations belong to the private company METRICA S.A. oper-ating Leica receivers (http://www.metricanet.gr). Twenty stationsbelong to KTIMATOLOGIO S.A., a state-owned company that opera-tes the HEPOS network (http://www.hepos.gr). Three belong tothe Corinth Rift Laboratory (CRL) project (http://crlab.eu), whosegeodetic data are publicly available at the moment, and two sta-tions belong to the COMET+/NTUA consortium. The data span ofthe analyzed continuously recording stations that produced thevelocity field exceeds 2.5 years with the majority of them oper-ating over five years, a time length which can be considered longenough to obtain reliable velocity estimates (Blewitt and Lavallee,2002; Caporali, 2003).

All data were processed in 24-h sessions using theGAMIT/GLOBK software package (Version 10.4; Herring et al.,2010) in a three step approach, which is based on the «quasi-observation» theory and the reference frame is not defined untilthe last step of the analysis (Feigl et al., 1993; Dong et al., 1998). Inthe first step, choosing the ionosphere-free linear combination, andfixing the ambiguities to integer values we obtain daily estimatesof station coordinates, the tropospheric zenith path delay at eachstation every 2 h, and the orbital and Earth Orientation parameters(EOP). We used precise orbits from the International GNSS Service(IGS) and absolute calibration values from IGS tables for modelingthe receiver and satellite antenna phase center variations. Theeffect of solid-earth tides was taken into account according to theIERS/IGS standard 2003 model (McCarthy and Petit, 2004), and weapplied the FES2004 model (e.g. Lyard et al., 2006) for ocean tidalloading corrections. Data from 21 well performing stations of IGS(inset of Fig. 3), which display low multipath effect and are notaffected by post-seismic relaxation, were included in the analysis.With this approach we tied our regional measurements to a globalreference frame and we improved ambiguity resolution by puttingtight constraints (5–8 cm) on these sites from their a priori coordi-nate values in the ITRF2008 (Altamimi et al., 2011). The coordinatesof the Greek permanent stations were allowed to vary freely by wayof very loose constraints (100–200 m). An automatic cleaning algo-rithm was applied to post-fit residuals in order to repair cycle slipsand to remove outliers. For each session, we obtained two solu-tions based on phase ambiguity resolution, one bias-free and onebias-fixed, along with the associated variance–covariance matrices.

In the second step, we combined our loosely constrained bias-fixed solutions of our regional network with analyzed global andregional solutions provided by SOPAC (http://sopac.ucsd.edu) intosingle day unconstrained solutions. We obtained station positiontime series in a common reference frame by considering the dailyloosely constrained estimates of station coordinates, orbits andEOP and their associated variance-covariance matrices as quasi-observations and passing them to GLOBK which employs theKalman filtering approach. The reference frame during the for-mation of these combined network solutions is loosely defineduntil the last processing step, where we realized a common ref-erence frame for our solution. This was achieved by applyinggeneralized constraints (Dong et al., 1998, 2002) while estimatinga six-parameter Helmert transformation (three network rotationsand three network translations), aligning each individual dailysolution to the 2008 realization of the ITRF-NNR frame. We min-imized, in the least-square sense, the departure from the a priorivalues determined in the ITRF2008 frame of the 21 IGS stationsincorporated in the GAMIT processing part. The generalized con-straints approach has the advantage that all the chosen referencesites are free to adjust, hence revealing bad data or coordinates. Weused five iterations to eliminate possible bad sites and to compute

station weights for the reference frame stabilization.

After deriving the position time series, we performed an anal-ysis for all GPS stations in central Greece with the aim of modelingthe constant velocity for each component of each station, together

K. Chousianitis et al. / Journal of Geodynamics 71 (2013) 1– 13 3

Fig. 1. Focal mechanism solutions of major earthquakes in Central Greece. Green beach balls have taken from Ambraseys and Jackson (1990) and from Papadimitriou andKarakostas (2003), while blue are from the CMT Catalog (1976–2005; Mw > 5) and from the NOA MT (Moment Tensor) Catalog (2005–present; 3.5 < Mw < 6.6). Gray linesshow mapped faults. Inset map showing the main structural features of the Hellenic Arc and the Aegean Sea. The red lines show the traces of the Hellenic Arc-Trench System(HAT), the Cephalonia Transform Fault (CTF), the Apulian Thrust (AT), the Volcanic Arc (VA) and the North Anatolian Fault (NAF). Also shown the North Aegean Trough (NAT),t e otheo pos Va( rred to

wioGaeDapieTtiddf

d

he South Cretan Trough (SCT), the Pliny Trench (PT) and the Strabo Trench (StT). Thf Evia; sGE, south Gulf of Evia; SI, Sporades Islands; PG, Pagasitikos Gulf; AV, AsoFor interpretation of the references to color in this figure legend, the reader is refe

ith the annual and semi-annual signals and the offsets observedn the series using TSVIEW software (Herring, 2003). These first-rder features have been identified in several studies of continuousPS time series and their successful modeling leads to accuratend realistic determination of geodetic velocities (e.g. Blewittt al., 2001; Van Dam et al., 2001; Blewitt and Lavallee, 2002;ong et al., 2002). The seasonal signals are commonly modeled as

