C. B. Papazachos

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.

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.

Empirical Peak Ground-Motion Predictive Relations for Shallow Earthquakes in Greece

In the present article new predictive relations are proposed for the peak values of the horizontal components of ground acceleration, velocity, and displace- ment, using 619 strong motion recordings from shallow earthquakes in the broader Aegean area, which are processed using the same procedure in order to obtain a homogeneous strong motion database. The data set is derived from 225 earthquakes, mainly of normal and strike-slip focal mechanisms with magnitudes 4.5 M 7.0 and epicentral distances in the range 1 km R 160 km that have been relocated using an appropriate technique. About 1000 values of peak ground acceleration (PGA), velocity (PGV), and displacement (PGD) from horizontal components were used to derive the empirical predictive relations proposed in this study. A term ac- counting for the effect of faulting mechanisms in the predictive relations is intro- duced, and the UBC (1997) site classification is adopted for the quantification of the site effects. The new relations are compared to previous ones proposed for Greece or other regions with comparable seismotectonic environments. The regression anal- ysis showed a noticeable (up to 30%) variance reduction of the proposed relations for predicting PGA, PGV, and PGD values compared to previous ones for the Aegean area, suggesting a significant improvement of predictive relations due to the use of a homogeneous strong motion database and improved earthquake parameter infor- mation.

Accelerated Preshock Deformation of Broad Regions in the Aegean Area

Abstract—Twenty-four regions where accelerating deformation has been observed for a few decades before corresponding strong (M = 6.0–7.5) mainshocks are identified in the broader Aegean area. To a first approximation these preshock regions have elliptical shapes and the radius, R (in km), of a circle with an area equal to the corresponding ellipse is related to the moment magnitude, M, of the mainshock by the equation:¶log R = 0.42 M-0.68.¶The dimension of each preshock region is about seven to ten times larger than the rupture zone (fault length) of the corresponding mainshock. The time variation of the cumulative Benioff strain was satisfactorily fitted by a power-law relation, which is predicted by statistical physics if the mainshock to which accelerating strain rates leads is considered as a critical point. The duration, t (in years), of the accelerating Benioff strain release period is given by the relation:¶¶log t = 5.94-0.75 log sr¶where sr is the mean Benioff strain rate release (per year for 104 km2) in the preshock region calculated by the complete available data (M≥5.2) for the entire instrumental period (1911–1998). The importance of identifying and investigating such regions for better understanding the dynamics of the active part of the lithosphere as well as for earthquake prediction and time-dependent seismic hazard assessment is discussed.

Active deformation on the shallow part of the subducting lithospheric slab in the southern aegean

Abstract Seismic strain rates estimated from volume-averaged moment tensor data are combined with the strike, dip angle and extent of the subducting lithosphere to study the slab deformation in the shallow part (40–100 km) of the southern Aegean Wadati—Benioff zone. The upper surface of this zone has an amphitheatrical shape, strikes parallel to the sedimentary arc and dips at a low angle (14°) from the outer (convex) side to the inner (concave) side of the Hellenic arc, that is, from the eastern Mediterranean Sea to the Aegean Sea. The fault plane solutions used in the summation, indicate reverse faulting with a considerable strike-slip component and T axis plunging more steeply than the subducting lithosphere. The results show that the subducting slab is in a state of down-dip extension which occurs along the dip of Wadati—Benioff zone at a rate of about 1 cm p.a., while a fast shortening, at a rate of about 3 cm p.a., occurs parallel to the general trend of the Hellenic arc.

Rayleigh wave group velocity tomography in the Aegean area

Data from a large-scale experiment which took place in Greece during the period January–July 1997 have been used to investigate the structure of the Aegean area using surface waves. During this experiment, 30 seismic broadband instruments were deployed throughout the whole Greek area. Additional data during the period 1996–2000 from other temporary networks have been included in the dataset. One hundred eighty-five events with magnitudes 4.0VMwV5.5 recorded by these stations have been collected and processed. The individual dispersion curves of the group velocity of Rayleigh waves for each source-station path have been calculated, producing more than 700 paths covering the studied region. These curves have been used to determine Rayleigh group velocity maps using a 2D-tomography method. On the basis of a regionalization of the dispersion measurements, local averaged dispersion curves have been obtained and non-linearly inverted to obtain models of shear-wave velocity versus depth. Since the dispersion curves in the period range 5 sVTV30 s are mostly affected by the crustal structure, the model velocities are estimated down to a depth of approximately 35–45 km. The results from the non-linear Hedhehog inversion as applied to a few local dispersion curves show a crustal thickness of approximately 32 km for the Northern Aegean Sea, and a relatively thin crust of approximately 22–24 km for the Southern Aegean Sea. D 2002 Elsevier Science B.V. All rights reserved.

