H.-G. Kahle

Global Positioning System constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus

We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1988–1997 at 189 sites extending east-west from the Caucasus mountains to the Adriatic Sea and north-south from the southern edge of the Eurasian plate to the northern edge of the African plate. Sites on the northern Arabian platform move 18±2 mm/yr at N25°±5°W relative to Eurasia, less than the NUVEL-1A circuit closure rate (25±1 mm/yr at N21°±7°W). Preliminary motion estimates (1994–1997) for stations located in Egypt on the northeastern part of Africa show northward motion at 5–6±2 mm/yr, also slower than NUVEL-IA estimates (10±1 mm/yr at N2°±4°E). Eastern Turkey is characterized by distributed deformation, while central Turkey is characterized by coherent plate motion (internal deformation of <2 mm/yr) involving westward displacement and counterclockwise rotation of the Anatolian plate. The Anatolian plate is de-coupled from Eurasia along the right-lateral, strike-slip North Anatolian fault (NAF). We derive a best fitting Euler vector for Anatolia-Eurasia motion of 30.7°± 0.8°N, 32.6°± 0.4°E, 1.2°±0.1°/Myr. The Euler vector gives an upper bound for NAF slip rate of 24±1 mm/yr. We determine a preliminary GPS Arabia-Anatolia Euler vector of 32.9°±1.2°N, 40.3°±1.1°E, 0.8°±0.2°/Myr and an upper bound on left-lateral slip on the East Anatolian fault (EAF) of 9±1 mm/yr. The central and southern Aegean is characterized by coherent motion (internal deformation of <2 mm/yr) toward the SW at 30±1 mm/yr relative to Eurasia. Stations in the SE Aegean deviate significantly from the overall motion of the southern Aegean, showing increasing velocities toward the trench and reaching 10±1 mm/yr relative to the southern Aegean as a whole.

GPS‐derived strain rate field within the boundary zones of the Eurasian, African, and Arabian Plates

We use the GPS velocity field (1988–1998) for eastern Mediterranean and Asia Minor to determine the crustal deformation strain rate field in an area bounded by 35°N and 43°N, and 20°E and 48°E. We calculate the normal and shear strain rate components associated with the major faults and compare these qualitatively with seismological data. Uncertainties in the calculation of the strain rates reach 50 nstrain yr−1 in sparsely observed parts of Anatolia, whereas we estimate errors <20 nstrain yr−1 in the Aegean and Marmara regions. The largest compressional strain rate components in the eastern part of the study area occur along the Greater Caucasus mountain front reaching 70 nstrain yr−1. (1 nstrain yr−1 = 0.0317 × 10−15 s−1). The North Anatolian Fault Zone is the clearest feature in the shear strain rate field. It is expressed as a pronounced dextral strike-slip fault zone, reaching rates of up to 170 nstrain yr−1. This holds true also for the Izmit area, where the August 17, 1999, earthquake occurred. Central Anatolia is almost strain-free, whereas extension prevails in western Anatolia. The principal axes of extension vary around the N-S direction with strain rates of up to 85 nstrain yr−1. These extensional areas coincide with graben features and normal faulting earthquakes. The central and southwestern Aegean Sea is strain-free with values far below 40 nstrain yr−1. The seismic cluster around the Dodekanissa islands, southeastern Aegean Sea, coincides with NW-SE oriented extension, attaining strain rates of up to 90 nstrain yr−1. This area of extension also exhibits recent active volcanism. The entire Hellenic arc shows compressional strain rates perpendicular to the arc. The Pliny-Strabo troughs along the eastern segment of the arc show left-lateral shear strain rates reaching 80 nstrain yr−1. Significant extension is found in central Greece, with a NNE-SSW oriented maximum of 120 nstrain yr−1 centered around the Gulf of Corinthos. The Kephalonia Fault Zone in NW Greece is a distinct dextral fault zone, separating Apulia from the rapidly moving Aegean microplate. Right-lateral shear strain rates reach 150 nstrain yr−1.

