1. Introduction

The Aegean, because of its geotectonic features, constitutes an active geological laboratory where important geological phenomena are in progress. As a result many geoscientists from all over the world are visiting the area in order to observe these phenomena when they actually happen. The reason why so many geotectonic phenomena are accumulated in this area should be sought in the wider geotectonic regime of the region and also in the general rules of tectonics.

The Hellenic area is found in the Mediterranean-Melanesian zone of the continental system of cracks. Greece belongs to the Alpine system and more precisely to the southern sector of the orogenic system of Tethys, the ocean that existed between Eurasia, the single continental area of Europe and Asia to the North, and Godwana, the single continent that included Africa, India and Australia to the south.

2. The Hellenic arc

Basic tectonic characteristic of the Hellenic space is the Hellenic arc. The characteristics of the Hellenic arc are the following: it has a length of approximately 1.500 km, it begins with morphotectonic direction North-Northwest-South-Southeast in Albania, South Yugoslavia and continental Greece, it is bent in direction West-East from Kythira to Crete and afterwards it bends in direction Northeast-Southwest in Dodecanese and Lysia of Southwest Asia Minor up to Isparta and Antalya. This leads to the formation of a right angle between the two parts of the arc.

The Greek arc is the only part of the entire Tethys system, where orogenesis is still in progress and has all the geodynamic features of an under development orogenetic arc. This is owed to the fact that the East Mediterranean Sea on which the Hellenic arc is still moving, is the last residual part of Tethys that has not participated in the evolving Alpine orogenesis and which represents the not deformated margin of the African plate. On the contrary, along the rest of the Alpine system of Tethys, the collision between Eurasia and the various parts of former continent Godwana has been completed. This particularity of the Greek arc gives the possibility of study to various orogenetic phenomena as they are in progress and to create models, which can be compared later with the results from scientists who work on the orogenetic phenomena of the Pacific Ocean and particular of the islands of Japan.

Morphological characteristics of tectonic importance related to the Aegean space are from the South to the North the Mediterranean ridge, the Hellenic arc and the structure of North Aegean. The Mediterranean ridge or Hellenic exterior ridge is a camber of the crust of East Mediterranean parallel to the Hellenic arc. Between the Mediterranean ridge and the Hellenic arc exists a system of trenches that is called Hellenic trench.

3. The lithospheric plates and the seismicity in the Aegean

The insular area of the Aegean Sea as well as the surrounding coastal areas of Greece (and western Turkey), constitute one of the most seismic regions of the planet with intense and most rapid deformation. This had as a consequence the creation of a big number of faults. The determination of an active field of tendencies is based mainly on the mechanisms of genesis of big earthquakes which have happened in Greece approximately during the last 30 years. The results of recent researches on the mechanisms of surface earthquakes genesis led to the conclusion that the region of the Aegean and the surrounding regions can be separated in two big parts: the exterior part of the arc where horizontal tendencies of compaction are practised and consequently reverse faults have been created, and the internal part of the arc where horizontal tensions are practised and regular faults have been created.

The most important geotectonic phenomena which can be observed on the surface of the Earth take place on two zones: the zones of the midoceanic ridge and the continental system of fracture zones.

The Aegean lies in the area where two large lithospheric plates (the African and the Eurasian) are colliding. Consequently, a fundamental cause of the earthquakes and other geological geodynamic phenomena (volcanism etc) in this area could be considered the collision of these two plates. However this area is seismotectonicaly more complicated. Recent researches showed that the earthquakes in the Aegean and the neighbouring areas are caused by: a)the compressional forces that are being pulled in the limits of this area by the three neighbouring lithospheric plates created due to their movement and b)the stretching forces that are practised in the lithosphere of this area caused by reasons that are found in the lithosphere of the Aegean.

3. 1. Tectonic behaviour of the external part of the Aegean

The external part of the Aegean has a different tectonic behaviour than the internal. In the external part compressional forces create inverse cracks, the lithosphere is thick and there is lack of earthquakes of intermediary depth as well as magmatic activity. In the internal part the field of tendencies is stretching and normal faulting can be observed. The lithosphere is thin, intense seismic activity exists, with earthquakes of intermediary depth and volcanism.

Thus the compressional forces in the lithosphere in the wider space of the Aegean are practised by the lithosphere of Eastern Mediterranean, the Apoulian (Adriatic) and by the Turkish plate.