combination of sinusoids with a fundamental period of one yearlus first harmonic mode of six-month period. The epoch offsets

n the GPS time series are in general caused either by nearbyarthquakes that affect the GPS station, or by antenna changes.he latter can occur when these changes are not well-modeled inhe processing software due to erroneous references during thenstallation time, or when the antenna models do not adequatelyescribe the measurements. The mathematical expression thatescribes the geodetic time series accounting for first-ordereatures may be written (Langbein, 2004) as:

i = a + bti +k0∑

vk(ti − Tk)H(ti − Tk) +j0∑

ojH(ti − Tj)

k=1 j=1

+m0∑

m=1

[am sin

(2�ti

Tm

)+ bm cos

(2�ti

Tm

)]+ ei (1)

r abbreviations in the main map are as follows: GC, Gulf of Corinth; nGE, north Gulflley; TB, Thiva Basin; P, Parnitha Mountain; SG, Saronic Gulf; SB, Sperchios Basin.

the web version of the article.)

where di is the measurement at time ti; ̨ is the site position; bis the linear rate; vk are rate changes starting at ti = Tk; H(ti − Tj) isthe Heaviside step function (Abramowitz and Stegun, 1972) whichequals 1 for ti ≥ Tj and 0 otherwise; oj are offsets in the time seriesat ti = Tj; ˛m and bm are sine and cosine amplitudes at period Tm andei denotes noise.

Moreover, because of the existence of abnormal outliers in dailysolutions, we performed editing in order to remove erroneous val-ues from contaminating the velocity solutions, and to retrieve cleantime series. We have used an automatic outlier function with a5 sigma rejection level. In some time series we detected offsets,which we modeled as step functions, by inspecting the data resid-uals in combination with the knowledge of each site history andnearby earthquakes. All of the observed offsets in the time seriescan be attributed to hardware changes within the time span ofthe processed data apart from the offsets mainly in the North andUp components of RLSO, and the HEPOS stations 012A and 030Awhich can be attributed to co-seismic deformation generated bythe nearby 2008 Achaia earthquake (Ganas et al., 2009).

Analyses of continuous GPS data have showed that there is

a significant amount of colored noise content within geodetictime series (e.g. Bock et al., 1997; Zhang et al., 1997). Thus, thewhite noise assumption that measurement errors are random anduncorrelated from one epoch to its next is not the case for GPS

4 K. Chousianitis et al. / Journal of Geodynamics 71 (2013) 1– 13

Fig. 2. Site locations of the continuous GPS stations used in this work. Yellow stars with red outline correspond to major events (M ≥ 6.0) of Central Greece taken fromAmbraseys and Jackson (1990, 1997, 1998). Intermediate depth events have been excluded as they do not impose seismic strains on the upper plate. Abbreviations correspondt LamiaA retativ

dpvuamc�twsa�GluimawG

b

wvsGc

o major active faults discussed in the text and are as follows: EF, Ekkara Fault; LF,

rkitsa Fault; AtF, Atalanti Fault; KpF, Kaparelli Fault; AvF, Avlonas fault. (For interpersion of the article.)

ata (e.g. Johnson and Agnew, 1995; Williams et al., 2004) and if aure white-noise model is used, it may result in unrealistically lowelocity uncertainties, especially for continuous data, that can benderestimated by a factor of 5 or more (e.g. Mao et al., 1999). Toccount for time-correlated errors in our processed time series, weodeled them using a first-order Gauss-Markov process. For each

oordinate component we estimate the increase in the normalized2 (chi-squared-per-degree of freedom) of successively longer

ime averages. For a white noise error model, the normalized �2

ould not depend on averaging time, but for non-white noisepectra, the normalized �2 increases with successively increasingveraging time. Next, the time-averaged values of the normalized2 were fit to the exponential function expected for a first-orderauss-Markov process so as to estimate a correlation time and the

ong-term variance. This model was used to predict site velocityncertainties based on the span of the time series. Since GLOBK

mplements a Kalman filter that is able to realize random walk noiseodel, which is a first-order Gauss–Markov model with infinite

veraging time, we calculated the random walk model values thatould predict the same velocity uncertainties as the first-orderauss–Markov model for the time series using the equation

2 = �RW T (2)

here b is the magnitude of the random walk noise, �RW is the

elocity uncertainty estimates and T is the time span of the timeeries (Zhang et al., 1997). These values were used as input toLOBK in order to add random walk to the error model andalculate “realistic” uncertainties for the velocity estimates.

