ANNALS OF GEOPHYSICS, 56, 6, 2013, S0674; doi:10.4401/ag-6238 Special Issue: Earthquake geology Geodetic evidence for passive control of a major Miocene tectonic boundary on the contemporary deformation field of Athens (Greece) Michael Foumelis1,*, Ioannis Fountoulis2, Ioannis D. Papanikolaou3, Dimitrios Papanikolaou2 1 European Space Agency (ESA-ESRIN), Frascati (Rome), Italy 2 National and Kapodistrian University of Athens, Department of Dynamics Tectonics and Applied Geology, Athens, Greece 3 Agricultural University of Athens, Department of Geological Sciences and Atmospheric Environment, Laboratory of Mineralogy and Geology, Athens, Greece Article history Received October 19, 2012; accepted May 20, 2013. Subject classification: Satellite geodesy, Crustal deformations, Geodynamics, Tectonics, Measurements and monitoring. ABSTRACT while there are sufficient data for the period after 1810 A GPS-derived velocity field is presented from a dense geodetic network [Ambraseys and Jackson 1990]. Reports on damage and (~5km distance between stations) established in the broader area of displacement of ancient monuments [Papanastassiou Athens. It shows significant local variations of strain rates across a major et al. 2000, Ambraseys and Psycharis 2012] suggest in inactive tectonic boundary separating metamorphic and non-metamor- turns that Attica region has experienced several strong phic geotectonic units. The southeastern part of Athens plain displays earthquakes in the past. It is interesting that despite the negligible deformation rates, whereas towards the northwestern part unexpected catastrophic seismic event of September 7, higher strain rates are observed, indicating the control of the inactive tec- 1999, Mw=6.0 [Papadimitriou et al. 2002], no further tonic boundary on the contemporary deformation field of the region. monitoring of the region was held. These findings are in agreement with previous geological observations, The contribution of previous geodetic GPS studies however, due to the dense local GPS network it was fatherly possible to lo- to examine the kinematic field of Attica are only lim- calize and quantify the effect of such a major inherited tectonic feature on ited to observations obtained from regional networks, the deformation pattern of the area. that are designed to monitor large-scale deformation rather than local tectonic movements [Clarke et al. 1998, 1. Introduction Veis et al. 2003]. With a limited number of stations Detailed instrumental observations of the tectonic within the region, only a gross view of the motion is movements in Athens Basin by geodetic or other meth- gained, while changes within are hardly traced. ods are absent, a fact attributed mainly to the limited In this study a comprehensive GPS-derived veloc- interest of the scientific community over areas exhibit- ity field for the broader area of Athens is presented. ing low deformation rates. Indeed, both historical and Variations of strain rates across a major tectonic bound- instrumental data for the broader area of Athens show ary occurring in the region are highlighted and the im- no large earthquakes since 1700, with an exception of plications on the contemporary kinematics and dynamics the 1705 event located on the northeastern flanks of of the region are further discussed. Parnitha Mt. [Ambraseys and Jackson 1997, Goldswor- thy et al. 2002]. The list of historical earthquakes in the 2. Geological setting region (480 B.C. - 1900) includes a limited number of The Athens basement belongs to alpine forma- reports of earthquakes that caused damage, while there tions outcropping in the mountains and the hills of the is no evidence of large events at distances smaller than area. Recent post-alpine sediments (syn-rift deposits) 30 km from Athens [Makropoulos et al. 1989, Papaza- often cover the slopes of the mountains as well as areas chos and Papazachou 1997]. of low altitude. It should be noted, however, that historical records The area presents a complex alpine structure com- are incomplete, particularly in the area of central Greece, prising mainly Mesozoic metamorphic rocks occurring S0674 FOUMELIS ET AL. Figure 1. Simplified neotectonic map of Attica showing the approximate location of the major tectonic boundary, separating metamorphic and non-metamorphic alpine rocks (modified from Papanikolaou et al. [1999]). at Pendeli and Hymmetus mountains and Mesozoic it is clear that this tectonic boundary has accommo- non-metamorphic rocks of the eastern Greece geot- dated more than 25 km of vertical displacement. It was ectonic unit, occurring at Parnitha, Poikilo and Aega- active throughout Middle-Late Miocene times and leo mountains [Katsikatsos et al. 1986, Papanikolaou gradually became inactive during Early Pliocene [Pa- 