The collision between the Eastern Mediterranean lithosphere, which forms the front part of the African lithospheric plate, and of the lithosphere of the wider area of the Aegean, which is the front part of the Eurasian lithospheric plate, is happening along the known convex part of the Hellenic arc (Zakynthos, southern coasts of Crete, southern Rhodes) with a consequence the subduction of the lithosphere of the Mediterranean under the lithosphere of the Aegean. This movement causes the surface seismic action along the Hellenic arc, the earthquakes of intermediary depth in south Aegean and the delimitation of the volcanic arc (Nisyros, Methana, Santorini, Milos ).

The left rotation of the Apoulian lithospheric plate has as a consequence the compressional forces and the genesis of surface earthquakes along the coasts of Albania and former Yugoslavia.

Finally the movement of the Turkish lithospheric plate to the West causes the big dextral fault of northern Anatolia. Its sectors extend up to the North Aegean.

3. 2. Tectonic behaviour of the internal part of the Aegean

The stretching in the area of the Aegean occurs due to the subduction of the Eastern Mediterranean lithosphere under the South Aegean lithosphere, which produces heat in the upper surface of the subducting lithosphere, due to friction. Consequently transportation currents are created in the asthenosphere and can be found between the subducting lithosphere and the lithosphere of the Aegean. Thus, hot material rises to the lithosphere of the Aegean and when it reaches the seabed it moves horizontally, it cools and sinks again. This hot material creates the non seismic space (without earthquakes) under the lithosphere of the Aegean (Cyclades region). During their horizontal movement, the transportation currents practise horizontal tangential pressure in the lower surface (seabed) of the Aegean consequently the growth of stretch forces in this (expansive forces) at direction N-S, its fracture and the infiltration of the hot material of the asthenosphere. This fracture causes the surface earthquakes that are observed in the wider area of the Aegean. The infiltration of hot material create the volcanic action and the geothermal events of this area.

It should be underlined that in the stretching field of the Aegean also contributes the model of the regression of the sinking area. According to this model the sinking of the cold and dense Mediterranean lithosphere causes traction to the South and dilation of the Aegean plate towards the same direction.

4. Geotectonic evolution of the Aegean

The Aegean, during its geotectonic evolution, was influenced by various stress fields which created or reactivated faults.

From Mid Pleistocene until today, the last 1 million years, the Aegean is found in an extensive stretching phase with main direction North-South, which is also confirmed by the analysis of mechanisms of genesis of surface earthquakes. The North Aegean is dominated by a North-South stretch field.

The main tectonic lines in Lesvos are determined by faults of North-East - Northwest-South-East direction, as well as from faults with East-West direction.

The tectonic lines that have been mentioned above can be considered as a result of big movements during Neogene (25-2,5 million years) that continue until today. They are directly connected with the creation of the north Aegean gap. They correspond to the analysis of mechanisms of genesis of earthquakes, that happened and they continue to happen in the same area. This proves that the above mentioned tectonic structure is still active.

A big part of the faults with direction North-Northeast-South-Southwest have been created during the tectonic movements in Pliocene, while they are reactivated as faults of dextrally horizontal shift during the tectonic movements of the Quaternary. The faults with direction Northwest-Southeast are also created during the enormous tectonic events of the Pliocene, while at the Pleistocene the faults are reactivated. Finally the faults and the cracks with direction East-West are normal faults that were created due to the well known stress fields of the Quaternary.

It is interesting that, although the faults in the wider Aegean area have various directions, the direction of the cracks on these faults have almost constantly direction North-South, an attribute which supports the opinion that the lithosphere of the Aegean is extended along the same direction. The only exception is observed only in the region of the basin of North Aegean and North Turkey (Anatolia fault), where a dextral movement prevails, creating dextral faults with small normal or inversion component.

Even if the relative movement of the Aegean in comparison to Eurasia is South-West with a speed of approximately 4cm/year, the relative dilation is approximately 1cm/year. The entire region of the Aegean is deformed by 3 cm/year in its northern part, by 1-2 cm/year in central mainland Greece and by 2 cm/year in southern Aegean along the ditch of Anatolia-North Aegean.

The intermediate depth earthquakes genesis mechanisms in the southern Aegean show that these earthquakes are caused by reverse faults, result that is in agreement with the subduction of the Mediterranean lithosphere under the lithosphere of the Aegean along the Hellenic arc. Another result of this subduction of the African plate under the microplate of the Aegean is also the creation of many volcanoes of calc alkaline constitution, which creates the volcanic arc of south Aegean. This volcanoes are Gyali, Nisyros, Kos, Santorini, Patmos, Milos, Antiparos, Krommyonia, Methana, Lichades, Kammena Vourla, Psathoura. The oldest of these volcanoes are 5 million years and some of them are active until today.