GPS and geologic estimates of the tectonic activity in the Marmara Sea region, NW Anatolia

A dense network of 52 Global Positioning System (GPS) sites is used to determine the velocity field and strain rate pattern at the western end of the North Anatolian Fault Zone (NAFZ) in the Marmara Sea region, NW Anatolia. Based on four GPS campaigns carried out biannually between 1990 and 1996, the detailed kinematic field of crustal motion is calculated. The southernmost station shows an average rate of 22±3 mm/yr oriented westward relative to Istanbul (Eurasia). This value is indicative of the dextral strike-slip motion of NW Anatolia relative to the Black Sea. The GPS velocity field reveals that the sites located on the Thrace-Black Sea block show negligible motion relative to Istanbul. The GPS-based deformation pattern of three sections crossing the westward prolongation of the NAFZ is compared with neotectonic data. Both the GPS and the neotectonic data give clear evidence that most of the deformation occurs along a relatively narrow E-W oriented zone. This belt extends from the single fault trace of the NAFZ through the Gulf of Izmit, the Marmara Sea, the Şarkoy region, and the Gulf of Saros into the North Aegean Trough (NAT). In addition, the cross sections show the increasing influence of the N-S oriented extension in the Aegean Sea region. From the comparison of the directions and rates of shear and normal strain deduced from GPS data with geologic and seismic observations we conclude that the most active zone of dextral shear follows a line along Mudurnu Valley, the northern Marmara Sea and the Saros Trough where it joins the NAT.

3D crustal structure from local earthquake tomography around the Gulf of Arta (Ionian region, NW Greece)

During summer of 1995 local seismicity was recorded in the area around the Gulf of Arta in northwestern Greece by a dense temporary seismic network. Of the 441 local events observed at 37 stations, 232 well locatable events with a total of 2776 P-phase readings were selected applying the criteria of a minimum of 6 P-observations and an azimuthal gap less than 180°. This data set is used to compute a minimum 1D velocity model for the region. Several tests are conducted to estimate model stability and hypocenter uncertainties, leading to the conclusion that relative hypocenter location accuracy is about 500 m in latitude and longitude and 1 km in depth. The minimum 1D velocity model serves as initial model in the non-linear inversion for three-dimensional P-velocity crustal structure by iteratively solving the coupled hypocenter–velocity problem in a least-squares sense. Careful analysis of the resolution capability of our data set outlines the well resolved features for interpretation. The resulting 3D velocity model shows generally higher average crustal velocities in the east, and the well resolved area of the eastern Gulf of Arta exhibits a homogeneous velocity around 6 km/s for the whole upper crust. A pronounced north–south trending zone of low velocities in the upper 5–10 km is observed in the area of the Katouna fault zone (KFZ). At greater depths (below 10 km) the KFZ is underlain by high-velocity material. E–W profiles suggest a horst–graben structure associated with the KFZ.

The strain rate field in the eastern Mediterranean region, estimated by repeated GPS measurements

We use the combined GPS velocity field of the eastern Mediterranean for the period 1988 to 1996 to determine crustal deformation strain rates in a region comprising the Hellenic arc, the Aegean Sea, and western Anatolia. We interpret the velocity field and determine the strain rate tensor by the spatial derivatives of the collocated motion vectors. The region following the line Marmara Sea, North Aegean Trough, northern central Greece, and the central Ionian islands is associated with strong right-lateral shear motion, with maximum shear strain rates of 180 nano-strain/a (180×10−9/a). In the central Aegean Sea, N–S-oriented extensional processes prevail, reaching 100 nano-strain/a. The southern Aegean is characterized by relatively small strain rates. Maximum extensional components of the strain rate tensor, reaching 150 nano-strain/a in a N–S direction, are found in central Greece. The Hellenic arc is associated with moderate arc-parallel extension and strong compression perpendicular to it. Projections of the strain rates parallel to the major fault zones reveal that the northern Aegean is governed by the westward continuation of the North Anatolian Fault Zone which is associated with strong dextral shearing (maximum 220 nano-strain/a), accompanied by numerous large earthquakes in this century.

Sea level in the Mediterranean: a first step towards separating crustal movements and absolute sea-level variations

Abstract The SELF (SEa Level Fluctuations: geophysical interpretation and environmental impact) project has been developed and realized in the framework of the Environment Programme designed by the Commission of the European Communities. The SELF project was aimed at providing a reliable base for the determination, in the Mediterranean area, of sea-level variations which could then be used as a possible indicator of climate changes and to study the interactions taking place among the ocean, the atmosphere: and the solid Earth. The project has made it possible to define a consistent network of well-established tide gauges encompassing the Mediterranean Basin as far as the Black Sea and to determine to centimeter accuracy the tide gauge benchmark heights in a global well-defined reference system such as the one provided by the SLR/VLBI space techniques. The SELF network constitutes, for the Mediterranean, the necessary prerequisite towards achieving the actual capability to separate vertical crustal movements from true sea-level variations. This has been accomplished through the use of space techniques namely SLR, VLBI and GPS in conjunction with Water Vapor Radiometer observations and absolute gravity measurements. The analysis of the available tide gauge records has shown a high spatial coherence of the annual to multidecadal sea-level variability. Sea-level fluctuations at periods longer than two months were found to be strongly correlated with air pressure. The seasonal cycle was found to be variable in time. Relative sea-level trends determined from records longer than 30 years are less than 1.5 mm/yr. Crustal movement rates as determined from the tide gauge records are in general of the order ± 1.0 mm/yr. The geological observations have shed light on the fact that a marked variability of crustal movements occurs on both the temporal and spatial scale, and it represents a major contribution to relative sea-level fluctuations. This fact has been verified for the selected sector which belongs to one of the more geodynamically active areas of the Central Mediterranean (Aeolian Archipelago). However, this work has shown that, at least at the tide gauges included in the present study, crustal movements are small compared to the decadal to multidecadal sea-level variability but of the same order as the long-term trend in sea level, thus necessitating a careful monitoring if crustal movement is to be separated from the oceanographic contribution to relative sea-level changes.

