Abstract: Determination of source, path and site characteristics in the Western Anatolia Region, which is one ofthe most seismically and tectonically active continental regions in the world, is a very important issue inthe sense of scaling the seismic hazard of a possible large earthquake. For that reason, the soil behaviorof the sedimentary fills of the Gediz Graben in the region was investigated. At first, site amplificationsand natural resonance frequencies of the two sites (IZM/zmir and BRN/Bornova stations) in Izmir weredetermined using micro and moderate sized earthquake data from two accelerometers operated by theTurkish National Strong Motion Network. After that, the detailed geology, stratigraphy, seismicreflection, gravity, resistivity, hydrogeology, water supply and well-logging studies in the graben wereexamined in order to obtain vertical distributions of average velocity and regional (anelastic)attenuation values for the sedimentary fill of the Gediz Graben. The Estimated vertical velocity andanelastic attenuation distributions were used as input parameters for the Quarter-WavelengthApproximation in order to determine an average site amplification function. Utilizing theseamplification values, the Peak Ground Acceleration (PGA) and Spectral Acceleration (5% dampedspectral acceleration, SA) values of a hypothetical earthquake, with Mw=7.0 and hypocentral depth=10km, were simulated in order to see the interaction between structures and this event, at differentdistances on the sedimentary fill of the Gediz Graben. The results implied that an obtained maximumPGA value is 0.42 g and the maximum horizontal SA value is 1.2 g at 0.3 sec at the epicentral locationof the hypothetical earthquake. Dominant periods are 0.3 sec for hypocentral distances of 10-30 km and0.5 sec for the hypocentral distance of 100 km in the Gediz Graben.