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New constraints on the rapid crustal motion of the Aegean region: recent results inferred from GPS measurements (1993–1998) across the West Hellenic Arc, Greece
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Earth and Planetary Science Letters 172 (1999) 39–47 www.elsevier.com/locate/epsl
New constraints on the rapid crustal motion of the Aegean region: recent results inferred from GPS measurements (1993–1998) across the West Hellenic Arc, Greece M. Cocard a,Ł , H.-G. Kahle a , Y. Peter a , A. Geiger a , G. Veis b , S. Felekis b , D. Paradissis b , H. Billiris b b
a Geodesy and Geodynamics Lab, Swiss Federal Institute of Technology, ETH Ho ¨ nggerberg, 8093 Zurich, Switzerland Higher Geodesy Lab, National Technical University of Athens, 9, Heroon Polytechnion Str., 15780 Zographos Athens, Greece
Received 10 May 1999; accepted 26 July 1999
Abstract In this paper we present the most recent observations of crustal motion across the entire West Hellenic Arc (WHA). These are based on repeated GPS measurements carried out in the period from 1993 to 1998. The results are presented in terms of trajectories and rates, relative to Eurasia. Within these five years southwestern Greece has moved to the southwest by an average rate of 30 mm=a, increasing from 10 mm=a at the island of Lefkada, in the center of the Ionian islands, to nearly 40 mm=a along the southwest part of the Peloponnesus and to 35 mm=a on the islands of Crete and Gavdhos. The data provide strong evidence that distributed shear strain starts at the Kephalonia Fault Zone (KFZ), were an anomalously high earthquake activity is also observed. A striking interruption of the motion is seen at the island of Strofades, at the southwest leading edge of the WHA, where a south-oriented displacement of 12 cm was detected, coincident with the M D 6:4 Strofades earthquake of Nov. 18, 1997. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Hellenic Arc; Greek Ionian Islands; Greece; crustal shortening
1. Introduction One of the key areas to understanding the kinematic processes at work in the Eastern Mediterranean is found in western Greece. Anderson and Jackson [1] have shown seismicity and fault-plane solutions along the southern Dalmatian coast (Montenegro, Albania, NW Greece), where thrusting mechanisms occur all the way to the central Ionian islands. This belt of NE–SW shortening is associated with transcurrent fault systems [2]. One of these is the Ł Corresponding
author. E-mail: [email protected]
Kephalonia Fault Zone (KFZ), which has been recognized as a major discontinuity between the Apulian platform and the transition to the West Hellenic Arc (WHA) (Fig. 1). Viewed from the south, Anderson and Jackson [1] suggested that the Hellenic subduction zone terminates against this major strike slip fault. The KFZ follows the submarine Kephalonia valley west of the island chain from the island of Lefkada to Kephalonia. From earlier GPS observations (1989–1993) made in southeast Apulia, on the northern Ionian Islands, in Epirus and on the Peloponnesus it was possible to identify two distinct kinematic fields of
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M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
Fig. 1. Seismicity of the Ionian and Aegean seas in the year 1997 (G. Stavrakakis, pers. commun., 1998) showing a high seismic activity south of the island of Zakynthos, the largest earthquake being the earthquake of Strofades (S) (M D 6:4). The inset summarizes the geodynamic framework around the Mediterranean Sea together with the major plates involved in the collision process. Superimposed on the counter-clockwise rotation of the African plate is a pronounced right-lateral motion of the Aegean=Anatolian plate towards the WSW. The large arrows describe the average motion of the African and Arabian plates as well as of the Anatolian and Aegean microplates, relative to the Eurasian plate. The Aegean microplate is bounded in the west by the Kephalonia Fault Zone (KFZ) and the West Hellenic Arc (WHA). Ab D Arkanania block, AnP D Anatolian microplate, AP D Apulian platform, CA D Calabrian arc, GA D Gulf of Arta, GP D Gulf of Patras, KF D Katouna fault, Lef D island of Lefkada, Zak D island of Zakinthos.
M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
recent crustal movements [3]. These are separated by a zone aligned with the KFZ, which is also the site of intense seismicity. Strong earthquakes (M > 7) near the island of Kephalonia occurred in 1953, 1972 and 1983. Source parameters of the 1983 Kephalonia island earthquake (M D 7:1) have revealed a dextral strike-slip mechanism [4], which agrees well with the strain field calculated from previous GPS observations [5]. In addition to the WHA sites re-measured in the last five years we have included in this paper also data from the southern Ionian islands Kithera and Antikithera, as well as from the islands of Crete and Gavdhos. The previous results have been geographically expanded and substantially improved upon. The kinematic field is much clearer now and shows distinct correlations with intermediate-size earthquakes. The purpose of this paper is to present rates and trajectories of crustal motion for the entire WHA and compare these with seismicity, with particular emphasis on detecting co-seismic motions.
2. Field campaigns and data analysis The network considered comprises the WHA including Epirus, the Ionian islands (from Othoni to Antikithera), the mainland of western Greece, the Peloponnesus and the islands of Crete and Gavdhos (Fig. 1). Major campaigns were carried out in 1993, 1994, 1996, and 1998. Some selected sites were also measured in 1995 and 1997. Table 1 shows the observation history for the WHA GPS sites. In addition selected data from the permanent GPS network [6] were integrated to calculate the velocity field. The analysis of the GPS data was performed using the Bernese GPS software 4.0 [7]. Every campaign was treated separately. Solutions for every session were calculated using final IGS orbits and integrating data from stations of the continuous IGS network. From the comparison with the final campaign solution, obtained by combining the normal equations of all individual sessions, it was concluded that a factor of 15 is an appropriate ratio between the realistic margin of error and the formal statistical one. This factor is dependent on the sampling rate and accounts for the high temporal correlation between the measurements, which is not taken into account
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in the GPS software. Therefore the uncertainty, deduced from the daily repeatabilities is more realistic than the formal statistical RMS error. The latter does not reflect contributions of systematic errors. The rates were then estimated based on the normal equations of the different campaign solutions. The coordinates and velocities of the IGS sites were constrained, and ITRF96 velocities of all sites were estimated. The velocity field obtained was then rotated from the ITRF96 to an Eurasia fixed reference frame using a rotation rate of 0.262º=Ma around a pole located at 59.0ºN and 97.1ºW (Heflin and Argus, pers. commun., 1998). For reasons of comparison it is noted that this reference frame is slightly different from the one used for the data set 1989 to 1993 in Kahle et al. [3], where the motion of Matera was fixed to zero velocity.
