GPS-derived estimates of crustal deformation in the central and north Ionian Sea, Greece: 3-yr results from NOANET continuous network data

Journal of Geodynamics 67 (2013) 62–71

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GPS-derived estimates of crustal deformation in the central and north Ionian Sea, Greece: 3-yr results from NOANET continuous network data A. Ganas a,∗ , A. Marinou b , D. Anastasiou b , D. Paradissis b , K. Papazissi b , P. Tzavaras b , G. Drakatos a a b

National Observatory of Athens, Institute of Geodynamics, Lofos Nymfon, Athens 118 10, Greece Higher Geodesy Laboratory, N.T.U.A., Faculty of Rural and Surveying Engineering, Athens, Greece

a r t i c l e

i n f o

Article history: Received 2 April 2011 Received in revised form 25 April 2012 Accepted 8 May 2012 Available online 22 May 2012 Keywords: GPS Strain Seismicity Ionian Sea Greece

a b s t r a c t Ionian Sea (western Greece) is a plate-boundary region of high seismicity and complex tectonics, dominated by frequent earthquake activity along the right-lateral Cephalonia transform fault. We present an analysis of 30-s GPS data from five (5) continuous stations of NOANET (NOA permanent GPS network) spanning the period 2007–2010. Our results show N-S crustal shortening onshore Lefkada island of the order of 2–3 mm/yr which is probably related to increased locking on the offshore Lefkada fault. We also calculated a large difference (1:3) in the principal strain rate amplitude between extension and shortening for the central Ionian Sea. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction The Ionian Sea is located between the Calabrian and Hellenic Arcs in the central Mediterranean Sea (Fig. 1). The region is characterized by high seismicity rates and occurrence of strong, shallow earthquakes (Figs. 1 and 2; Makropoulos and Kouskouna, 1994; Fokaefs and Papadopoulos, 2004; Papathanassiou et al., 2005). In instrumental times (post-1911), the strongest events (6 < M < 7.4; Qi Cheng et al., 2007) were the 27 November 1914 event offshore south Lefkada, the 22 April and 30 June 1948 events offshore north (and south) Lefkada, the early August 1953 events offshore Ithaka and Cephalonia, the 17 January 1983 event to the SW of Cephalonia and the 14 August 2003 event offshore Lefkada. 6988 events with M > 3.5 have occurred since 1964 (National Observatory of Athens – NOA declustered catalogue; Fig. 2). An earthquake density map (Fig. 2) shows that the areas of highest seismicity are SW of Cephalonia and south of Zakynthos with almost 280 events since 1964 or about 6 events with M ≥ 3.5 per year. Deformation is complex due to the activity of large faults with different kinematics (from 40◦ North to 38◦ N, Fig. 1): In the region near Corfu (Kerkyra) and Paxoi (North Ionian Sea) there is continental collision between Apulian microplate (former Nubia – Africa) and Eurasia (Louvari et al., 2001; van Hinsbergen et al., 2006), in the region from Lefkada to Cephalonia (central Ionian Sea) there is

∗ Corresponding author. E-mail addresses: [email protected] (A. Ganas), [email protected] (A. Marinou). 0264-3707/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jog.2012.05.010

strike-slip (horizontal) motion and in the region south of Zakynthos (south Ionian Sea) there is oceanic subduction (e.g. Clement et al., 2000). The dominant fault system is the Cephalonia transform fault (Ganas and Parsons, 2009; Louvari et al., 1999) striking N10◦ –20◦ E and dipping to the east. In the central Ionian Sea the deformation of the upper plate (Eurasia: mainland Greece) is accommodated by a combination of normal and strike-slip faults (Baker et al., 1997; Haslinger et al., 1999; Ganas et al., 2009; Feng et al., 2010). The variety of tectonic styles along a 200 km long plate boundary poses a challenge to understand how deformation is distributed in such complex settings. Previous works (Kahle et al., 1995; Anzidei et al., 1996; Peter et al., 1998; Rossikopoulos et al., 1998; Kahle et al., 2000; Hollenstein et al., 2006, 2008; Rontogianni, 2010; Floyd et al., 2010; Cocard et al., 1999) have provided results, with respect to a fixed Europe, for both permanent and non-permanent GPS stations in this region. North of Lefkada Hollenstein et al. (2008) measured west to northwest motion at 2–4 mm/yr. South of Lefkada the motion changes to southwest with rates of 7–30 mm/yr (depending on station latitude). Rates increase systematically from North to South. To understand the space variability of strain patterns we need to produce time series of positions along the plate boundary, such as those provided by continuous GPS stations with stable telemetry. The National Observatory of Athens during the last 5 years has established several permanent GPS stations in central and western Greece in order to monitor the tectonic motion (Fig. 3; Ganas et al., 2008). In western Greece there are six stations that operate since early 2006–mid 2007 (VLSM, RLSO, PONT, SPAN, KASI, KIPO,

