The distribution of seismicity, faulting pattern and its effect on local geomorphology is examined for the islands of Lemnos, Aghios Efstratios, Lesvos, Chios, Samos and Ikaria of north-eastern ...Aegean Sea, Greece. The main active faults on each island are described in terms of their geometrical characteristics and geomorphology. Faults that comply with specific criteria (geological age, effect on relief, their geometrical relationship to the active stress field) have been characterized as active. We evaluated and reviewed published information, augmented with new field data for onshore faults, while the effects of faulting on the sea-floor and their probable association with recorded earthquakes were used to determine offshore faulting. The relation of active faulting to the stress pattern has been examined as well. It is shown that as the deformation changes gradually from transtensional in the north to extensional in the south, so does the active faulting pattern. The effect of the westernmost splays of the North Anatolian Fault Zone, the largest of which is the~300km long, North Aegean Trough, is profound due to their close vicinity, causing shearing in good agreement with the modeled principal displacement zone deformation pattern. Faulting in the area is controlled by the distance from the main dextral principal displacement zones: the northern part of the area is directly affected by the North Anatolian Fault Zone and its splays, while this effect gradually weakens in the central and southern areas. The geomorphology responds to this faulting, causing the formation of fault-parallel gulfs near Lesvos and Lemnos and fault-defined shorelines in the rest of the islands. Ikaria exhibits a notable fault-controlled tilted topography as the result of footwall uplift.
► We identify the main active fault zones of the northeastern Aegean islands (Greece) ► Faulting pattern changes gradually from North to South ► Faults may be explained as shears of the splayed North Anatolian Fault Zone ► Focal mechanisms are in good agreement with this deformation model ► The geomorphology of the islands is largely controlled by neotectonics
We combine probabilistic and deterministic approaches in an attempt to tackle a challenging topic in engineering seismology, that of the prediction of strong ground motion in urban areas located ...close to active faults, with sparse modern seismicity. The case we study is the city of Xanthi in northern Greece and we generate bedrock synthetic time series to be used as input motions into site-specific site effect analysis. We re-assess the seismic hazard using the probabilistic method and then perform hazard deaggregation in order to define realistic earthquake scenarios, which we examine in a deterministic way. The scenarios examined are: (a) a reference earthquake of M6.7, return period 100 years and at ~30 km distance from the city and (b) a reference earthquake of M5.8, return period 475 years and at ~3 km distance from the city of Xanthi. To compute synthetic time series for the distant source, we apply the stochastic method for finite faults. For the near-fault earthquake scenario we adopt a hybrid deterministic-stochastic approach that includes the semi-analytical method implemented in the COMPSYN code for the low frequency (~<3 Hz) part of the spectrum and the stochastic method for the high-frequency (~>3 Hz). The results of the probabilistic and the deterministic approaches converge to a satisfactory degree.
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.
Gephart & Forsyth's method has been applied to estimate stress orientations from earthquake fault-plane solutions of the westernmost North Anatolian Fault zone (NAFZ), its westward continuation in ...the Northern Aegean Sea, where it interacts with the normal faulting of central Greece, and of the western coastal parts of Anatolia. In total, 163 fault-plane solutions were used to invert for the stress tensor of 9 seismotectonic subregions. Magnitudes range from 1.1 to 7.4. All events have shallow focal depths; most occurred in the uppermost 15 km and very few in the 15 to 40 km depth range. The region studied is characterized by strike-slip (NAFZ), by transtension (Central Greece and western Anatolia) and by a strong signature of transpression in the Marmara area. From NAFZ to central Greece, the σ3 axis changes, continuously and smoothly, its orientation from N35°E in the westernmost NAFZ to nearly North–South in continental central Greece, and the maximum horizontal stress, σHmax changes from WNW–ESE to ∼E–W. The total angle of lateral variation in the direction of either σHmax or σ3 is ∼40°. In central western Anatolia the orientation of σ3 is NNE–SSW (N10°E) while in southern western Anatolia it is NNW–SSE (N33°W). In both cases the development of the major grabens (Simav, Gediz, Menderes) is in agreement with the resolved stress tensor. At the termination of the NAFZ against central Greece the regime is complex. The dextral shear motion transferred from the east crosses the Pilion peninsula and northern Evia Island where it interacts with the major normal faults of continental Greece. These faults have variable orientations. The E–W trending Quaternary fault system and the NW–SE trending fault system, which is inherited from the past. The latter faults under the presently acting stress field are reactivated either as strike-slip faults with sinistral strike-slip component (when oriented at small angle in respect to σ3) or as normal faults with small sinistral strike-slip component (when oriented at high angle in respect to σ3). The occurrence of the recent 2001 July 26 Skyros event, which was clearly connected to a NNW–SSE fault with sinistral strike-slip motion, provides strong seismological evidence that the old structures can equally well be reactivated under the presently active stress field.