3. Trajectories In order to visualize the discrete displacements in the time periods between the observation campaigns we plotted the trajectories of crustal motion, relative to Eurasia (Fig. 2). Three main features are clearly evident: relatively small displacements in the northern part, with irregular directions, but mainly towards the west; intermediate-size displacements from the Gulf of Arta to the island of Kefalonia, mainly SW directed (compare inset in Fig. 2); and rapid motion in the south, towards the southwest. A new striking feature, seen in the data set 1993– 1998, is the abrupt dislocation at the island of Strofades, between Oct. 1997 and Dec. 1997. In this period Strofades has moved by an amount of 12 cm towards the south. Also the station at LOGO, at the southern tip of the island of Zakynthos, has moved rather rapidly in this time frame by about 1.5 cm towards the southwest. Taking a view at the seismicity of 1997 (G. Stavrakakis, pers. commun., 1998) it is evident that the area between the islands of Zakynthos and Strofades was struck by a cluster of earthquakes, with a main event, on Nov. 18, 1997, near the island of Strofades. On the other hand it is clearly seen that the relative displacements prior to 1997 are quite homogeneous, with average displacements on the order of 25 mm=a. It is obvious that co-seismic displacements such as the ones seen
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M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
Table 1 Observation history and annual displacement rates (v, in mm=a) relative to Eurasia Site
Abbr.
1
2
3
4
5
6
7
a
b
c
d
Lat.
Long.
Matera (I) Kastoria Specchia Christi (I) Othoni Pantokrator Karitsa Hania Terovou Parga Paxi Arta Vonitsa Amfilochia Kavallos Lepenou Vasiliki Doukato Astakos Dionysos Skinari Doxa Logos Zaharo Strofades Aetos Sparta Chrisokellaria Velies Gerolimenas Kithira Antikithira Souda Sfinari Roumeli Kiriamadi Damnoni Gavdhos
MATE KAST SPEC OTHO PNTN KARI TERO PARG PAXI ARTA VONI AMFI 1KVL L2PE VASI DUKA ATKO DION SKIN DOXA LOGO ZAHA STRO AETO SRTA XRIS VELI GERM KITH AKIT SOUD SFIN ROUM KIRI DAMN GVDO
ž
ž
ž
ž
ž
ž
ž
ž ž ž ž
ž ž
ž ž
ž
ž ž
ž ž
ž
ž
ž
ž ž
ž ž
ž ž
ž
ž
ž
ž
ž
ž ž
ž ž
ž
ž ž
ž
ž
ž
ž
ž
ž
40.649º 40.519º 40.063º 39.856º 39.749º 39.734º 39.463º 39.292º 39.236º 39.166º 38.910º 38.864º 38.791º 38.697º 38.607º 38.564º 38.494º 38.079º 37.931º 37.702º 37.675º 37.490º 37.250º 37.244º 37.034º 36.791º 36.718º 36.475º 36.307º 35.873º 35.487º 35.422º 35.404º 35.305º 35.171º 34.837º
16.704º 21.269º 18.458º 19.379º 19.861º 20.665º 20.880º 20.473º 20.128º 20.988º 20.846º 21.165º 20.656º 21.291º 20.573º 20.543º 21.120º 23.933º 20.702º 21.925º 20.799º 21.644º 21.016º 21.835º 22.388º 21.878º 22.948º 22.413º 22.984º 23.296º 24.083º 23.566º 24.694º 26.297º 24.419º 24.074º
ž ž ž
ž ž ž ž ž
ž ž ž ž ž ž ž ž ž ž ž ž ž ž
ž ž
ž ž ž
ž
ž
ž
ž
ž
ž
ž ž
ž ž ž
ž
ž ž
ž ž
ž ž
ž ž
ž
ž
ž
ž
ž ž
ž ž ž ž ž ž ž
ž
ž ž ž
ž ž ž ž ž ž ž ž ž ž ž ž ž
ž ž
vnorth 4.8 š 0.0 1.6 š 1.4 5.3 š 1.3 0.6 š 1.0 1.5 š 1.2 2.9 š 1.0 4.3 š 1.5 1.1 š 1.3 5.6 š 0.5 5.7 š 1.2 11.6 š 1.2 4.8 š 1.0 10.9 š 1.1 9.5 š 1.4 7.7 š 1.1 4.9 š 0.7 16.4 š 1.4 27.8 š 0.3 12.5 š 1.3 28.3 š 1.2 15.8 š 0.6 30.8 š 1.2 19.8 š 1.4 27.0 š 1.1 30.3 š 1.4 26.7 š 0.7 25.1 š 0.7 26.7 š 1.6 26.8 š 1.1 30.1 š 1.2 33.3 š 1.9 27.0 š 1.0 26.6 š 1.0 29.8 š 2.1 22.9 š 1.2 28.7 š 0.8
veast 0.9 š 0.0 1.1 š 1.1 11.1 š 1.1 6.5 š 0.8 4.3 š 1.0 3.2 š 0.8 6.1 š 1.2 4.4 š 1.0 0.9 š 0.4 6.1 š 1.0 5.0 š 0.9 7.0 š 0.9 6.3 š 0.9 8.5 š 1.1 5.1 š 0.8 3.1 š 0.6 8.8 š 1.0 13.0 š 0.3 14.5 š 1.0 21.3 š 1.0 13.4 š 0.5 25.3 š 0.9 18.0 š 1.2 23.5 š 0.9 18.4 š 1.1 20.8 š 0.6 16.4 š 0.6 24.3 š 1.2 19.8 š 1.0 18.1 š 1.0 15.7 š 1.5 15.1 š 0.8 18.4 š 0.9 13.1 š 1.9 18.2 š 1.1 20.2 š 0.7
vnorth positive to the north, veast positive to the east. The station Matera is an IGS site which was constrained. The errors given are 15 times the formal statistical errors. 1–7 D data from campaigns: 1 D Sep. – Oct. 1993; 2 D Sep. 1994,; 3 D Nov. 1995; 4 D Sep. 1996; 5 D Nov. 1997; 6 D Jul. 1998; 7 D Sep. 1998; a–d D data from the continuous network: a D Sep. 1995; b D Sep. 1996; c D Oct. 1997; d D Dec. 1997.