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Fig. 1. Relief map of western Greece showing major (M > 6) shallow earthquakes (depth less than 40 km) since 1964. Beach balls indicate focal mechanism of earthquakes with compressional quadrant in black. Focal mechanism data are after the global CMT project (best double-couple solutions from full MT) and Baker et al. (1997). Black lines are active faults. Inset box shows location of study area within Greece.

Fig. 3; Table 1). This paper presents new results from data provided by five (5) permanent GPS stations as station KIPO has not produced enough data, yet (Fig. 3; the RLSO receiver is owned by the University of Athens). Our results include: time series of daily positions for stations KASI, SPAN, PONT, VLSM and RLSO, velocity vectors of above stations with respect to fixed Europe and strain tensor estimates for the central Ionian Sea region. 2. The geodetic network and data analysis Geodetic data is crucial for understanding seismicity patterns in the Ionian Sea, where the plate boundary evolves from collision

to horizontal shear to subduction (from North to South). Good quality GPS observations are necessary to: (a) identifying crustal blocks on the upper (Eurasian) plate (b) measuring velocity vectors of crustal blocks (c) mapping strain patterns in the area from Corfu to Cephalonia, including block rotations. The NOA stations operate on 1-s mode and raw, binary data are transferred to Athens in hourly intervals via the Internet. Station profiles can be viewed at www.gein.noa.gr/gps.html. The acquisition software in Athens archives the raw GPS and GLONASS observations (from station KIPO) and produces daily 30-s data in RINEX format. For the Ionian Sea preliminary results have been published by Anastasiou et al. (2009a, 2009b) and Papanikolaou et al. (2010).

Table 1 Table showing station name, period of observation, receiver type and antenna type. Site

Session start

Session stop

Receiver type

Antenna type

NOA1 KASI PONT RLSO VLSM SPAN

2007 001 00 00 00 2007 91 00 00 00 2007 130 00 00 00 2007 001 00 00 00 2007 001 00 00 00 2007 142 00 00 00

2010 154 23 59 59 2010 154 23 59 59 2010 154 23 59 59 2010 154 23 59 59 2010 154 23 59 59 2010 154 23 59 59

LEICA GRX1200PRO LEICA GRX1200PRO LEICA GRX1200PRO LEICA GRX1200 LEICA GRX1200PRO LEICA GRX1200PRO

LEIAT504 LEIS LEIAX1202GG LEIAX1202GG LEIAX1202 LEIAX1202 LEIAX1202GG

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Fig. 2. Map of shallow seismicity for central and north Ionian Sea from NOA catalogue (period 1964–2009, M ≥ 3.5 local). The catalogue has been declustered to show background seismic activity. The area has been gridded in 235 cells (white triangle indicates cell centre). Event density is shown in rainbow colours with lowest density in violet and highest density in red. The areas of highest seismicity are SW Cephalonia and S Zakynthos with 272/283 events, respectively, or about 6 events with M ≥ 3.5 per year. Narrow red lines indicate block boundaries (possibly active fault lines) illuminated by seismicity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

