S U M M A R YTo provide quantitative information on the shear wave velocity structure of theİzmit Bay area, we conduct active-passive array surface wave and single-station microtremor measurements at 60 sites. We process these array measurements to produce combined Rayleigh wave dispersion curves that span a broad-frequency range (0.5-20 Hz). We also make use of horizontal-to-vertical spectral ratio (HVSR) curves obtained from the single-station microtremor measurements to benefit from their close relation to the ellipticity of Rayleigh waves. Using the dispersion curve sensitivity to the absolute velocities and the shape of the HVSR curve sensitivity to the velocity contrasts of the velocity-depth model, we have applied a combined inversion technique to derive better constrained shear velocity models at each site. The derived shear velocity models are used to provide a V s 30 site classification map and information on the sediment-bedrock structure for theİzmit Bay area. The V s 30 map shows that the entire shoreline regions including artificial infill areas of theİzmit Bay have V s 30 values less than 200 m s −1 , locally as low as 80 m s −1 . The older sediment areas of theİzmit basin have V s 30 values ranging between 250 and 350 m s −1 . The highest V s 30 values are associated with rock sites reaching about 950-1300 m s −1 . We also present a high correlation between the V s 30 values and the phase velocity of Rayleigh wave corresponding to the 40 m wavelength (C40). The important implication of this correlation is that V s 30 may be estimated from C40 without inverting the dispersion curve. The inferred sediment-bedrock interface along a crosssection shows an antisymmetric V shaped basin with a sedimentary cover thickness reaching about 1200 m at the deepest part of theİzmit basin. This deepest part coincides in the map view where the North Anatolian fault zone crosses the basin in the east-west direction. The sedimentary cover thickness is found to be 750 m in the Gölcük-Derince basin.

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