at LOGO and at STRO, in particular, yield biased rates if these are estimated from the entire time series used in the analysis. It is therefore necessary to exclude the Strofades event in the determination of the “interseismic” motion. The velocities estimated from the 1993 to 1998 data set are shown in Fig. 3 (except for LOGO and STRO for which the time period Sep. 1993 to Oct. 1997 was used). The error ellipses are based on the
formal errors, obtained by error propagation using the respective variance–covariance matrices for the coordinates of the individual epochs under consideration. The formal errors were then multiplied by a factor of 15, deduced from a comparison of session-to-session solutions (see Section 2), in order to obtain a realistic error estimate.
M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
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Fig. 2. Trajectories of crustal motion relative to Eurasia for the time period 1993–1998.
4. The velocity field All sites north of the island of Lefkada and the Gulf of Arta have rates around 5 mm=a, the smallest at Kastoria (KAST) in northern Epirus with 2.4
mm=a. Following the line from KARI (4.4 mm=a) to ATKO (18.6 mm=a) we notice a continuous southward increase in the velocities (Table 1, Fig. 3). The sites on the northern Ionian islands PAXI, PNTN, and OTHO show rates between 4 and 7 mm=a; how-
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M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
Fig. 3. Annual displacement rates along the West Hellenic Arc relative to Eurasia, based on repeated GPS measurements between 1993 and 1998, except for the sites LOGO and STRO which include only data up to Sep. 1997.
ever, they are more northwesterly oriented than the sites in Epirus. This is particularly obvious between the sites PAXI (5.8 mm=a to the NNW) and PARG
(4.6 mm=a to the WSW). ATKO and SKIN are moving rapidly at a rate of nearly 19 mm=a to the southwest. A conspicuous velocity gradient is seen
M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
on the island of Zakynthos between the sites SKIN (18.9 mm=a to the west-southwest) and LOGO (24.3 mm=a to the southwest). This gradient corresponds to a shear strain rate of about 120 nstrain=a. The island of Strofades and the sites on the Peloponnesus show a pronounced southwest-oriented movement (average 35 mm=a). Compared to the results of the 1989–1993 data [3], the analysis of the 1993–1998 observations shows a more coherent pattern of motion for southwest Greece, which holds true all the way to the islands of Crete and Gavdhos. The kinematic field of the central Ionian islands clearly indicates that this area forms a transition zone between western central Greece, northern Epirus and the Apulian platform. The two kinematic fields are clearly separated by the NNE–SSW-striking KFZ. An interesting feature is seen by comparing the site at ARTA with the one at VONI and L2PE with ATKO. These pairs are separated by the Gulf of Arta and the Katouna fault, respectively. There is increasing evidence now that baseline lengthening occurs across this boundary. This could be an indication for the Akarnania block separating from central Greece, as was discussed by [8]. It is furthermore interesting to note that also the rates for the sites on the southern Peloponnesus are indicative for baseline lengthening on the order of 5 mm=a in a NE–SW direction (VELI, SE Peloponnesus, 30.0 mm=a towards southwest, versus SRTA 35.4 mm=a and GERM 36.1 mm=a, southwest Peloponnesus, also towards southwest). It remains an open question whether this observation is due to normal faulting and rifting caused by the fault system near the city of Sparta. The western part of the Peloponnesus and the southern Ionian islands of Kithera (KITH) and Antikithera (AKIT) show larger rates in recent times, in contrast to previous results. We cannot exclude the possibility that also these stations have been influenced by the Strofades earthquakes, but as seen on the seismicity map (Fig. 1) these events were more restricted to the area between Zakinthos and Strofades. In recent times paleomagnetists suggested that the central Ionian islands have undergone a strong clockwise rotation between 0.77 Ma and the present [9]. The interpolation and analysis techniques used in this study allow to consistently separate the ve-
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locity field into pure deformation and rotation. The local rotations calculated from the velocity field may be compared with rotation parameters locally determined by paleomagnetic methods. Fig. 4 shows the rotational part of the velocity field. While the symmetrical part of the tensor of spatial derivatives of the velocities corresponds to the strain reflecting the deformation, the antisymmetrical part is equivalent to a local rotation around a pole centered at the location itself. Near the islands of Kephalonia and Zakinthos a relatively strong clockwise rotation of up to 8º=Ma is found. More to the south (southern Peloponnesus down to Crete) a small counterclockwise rotation up to 1º=Ma is prevailing. Compared to the uncertainty estimated at 1–1.5º=Ma this counterclockwise rotation is negligible.