All data are processed with GAMIT/GLOBK 10.35 (Herring et al., 2006). The time period of observation as well as the data gaps for each station is presented in Fig. 4. The local reference frame was realized using 18 IGS (International GPS Service) stations in International Terrestrial Reference Frame 2005, ITRF 2005 (Fig. 5). The selected 18 IGS and other stations were chosen to provide a local reference at a continental scale, considering the fact that very few IGS stations are available to the south of the study area. IGS precise orbits from the SOPAC web site and earth rotation parameters were used for the calculation. All the available GPS data is processed on a daily basis. In addition, NOANET antennas are calibrated according to IGS standards. In GAMIT processing IGS provided absolute antenna calibration values via an atx file. GAMIT is used in the first part of the analysis where the 30-s GPS data of all NOA stations and 30-s data from 18 reference sites are processed all together to produce (a) daily loose-constrained estimates of station coordinates, (b) orbital parameters, (c) EOP (Earth Orientation Parameters) (d) estimates of tropospheric zenith-delay parameters, (e) phase ambiguities and the associated covariance

matrix. In the second part of the analysis the daily, loose “quasiobservations” and the associated covariance matrix are used in GLOBK to produce daily constrained estimates of station coordinates, time series of station coordinates, station secular velocities, in a local reference frame, realized by using fiducial IGS stations. In the process of combination, GLOBK uses a Kalman filter which weights each “quasi-observation” derived from GAMIT relative to its standard error (e.g. Dong et al., 1998). Our analysis has resulted in the following products: (a) station velocities with respect to the local reference frame (aligned to ITRF2005) (Table 2) (b) station velocities with respect to fixed Europe (Table 3) using the European plate velocity after Yannick et al. (1998); Yannick (2000) and (c) strain tensors. With respect to a fixed Europe, the station velocities range from 4.5 mm/yr (KASI) to 28.6 mm/yr (NOA1). The azimuths of the velocities range from N198◦ E (SPAN) to N306◦ E (KASI). A map of station velocities with respect to fixed Europe is shown in Fig. 6, as well as velocities reported in literature. The strain tensor is presented in Fig. 7 (based on all stations in central Ionian region). Figs. 8 and 9 show ITRF 2005

Table 2 Table of station velocities in the local reference frame (aligned to ITRF 2005) and uncertainties. Units in m/yr. Site

Vx

Vy

Vz

sVx

sVz

sVz

NOA1 RLSO SPAN VLSM PONT KASI

0.0040 0.0035 −0.0098 −0.0095 −0.0138 −0.0166

0.0096 0.0099 0.0189 0.0149 0.0158 0.0151

−0.0094 −0.0018 0.0033 0.0022 0.0041 0.0096

0.0001 0.0002 0.0002 0.0002 0.0002 0.0002

0.0001 0.0001 0.0001 0.0001 0.0001 0.0001

0.0001 0.0001 0.0002 0.0001 0.0002 0.0001

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Fig. 3. Map of Greece showing permanent GPS stations of NOA and NTUA (as of March 2011). Station locations are shown as black triangles with name of station beneath.

Fig. 4. Time diagram of 30-s GPS data used in processing. Data gaps are shown in white.

Fig. 5. Stations used for the realization of the reference frame.

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Table 3 Station velocities and standard deviations with respect to a fixed Europe (Yannick et al., 1998) in mm/yr. Az is azimuth in degrees (clockwise from North), Norm is norm of horizontal velocity vector. Location of stations is shown in Fig. 3. SITE

Vn

␴Vn

Ve

␴Ve

Vu

␴Vu

Norm

Az

NOA1 RLSO SPAN VLSM PONT KASI

−23.4 −17.1 −7.3 −7.5 −3.6 2.7

±0.05 ±0.06 ±0.08 ±0.06 ±0.07 ±0.06

−16.5 −15.7 −2.4 −6.4 −4.0 −3.7

±0.05 ±0.06 ±0.08 ±0.07 ±0.07 ±0.06

0.2 4.3 0.1 −1.5 −3.2 −1.9

±0.17 ±0.22 ±0.27 ±0.24 ±0.28 ±0.23

28.63 23.21 7.68 9.86 5.38 4.58

215.18 222.55 198.19 220.47 228.01 306.11

time-series of positions for two stations onshore Lefkada, namely SPAN (north) and PONT (south) (Table 4). 3. Results and discussion 3.1. Motion of stations SPAN – PONT (Lefkada) Our results (Table 3 and Fig. 6) show that SPAN is moving faster than PONT (with respect to Eurasia) which is unexpected.

