The May 24, 2014 (Mw6.8) earthquake occurred in the North Aegean Trough (NAT), along a large transcurrent fault zone, the North NAT fault zone (GRCS290). The broader area is rich in strong events since historical times; however, two fault segments were defined in this zone before the recent event: the Saros (GRIS290) and the Samothraki SE (GRIS291) faults. The last earthquake filled in a gap at the westernmost part of the fault zone.
The large tectonic structure of the NAT is not confined only by this fault zone, but it is represented by a crustal-scale negative flower structure affecting the sea floor between Kallipoli (Gelibolu) peninsula, Lemnos and Imvros (Gökçeada) Islands to the south and Samothraki Island to the north. Thus, another fault zone exists at the southern border of the graben, the South NAT fault zone (GRCS800) which probably extends northeastwards cross cutting the Kallipoli (Gelibolu) peninsula.
At the eastern side of the NAT, another regional-scale tectonic structure occurs, i.e. the North Aegean Basin (NAB). The NAB is situated between the Chalkidiki peninsula, Thessalian coast and Sporades Islands. Along strike, it is bordered by two major fault zones which are mainly characterized by oblique-slip kinematics: the South Chalkidiki offshore fault zone (GRCS280) and North Aegean Basin (GRCS810). Along the former, one ISS (the Athos fault, GRIS282) has been recognized in GreDaSS, but due to its doubtful connection with the November 8, 1905 earthquake it is not included in the stress transfer modelling. Along the latter fault zone, two fault segments have been recognized: Segment A (GRIS810) to the east and Segment B (GRIS811 to the west, related with the August 6, 1983 and the January 18, 1982 earthquakes respectively.
One fault of different seismotectonic behaviour occurs along the northern coast of Samothrace island (North Samothraki, GRIS288) and is associated with the February 9, 1893 earthquake. From a mechanical point of view, this fault can be interpreted as a normal dip-slip secondary structure of the NAT.
The parametric data of the aforementioned active tectonic structures can be found in GreDaSS.
When an earthquake occurs, the average value of stress on the fault that slipped is reduced, while stress is increased at the tips of its plane and at sites around it. An immediate result of this stress transfer is the generation of aftershocks. The accumulation and release of stress on a fault are controlled not only by the regional stress field and rock property, but also by its surrounding faults. Earthquake will cause stress to increase or decrease at other faults, and thereby trigger or delay earthquake on them; this is called the effect of earthquake triggering and delaying. For bibliographic references see the PhD thesis of Sboras (2012).
For the stress change models and calculations, the Coulomb 3 software was used by Toda et al. (2011).
The fault model of the GRIS292 fault, that caused the 2014 earthquake, is based on the seismological data of NOA (epicentre, magnitude, moment tensors). Accordingly, the given parameters are:
strike = 72°, dip = 85°, rake = -167°, length = 44 km, width = 12 km, min depth = 0 km and average slip = 0.92 m.
For the rest faults the ISSs of GreDaSS are used.
Fault numbering represents the following ISSs:
1 = GRIS292
2 = GRIS291
3 = GRIS290
4 = GRIS810
5 = GRIS811
6 = GRIS288
The Coulomb stress change pattern is shown in the following figures. It is calculated for various receiver faults and for depths of 10 and 15 km. The source fault is GRIS 292.
The Coulomb 3 software (Toda et al., 2011) can also calculate the displacement pattern of a given reactivated fault based on the Okada model (1992). Thus, the horizontal and vertical displacements on the surface (0 km depth) are also calculated for the May 24, 2014 event.
Okada Y. (1992): Internal deformation due to shear and tensile faults in a half-space. Bull. Seismol. Soc. Am., 82(2), 1018-1040.
Sboras S. (2012): The Greek Database of Seismogenic Sources: seismotectonic implications for North Greece. PhD Thesis, Università degli studi di Ferrara, 254 pp.
Toda S., Stein, R.S., Sevilgen V. and Lin J. (2011): Coulomb 3.3 Graphic-rich deformation and stress-change software for earthquake, tectonic, and volcano research and teaching – user guide: U.S. Geological Survey Open-File Report 2011-1060, 63 p., available at http://pubs.usgs.gov/of/2011/1060/.