5. Discussion and conclusions In contrast to earlier campaigns (1989–1993) it has been possible to analyze the new GPS data (1993–1998) in a consistent Eurasian reference frame using high-precision IGS orbits and to expand the study area along the entire WHA. The trajectories of crustal motion, referred to Eurasia, clearly show a pronounced change of motion pattern along a zone between the Gulf of Arta and the central Ionian islands of Lefkada and Kephalonia. We note consistency between the azimuths of earthquake slip vectors found by Baker et al. [10] and our GPS results. The most striking feature of the trajectory plot is the abrupt change of motion at the island of Strofades. It is clearly associated with the Strofades earthquake of Nov. 18, 1997, which has caused a relatively large co-seismic slip of 12 cm towards the south. The rates estimated from the entire data set confirm to a large degree the features seen in the trajectories. The Akarnania block bordered by the Gulf of Arta, the island of Lefkada and the Katouna Fault Zone has larger SW-oriented rates than the sites to the north and east of it. This corresponds to sinistral-type of displacements at the eastern boundary of this block and corroborates suggestions based on geological studies [11]. Baseline lengthening between the Arkanania block and central Greece amounts to 5 mm=a on average. Baseline lengthening on the order
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M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
Fig. 4. Differential rotation of collocated velocity field relative to Eurasia. The pronounced clockwise rotation around the central Ionian islands amounts to 8º=Ma. The uncertainty is estimated at 1–1.5º=Ma. The counterclockwise rotation seen over the southeastern Peloponnesus is small and within the error margin estimated.
of 5 mm=a in a NE–SW direction is also seen on the southern Peloponnesus which is concordant with geologic evidence of E–W spreading on the order of 4–8 mm=a [12]. The rotation pattern shows a relatively high rotation rate of 8º=Ma in the region of the central Ionian islands. This would support suggestions that this
area still undergoes a significant clockwise rotation, as was already surmized by Le Pichon and Angelier [13] and recently re-emphasized by Duermeijer and Langereis [9]. Finally we would like to note that the concept of calculating average rates in seismic areas from GPS campaigns separated by a few years can easily
M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
introduce large biases due to co-seismic events. Only control from continuously operating networks, such as the one integrated in this study [6,14] allows to deduce the “interseismic” motion, and separate it from co-seismic slip.
Acknowledgements We thank Babis Anemoyianis, Spiros Aronis, Lambis Calofonos, Costas Condogouris, Stella Condopoulos, Panayiotis Frangoyianis, Papa Gregoris, Nikos Kefalinos, Constantina Koyiadi, Gelli Koyiadi, Maria Koyiadi, Panayiotis Koyiadi, Dimitri Liveris, Costas Marinos, Dionisis Marinos, Giorgos Mourelatos, Yanis Perikles, Andreas Petrou, Alexander Politis, Gerasimos Potamitis, Giorgos Psaraftis, Antonis Robotis, Yanis Robotis, Giorgos Spinos, Panayiotis Spinos, Dimitris Vlachopoulos, Yanis Vlachopoulos, and the prefect of the city of Amfilochia who helped us by providing site-hosting facilities for the GPS-stations. In addition, we are grateful for the help of Mr. Papadopoulos, Director of the Lighthouse Authority of the Hellenic Navy.This study benefited greatly from discussions with our partners in the EU project ‘Seismic strain in Greece (SING)’: B. Ambrosius, P. Briole, P. Clark, P. Cross, P. Cruddace, P. England, H. Lyon-Caen, B. Parson, and G. Stavrakakis. Reviews by P. Meijer, University of Utrecht, and an anonymous reviewer were especially useful and helped to improve the manuscript. Funds were kindly provided by ETH grant No. 41-2647.5 in the frame of the project ‘Present-day crustal dynamics in the Adriatic–Aegean plate boundary zone based on GPS, INSAR and microseismic studies’, and EU grant No. ENV4-CT970519 in the frame of project SING provided by the Swiss Federal Office of Education and Science. [AC]
References [1] H. Anderson, J.A. Jackson, Active tectonics of the Adriatic region, Geophys. J.R. Astron. Soc. 91 (1987) 937–983.
