Συμβολή στον προσδιορισμό τρισδιάστατου εδαφικού προσομοιώματος για τη μελέτη της σεισμικής απόκρισης: εφαρμογή στην ιζηματογενή Μυγδόνια λεκάνη

Abstract

The geometry and the dynamic properties of the surface soil material are the most important parameters for the seismic behavior of a region. The main parameter for the assessment of the local site effects in seismic motion is the shear wave velocity (Vs). Its distribution from the surface down to bedrock is the requisite parameter in seismic response analysis into different dimensions (1D, 2D and 3D). The scope of the present study is the definition of a three-dimensional model of the Mygdonian basin in which the geometry of the main geological formations is accurately described and enhanced with all the necessary dynamic soil parameters for site response analysis. Mygdonian basin is located at about 30 km E-NE of the city of Thessaloniki, it is the closest active seismic zone affecting it (Figure 2.3). The destructive Stivos earthquake sequence of 20 June 1978, Ms = 6.5R occurred in the north part of the basin (Figure 2.9), was the ignition of many researches. In the region one of the biggest and well established experimental sites in Europe is Euroseistest (http://euroseis.civil.auth.gr). It is suited at the central part of the graben and consist of an array of 18 3D free-field and down-hole accelerometers located in roughly N-S and E-W directions covering an area of about 48 km2 (Figure 2.16). The preliminary 3D model of the Mygdonian basin was constructed by the French company Bureau de Recherche Geologiques et Minieres during years 1971-1972 (BRGM, 1971). This study due to its objective (water supply purposes) was concentrated mainly on the bedrock interface using P-wave refraction surveys, electrical soundings and drillings (Figure 2.19). The synthesis of these data provided a 3D model which described only the geometry of the interface between sediments - bedrock and not of its intermediate layers with Vp and not Vs velocities which are necessary input parameters for site response analysis. According to this structure the bedrock between Lagada and Volvi Lakes form a syncline with its axis parallel to E-W direction. It lies at a depth of 400 m at the western part, approximately 100 - 120 m at the central part and less than 100 m at the eastern part of the basin. In contrast to BRGM geometry, the extensive program of geophysical and geotechnical surveys performed across Profitis-Stivos villages (N-S direction) in the framework of the Euroseistest and Euroseismod projects (1993-1999) show thicker soil layers at the central part of the basin (about 150 m above sea level at TST site) and a shape that is different from the syncline-shape geometry (Figure 2.25). Posterior to this model, other 3D geometries were also constructed based mainly on electrical prospecting results (Thanasoulas, 1983; IGME, 2001; Tournas, 2005) and reproducing the BRGM model with small differences. None of the aforementioned studies succeeded defining a 3D geometry adequate for site response analysis. This necessitated new geophysical and geotechnical surveys which were conducted within the framework of the European project Euroseisrisk (EVG1-CT-2001-00040, 2002-2005). These surveys included seismic refraction and electrical surveys, array microtremor measurements, geotechnical boreholes, laboratory and in-situ tests such as Standard and Cone Penetration Tests (SPT, CPT respectively).