EURO-SEISTEST - DETERMINATION OF THE GEOLOGICAL STRUCTURE OF THE VOLVI BASIN AND VALIDATION OF THE BASIN RESPONSE

The Volvi basin (northern Greece) is a 5-km-wide graben of Neogene age . It has been chosen by European Union (Directorate General XII for Sc ience, Research and Development in Climatology and Natural Hazards) as a test site (EURO-SEISTEST) for studying site effects in one of Europ e's most seismic active areas. The site is located in the epicentral a rea of the 1978, Ms = 6.5, earthquake that struck the city of Thessalo niki. The geometry, velocity, and stratigraphy of the basin were not w ell known. Consequently, a number of geophysical and geotechnical stud ies were performed to determine the 2D geometry and the dynamic charac teristics (Vp, Vs, Qs) of the valley. Investigations included 78 refra ction tests, 20 surface-wave inversions, and 8 borehole seismic tests as well as geotechnical tests. This article presents the methodology u sed for the geophysical investigation and the results. In its center, the graben depth reaches 190 m, and at least four normal faults have b een located along the section. The overall graben structure and the fa ult locations have been confirmed by the records of a shot generated a t one edge of the valley and by the measurements of a local earthquake . Wave propagation appears to be strongly influenced by the thickness of the sediments, which in turn is controlled by fault displacements. Simple 2D modeling of the basin response was performed, and the result s are in good agreement with weak-motion data, confirming the validity of the seismic structure inferred from seismic prospecting.