The January 2010 Efpalio earthquake sequence provides some key elements to enhance our view on the western Corinth Gulf tectonics. The sequence lasted almost six months, and included two Mw>5 strong ...events, both exhibiting normal faulting along ~E–W trending planes. This paper attempts to construct a unified seismotectonic model of the sequence jointly interpreting earthquake locations, moment-tensors and slip inversions in terms of the possible activated fault planes. Previous studies have connected the prevailing microseismic activity to a major low-angle, north-dipping structure under the Corinth Gulf and the Efpalio sequence favors such a general trend. Moreover, it clearly shows the significance of the shallow activity, so far less recognized, and possibly connected to the relatively steep faults outcropping on the northern coast. The first 18 January 2010 Mw>5 event had almost no on-fault aftershocks and most likely it occurred on a 55° south-dipping nodal plane. The early off-fault aftershocks formed two clusters roughly E–W trending, both of which are connected with normal faulting. Cross-sections revealed that the northernmost cluster is connected with a north-dipping structure, where the second 22 January 2010 Mw>5 event occurred. In addition, the very shallow parts of the faults (<4km) were mainly aseismic, probably due to their creeping behavior. Interestingly, both clusters, at their western and eastern ends, are bounded by NE-SW trending strike-slip faults, a pattern previously observed in normal-faulting structures in Greece. This observation further invokes the role of transfer faults in the western termination of Corinth Gulf, which provide the link with regional structures, such as the Trichonis and Rion-Patras fault systems. Most of all the 2010 Efpalio sequence enhanced the complex mechanical interactions within the Corinth Gulf fault network, with many earthquake generating cluster centers, an observation which has strong implications for the seismic hazard of this densely populated region.
► We study an earthquake sequence in Corinth Gulf, intra-continental rift. ► High-quality seismic data are processed by a variety of methods and codes. ► Hypocenters and centroids are linked by a method recently developed by the authors. ► Coulomb stress analysis and GPS data provide further constraints on the model. ► Seismotectonic model links normal and strike-slip faults with a low-angle structure.
The 8 January 2013 Mw5.8 earthquake sequence south of Lemnos Island (East Aegean Sea) is investigated. The focal mechanisms of the strongest events, obtained by waveform modelling, together with the ...HypoDD relocated epicentres, clearly show the activation of a dextral strike-slip fault, no more than 14km in length, trending N60°E, and extending in depth from approximately 3 to 25km, with the best determined focal depths in the range 3 to ~13km. The distribution of slip onto the fault plane shows a single patch of 7×8km2, where the average and the peak slip are equal to 30cm and 130cm, respectively. The locus of the peak slip is well resolved to be ~2.5km away from the hypocentre, towards SW. The slip model was used to forward model the distribution of Peak Ground Velocity (PGV in cm/s) in the near source. The PGV contours imply that if any directivity is associated with this event, this is towards SW. The sequence in study, is located in the eastward extension of the Agios Efstratios Fault Zone, which was the locus of the February 19, 1968 Mw7.2 earthquake, which produced surface rupture on the island itself. The Agios Efstratios Fault Zone terminates off shore, south of Lemnos Island. The 2013 sequence is not connected, in a strict sense, with the Agios Efstratios Fault. It indicates rupture of another fault, approximately parallel to it, and the characteristics of the sequence are in accordance with the regional tectonics and previous knowledge. The Coulomb stress change associated with the mainshock, is also examined to evaluate any significant enhance of stresses along the Agios Efstratios Fault Zone.
•Evidence for strike-slip faulting in East Aegean Sea•Confirmation of previews knowledge•Implication for westward rupture directivity
We investigate the properties of the April 2007 earthquake swarm (Mw 5.2) which occurred at the vicinity of Lake Trichonis (western Greece). First we relocated the earthquakes, using P- and S-wave ...arrivals to the stations of the Hellenic Unified Seismic Network (HUSN), and then we applied moment tensor inversion to regional broad-band waveforms to obtain the focal mechanisms of the strongest events of the 2007 swarm. The relocated epicentres, cluster along the eastern banks of the lake, and follow a distinct NNW–ESE trend. The previous strong sequence close to Lake Trichonis occurred in June–December 1975. We applied teleseismic body waveform inversion, to obtain the focal mechanism solution of the strongest earthquake of this sequence, i.e. the 31 December 1975 (Mw 6.0) event. Our results indicate that: a) the 31 December 1975 Mw 6.0 event was produced by a NW–SE normal fault, dipping to the NE, with considerable sinistral strike-slip component; we relocated its epicentre: i) using phase data reported to ISC and its coordinates are 38.486°N, 21.661°E; ii) using the available macroseismic data, and the coordinates of the macroseismic epicentre are 38.49°N, 21.63°E, close to the strongly affected village of Kato Makrinou; b) the earthquakes of the 2007 swarm indicate a NNW–SSE strike for the activated main structure, parallel to the eastern banks of Lake Trichonis, dipping to the NE and characterized by mainly normal faulting, occasionally combined with sinistral strike-slip component. The 2007 earthquake swarm did not rupture the well documented E–W striking Trichonis normal fault that bounds the southern flank of the lake, but on the contrary it is due to rupture of a NW–SE normal fault that strikes at a ∼
45° angle to the Trichonis fault. The left-lateral component of faulting is mapped for the first time to the north of the Gulf of Patras which was previously regarded as the boundary for strike-slip motions in western Greece. This result signifies the importance of further investigations to unravel in detail the tectonics of this region.
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
U
33 of the velocity tensor.
Stochastic finite-fault simulation is used to simulate the acceleration time histories of two intermediate depth (
h∼90
km) earthquakes, which occurred on 30 May 1990 in Vrancea (Romania) and on 22 ...January 2002 near Karpathos island in Greece. Site amplification functions were used for all stations included in the simulation process. For the Romanian earthquake, peak acceleration values were observed at the more distant from the epicentre stations. In Romania, strong anisotropy in
Q-values was observed along the NE–SW direction (along strike of the fault) compared to the normal to the strike of the fault direction. Specific site corrections were required for the station in Bucharest, employing the
H/
V ratio technique, to refine the site-specific amplification function and achieve better match of the simulated spectra. In the Aegean Sea region, average values for attenuation and site amplification functions were used for all stations with overall satisfactory results. Any unmatched discrepancies between observed and simulated spectra would require further accurate knowledge of the site characteristics.
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