Fault; KVF, Kammena Vourla Fault; KlF, Kallidromo Fault; PF, Parnassos Fault; ArF,on of the references to color in this figure legend, the reader is referred to the web

After the above time series analysis, our final ITRF2008velocity solution accounting for seasonal signals, offsets and time-correlated noise content was produced by combining the individualepoch-by-epoch solutions into one “stacked” solution by meansof the GLOBK’s Kalman filter. In order to highlight crustal defor-mation within Greece we subsequently defined a Eurasian-fixedreference frame by minimizing the departure in position and veloc-ity of the same 21 IGS stations with respect to their a priori valuesthat were generated by rotating their ITRF2008 values by meansof the Euler vector for Eurasia (Altamimi et al., 2012). The resultingstation velocities, for both the ITRF2008 and the Eurasian-fixed ref-erence frame, together with the estimated uncertainties are listedin Table 1 and the corresponding computed horizontal velocity fieldis visualized in Fig. 3. The north, east and up position time seriesare shown in Fig. A1 of the auxiliary material.

To locate discontinuities and/or gradients in the GPS velocityfield we constructed two velocity profiles along a NW–SE direction,i.e. normal to the regional velocity field (Fig. 4). The profiles azimuth(N120◦ E) is suitably oriented to allow us to draw conclusions forcrustal deformation patterns in central Greece. In the first profile(upper panel of Fig. 4) we incorporated only the stations of centralGreece, excluding those of Peloponnese. By looking at the corre-sponding cross-profile it is evident that the rates of these stationsfrom 071A and KLOK, through ATAL, THIV, NOA1 and finally ANAV

and 006A are illustrating a velocity increase from NW to SE, indi-cating an extensional strain regime accompanied by rotation of theentire region north of the Gulf of Corinth (Fthiotida–Viotia–Evia).We fit the cross-velocity profile in two steps, one incorporating

K. Chousianitis et al. / Journal of Geodynamics 71 (2013) 1– 13 5

F r ellipt locity

sfuotfopEisttfpt“Pf

ig. 3. GPS-derived horizontal velocity field with respect to stable Eurasia. The errohe location of the frame defining IGS stations during the calculation of the final ve

tations up to 072A with a cross-rate of about 8 mm/yr and a secondrom LIDO to 024A with a cross rate of about 10 mm/yr, makingse of the proposed E–W boundaries of the published block modelf Reilinger et al. (2006). In addition, this trend of velocity increaseerminates toward station 024A, while for the cluster of stationsrom 007A to 006A (Attica), it disappears, indicating that this regionf central Greece is more stable. In the second set of profiles (loweranel of Fig. 4), we included also the cGPS stations of Peloponnese.xtension is indicated by the result that the Fthiotida area is mov-ng more slowly toward the SW than the NW Peloponnese at theame distance along the profile. It is also evident that the cluster ofhe northwest Peloponnese stations behaves differently comparingo the stations of the southeast coast of the Gulf of Corinth, namelyrom KRYO to 010A. The latter, demonstrate a more coherenticture of the velocity field, which additionally is similar to that of

he stations located in Attica. In both areas we observe a velocityplateau” slightly above 30 mm/yr (reported previously for NEeloponnese by Avallone et al., 2004), which differentiates itrom Viotia (velocities of THIV, 014A and 024A between 25 and

ses represent the 95% confidence region. The map in the upper right corner showssolution.

30 mm/yr) and from NW Peloponnese. This finding indicates thatthe areas of Attica and NE Peloponnese belong to the same crustalblock in accordance with the findings of Mattei et al. (2004). Weexplore this result with a more rigorous approach below.

3. Discussion

3.1. Strain pattern

We investigate strain accumulation in central Greece by firstestimating 1D strain rates from GPS baseline vectors. Knowing theyearly rate of change between stations and their relative distances(baseline length) we can compute 1-D strain per year (�L/L). Table 2shows the strain estimates for 46 baselines in central Greece (Fig. 5).The range of baseline lengths is from 11 up to 132 km and the range

of length changes is from −1.95 mm/yr up to 14.14 mm/yr. Largestrains of positive sign (extension) are obtained across the Gulfof Corinth, across the Sperchios Valley rift and across the southGulf of Evia and Parnitha region. Extension is also obtained across

6 K. Chousianitis et al. / Journal of Geodynamics 71 (2013) 1– 13

Table 1Horizontal GPS velocities in ITRF2008 and Eurasian reference frames (in mm/yr) together with associated 1-sigma uncertainties (in mm/yr) that have been estimated byadding a random walk to the error model used by GLOBK. RHO is the correlation coefficient between the E (east) and N (north) estimates, which are necessary to computehorizontal uncertainty ellipses.

Station Longitude Latitude ITRF2008 Stable Europe RHO Uncertainty (mm/yr)

