Anastasia Kiratzi

A DETAILED STUDY OF THE ACTIVE CRUSTAL DEFORMATION IN THE AEGEAN AND SURROUNDING AREA

Abstract A detailed study of the active crustal deformation in the Aegean sea and surrounding area is presented. The estimation of the deformation velocities is not performed in broad seismic zones but in seismogenic sources of limited spatial extent, where the seismic energy is practically released, in order to describe the spatial variations of the components of the strain rate tensor. The deformation analysis is based on previous work where the seismicity and the available focal mechanisms are used separately for estimating the rate and shape of the deformation, respectively. The calculations were performed for 63 seismogenic sources identified in the Aegean sea and surrounding area which belong to eight belts with an almost uniform orientation of the stress field. Along the coasts of Albania and northwestern Greece (up to Leukada island) the collision of the Adria plate with Eurasia results in a compressional velocity of about 4 mm/yr in a direction (N49°E) almost normal to the coast line. The dextral movement with almost 3 cm/yr along the Cephalonia transform fault with strike N45°E is expressed as an almost equal extension and compression in N-S and E-W direction, respectively. Along the convex (outer) part of the Hellenic arc a crustal shortening of 1.3 cm/yr is observed in a mean direction N34°E. The direction of this shortening is almost constant from the southern Ionian islands (Zakynthos) up to Rodos while the shortening rate does not significantly change from the outer sources to the inner ones. The Aegean sea and surrounding area is characterised by an extensional deformation. The direction of the maximum extension in the northern part of this area shows an anticlockwise rotation from east to west. Thus, this extension has a NNE direction in northwestern Turkey (N11°E), a NNW direction in central and northern Greece as well as along the southern Aegean volcanic arc and its extension to southwestern Turkey (N12°W) and a WNW direction in the westernmost part of this extensional area (N72°W). The mean value of the extensional velocities in this area is 5 mm/yr. The northern part of Asia Minor and of the Aegean sea is characterised by the dextral strike-slip movement of about 2 cm/yr of the North Anatolian fault and its continuation in the northern Aegean, where this movement gradually decreases to about 6 mm/yr. The vertical crustal (upper 10 km of the crust) thickening is about 2 mm/yr along the convex side of the Hellenic arc and the vertical crustal thinning is about 1 mm/yr in the Aegean sea. These values were obtained from the component U33 of the velocity tensor.

THE CEPHALONIA TRANSFORM FAULT AND ITS EXTENSION TO WESTERN LEFKADA ISLAND(GREECE)

The central area of the Ionian Islands is dominated by the existence of a major tectonic structure called the Cephalonia Transform Fault (CTF). Its main part (Cephalonia segment) has been identified by previous work based on the spatial distribution of earthquake foci, fault plane solutions of strong earthquakes, active tectonics, structural studies and geodetic measurements. This part (Cephalonia segment) which exhibits strike-slip motion with a thrust component, strikes in a northeast direction, dips to the southeast and has a length of ∼90 km. In the present paper information concerning new fault plane solutions, orientation of isoseismals, sea bottom topography and recent GPS results are used to further check the properties of this southern part of the CTF and to explore its northeastward prolongation to Lefkada Island. It is shown that the CTF is extended to the western coast of Lefkada. This northern branch (Lefkada segment) of the CTF which is also characterized by strike-slip motion with a thrust component, strikes in a north-northeast direction, dips to the east-southeast and has a length of ∼40 km. These two segments of the CTF form a major kinematic boundary where the slip rate is 2–3 cm/yr.

Focal mechanisms of shallow earthquakes in the Aegean Sea and the surrounding lands determined by waveform modelling: a new database

Abstract Source parameters for 40 earthquakes with epicentres in the Aegean Sea and the surrounding lands have been determined using P and SH body waveform inversion of digital waveforms from the Global Seismograph Network (GSN). These results are combined with 145 previous solutions, 41 of which were determined by first motion polarities and 104 by waveform modelling. Thus, a new updated database of source parameters for earthquakes for the period 1953–1999 is available. These fault plane solutions confirm our previous knowledge about the seismotectonics of the Aegean Sea and the surrounding lands. They also provide strong evidence that the western part of the Peloponnese (Gulf of Patras to Gulf of Kyparissia) is mainly deforming by strike-slip faulting, parallel in strike to the Cephalonia Fault and are in favour of the models that suggest a connection of the North Anatolian Fault (NAF) zone with the transform zone of the Ionian Islands. The sedimentary arc (inner part of the Hellenic trench) is deforming by normal faulting with the T-axes trending ∼E–W. We did not determine any fault plane solutions at the eastern part of the Hellenic arc indicating pure strike-slip motions as expected from the presence of the Pliny and Strabo trenches. However, we did determine solutions that indicate normal faulting combined with a considerable strike-slip component. In the central Aegean Sea the newly determined focal mechanisms support the conclusion that the area, extending from the eastern coasts of Evia up to the coasts of Turkey, is exhibiting a clear strike-slip character. Thus, the effect of the propagating tip of the North Anatolian fault into the Aegean is very clearly pronounced. The focal mechanisms in the back-arc Aegean area confirm the existence of normal faulting with T-axes trending NNE–SSW in western Anatolia and NNW–SSE in Greece. A zone showing NNE–SSW extension that runs parallel to Northern Aegean branch of the NAF zone up to the Corinth Gulf is depicted from the mechanisms of few earthquakes.

