As one of the ways to identify seismological characteristics in the region, determining the quality factor of seismic mapping can provide valuable information about inside the earth. This study investigates local site effects as a function of frequency and presents a new relationship for determining the quality factor in northwestern Iran with regard to local site effects. These sites are selected so that their signal-to-noise ratio (SNR) is greater than 5. This study uses the Short-Time Fourier Transform (STFT) method in which a fixed time window and its multiplication by a given signal are used. The coefficients resulting from this transformation are considered as wave amplitudes at any frequency by performing a short-time Fourier transform. The amount of power spectrum decay is used instead of the ground displacement amplitude decay. Local site effects and kappa, a function of the path and site effects, were investigated and became the basis of spectral decay calculations. The results of this study were compared with those of the previous study based on conventional and classical methods and the accuracy of the methods was evaluated using standard deviation (SD) values. Finally, the quality factor equations were obtained for the North-South component (N-S) as Q(f)=(78±2)f^((1.37±0.02)), for the East-West component (E-W) as Q(f)=(62±2)f^((1.5±0.03)), and for the vertical component (Z) as Q(f)=(87±2)f^((1.29±0.03)). Manuscript profile