ve vn ve vn �ve �vn

ARK2 23.033 38.755 13.79 −5.36 −10.13 −19.50 0.007 0.14 0.11LIDO 22.201 38.529 11.40 0.25 −12.42 −14.01 0.063 0.13 0.16KOUN 22.045 38.209 8.87 −11.36 −14.97 −25.65 −0.022 0.12 0.14LAMB 21.973 38.320 10.96 −7.77 −12.86 −22.06 0.179 0.18 0.08KRYO 22.618 37.972 6.74 −13.34 −17.22 −27.54 −0.036 0.17 0.08NOA1 23.864 38.047 7.49 −11.39 −16.65 −25.40 0.016 0.18 0.23ATAL 22.999 38.653 12.96 −4.84 −10.97 −18.99 0.025 0.22 0.25KLOK 22.014 39.564 20.96 5.87 −2.68 −8.42 −0.146 0.12 0.13ANAV 23.904 37.733 6.92 −11.86 −17.28 −25.87 −0.007 0.21 0.18MET0 23.762 38.065 8.15 −11.57 −15.97 −25.60 0.016 0.14 0.14THIV 23.317 38.316 7.77 −8.12 −16.25 −22.22 −0.004 0.19 0.18SPET 23.155 37.267 6.89 −14.40 −17.24 −28.52 −0.001 0.22 0.25KORI 22.931 37.941 7.49 −11.97 −16.53 −26.13 −0.013 0.12 0.13023A 24.322 38.046 7.63 −12.41 −16.63 −26.35 0.006 0.47 0.47025A 24.113 38.574 10.14 −10.61 −14.02 −24.58 0.003 0.37 0.37006A 24.046 37.671 6.81 −12.56 −17.46 −26.54 0.006 0.38 0.38008A 23.981 38.112 7.72 −11.31 −16.48 −25.30 −0.027 0.52 0.53098A 23.803 38.007 7.74 −11.38 −16.45 −25.40 0.013 0.40 0.39061A 23.727 39.122 14.75 −5.94 −9.27 −19.97 −0.033 0.39 0.40024A 23.648 38.471 9.47 −9.29 −14.63 −23.34 0.010 0.37 0.37007A 23.541 38.042 8.09 −11.28 −16.06 −25.34 −0.010 0.45 0.45010A 23.452 37.500 7.41 −13.01 −16.79 −27.08 −0.035 0.43 0.44014A 23.386 38.294 8.14 −10.88 −15.95 −24.97 −0.020 0.45 0.44026A 22.976 38.863 15.38 −2.38 −8.57 −16.53 −0.056 0.51 0.49043A 22.761 37.976 8.16 −14.03 −15.87 −28.21 −0.002 0.45 0.46002A 22.739 37.589 6.70 −13.40 −17.39 −27.58 0.005 0.39 0.38015A 22.732 38.516 14.89 −5.60 −9.06 −19.79 −0.033 0.41 0.40072A 22.371 38.903 15.36 3.11 −8.48 −11.13 0.006 0.39 0.39011A 22.316 38.152 9.91 −10.02 −14.03 −24.26 0.020 0.37 0.38013A 22.124 37.899 8.24 −11.04 −15.71 −25.32 −0.010 0.48 0.50

2

9

1

wS0−o

sa(l0bStnsoa0srp4nAeqoAfa

071A 21.843 39.559 20.57 7.3012A 21.789 38.295 8.20 −6.2027A 21.732 38.842 15.72 6.2

estern Thessaly, Attica, Saronic Gulf and northern Peloponnese.hortening is obtained for two baselines, namely 061A–026A and24A–026A, in north Evia and Fthiotida regions. Strain ranges from27 to 226 nstrain per year (ns/yr), with uncertainties of the orderf ±10–15 ns/yr.

Across Evia island our 1D strain estimates point to a N–S exten-ion of 53 ns/yr (baseline between station 061A and 024A) becauset angles 35–45◦ on either side 1D strain decreases considerablythese are relative angles with respect to the orientation of the base-ine 061A–024A given station 061A as apex; 061A–025A = 27 ns/yr,61A–ATAL = 7 ns/yr). The 061A–024A extension is accommodatedy the coastal fault system of N. Evia (Roberts and Jackson, 1991;tiros et al., 1992), offshore oblique-slip faults such as the fault rup-ured on 14 October 2008 (Roumelioti and Kiratzi, 2010) and of theormal faults in the Psachna area which produced the earthquakewarm of 2002–2003 (Benetatos et al., 2004), about 15 km northf station 024A. This extension rate continues further to the south,cross the south Gulf of Evia, where the baseline between stations24A and 007A extends with a rate of 48 ns/yr. Further east, exten-ion drops about 50% to 18–23 ns/yr (NOA1–025A and 025A–008A,espectively), which is indicative of the younger age of the riftingrocess along the south Gulf of Evia. Moreover, we suggest that the8 ns/yr extension is mostly accommodated by the Parnitha regionormal faults, which strike E–W ±20◦ and dip to the North (e.g. thevlonas fault, Ganas et al., 2004; see also fault maps of Goldsworthyt al., 2002). The latest major event on this area was the 1938 earth-uake (Ms = 6.1; Ambraseys and Jackson, 1990), while the previous

ne was the 1694 M = 6.5 event (Ambraseys and Jackson, 1997).s further evidence of (a) the strike variation of the active normal

aults and (b) the oblique slip component of slip vector we note themount of strain resolved along the NNW–SSE baseline 014A–007A

−3.08 −6.99 −0.002 0.39 0.39−15.64 −20.62 0.001 0.44 0.44−8.03 −8.13 −0.075 0.44 0.42

of 17 ns/yr and along the NW–SE baseline NOA1-THIV of 19 ns/yr.South of 007A, N–S extension drops to 26 ns/yr, a non-negligibleamount and clear indication that the area is active and seismicactivity need to be expected. The candidate (active) faults havebeen mapped by Makris et al. (2004) and Drakatos et al. (2005). Ofequal interest is the strain rate resolved to the east, along the base-line 008A–006A of 22 ns/yr. This amount represents internal strainof the “rigid” Attica block and it is not clear along which faults isaccommodated because geological data are lacking. Some minorextension is observed along the NE-SW direction east of Attica(baseline 006A–023A of 13 ns/yr), while no changes are observedalong the E–W orientation between Attica and Peloponnese (seebaselines 008A–007A, 007A–043A, 006A–010A and KORI–010A).