Table 4 Strain rate tensor parameters. The principal axes of the tensor are shown in Fig. 7. The strain rate in the central Ionian Sea consists of a small amount of extension in the direction approximately N12◦ W, and about three times more compression at N78◦ E. Area

Kmax (ppm)

Kmin (ppm)

 (␮ strain/yr)

Az (degrees)

Whole St. Dev.

+0.038 ± 0.040

−0.109 ± 0.042

+0.147 ± 0.057

168.372 ± 9.649

Fig. 6. Map of western Greece showing NOA station velocities with respect to fixed Europe. Velocities and azimuths are reported in Table 3. Black lines are faults.

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Fig. 7. Strain rate tensor for the whole study area derived from continuous GPS data (white solid circles indicate station locations). Station KASI was excluded (see text for discussion). Red arrows indicate extension, blue arrows compression. 10 n strain/yr corresponds to an annual motion of 1 mm over a distance of 100 km. Black lines are faults. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

The crustal shortening onshore Lefkada amounts between 2 and 3 mm/yr. Station SPAN is located in North-central Lefkada near the village Spanohori (latitude 38◦ 46 52.69841, longitude 20◦ 40 25.11708, elevation 447.85 m). The local geology is sandstone of Miocene age (Ionian zone). Station PONT is located on the west coast of Vassiliki bay in south Lefkada (latitude 38◦ 37 08.33042, longitude 20◦ 35 06.66228, elevation 48.52 m). The local geology is limestone (Paxos zone). We suggest that the reduction of velocities from North to South is reliable and is not due to (a) a local effect influencing the velocity of a station (i.e. slope instability at SPAN and/or PONT) as our own field work has shown or (b) to a different number of days processed for the two stations (note that both stations are processed about the same time period, Fig. 4). Further more, the GPS data were collected after May 2007, long after the August 2003 earthquake near Lefkada’s western coast (Fig. 1; Papadopoulos et al., 2003; Papadimitriou et al., 2006). Modeling of seismological data (Benetatos et al., 2007) showed that the rupture did not reach the surface while InSAR data (Lagios et al., 2007) indicate a co-seismic motion (Line of sight uplift) of 3–5 cm onshore Lefkada (central and north part). Postseismic deformation studies for strike-slip earthquakes with 6 < M < 7 indicate that the postseismic signal across the fault does not extend further than 3 years (e.g. Fielding et al., 2009). In addition, SPAN is located about 10 km to the east of the 2003 earthquake and PONT about 21 km to the south, respectively. So, we believe that there is no 2003 post-seismic signal (strain) present at the SPAN & PONT time series. Previous work by Hollestein et al. (2008) has reported GPS results for two non-permanent stations on-shore Lefkada (1KVL

and VASI; Figure 6). These stations show Europe-fixed velocities of 10.3 and 10.6 mm/yr, respectively. A third station (DUKA; Fig. 6) also reported by Hollenstein et al. (2008) has a velocity of 6.9 mm/yr. Station DUKA is a permanent station (operated by ETHZ) and we consider its velocity more reliable. We note that DUKA’s velocity is close to the one we obtained for station PONT (5.3 mm/yr) which is located about 7.5 km away. There are two possible explanations for the observed velocity difference (a) an existing discontinuity separating two blocks onshore Lefkada taking up crustal shortening or (b) there is a local gradient in bulk deformation related to the offshore plate boundary. First, a possible interpretation of the velocity pattern onshore Lefkada is that it is a manifestation of motion of two upper crustal blocks (north vs. south) that are separated by a discontinuity that cannot be pin-pointed by our continuous GPS stations. A local GPS network is necessary to be installed and measured for at least 3 years (4–6 campaigns) so that strain patterns onshore can be mapped with satisfactory accuracy. Moreover, from our knowledge of local tectonics it may be hypothesized that the “GPSsensed” discontinuity may be related to the reactivation of a thrust fault beneath Lefkada under the present day stress field (horizontal compression oriented ENE-WSW, see strain tensor in Fig. 7; also Serpelloni et al., 2005). It is possible that the thrust brings in contact rocks of the Ionian and Paxoi isopic zones of the external Hellenides or it is a thrust fault cutting across older structures. For example, Karakostas and Papadimitriou (2010) report a NW-SE striking “blind”, active fault in central Lefkada. A major thrust structure is outcropping in central Lefkada (Lekkas et al., 2001); however, there is no seismological or geological evidence that is active today.