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[2] E. Mantovani, D. Albarello, D. Babbucci, and C. Tamburelli, Recent geodynamic evolution of the Central Mediterranean region. Technical Report, Dept. of Earth Sciences University of Siena, 1992, 88 pp. [3] H.-G. Kahle, M.V. Mu¨ller, A. Geiger, G. Danuser, S. Mu¨ller, G. Veis, H. Billiris, D. Paradissis, The strain field in NW Greece and the Ionian Islands: results inferred from GPS measurements, Tectonophysics 249 (1995) 41–52. [4] A.A. Kiratzi, C.A. Langston, Moment tensor inversion of the 1983 Jan. Keffalinia event of Ionian islands (Greece), Geophys. J. Int. 105 (1991) 529–535. [5] H.-G. Kahle, C. Straub, R. Reilinger, S. McClusky, R.W. King, K. Hurst, G. Veis, K. Kastens, P. Cross, The strain field in the Eastern Mediterranean, estimated by repeated GPS measurements, Tectonophysics 294 (1998) 237–252. [6] Y. Peter, H.-G. Kahle, M. Cocard, G. Veis, S. Felekis, D. Paradissis, Establishment of a permanent GPS network across the Kephalonia Fault Zone, Ionian Islands, Greece, Tectonophysics 294 (1998) 253–260. [7] M. Rothacher, L. Mervart, Bernese GPS Software Version 4.0, 1996. [8] F. Haslinger, E. Kissling, J. Ansorge, D. Hatzfeld, E. Papadimitriou, V. Karakostas, K. Makropoulos, H.-G. Kahle, Y. Peter, 3D structure from local earthquake tomography around the Gulf of Arta (Ionian region, NW Greece), Tectonophysics 304 (1999) 201–218. [9] C.E. Duermeijer, C.G. Langereis, Neogene to recent geodynamic evolution of the Aegean area: an integrated paleomagnetic study, EGS Geophys. Abstr. 1 (1) (1999) 80. [10] C. Baker, D. Hatzfeld, H. Lyon-Caen, E. Papadimitriou, A. Rigo, Earthquake mechanisms of the Adriatic Sea and Western Greece: implications for the oceanic subduction– continental collision transition, Geophys. J. Int. 131 (3) (1997) 559–594. [11] G. King, D. Sturdy, J. Whitney, The landscape geometry and active tectonics of northwestern Greece, Geol. Soc. Am. Bull. 105 (1993) 137–161. [12] R. Armijo, H. Lyon-Caen, D. Papanastassiou, E–W extension and Holocene normal-fault scarps in the Hellenic are, Geology 20 (1992) 491–494. [13] X. Le Pichon, J. Angelier, The Hellenic Arc and Trench system: a key to the neotectonic evolution of the Eastern Mediterranean area, Tectonophysics 60 (1979) 1–42. [14] Y. Peter, H.-G. Kahle, M. Cocard, G. Veis, S. Felekis, D. Paradissis, Continuous GPS monitoring across the subduction zone of the West Hellenic Arc, Greece. Eos 80 (17), S77, 1999.
Earth and Planetary Science Letters 172 (1999) 39–47 www.elsevier.com/locate/epsl
New constraints on the rapid crustal motion of the Aegean region: recent results inferred from GPS measurements (1993–1998) across the West Hellenic Arc, Greece M. Cocard a,Ł , H.-G. Kahle a , Y. Peter a , A. Geiger a , G. Veis b , S. Felekis b , D. Paradissis b , H. Billiris b b
a Geodesy and Geodynamics Lab, Swiss Federal Institute of Technology, ETH Ho ¨ nggerberg, 8093 Zurich, Switzerland Higher Geodesy Lab, National Technical University of Athens, 9, Heroon Polytechnion Str., 15780 Zographos Athens, Greece
Received 10 May 1999; accepted 26 July 1999
Abstract In this paper we present the most recent observations of crustal motion across the entire West Hellenic Arc (WHA). These are based on repeated GPS measurements carried out in the period from 1993 to 1998. The results are presented in terms of trajectories and rates, relative to Eurasia. Within these five years southwestern Greece has moved to the southwest by an average rate of 30 mm=a, increasing from 10 mm=a at the island of Lefkada, in the center of the Ionian islands, to nearly 40 mm=a along the southwest part of the Peloponnesus and to 35 mm=a on the islands of Crete and Gavdhos. The data provide strong evidence that distributed shear strain starts at the Kephalonia Fault Zone (KFZ), were an anomalously high earthquake activity is also observed. A striking interruption of the motion is seen at the island of Strofades, at the southwest leading edge of the WHA, where a south-oriented displacement of 12 cm was detected, coincident with the M D 6:4 Strofades earthquake of Nov. 18, 1997. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Hellenic Arc; Greek Ionian Islands; Greece; crustal shortening
1. Introduction One of the key areas to understanding the kinematic processes at work in the Eastern Mediterranean is found in western Greece. Anderson and Jackson [1] have shown seismicity and fault-plane solutions along the southern Dalmatian coast (Montenegro, Albania, NW Greece), where thrusting mechanisms occur all the way to the central Ionian islands. This belt of NE–SW shortening is associated with transcurrent fault systems [2]. One of these is the Ł Corresponding
author. E-mail: [email protected]
Kephalonia Fault Zone (KFZ), which has been recognized as a major discontinuity between the Apulian platform and the transition to the West Hellenic Arc (WHA) (Fig. 1). Viewed from the south, Anderson and Jackson [1] suggested that the Hellenic subduction zone terminates against this major strike slip fault. The KFZ follows the submarine Kephalonia valley west of the island chain from the island of Lefkada to Kephalonia. From earlier GPS observations (1989–1993) made in southeast Apulia, on the northern Ionian Islands, in Epirus and on the Peloponnesus it was possible to identify two distinct kinematic fields of
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M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
Fig. 1. Seismicity of the Ionian and Aegean seas in the year 1997 (G. Stavrakakis, pers. commun., 1998) showing a high seismic activity south of the island of Zakynthos, the largest earthquake being the earthquake of Strofades (S) (M D 6:4). The inset summarizes the geodynamic framework around the Mediterranean Sea together with the major plates involved in the collision process. Superimposed on the counter-clockwise rotation of the African plate is a pronounced right-lateral motion of the Aegean=Anatolian plate towards the WSW. The large arrows describe the average motion of the African and Arabian plates as well as of the Anatolian and Aegean microplates, relative to the Eurasian plate. The Aegean microplate is bounded in the west by the Kephalonia Fault Zone (KFZ) and the West Hellenic Arc (WHA). Ab D Arkanania block, AnP D Anatolian microplate, AP D Apulian platform, CA D Calabrian arc, GA D Gulf of Arta, GP D Gulf of Patras, KF D Katouna fault, Lef D island of Lefkada, Zak D island of Zakinthos.