1986, Papanikolaou et al. 2004] (Figure 1). The bound- panikolaou and Royden 2007, Royden and Papaniko- ary between the metamorphic and non-metamorphic laou 2011]. However, it forms a major boundary that geotectonic units, although generally accepted to be separates the E-W trending higher slip-rate active faults of tectonic origin, its exact geometric and kinematic in the western part of Attica from the NW-SE trending characteristics are yet to be determined since no direct lower slip-rate faults in the eastern part [Mariolakos and geological mapping could be undertaken because it is Papanikolaou 1987, Papanikolaou et al. 2004, Pa- covered with Neogene and Quaternary deposits (Fig- panikolaou and Papanikolaou 2007]. ure 2). It is traced northeastwards from the Aegean coast of southern Evia, through Aliveri to Kalamos in 3. GPS network establishment northeast Attica and continues to the southwest into Local geodetic networks offer a unique opportu- the plain of Athens. Within the area of interest, its lo- nity for understanding the fragmentation pattern cations coincide approximately with the riverbed of Ki- within a small part of the crust. Most often they are es- fissos River [Papanikolaou et al. 1999, Mariolakos and tablished in relationship to existing geodetic reference Fountoulis 2000, Xypolias et al. 2003], also confirmed frames, as well as independent networks serving highly by geophysical investigations at the northern part of accurate geodetic control in specific regions. Among the basin [Papadopoulos et al. 2007]. Results of seismic the advantages is that these networks are designed with tomography indicate the presence of abnormally high a specific geodynamic regime in mind and that major seismic velocities in the central part of the basin, most error sources (e.g. ionospheric) are limited compared likely related to this major boundary, extending to- to regional networks. wards the southwest at the Saronikos Gulf [Drakatos Given the lack of previous instrumental observa- et al. 2005]. tions, the design of the geodetic network used in this According to Papanikolaou and Royden [2007] this study was primarily focused on the investigation of the boundary represents a broad extensional detachment local tectonic regime. The minimum number of survey with a significant portion of dextral shear, whereas points required is imposed by the tectonic complexity of opinions of a right-lateral strike slip fault zone have also the region and the degree of fragmentation of the crust. been reported [Mariolakos and Fountoulis 2000, Krohe The established Athens Geodetic Network (AGNET) et al. 2009]. Considering a depth of about 30 km for the consisted of a total number of 41 campaign GPS sites tectono-metamorphic input of the metamorphics in (Figure 3) including already available benchmarks of Oligocene times [Lozios 1993], that are now at surface, the Hellenic Military Geographical Service (HMGS), as 2 CONTEMPORARY DEFORMATION OF ATHENS (GREECE) Figure 2. Geological map of the study area, (1) post-Alpine sediments, (2) non- metamorphic rocks (eastern Greece geotectonic unit), (3) allochthonous system (Athens schists), (4) metamorphic rocks (Attica geotectonic unit), (5) fault zone and (6) major tectonic boundary. well as sites previously installed by the Hellenic Map- more on 2008. Measurements were conducted using ping and Cadastral Organization (HEMCO) and the LEICA geodetic GPS receivers equipped with SR299/399, National Technical University of Athens (NTUA). Con- AT202/302 and Ach1202Pro antennas. Carrier phase tinuous (real-time) GPS stations operate in the region observations were recorded every 10 seconds from each by Metrica S.A. (MET0), National Observatory of Athens station for a period of at least four hours. In an effort to (NOA1) and National and Kapodistrian University of achieve optimal results, selected stations were occupied Athens (UOA1), and despite their relatively limited ob- for several days per epoch (independent sessions). To servations at the time, they were incorporated in the avoid large tropospheric errors, an initial elevation cut- analysis as well. The network covers essentially both off angle of 10° was used. the Athens and Thriassio basins as well as their bor- Collected data were processed using Leica Geo Of- dering mountain ranges, showing a relatively uniform fice v.1.1 and Bernese ver. 4.2 [Hugentobler et al. 2001]. spatial distribution. With an average distance of ap- The realization of the reference frame was performed proximately 5 km between stations, a sufficient sam- using the coordinates and velocity of Dionysos (DION) pling and a high resolution view of the local deformation continuous GPS station, located on the metamorphic field is accomplished. alpine basement. DION was tied to the ITRF2000