4 th International Conference on Earthquake Geotechnical Engineering June 25-28, 2007 Paper No. 1610 NON-SEISMIC SURFACE FAULTING: THE PERAIA FAULT CASE STUDY (THESSALONIKI, N. GREECE) Zervopoulou A. 1 , Chatzipetros A. 1 , Tsiokos L. 2 , Syrides G. 1 , Pavlides S. 1 ABSTRACT During the last months of 2005 a surface rupture started to form in Peraia suburb southwest of Thessaloniki city, located at the southern shore of Thermaikos bay in northern Greece. The rupture is concave, with one branch trending WSW – ENE, while the other one trends WNW – ESE. It dips towards the N and is located along the scarp that defines two morphological plateaus. These two plateaus are the top surfaces of the hangingwall and the footwall respectively of the well known, mapped and studied Anthemountas fault. This is a large active normal fault that bounds the southern margin of Anthemountas valley and controls the shoreline at its western extension. It is believed to be associated with certain large historical earthquakes, while its recent activity is documented by microseismicity. It is one of the most hazardous earthquake sources for the city of Thessaloniki. Four boreholes (G1-G4) have been drilled on both sides of the rupture, in order to examine the stratigraphical and geotechnical properties of the faulted geological substratum as well as possible water level fluctuations. Furthermore, correlation of boreholes G1 and G2 shows that there is a vertical displacement of at least 35 m during the Quaternary. Trenching along the fault has also proved that the surface rupture coincides with the main fault zone, showing a very large vertical displacement, non measurable in the trench. Periodic measurements of the water level in a number of boreholes used for irrigation have shown that there has been a significant fall of the water level during recent years. In conclusion, it seems that the Peraia rupture has been formed along the already existing Anthemountas fault and at least a part of the total displacement is attributed to overpumping. Keywords: surface ruptures, active faults, creep INTRODUCTION During the 2005-2006 seasons, a set of surface ruptures has been observed at Upper Peraia urban area of Municipality of Thermaikos (suburb of Thessaloniki, Macedonia, northern Greece); the fractures are deforming both streets and buildings. These ruptures were studied using surface mapping, borehole data and stratigraphic correlation in order to define their origin, growth and slip rate. Ruptures appear at the lower part of the scarp between the hilly upper Peraia and the coastal lower Peraia plateaus. It is believed that the ruptures are caused by either the aseismic creep of Anthemountas Fault (Peraia segment) or the overpumping for water supply of Peraia town, but probably by both factors. SURFACE RUPTURES The surface ruptures extend at a length of ca. one kilometre in Peraia urban area close to the Thessaloniki – N. Michaniona road. They are traceable from Pissalides Clinic to the east, up to Kyprou Street at a WSW-ENE direction, whereas from Kyprou up to Mideias street (Lower Peraia) its direction changes to WNW-ESE (Fig. 1). The maximum vertical displacement of the ruptures has been measured at the Agnostou Stratioti and Megalou Alexandrou streets. Surface ruptures have also been 1 University of Thessaloniki, Greece, [annzervo, ac, syrides, pavlides] @geo.auth.gr ] 2 University of Aegean, Greece. ltsiokos@env.aegean.gr