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Fig. 8. Time series (2007–2010) of PONT station (North Lefkada).

Onshore Lefkada, many neotectonic faults striking NNE-SSW or EW, cross-cut the island (Lekkas et al., 2001; Papathanassiou et al., 2005). They are mainly normal or strike-slip faults with a sinistral sense of shear. It is not clear how many of these faults are active today so more geological field work is necessary. So, it seems that the thrust-fault scenario cannot explain the onshore geological observations. Nevertheless, the need for local campaigns is evident. A second scenario involves the dependence of strain buildup onshore Lefkada due to its proximity to the plate boundary. The plate boundary dips to the east at a high-angle (see focal plane solutions of the 14 August 2003 earthquake in Papadopoulos et al., 2003), while the strike of the boundary changes south of Lefkada and north of Cephalonia (Fig. 6; Louvari et al., 1999). The reduced velocity of station PONT (south Lefkada) may reflect resistance to southward motion along the plate boundary due to its locking. In

this bulk-deformation scenario we expect to measure a similar velocity for both Lefkada GPS stations only after a major earthquake offshore south Lefkada releases accumulated strain. We note the last earthquake offshore south Lefkada occurred in 1948, while two events occurred in the north part of the Lefkada segment of the Cephalonia transform (1948 and 2003; see Papadopoulos et al., 2003 for earthquake chronology). 3.2. Motion of station KASI (Corfu) Station KASI (latitude 39◦ 44 46.92506, longitude 19◦ 56 07.95572, elevation 103.85 m) is located near the village Kassiopi upon a building founded on limestone bedrock. Our analysis shows that this station is moving to the NW (4.5 mm/yr, N306◦ E with respect to fixed Europe) in broad agreement to previous works of Hollenstein et al., 2008, (3.9 mm/yr, point PNTN 5.6 km to the

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Fig. 9. Time series (2007–2010) of SPAN station (North Lefkada).

west of KASI), and Serpelloni et al., 2005, (3.2 mm/yr, point IGOU 35.8 km to the SE of KASI; Fig. 6). Furthermore, Hollenstein et al. (2008) reported a velocity of 4.7 mm/yr for non-permanent station AMAT (located about 25.3 km to the SW of KASI onshore Corfu; Fig. 6). In terms of tectonics this similarity is important as the Westward motion of those stations at comparable rates implies no significant strain of the Eurasia crust between Corfu and Epirus. We note the similarity between the KASI NW-motion (N306◦ E) and the strike of average thrust-type mechanism of recent earthquakes from Louvari et al. (2001; N336◦ E ±16◦ ) which implies a significant oblique-slip component of the hanging wall block where KASI belongs to (assuming that the plate boundary thrust is located offshore Corfu to the west). We can speculate that for seismic hazard purposes the only important source is found offshore Corfu (earthquake potential for Corfu is about 6.9; Papaioannou and Papazachos, 2000).

3.3. Orientation of strain axes and amount of strain (elongation-shortening) Strain tensors were calculated considering that earth is deforming in two dimensions and that the earth’s crust is a thin deformable calotte on a spherical earth (Veis et al., 1992; Agatza-Balodimou et al., 1994). Space (faults) and time (earthquakes) discontinuities are not included in the calculation. The only event that caused a mm-size, static offset within the network was the 8 June 2008 (Mw = 6.4; Ganas et al., 2009) earthquake near station RLSO. In addition, for areas less that 5 degrees times 5 degrees mapping distortions do not exceed 10−3 in scale and therefore can be ignored without any practical loss in rigour. The deformation of the region appears graphically by the convention used by the geophysics community: we plot the strain rates directly where a line shows the orientation of each principal axes (with a convention to tell us