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recent crustal movements [3]. These are separated by a zone aligned with the KFZ, which is also the site of intense seismicity. Strong earthquakes (M > 7) near the island of Kephalonia occurred in 1953, 1972 and 1983. Source parameters of the 1983 Kephalonia island earthquake (M D 7:1) have revealed a dextral strike-slip mechanism [4], which agrees well with the strain field calculated from previous GPS observations [5]. In addition to the WHA sites re-measured in the last five years we have included in this paper also data from the southern Ionian islands Kithera and Antikithera, as well as from the islands of Crete and Gavdhos. The previous results have been geographically expanded and substantially improved upon. The kinematic field is much clearer now and shows distinct correlations with intermediate-size earthquakes. The purpose of this paper is to present rates and trajectories of crustal motion for the entire WHA and compare these with seismicity, with particular emphasis on detecting co-seismic motions.
2. Field campaigns and data analysis The network considered comprises the WHA including Epirus, the Ionian islands (from Othoni to Antikithera), the mainland of western Greece, the Peloponnesus and the islands of Crete and Gavdhos (Fig. 1). Major campaigns were carried out in 1993, 1994, 1996, and 1998. Some selected sites were also measured in 1995 and 1997. Table 1 shows the observation history for the WHA GPS sites. In addition selected data from the permanent GPS network [6] were integrated to calculate the velocity field. The analysis of the GPS data was performed using the Bernese GPS software 4.0 [7]. Every campaign was treated separately. Solutions for every session were calculated using final IGS orbits and integrating data from stations of the continuous IGS network. From the comparison with the final campaign solution, obtained by combining the normal equations of all individual sessions, it was concluded that a factor of 15 is an appropriate ratio between the realistic margin of error and the formal statistical one. This factor is dependent on the sampling rate and accounts for the high temporal correlation between the measurements, which is not taken into account
41
in the GPS software. Therefore the uncertainty, deduced from the daily repeatabilities is more realistic than the formal statistical RMS error. The latter does not reflect contributions of systematic errors. The rates were then estimated based on the normal equations of the different campaign solutions. The coordinates and velocities of the IGS sites were constrained, and ITRF96 velocities of all sites were estimated. The velocity field obtained was then rotated from the ITRF96 to an Eurasia fixed reference frame using a rotation rate of 0.262º=Ma around a pole located at 59.0ºN and 97.1ºW (Heflin and Argus, pers. commun., 1998). For reasons of comparison it is noted that this reference frame is slightly different from the one used for the data set 1989 to 1993 in Kahle et al. [3], where the motion of Matera was fixed to zero velocity.
3. Trajectories In order to visualize the discrete displacements in the time periods between the observation campaigns we plotted the trajectories of crustal motion, relative to Eurasia (Fig. 2). Three main features are clearly evident: relatively small displacements in the northern part, with irregular directions, but mainly towards the west; intermediate-size displacements from the Gulf of Arta to the island of Kefalonia, mainly SW directed (compare inset in Fig. 2); and rapid motion in the south, towards the southwest. A new striking feature, seen in the data set 1993– 1998, is the abrupt dislocation at the island of Strofades, between Oct. 1997 and Dec. 1997. In this period Strofades has moved by an amount of 12 cm towards the south. Also the station at LOGO, at the southern tip of the island of Zakynthos, has moved rather rapidly in this time frame by about 1.5 cm towards the southwest. Taking a view at the seismicity of 1997 (G. Stavrakakis, pers. commun., 1998) it is evident that the area between the islands of Zakynthos and Strofades was struck by a cluster of earthquakes, with a main event, on Nov. 18, 1997, near the island of Strofades. On the other hand it is clearly seen that the relative displacements prior to 1997 are quite homogeneous, with average displacements on the order of 25 mm=a. It is obvious that co-seismic displacements such as the ones seen