Seismological and GPS evidence for the Aegean‐Anatolia Interaction

Seismological and GPS data are used in order to examine the seismotectonic setting and the plate motions in the Eastern Mediterranean. The main objective is to reevaluate the recently proposed idea of a single Anatolia-Aegean microplate, which performs a rigid body rotation about an Euler pole located in northern Africa. Both data sets show a more complicated setting. South of the North Anatolia Fault (NAF) the plate motion of Anatolia relative to Europe can be adequately described by a simple rigid body rotation with a dominant East-West component and an average velocity of ∼22mm/yr. However, both types of data show a significant increase of the deformation velocities in the Aegean area (30–35mm/yr) and exhibit a much larger N-S component (up to 20mm/yr) than the one predicted from rigid body rotation. This deformation pattern is more consistent with models which consider the Aegean as a separate microplate.

Anisotropic radiation modelling of macroseismic intensities for estimation of the attenuation structure of the upper crust in Greece

A method is suggested for the analysis of macroseismic intensity data in order to accurately determine an “average” attenuation structure of the upper part of the crust in an area. The method is based on a model which assumes that the observed intensities depend on source properties (radiation pattern, size, focal depth), geometrical spreading and anelastic attenuation. The method is applied to 13,008 intensity values, observed in corresponding sites of Greece and grouped (in 4228 groups), according to their spatial clustering in order to diminish observational errors and site effects. An average intensity attenuation coefficient,c=−0.0039±0.0016, corresponding to a quality factor, Q=350±140, is determined for the upper 20 km of the crust in this area. This value is relatively low, in good agreement with the relatively high heat flow and high seismic activity of this area. A byproduct of the present study is the determination, for each earthquake, of a macroseismic focal depth and of a “macroseismic size,” which is strongly correlatted with both the earthquakes magnitude and its seismic moment determined by independent methods.

Rates of active crustal deformation in the Aegean and the surrounding area

Abstract Active crustal deformation is calculated for 26 zones of shallow seismicity in the Aegean and the surrounding regions. The data analysis is based on a procedure developed in a previous paper (Papazachos and Kiratzi, 1993). This procedure takes advantage of all the available historical and instrumental data for the calculation of the “size” of the deformation in a seismic zone and of all reliable fault plane solutions which are available for the broader seismic belt for the determination of the “shape” of the deformation. The Aegean and its surroundings has been divided into 11 such seismic belts which may consist of one or more seismic zones that share earthquakes with similar focal mechanisms. This data analysis showed that along the coastal region of Albania, Yugoslavia and western Greece the deformation is taken up by compression in a direction perpendicular to the coast line (47°E) at a rate of about 2 mm/a. In the Ionian islands (Leukada, Cephalonia, Zante) compression occurs at a rate of 10 mm/a in an almost EW direction (N83°E) and extension at a rate of 11 mm/a in an almost NS direction (N174°E). Along the convex side of the Hellenicarc (south of Peloponese, Crete, Rodos), the upper crust is compressed at a rate of about 6 mm/a in a direction N34°E. In the Aegean Sea and the surrounding lands (mainland of western and northern Greece, southern Yugoslavia and Bulgaria, western Turkey) the seismic deformation is taken up by an almost NS extension at an average rate of 5 mm/a. In northwestern Anatolia and the northern Aegean fault zones deformation is controlled by the westward movement along the North Anatolian fault. Northern Anatolia is undergoing a N115°E compression at a rate of 22 mm/a which is relieved by a N25°E extension at a rate of 19 mm/a, and the Northern Aegean is undergoing EW compression at a rate of 16 mm/a and NS extension at a rate of 8 mm/a. The vertical crustal thickening along the external compressional zones ranges from 0.2 to 0.5 mm/a with an average of 0.3 mm/a and the vertical crustal thinning in the inner back-arc extensional area ranges from 0.1 to 2.4 mm/a with an average equal to 0.8 mm/a. In the western part of the area and between the external compressional field and the internal extensional field, a belt exists where the deformation is expressed as extension at an average rate of 1.6 mm/a in a N112°E direction.

Rupture zones in the Aegean region

Rupture zones of strong shallow earthquakes can be investigated by field observations of surface fault traces (SF), accurate location of spatial clusters of aftershocks or other small earthquakes (CL), reliable fault plane solutions (FP) and information on the spatial distribution of sites with high ( I VIII) macroseismic intensities (MA). The validity of these four techniques was successfully tested by comparing the results of their application to several cases of strong shallow earthquakes. These techniques were then applied to spatially define the rupture zones of 150 strong.M 6:0/ shallow earthquakes in the Aegean and surrounding area. The type of faulting was determined for all these zones by the use of reliable fault plane solutions, which are available for this area. The orientation of most of these rupture (fault) zones is in good agreement with the presently acting stress field with some exceptions which are attributed to the rupture of pre-existing faults. The tectonic implications of these zones are discussed.