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whether it is extensional or contractional; red for the former and blue for the latter) and a scale (Fig. 7). In the Ionian Sea, we are interested in identifying the change in azimuth of the extensional and/or shortening strain axis as this reflects the change in azimuth of major tectonic and possibly seismogenic structures. However, the northern strain-rate tensor may be highly uncertain as the three points (KASI, SPAN, PONT) form a very elongated triangle (base about 1/6 its height); propagation of the errors may yield uncertainties equivalent to the strain rates. Therefore, it would be better to form a single estimate of the strain rate from stations VLSM, RLSO, PONT, and SPAN. We included station RLSO (Riolos, Fig. 3) in the strain analysis to include the region between Cephalonia and Peloponnese in our present study. Our results (Fig. 7, Table 4) indicate that the orientation of the minor principal strain axis is N168◦ E (±9.6◦ ) in the central Ionian Sea (this is the extension direction). This orientation has to change in the North Ionian Sea because of the change in strike of the plate boundary (NE-SW offshore Cephalonia to NW-SE offshore Corfu; Fig. 1). We will obtain a better estimate of the strain rate sensor in this region within the next two years as new continuous GPS stations from private networks are archived at NOA since 2010. We obtain a strain rate of about 140 n strain/yr with the minor principal axis indicating 0.038 ppm of extension and the major axis as 0.109 ppm of shortening (orientation N258◦ E). The large difference of eigenvalues indicates the predominance of compression. Seismological data show dominance of compression on-shore western Peloponnese (e.g. Ganas et al., 2009) while extension dominates in the region of Amvrakikos Gulf (e.g. Adamova et al., 2009), to the northeast of Lefkada. The western boundary of compression may be traced up to the Agrinion-Patras line (Fig. 1) where recent earthquake activity provided evidence for horizontal shear (Kiratzi et al., 2008). 4. Conclusions This paper reports new cGPS data from one of the most highstrain-rate parts of the Aegean plate-boundary zone. The main conclusions are the following: 1. GPS station velocities in the north and central Ionian Sea range from 4.5 mm/yr (KASI) to 23.2 mm/yr (RLSO) with respect to fixed Europe. 2. The azimuths of the station velocities (w.r.t stable Europe; Fig. 6) range from N198◦ E (SPAN)–N306◦ E (KASI). 3. Station SPAN in northern Lefkada Island is moving faster (7.6 mm/yr) than PONT (5.3 mm/yr south Lefkada) which is unexpected. This result indicates on-going shortening onshore Lefkada. 4. Station KASI (Northern Corfu Island) is moving to the NW (4.5 mm/yr, N306◦ E with respect to fixed Europe) in broad agreement to previous works of Hollenstein et al. (2008) and Serpelloni et al. (2005). 5. The orientation of the minor principal strain axis is N168◦ E in the central Ionian Sea (Fig. 7). The major principal strain is shortening at N258◦ E. Ratio of extension to shortening is 1:3. Acknowledgements We are indebted to NOA colleagues P. Argyrakis, M. Papanikolaou, N. Melis, K. Boukouras, and V. Karastathis and for network maintenance and trouble shooting. We thank G. Stavrakakis, C. Zerefos and K. Makropoulos for encouragement and support. We also thank Bob King, K. Palamartchouk, M. Anzidei, E. Serpelloni, K. Chousianitis, V. Zacharis and X. Papanikolaou for help with data analysis. We thank Philip England and one anomynous reviewer

for comments and suggestions. The NOA GPS network set-up and operation in the Ionian Sea was supported by E. Skassis, P. Grigorakakis, A. Antonakakis, Zoel van Cranenbroek, D. Apostolatos (VLSM), A. Galanopoulos (RLSO), G. Kourtis (SPAN), and G. Pantazis (KASI). The OTE people helped us with data telemetry many times. Prof. E. Lagios (NKUA) made available the data from RLSO. Figures were created using GMT (Wessel & Smith, 1991). Funding was provided by EU-FP6 contract 516172, and the 3rd Community Support Framework 2000–2006. 30-s NOA data is made available from www.gein.noa.gr/gps.html. References Adamova, P., Sokos, E., Zahradnik, J., 2009. Problematic non-double-couple mechanism of the 2002 Amfilochia M(w)5 earthquake, Western Greece. Journal of Seismology 13 (1), 1–12. Agatza-Balodimou, A.M., Mitsakaki, C., Papazissi, K., Veis, G., 1994. Tectonic deformation in the Corinthian Gulf from geodetic data. Technika Chronika 14 (3), 271–285. 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