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M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
Table 1 Observation history and annual displacement rates (v, in mm=a) relative to Eurasia Site
Abbr.
1
2
3
4
5
6
7
a
b
c
d
Lat.
Long.
Matera (I) Kastoria Specchia Christi (I) Othoni Pantokrator Karitsa Hania Terovou Parga Paxi Arta Vonitsa Amfilochia Kavallos Lepenou Vasiliki Doukato Astakos Dionysos Skinari Doxa Logos Zaharo Strofades Aetos Sparta Chrisokellaria Velies Gerolimenas Kithira Antikithira Souda Sfinari Roumeli Kiriamadi Damnoni Gavdhos
MATE KAST SPEC OTHO PNTN KARI TERO PARG PAXI ARTA VONI AMFI 1KVL L2PE VASI DUKA ATKO DION SKIN DOXA LOGO ZAHA STRO AETO SRTA XRIS VELI GERM KITH AKIT SOUD SFIN ROUM KIRI DAMN GVDO
ž
ž
ž
ž
ž
ž
ž
ž ž ž ž
ž ž
ž ž
ž
ž ž
ž ž
ž
ž
ž
ž ž
ž ž
ž ž
ž
ž
ž
ž
ž
ž ž
ž ž
ž
ž ž
ž
ž
ž
ž
ž
ž
40.649º 40.519º 40.063º 39.856º 39.749º 39.734º 39.463º 39.292º 39.236º 39.166º 38.910º 38.864º 38.791º 38.697º 38.607º 38.564º 38.494º 38.079º 37.931º 37.702º 37.675º 37.490º 37.250º 37.244º 37.034º 36.791º 36.718º 36.475º 36.307º 35.873º 35.487º 35.422º 35.404º 35.305º 35.171º 34.837º
16.704º 21.269º 18.458º 19.379º 19.861º 20.665º 20.880º 20.473º 20.128º 20.988º 20.846º 21.165º 20.656º 21.291º 20.573º 20.543º 21.120º 23.933º 20.702º 21.925º 20.799º 21.644º 21.016º 21.835º 22.388º 21.878º 22.948º 22.413º 22.984º 23.296º 24.083º 23.566º 24.694º 26.297º 24.419º 24.074º
ž ž ž
ž ž ž ž ž
ž ž ž ž ž ž ž ž ž ž ž ž ž ž
ž ž
ž ž ž
ž
ž
ž
ž
ž
ž
ž ž
ž ž ž
ž
ž ž
ž ž
ž ž
ž ž
ž
ž
ž
ž
ž ž
ž ž ž ž ž ž ž
ž
ž ž ž
ž ž ž ž ž ž ž ž ž ž ž ž ž
ž ž
vnorth 4.8 š 0.0 1.6 š 1.4 5.3 š 1.3 0.6 š 1.0 1.5 š 1.2 2.9 š 1.0 4.3 š 1.5 1.1 š 1.3 5.6 š 0.5 5.7 š 1.2 11.6 š 1.2 4.8 š 1.0 10.9 š 1.1 9.5 š 1.4 7.7 š 1.1 4.9 š 0.7 16.4 š 1.4 27.8 š 0.3 12.5 š 1.3 28.3 š 1.2 15.8 š 0.6 30.8 š 1.2 19.8 š 1.4 27.0 š 1.1 30.3 š 1.4 26.7 š 0.7 25.1 š 0.7 26.7 š 1.6 26.8 š 1.1 30.1 š 1.2 33.3 š 1.9 27.0 š 1.0 26.6 š 1.0 29.8 š 2.1 22.9 š 1.2 28.7 š 0.8
veast 0.9 š 0.0 1.1 š 1.1 11.1 š 1.1 6.5 š 0.8 4.3 š 1.0 3.2 š 0.8 6.1 š 1.2 4.4 š 1.0 0.9 š 0.4 6.1 š 1.0 5.0 š 0.9 7.0 š 0.9 6.3 š 0.9 8.5 š 1.1 5.1 š 0.8 3.1 š 0.6 8.8 š 1.0 13.0 š 0.3 14.5 š 1.0 21.3 š 1.0 13.4 š 0.5 25.3 š 0.9 18.0 š 1.2 23.5 š 0.9 18.4 š 1.1 20.8 š 0.6 16.4 š 0.6 24.3 š 1.2 19.8 š 1.0 18.1 š 1.0 15.7 š 1.5 15.1 š 0.8 18.4 š 0.9 13.1 š 1.9 18.2 š 1.1 20.2 š 0.7
vnorth positive to the north, veast positive to the east. The station Matera is an IGS site which was constrained. The errors given are 15 times the formal statistical errors. 1–7 D data from campaigns: 1 D Sep. – Oct. 1993; 2 D Sep. 1994,; 3 D Nov. 1995; 4 D Sep. 1996; 5 D Nov. 1997; 6 D Jul. 1998; 7 D Sep. 1998; a–d D data from the continuous network: a D Sep. 1995; b D Sep. 1996; c D Oct. 1997; d D Dec. 1997.
at LOGO and at STRO, in particular, yield biased rates if these are estimated from the entire time series used in the analysis. It is therefore necessary to exclude the Strofades event in the determination of the “interseismic” motion. The velocities estimated from the 1993 to 1998 data set are shown in Fig. 3 (except for LOGO and STRO for which the time period Sep. 1993 to Oct. 1997 was used). The error ellipses are based on the
formal errors, obtained by error propagation using the respective variance–covariance matrices for the coordinates of the individual epochs under consideration. The formal errors were then multiplied by a factor of 15, deduced from a comparison of session-to-session solutions (see Section 2), in order to obtain a realistic error estimate.
M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
43
Fig. 2. Trajectories of crustal motion relative to Eurasia for the time period 1993–1998.
4. The velocity field All sites north of the island of Lefkada and the Gulf of Arta have rates around 5 mm=a, the smallest at Kastoria (KAST) in northern Epirus with 2.4
mm=a. Following the line from KARI (4.4 mm=a) to ATKO (18.6 mm=a) we notice a continuous southward increase in the velocities (Table 1, Fig. 3). The sites on the northern Ionian islands PAXI, PNTN, and OTHO show rates between 4 and 7 mm=a; how-
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M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
Fig. 3. Annual displacement rates along the West Hellenic Arc relative to Eurasia, based on repeated GPS measurements between 1993 and 1998, except for the sites LOGO and STRO which include only data up to Sep. 1997.
