Subsurface structure model is constructed by simulating dispersion curves usually by GA with reference data such as PS logging. When the reference data is not available or micro-tremor array radius is inadequate for the wavelength of interest, method to infer a simpler subsurface model such as a two-layered model is desirable. We propose the method to deduce surface layer thickness and shear wave velocity of the model by resorting to quarter wavelength theory and the concept of wavelength-thickness ratio. The method itself and the wavelength-thickness ratio is tested by PS logging data at K-NET and KiK-net observation sites.