Seismic faults in the Aegean area

Abstract Reliable fault plane solutions of shallow earthquakes and information on surface fault traces in combination with other seismic, geomorphological and geological information have been used to determine the orientation and other properties of the seismic faults in the Aegean and surrounding area. Thrust faults having an about NW-SE strike occur in the outer seismic zone along western Albania-westernmost part of mainland of Greece-Ionian Sea-south of Crete-south of Rhodes. The inner part of the area is dominated by strike-slip and normal faulting. Strike-slip with an about NE-SW slip direction occurs in the inner part of the Hellenic arc along the line Peloponnesus-Cyclades-Dodecanese-southwest Turkey as well as along a zone which is associated with the northern Aegean trough and the northwesternmost part of Anatolia. All other regions in the inner part of the area are characterized by normal faulting. The slip direction of the normal faults has an about SW-NE direction in Crete (N38°E) and an about E-W direction (N81°E) in a zone which trends N-S in eastern Albania and its extension to western mainland of Greece. In all other regions (central Greece-southern Yugoslavia and Bulgaria, western Turkey) the slip of the normal faults has an about N-S direction.

Active crustal deformation from the Azores triple junction to the Middle East

Abstract The co-seismic crustal deformation along the Africa-Eurasia, Eurasia-Arabia plate boundary, from the Azores triple junction (30°W) up to the Karliova triple junction (41°E) is examined, based on the moment tensor summation of mainly post-1964 earthquakes with MS ⩾ 5.0. Strain rates range from 10−8/yr to 10−6/yr, while values exceeding 10−7/yr were calculated for the back-arc Aegean area and for the North Anatolian Fault Zone. A remarkable stability in the azimuth of the component of compression and of extension is observed in the area that extends west of Italy (17°E) up to the Azores triple junction. Along this region the deformation is taken up by compression along a mean direction of N140°, with rates from 1.1 to 2.8 mm/yr, and by extension along a mean direction of N50°, with rates from 2.4 to 12.0 mm/yr. At the Gloria fault and in the surrounding area dextral strike-slip motion occurs at a rate of 11 mm/yr. From Sicily up to the eastern end of the Hellenic Arc the crust is compressed at azimuths in the range N20–N31°, with an average value of 26 ± 5°. This compression is occurring at a rate of 1–21 mm/yr with an average value of 9 ± 8 mm/yr. In the back-arc Aegean area extension is occurring along a mean azimuth of N156° with a rate of 5.2 ± 3.8 mm/yr. The average shortening rate along the convex side of the Hellenic arc is 12 mm/yr. From the eastern end of the North Anatolian Fault (41°E), up to the Northern Aegean trough, extension along N68–N179° and compression along N158-N89° occurs, which results in dextral strike-slip motion. The rate of extension reduces from east to west from 30 mm/yr to 5 mm/yr, while the rate of compression reduces from 17 mm/yr to 5 mm/yr. The northwards motion of Arabia results in a N31° trending compression at a rate of 7 mm/yr along the East Anatolian Fault Zone.

A study on the active crustal deformation of the north and east anatolian fault zones

Abstract Active crustal deformation in the North Anatolian Fault Zone (NAFZ) and the East Anatolian Fault Zone (EAFZ), based on seismicity data, is examined. The results show that the deformation in the NAFZ is taken up by extension on an azimuth of 59° at a rate of 27 mm/yr and as compression on an azimuth of 149° and a rate of 20 mm/yr. The average dextral lateral motion on the NAF is about 23 mm/yr. This motion in the western part of the NAF is 16 mm/yr; while it increases in the eastern part of the fault to 27 mm/yr. In the EAFZ the deformation is taken up as nearly N-S compression (N7°E) at a rate of 5 mm/yr and as nearly E-W extension (N97°) at a rate of about 9 mm/yr. The average sinistral-lateral motion has a rate of about 6 mm/yr. The expected slip rates, from plate motions, along the North Anatolian Fault are of the order of 38 mm/yr. The predicted slip rates for East Anatolian Fault are 19 mm/yr or 29 mm/yr. The seismicity of the last 140 years can account for the expected deformation along the NAFZ: for the EAFZ, however, it is possible that part of the total deformation has been expressed aseismically.