ever, they are more northwesterly oriented than the sites in Epirus. This is particularly obvious between the sites PAXI (5.8 mm=a to the NNW) and PARG
(4.6 mm=a to the WSW). ATKO and SKIN are moving rapidly at a rate of nearly 19 mm=a to the southwest. A conspicuous velocity gradient is seen
M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
on the island of Zakynthos between the sites SKIN (18.9 mm=a to the west-southwest) and LOGO (24.3 mm=a to the southwest). This gradient corresponds to a shear strain rate of about 120 nstrain=a. The island of Strofades and the sites on the Peloponnesus show a pronounced southwest-oriented movement (average 35 mm=a). Compared to the results of the 1989–1993 data [3], the analysis of the 1993–1998 observations shows a more coherent pattern of motion for southwest Greece, which holds true all the way to the islands of Crete and Gavdhos. The kinematic field of the central Ionian islands clearly indicates that this area forms a transition zone between western central Greece, northern Epirus and the Apulian platform. The two kinematic fields are clearly separated by the NNE–SSW-striking KFZ. An interesting feature is seen by comparing the site at ARTA with the one at VONI and L2PE with ATKO. These pairs are separated by the Gulf of Arta and the Katouna fault, respectively. There is increasing evidence now that baseline lengthening occurs across this boundary. This could be an indication for the Akarnania block separating from central Greece, as was discussed by [8]. It is furthermore interesting to note that also the rates for the sites on the southern Peloponnesus are indicative for baseline lengthening on the order of 5 mm=a in a NE–SW direction (VELI, SE Peloponnesus, 30.0 mm=a towards southwest, versus SRTA 35.4 mm=a and GERM 36.1 mm=a, southwest Peloponnesus, also towards southwest). It remains an open question whether this observation is due to normal faulting and rifting caused by the fault system near the city of Sparta. The western part of the Peloponnesus and the southern Ionian islands of Kithera (KITH) and Antikithera (AKIT) show larger rates in recent times, in contrast to previous results. We cannot exclude the possibility that also these stations have been influenced by the Strofades earthquakes, but as seen on the seismicity map (Fig. 1) these events were more restricted to the area between Zakinthos and Strofades. In recent times paleomagnetists suggested that the central Ionian islands have undergone a strong clockwise rotation between 0.77 Ma and the present [9]. The interpolation and analysis techniques used in this study allow to consistently separate the ve-
45
locity field into pure deformation and rotation. The local rotations calculated from the velocity field may be compared with rotation parameters locally determined by paleomagnetic methods. Fig. 4 shows the rotational part of the velocity field. While the symmetrical part of the tensor of spatial derivatives of the velocities corresponds to the strain reflecting the deformation, the antisymmetrical part is equivalent to a local rotation around a pole centered at the location itself. Near the islands of Kephalonia and Zakinthos a relatively strong clockwise rotation of up to 8º=Ma is found. More to the south (southern Peloponnesus down to Crete) a small counterclockwise rotation up to 1º=Ma is prevailing. Compared to the uncertainty estimated at 1–1.5º=Ma this counterclockwise rotation is negligible.
5. Discussion and conclusions In contrast to earlier campaigns (1989–1993) it has been possible to analyze the new GPS data (1993–1998) in a consistent Eurasian reference frame using high-precision IGS orbits and to expand the study area along the entire WHA. The trajectories of crustal motion, referred to Eurasia, clearly show a pronounced change of motion pattern along a zone between the Gulf of Arta and the central Ionian islands of Lefkada and Kephalonia. We note consistency between the azimuths of earthquake slip vectors found by Baker et al. [10] and our GPS results. The most striking feature of the trajectory plot is the abrupt change of motion at the island of Strofades. It is clearly associated with the Strofades earthquake of Nov. 18, 1997, which has caused a relatively large co-seismic slip of 12 cm towards the south. The rates estimated from the entire data set confirm to a large degree the features seen in the trajectories. The Akarnania block bordered by the Gulf of Arta, the island of Lefkada and the Katouna Fault Zone has larger SW-oriented rates than the sites to the north and east of it. This corresponds to sinistral-type of displacements at the eastern boundary of this block and corroborates suggestions based on geological studies [11]. Baseline lengthening between the Arkanania block and central Greece amounts to 5 mm=a on average. Baseline lengthening on the order
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M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
Fig. 4. Differential rotation of collocated velocity field relative to Eurasia. The pronounced clockwise rotation around the central Ionian islands amounts to 8º=Ma. The uncertainty is estimated at 1–1.5º=Ma. The counterclockwise rotation seen over the southeastern Peloponnesus is small and within the error margin estimated.
of 5 mm=a in a NE–SW direction is also seen on the southern Peloponnesus which is concordant with geologic evidence of E–W spreading on the order of 4–8 mm=a [12]. The rotation pattern shows a relatively high rotation rate of 8º=Ma in the region of the central Ionian islands. This would support suggestions that this
area still undergoes a significant clockwise rotation, as was already surmized by Le Pichon and Angelier [13] and recently re-emphasized by Duermeijer and Langereis [9]. Finally we would like to note that the concept of calculating average rates in seismic areas from GPS campaigns separated by a few years can easily
M. Cocard et al. / Earth and Planetary Science Letters 172 (1999) 39–47
introduce large biases due to co-seismic events. Only control from continuously operating networks, such as the one integrated in this study [6,14] allows to deduce the “interseismic” motion, and separate it from co-seismic slip.
Acknowledgements We thank Babis Anemoyianis, Spiros Aronis, Lambis Calofonos, Costas Condogouris, Stella Condopoulos, Panayiotis Frangoyianis, Papa Gregoris, Nikos Kefalinos, Constantina Koyiadi, Gelli Koyiadi, Maria Koyiadi, Panayiotis Koyiadi, Dimitri Liveris, Costas Marinos, Dionisis Marinos, Giorgos Mourelatos, Yanis Perikles, Andreas Petrou, Alexander Politis, Gerasimos Potamitis, Giorgos Psaraftis, Antonis Robotis, Yanis Robotis, Giorgos Spinos, Panayiotis Spinos, Dimitris Vlachopoulos, Yanis Vlachopoulos, and the prefect of the city of Amfilochia who helped us by providing site-hosting facilities for the GPS-stations. In addition, we are grateful for the help of Mr. Papadopoulos, Director of the Lighthouse Authority of the Hellenic Navy.This study benefited greatly from discussions with our partners in the EU project ‘Seismic strain in Greece (SING)’: B. Ambrosius, P. Briole, P. Clark, P. Cross, P. Cruddace, P. England, H. Lyon-Caen, B. Parson, and G. Stavrakakis. Reviews by P. Meijer, University of Utrecht, and an anonymous reviewer were especially useful and helped to improve the manuscript. Funds were kindly provided by ETH grant No. 41-2647.5 in the frame of the project ‘Present-day crustal dynamics in the Adriatic–Aegean plate boundary zone based on GPS, INSAR and microseismic studies’, and EU grant No. ENV4-CT970519 in the frame of project SING provided by the Swiss Federal Office of Education and Science. [AC]
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