Across the Gulf of Corinth we obtain the largest estimates ofstrain as expected from previous investigations (Clarke et al., 1998;Briole et al., 2000; Avallone et al., 2004; Floyd et al., 2010) inits central and western part. Baselines 027A–012A, 011A–LIDO,015A–011A and 015A–043A show strain rates of 192, 226, 122 and135 ns/yr, respectively. Across the eastern part of the Gulf (N–Sbaseline KORI–ATAL, 99 ns/yr) strain rate is reduced to about 30%with respect to the central part of the Gulf, and 50% with respectto its western part, while across the Kaparelli fault, strain rate isfurther reduced to 89 ns/yr (baseline KORI–THIV). However, (a) theorientation of the latter baseline is not orthogonal to the strike ofKaparelli fault, (b) the strike of the fault switches toward SW at itswestern termination (Kokkalas et al., 2007) while (c) the slip vec-tor azimuth of the 4 March 1981 earthquake ruptures is between

N010◦ and 030◦ E (Jackson et al., 1982). Given this obliquity, it isprobable that strain in the Kaparelli region is slightly larger, simi-lar to the strain sampled along the baseline KORI–ATAL. Data fromlocal GPS networks will better determine strain in this critical area

K. Chousianitis et al. / Journal of Geodynamics 71 (2013) 1– 13 7

Fig. 4. Horizontal GPS velocities with 1-sigma uncertainties plotted as a function of distance along profiles whose directions are shown by the black dotted lines in the velocitymaps on the left. In each profile, only the sites with blue vectors and blue station name have been taken into account. In each case, upper panels present the profile-parallela ationso ocity me reader

oss5tmtwic

satAmaineo0

nd lower panels present the profile-normal velocity components of the selected stf the line. The two vertical black dotted lines in the cross-profile of the upper velt al. (2006). (For interpretation of the references to color in this figure legend, the

f the rift. Another important observation is the amount of strainampled along the northern coast of Peloponnese as far south astation 013A (nearly 40 km from the coast) which ranges between4 and 71 ns/yr (baselines 012A–013A and 011A–013A, respec-ively). This extension possibly accommodated by normal faults of

ixed orientations and with an oblique slip component. Moreover,he mapped extension demonstrates that significant strain occursithin the “rigid” north-Peloponnese block taken by normal faults

n the region spanning from Kalavryta and further N–NW, until theoastline of the Gulf of Corinth.

Across the Spechios valley–Kammena Vourla rift system strainhows a west-to-east reduction from 104 to 119 ns/yr sampledlong baselines LIDO–072A and 015A–072A to 68–54 ns/yr alonghe baselines 015A–026A and ATAL–026A. Comparing strain alongTAL–026A (54 ns/yr) to ATAL–ARK2 (59 ns/yr) we observe thatost of extensional strain across the Northern Gulf of Evia, is

ccommodated by the Arkitsa normal fault as the ATAL stations located at the hanging wall of the Atalanti normal fault and

o offshore faults are sampled, as well. This is surprising as wexpected significant strain accommodation along the normal faultsn the northern side of the Northern Gulf of Evia (near station26A). Moreover, the baselines 026A–ATAL (oriented N–S) and

. The starting point of each profile is the NW edge and the end point is the SE edgeap, correspond to the E–W boundaries of the published block model of Reilinger

is referred to the web version of the article.)

ARK2-ATAL (oriented N25◦ E) extend at mean rates 1.28 and0.69 mm/yr, respectively. Assuming that these figures representthe horizontal component of slip along E–W striking normalfaults dipping to the north and using simple trigonometry(for 45 and 60 degrees dip angles) we obtain mean fault sliprates of 0.97–1.38 mm/yr (for the 0.69 mm/yr baseline) and1.81–2.56 mm/yr (for the 1.28 mm/yr baseline). This geodeticestimate places a lower constraint of about 1–1.3 mm/yr mean sliprate along the Arkitsa fault, assuming that this fault accommodatesall extension in this area of the rift. The upper constraint is about2.5 mm/yr for the Arkitsa fault unless two or more faults areactive in this area sharing the remaining 1.2–1.5 mm/yr slip rate(including one offshore fault suggested by Cundy et al., 2010). Thebaseline 015A–026A extends at 68 ns/yr or about 25% higher than026A–ATAL. It is reasonable to assume that most of this extension isaccommodated by the coastal Kammena Vourla fault while a largepart of strain (50%?) is taken by slip along the Kallidromo–Parnassosand offshore faults. This is because the geodetic extension rate is

3.03 mm/yr, obviously much larger than the slip rate assumed forthe coastal fault system (∼1 mm/yr; Walker et al., 2010).