Shear wave anisotropy in the upper mantle beneath the Aegean related to internal deformation

Seismic anisotropy, deduced from SKS splitting measured at 25 stations installed in the Aegean, does not show a homogeneous pattern. It is not restricted to the North Anatolian Fault but is distributed over a region several hundreds kilometers wide. Little anisotropy is observed in continental Greece or along the Hellenic arc; however, significant anisotropy is observed in the north Aegean Sea. Large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy. Our results, both in fast polarization directions and in values of delay time, do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present-day Aegean motion relative to an absolute frame. In contrast, the direction of fast polarization and the magnitude of delay times correlate well with the present-day strain rate observed at the surface deduced from both geodetic measurements and seismicity. This anisotropy is not horizontally restricted to major surface faults but is spread over a wide region.

Regional Focal Mechanisms for Earthquakes in the Aegean Area

The distribution of the focal mechanisms of the shallow and intermediate depth (h>40 km) earthquakes of the Aegean and the surrounding area is discussed. The data consist of all events of the period 1963–1986 for the shallow, and 1961–1985 for the intermediate depth earthquakes, withMs≥5.5. For this purpose, all published fault plane solutions for each event have been collected, reproduced, carefully checked and if possible improved accordingly. The distribution of the focal mechanisms of the earthquakes in the Aegean declares the existence of thrust faulting following the coastline of southern Yugoslavia, Albania and western Greece extending up to the island of Cephalonia. This zone of compression is due to the collision between two continental lithospheres (Apulian-Eurasian). The subduction of the African lithosphere under the Aegean results in the occurrence of thrust faulting along the convex side of the Hellenic arc. These two zones of compression are connected via strike-slip faulting observed at the area of Cephalonia island. TheP axis along the convex side of the arc keeps approximately the same strike throughout the arc (210° NNE-SSW) and plunges with a mean angle of 24° to southwest. The broad mainland of Greece as well as western Turkey are dominated by normal faulting with theT axis striking almost NS (with a trend of 174° for Greece and 180° for western Turkey). The intermediate depth seismicity is distributed into two segments of the Benioff zone. In the shallower part of the Benioff zone, which is found directly beneath the inner slope of the sedimentary arc of the Hellenic arc, earthquakes with depths in the range 40–100 km are distributed. The dip angle of the Benioff zone in this area is found equal to 23°. This part of the Benioff zone is coupled with the seismic zone of shallow earthquakes along the arc and it is here that the greatest earthquakes have been observed (Ms∼8.0). The deeper part (inner) of the Benioff zone, where the earthquakes with depths in the range 100–180 km are distributed, dips with a mean angle of 38° below the volcanic arc of southern Aegean.

Source parameters of some large earthquakes in Northern Aegean determined by body waveform inversion

AbstractAverage source parameters for three large North Aegean events are obtained from body wave inversion for the moment tensor. The parameters for the events are as follows: The events exhibit dextral strike-slip faulting with theT axis striking NS and nearly horizontal, implying extension in this direction. The focal mechanisms obtained are in agreement with the seismotectonic regime of the North Aegean. It is known that the region is tectonically controlled by the existence of the strike-slip Anatolian fault and its westward continuation in the Aegean, as well as the NS extension the whole Aegean area undergoes.The components of the moment tensor show that the region is dominated by compression in the EW direction which is encompassed by extension in the NS direction. All the events were found to be shallow (≤10 km) with a source time function of approximately 8 s duration and small stress drop values.The teleseismic long period verticalP-waves exhibited distortions, that could be attributed to lateral inhomogeneities in the source structure or more probably to a nonflat water-crust interface.

Surface Fault Traces, Fault Plane Solution and Spatial Distribution of the Aftershocks of the September 13, 1986 Earthquake of Kalamata (Southern Greece)

A shallow earthquake ofMS=6.2 occurred in the southern part of the Peloponnesus, 12 km north of the port of the city of Kalamata, which caused considerable damage. The fault plane solution of the main shock, geological data and field observations, as well as the distribution of foci of aftershocks, indicate that the seismic fault is a listric normal one trending NNE-SSW and dipping to WNW. The surface ruptures caused by the earthquake coincide with the trace of a neotectonic fault, which is located 2–3 km east of the city of Kalamata and which is related to the formation of Messiniakos gulf during the Pliocene-Quaternary tectonics. Field observations indicate that the earthquake is due to the reactivation of the same fault.A three-days aftershock study in the area, with portable seismographs, recorded many aftershocks of which 39 withMS≥1.7 were very well located. The distribution of aftershocks forms two clusters, one near the epicenter of the main shock in the northern part of the seismogenic volume, and the other near the epicenter of the largest aftershock (MS=5.4) in the southern part of this volume. The central part of the area lacks aftershocks, which probably indicates that this is the part of the fault which slipped smoothly during the earthquake.