Active crustal deformation in the Marmara Sea Region, NW Anatolia, Inferred from GPS measurements

Deformation in the boundary zone between the Anatolian and Eurasian plates has been determined by means of repeated Global Positioning System (GPS) measurements. Three campaigns (1990, 1992, 1994) were carried out around the Marmara Sea. The network lies within the active earthquake belt of NW Anatolia, extending from the western part of the North Anatolian Fault Zone (NAFZ) to the Aegean extensional province. Results of the dense network consisting of 52 sites clearly reveal the westward motion of Anatolia. The rates amount to 17 mm/a ±3 mm/a on average relative to Istanbul, located at the northeastern edge of the Sea of Marmara. Based on the trajectories for the displacements of the observation sites it is concluded that the Marmara Sea forms a transition zone between the Europe fixed region of Thrace/Black Sea (Eurasia) and NW Anatolia. At the western extremity of NW Anatolia the motion bends from W to WSW(247°). The strain field calculated yields an E-W oriented right-lateral strike-slip movement (average 0.12 ppm/a, maximum 0.41 ppm/a) accompanied by NE-SW extension (average −0.12 ppm/a, maximum −0.52 ppm/a) at the eastern end of the Marmara Sea and N-S extension at the Aegean shore line. Comparisons with the directions of T axes derived from 65 fault plane solutions of earthquakes show remarkable agreement.

The GPS strain rate field in the Aegean Sea and western Anatolia

The GPS velocity field of the Aegean Sea and western Anatolia is used to determine crustal deformation strain rates for the period 1988 to 1996. The zone from the Marmara Sea to the North Aegean Trough is associated with strong right-lateral shear motion, with maximum strain rates of 170 nstrain/a. In the northern Aegean Sea the extensional deviatoric axes are oriented NNE-SSW, associated with strain rates reaching 150 nstrain/a, and in western Anatolia varying around N-S with rates of up to 95 nstrain/a. The extensional areas mostly coincide with active graben features. The south-western Aegean Sea is almost strain-free. In general, the areas of high geodetic strain rates are accompanied by distinct seismic clusters whereas the strain-free regions are nearly aseismic.

Global Positioning System (GPS) estimates of crustal deformation in the Marmara Sea region, Northwestern Anatolia

Abstract In the Marmara Sea region the relative motion between the Anatolian and Eurasian lithosphere has been measured by means of the modern techniques of space geodesy using the Global Positioning System (GPS). In order to resolve in detail the kinematic field within the active earthquake belts of Northwestern Anatolia, two GPS campaigns were carried out across a dense network consisting of 45 sites. Preliminary results reveal horizontal westward motion of Anatolia relative to Eurasia of 2.4 cm/y on average that runs along the western end of the North Anatolian Fault Zone. E-W oriented right-lateral strike-slip movement (average 0.06 ppm/y, maximum 0.13 ppm/y) (1ppm/y= 1 μstrain/y and NE-SW extension (average 0.06 ppm/y, maximum 0.21 ppm/y) can be obtained from the calculated strain field. In the west strike-slip motion trending WSW-ENE is observed. These findings are compared to the stress pattern derived from seismology and neotectonics.

THE KEPHALONIA TRANSFORM FAULT AND THE ROTATION OF THE APULIAN PLATFORM : EVIDENCE FROM SATELLITE GEODESY

This paper describes a collaborative GPS-Project to determine recent crustal movements and deformations across the NE Ionian Sea. This region is characterized by rifting in the African Foreland (Pelagian Sea), subduction along the Calabrian and West Hellenic Arcs, rotation of the Italian peninsula and distributed normal faulting in Central Greece. From GPS-campaigns on the Ionian Islands and in NW Greece performed in 1989 and 1991 a kinematic field is deduced which is controlled by the deep trough following the NW coast of Kephalonia island. It is concluded that this prominent bathymetric feature resembles an active fault system (Kephalonia fault) which has the signature of a right-lateral transform fault.