Large extension is obtained for two baselines across the Sper-chios rift, where the last major earthquake occurred in 1740

8 K. Chousianitis et al. / Journal of Geodynamics 71 (2013) 1– 13

Table 2Rates of baseline length changes for lines reported in the text and associated strain accumulation along each corresponding line (change in baseline length/baseline length).

No. Baseline Distance (m) Rate (m/yr) Strain rate (ns/yr)

1 015A–011A 54,317.734 0.00664 122.2442 015A–072A 56,242.043 0.00671 119.3063 025A–061A 69,481.415 0.00185 26.6264 026A–ATAL 23,445.188 0.00128 54.5955 027A–LIDO 53,584.408 −0.00045 −8.3986 061A–026A 70,970.241 −0.00195 –27.4767 072A–ATAL 62,896.840 0.00130 20.6698 072A–LIDO 47,696.826 0.00499 104.6199 ARK2–ATAL 11,729.779 0.00069 58.825

10 KLOK–027A 83,875.053 0.00111 13.23411 KLOK–072A 76,295.389 −0.00016 −2.09712 KLOK–ATAL 132,282.287 0.00277 20.94013 KORI–ATAL 79,338.992 0.00793 99.95114 KRYO–ATAL 82,690.552 0.00856 103.51815 NOA1–THIV 56,411.053 0.00108 19.14516 NOA1–025A 62,496.748 0.00113 18.08117 THIV–ATAL 46,552.571 0.00011 2.36318 THIV–KORI 53,622.494 0.00479 89.32819 061A–024A 72,653.881 0.00387 53.26620 024A–007A 48,659.628 0.00234 48.08921 015A–043A 60,104.318 0.00816 135.76422 007A–043A 68,739.520 0.00026 3.78223 007A–010A 60,730.886 0.00160 26.34624 025A–008A 52,568.455 0.00122 23.20825 008A–006A 49,387.082 0.00110 22.27326 006A–010A 55,732.783 −0.00033 −5.92127 025A–024A 41,979.493 0.00040 9.52828 008A–007A 39,301.059 −0.00033 −8.39729 026A–072A 52,998.774 0.00057 10.75530 071A–027A 80,265.046 0.00187 23.29831 071A–072A 82,821.768 0.00076 9.17632 THIV–007A 31,194.495 0.00054 17.31133 011A–013A 32,848.605 0.00178 54.18834 015A–026A 43,997.773 0.00303 68.86735 026A–024A 72,826.240 −0.00062 −8.51336 006A–007A 60,557.305 −0.00021 −3.46837 061A–ATAL 81,775.213 0.00061 7.45938 023A–025A 61,419.591 0.00089 14.49039 023A–006A 48,202.306 0.00063 13.07040 KORI–010A 67,162.775 −0.00047 −6.99841 KORI–SPET 77,529.034 0.00077 9.93242 027A–012A 61,090.947 0.01178 192.82743 027A–013A 110,391.436 0.01414 128.090

075630

198

(Ltaaeacbowrt0ptwtnKmai

44 012A–013A 52,930.45 LIDO–011A 43,038.46 071A–LIDO 118,664.

M = 6.6, Ambraseys and Jackson, 1997). Baseline 015A–072A andIDO–072A cross the N100◦ E Sperchios rift at high-angles sohe strain measurements (104–119 ns/yr) are reliable and arettributed to the N–S extension, well established from geologicalnd seismological data (e.g. Eliet and Gawthorpe, 1995; Burtont al., 1995; Hatzfeld et al., 1999; Kilias et al., 2008). A certainmount of strain is accommodated by normal faults south of Sper-hios where a historical earthquake of M = 6.1 (1852) is reportedy Ambraseys and Jackson (1997), while a normal-slip eventccurred on 13 December 2008 (Chouliaras, 2009). Toward theest, extension continues to dominate but at considerably lower

ates, as the baseline LIDO–071A extends at a rate of 35 ns/yr whilehe baseline 027A–071A at a rate 23 ns/yr, respectively. Station71A is located in western Thessaly, a well-known extensionalrovince (e.g. Caputo and Pavlides, 1993) so the two aforemen-ioned baselines cross a number of active faults, including theestern Sperchios rift structures and the western prolongation of

he 1954 seismic fault in the Ekkara area (Palyvos et al., 2010). Weote that the Ekkara fault is also crossed by the 072A–071A and

LOK–072A baselines, that map a minor extension (9 ns/yr at theost), indicating that the principal extensional strain axis in that

rea is oriented at high-angles to the two baselines. A similar results obtained for baseline KLOK–027A that extends at a rate of 13 ns/yr

0.00381 71.9820.00974 226.3080.00422 35.563

and is oriented NNE–SSW. It is clear that tectonic strain in westernThessaly is much less (10–20%) than that of Sperchios rift.

