Thessaloniki – Gerakarou Fault Zone (TGFZ): the western extension of the 1978 Thessaloniki earthquake fault (Northern Greece) and seismic hazard assessment Markos D. Tranos a, * , Eleftheria E. Papadimitriou b , Adamantios A. Kilias a a Department of Geology and Palaeontology, University of Thessaloniki, GR-54124 Thessaloniki, Greece b Geophysics Department, University of Thessaloniki, GR-54124 Thessaloniki, Greece Received 31 October 2001; received in revised form 1 August 2002; accepted 17 February 2003 Abstract Active faulting and seismic properties are re-investigated in the eastern precinct of the city of Thessaloniki (Northern Greece), which was seriously affected by two large earthquakes during the 20th century and severe damage was done by the 1759 event. It is suggested that the earthquake fault associated with the occurrence of the latest destructive 1978 Thessaloniki earthquake continues westwards to the 20-km- long Thessaloniki – Gerakarou Fault Zone (TGFZ), which extends from the Gerakarou village to the city of Thessaloniki. This fault zone exhibits a constant dip to the N and is characterised by a complicated geometry comprised of inherited 1008-trending faults that form multi- level branching (tree-like fault geometry) along with NNE- to NE-trending faults. The TGFZ is compatible with the contemporary regional N–S extensional stress field that tends to modify the pre-existing NW–SE tectonic fabric prevailing in the mountainous region of Thessaloniki. Both the 1978 earthquake fault and TGFZ belong to a ca. 65-km-long E – W-trending rupture fault system that runs through the southern part of the Mygdonia graben from the Strymonikos gulf to Thessaloniki. This fault system, here called Thessaloniki – Rentina Fault System (TRFS), consists of two 17 – 20-km-long left-stepping 1008-trending main fault strands that form underlapping steps bridged by 8 – 10-km-long ENE – WSW faults. The occurrence of large (M ^ 6.0) historical earthquakes (in 620, 677 and 700 A.D.) demonstrates repeated activation, and therefore the possible reactivation of the westernmost segment, the TGFZ, could be a major threat to the city of Thessaloniki. Changes in the Coulomb failure function (DCFF) due to the occurrence of the 1978 earthquake calculated out in this paper indicate that the TGFZ has been brought closer to failure, a convincing argument for future seismic hazard along the TGFZ. q 2003 Elsevier Ltd. All rights reserved. Keywords: Active faulting; Seismic activity; Fault segmentation; Earthquake triggering; Coulomb stress; Thessaloniki; Northern Greece 1. Introduction Destructive earthquakes occurring close to populated areas demonstrate the necessity of additional efforts to improve existing scientific knowledge on active faulting and its relationship with the seismicity in the area of interest, the ultimate goal being the assessment of the future seismic hazard. Many attempts have been made in seismically active areas to evaluate the behaviour of the earthquake faults and the possible occurrence of earthquakes on neighbouring faults, taking into account fault interaction. Faults are not mechanically isolated structures, but they occur within a population of faults and they may interact with other faults through their stress field. Many recent earthquake sequences involved slip on interacting fault segments showing that fault interaction can profoundly affect rupture sequences, such as in the case of the 1999 Izmit and Duzce mainshocks (Hubert-Ferrari et al., 2000; Parsons et al., 2000; Papadimi- triou et al., 2001). This study refers to the Thessaloniki area, which has recently been affected by the 1978 destructive earthquake sequence (Papazachos and Papazachou, 1997). Since then, an intense neotectonic and seismological investigation of the broader area has been carried out (Papazachos et al., 1979, 1982; Mercier et al., 1983; Mountrakis et al., 1983, 1996; Hatzfeld et al., 1987; Pavlides and Kilias, 1987; Pavlides et al., 1990; Tranos, 1998). The results contribute to the better understanding of the active faulting and seismicity of the study area. However, the identification and 0191-8141/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0191-8141(03)00071-3 Journal of Structural Geology 25 (2003) 2109–2123 www.elsevier.com/locate/jsg * Corresponding author. Tel./fax: þ 30-310-998482. E-mail address: tranos@geo.auth.gr (M.D. Tranos).