Additionally, we calculated the horizontal strain and rotationrate field. To do so, we discretized the study area into a grid of,as much was possible, equilateral triangles using the Delaunay tri-angulation. Singular value decomposition was used to solve thelinear least-squares problem and generate the velocity gradienttensor along with the associated errors (diagonal elements of thecovariance matrix). In Fig. 6a we show the axes of maximum ratesof contraction and extension within each triangle. The strain mapshows rates of extension of 250 ns/yr across the western Gulf ofCorinth, with decreasing strain rates toward the east (170 ns/yr).The greater Kalavryta region shows strain rate of 74 ns/yr, whilethe region to the S and SE of Corinth shows strain rates around20–30 ns/yr. The overall orientation of the extension in the studyarea is N–S (±20◦). However, systematic NNW-orientations areobtained across the Sperchios rift valley with maximum strain ratesof the order of 160 ns/yr. Further north (southwest Thessaly), weobtain considerably less strain rates (about 10 ns/yr) indicating that

the area north of the major tectonic units of Sperchios rift is lessactive. We also present the rotation rates for each triangle (Fig. 6b),illustrating an overall picture of clockwise rotation. However, thelargest rotations are obtained across the central Gulf of Corinth and

K. Chousianitis et al. / Journal of Geodynamics 71 (2013) 1– 13 9

Fig. 5. Map showing the rates of change in baseline length between GPS stations which are converted to strain accumulation along each corresponding line in Table 2.

Fig. 6. (a) Map of the principal axes of the 2D strain rate tensor shown at the centroid of each triangle. Convergent arrows (blue) denote contraction while divergent arrows(red) denote extension. (b) Map of the rotation rates of the velocity gradient tensor. Red wedges denote clockwise rotation while blue wedges show counterclockwise rotation.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

1 nal of

amoitT

3

tfieati

[

wlasi0igbrtctRt

FFb

0 K. Chousianitis et al. / Jour

cross the Sperchios rift (exceeding 5◦/Ma), while this clockwiseotion is decreasing rapidly in the areas of Corinth and Attica. The

rigin of clockwise rotations in central Greece maybe the dominat-ng right-lateral shear of the north and central Aegean Sea due tohe existence of the two branches of the North Anatolian Fault (e.g.aymaz et al., 1991; Goldsworthy et al., 2002).

.2. Crustal block boundary between Attica and Viotia

To assess the rigidity of the Attica–Corinth block suggested inhe previous section and to better visualize the relative velocityeld of the Viotia block we estimated the three Euler pole param-ters for the rotation of the former domain. We minimized thedjustments of the horizontal velocity components of the stationshat belong at the Attica–Corinth block with weighted least-squaresnversion of the following equation:

vn

ve

]=

[r sin � −r cos � 0

−r sin � cos � −r sin � sin � r cos �

]⎡⎢⎣

˝x

˝y

˝z

⎤⎥⎦ (3)

here vn and ve are the north and east velocity, � and ϕ are theongitude and latitude, r is the Earth radius and ˝x, ˝y, and ˝z

re the components of the rotation vector. In order to better con-train the Euler pole and because our data set contains fourteenntra-block GPS sites (NOA1, MET0, ANAV, KORI, KRYO, SPET, 002A,10A, 006A, 007A, 008A, 013A, 043A and 098A), we incorporated

n the inversion, the geodetic velocities of Floyd et al. (2010) thateographically belong to the Attica–Corinth block. So as to alignoth data sets into a common Eurasia-fixed reference frame, weotated the Floyd et al. (2010) velocity field through the estima-ion of a six-parameter transformation (six components of rate of

hange of translation and rotation), by minimizing the horizon-al velocity residuals between the co-located stations. The resultedMS fit between 14 co-located GPS stations was 1.84 mm/yr. Afterhat, all the intra-block GPS sites of the two data sets were used

ig. 7. Residual vectors obtained after removing the motion predicted by the Euler pole

loyd et al. (2010) velocity field along with that of the present study. The former was rotefore making the calculations.

Geodynamics 71 (2013) 1– 13

to calculate the Euler pole parameters of the Attica–Corinth block.The resulting parameters with respect to Eurasia are rotation rateof 0.559 ± 0.144◦/Myr around a pole located at 18.530 ± 0.196◦ Sand 50.033 ± 0.423◦ E. To compare predicted–observed velocitieswe computed the normalized root-mean-square misfit function

MF =

√√√√ 1N − f

N∑i=1

(i − pi)2

�i2

(4)

where N is number of velocity vectors, f is degrees of freedom, viis the GPS velocity vector at the ith site, pi is the predicted motionat ith site and �i is the standard deviation of i. For a total numberof 41 stations, the NRMS misfit was found to be 3.48 mm/yr. Theresidual velocities, obtained by applying the deduced Euler poleat the observed GPS velocity field, are displayed in Fig. 7 show-ing a clockwise motion of stations in Viotia and Fokida to the NE.In addition, NW Peloponnese stations follow the same pattern asFokida stations, however, at much lower rates given the exten-sion across the Corinth Gulf. It is inferred that a boundary betweentwo crustal blocks exists in central Greece, separating GPS stationsin Attica and Corinth, from stations in Viotia region. A first indi-cation of such a boundary was suggested by Nyst and Thatcher(2004) and Reilinger et al. (2006) using mostly campaign GPS data.Given the available geological data in this area (Jackson et al., 1982;Ganas et al., 2004, 2007; Tsodoulos et al., 2008), it is reasonable toplace this boundary line along the Kaparelli–Asopos valley faultsand further toward the east. In NW-Peloponnese, the GPS stationsshow different pattern from those of the Corinth area, indicatingsome kinematic variation between these domains. In this area,the western boundary of the proposed Attica–Corinth block needsmore geodetic data to be constrained due to the lack of stations inthe intermediate area between the Corinth and NW-Peloponnese

regions. However, it is evident that west of Kaparelli fault, thisboundary passes through the Gulf of Corinth across the existingactive coastal faults and crosses the area between Kalavryta andCorinth.

that minimized the velocities of the sites whose names are depicted. We used theated in order to align both data sets into a common Eurasia-fixed reference frame

nal of

4

1

2

3

4

5

6

7

K. Chousianitis et al. / Jour

. Conclusions

. In this paper we present an updated velocity field in centralGreece based solely on continuously operating GPS stations.We processed IGS stations together with the Greek stations inorder to optimize the network internal constraints. The veloc-ity field was produced by combining the individual solutionsusing the Kalman filter approach. Outlier editing and modelingof the first-order features of the time series was also performed,while temporally correlated noise was taken into account, so asto ensure the precise determination of the presented geodeticvelocities.

. The overall orientation of the extension in the study area is N–Swith some small variability however (±20◦). Systematic NNW-orientations are obtained across the Sperchios rift valley. Wealso illustrate a picture of clockwise rotation that dominates thestudy area, with the largest values exceeding 5◦/Ma across thecentral Gulf of Corinth and across the Sperchios rift. This clock-wise motion is decreasing rapidly in the areas of Corinth andAttica.

. We observe a velocity “plateau” between 30 and 33 mm/yr forstations located in Attica and on the SE coast of the Gulf of Corinth(in the latter case confirming the results of Avallone et al., 2004).The coherent picture of the velocity for Attica and Corinth GPSstations indicates that these areas belong to the same crustalblock. From Euler pole estimation modeling it is inferred thatthe boundary between this block and the adjacent Viotia block,consists of the Kaparelli–Asopos valley faults. To the west, thisboundary passes through the Gulf of Corinth across the exist-ing active coastal faults and crosses the area between Kalavrytaand Corinth onshore Peloponnese. Also, the NW Peloponnesestations follow the same pattern as the Fokida stations thoughat much lower rates due to the extension across the CorinthGulf.

. The analysis of the rates of the baseline length changes in centralGreece provides geodetically an evidence of crustal deforma-tion in that area, giving strain rate values that range from−27 ns/yr up to 226 ns/yr. The largest extension (192–226 ns/yr)is observed in the western and central part of the Corinth rift.Extension rates of 80–120 ns/yr are obtained across the contin-uation of the Corinth rift to the east (i.e. the eastern part of theCorinth gulf and in the south Viotia region). Extension decreasesfurther to the east (including Evia island and NE Attica). We mea-sure N–S extension of 53 ns/yr across central Evia Island which isaccommodated by the coastal fault system of north Evia and bythe normal faults in the Psahna area. Further southeast onshorethe Evia Island, extension drops about 50% which is indicativeof the younger age of the rifting process along the south Gulf ofEvia.

. Large NNW–SSE extension (119 ns/yr) is obtained across theSperchios rift, accommodated by the major north-dipping Sper-chios valley–Lamia fault as well as by normal faults south ofSperchios rift. Toward the west, extension continues to domi-nate but at considerably lower rates. North of the latter region, inwestern Thessaly, we also map extension, but the tectonic strainis much less (10–20%) than that of Sperchios rift, indicating a lessactive domain.

. We also find “internal” extension across “rigid” areas, such as thenon-negligible rate of the order of 25 ns/yr across Attica and thelarger 54–71 ns/yr across northern Peloponnese.

. Most of extensional strain across the southern coast of theNorthern Gulf of Evia, is accommodated by the coastal fault

system (Arkitsa fault and Kammena Vourla–Agios Konstanti-nos fault). In the case of the Arkitsa normal fault, ourgeodetic results provide slip rate estimates between 1 and2.5 mm/yr.

Geodynamics 71 (2013) 1– 13 11

Acknowledgements

We would like to thank Ktimatologio S.A. for providing RINEXfiles from HEPOS, their CGPS network, COMET+/NTUA for providingthe KRYO and ARK2 station data and METRICA S.A. for providingdata of their network. We thank Robert King for discussions on GPSdata processing. This study benefited from careful and thoroughcomments by Robert Reilinger and an anonymous reviewer. Mapswere generated using the Generic Mapping Tools (GMT) (Wesseland Smith, 1998).

Appendix A. Supplementary data

Supplementary material related to this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jog.2013.06.004.

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