Site response evaluation through measuring the ambient noise (case study: Iran, Babol City)

One major step in evaluating the site response is estimation of the shear wave velocity and fundamental frequency. Various methods are available for evaluating the site effect, of which the ambient noise measurement method is a prevalent one. The method is quick with low cost; it is nondestructive and gives precise results. In the present research, to assess the site response in Babol City, measurements of ambient noise were performed at 60 points and the data were analyzed by Nakamura method. Also, at 8 sites, measurements of the ambient noise were done by applying SPAC method and the data were analyzed to obtain the shear wave velocity. In addition, equivalent-linear method was performed using the shear wave velocity as well as geotechnical and geological data of those 8 sites. Furthermore, the artificial neural network (ANN), with different input data, was trained and then was utilized to predict the fundamental frequency. The fundamental frequencies acquired from ambient noise, ANN and equivalent-linear method were compared to each other to investigate reliability of the results. The results showed that most parts of the study area have a fundamental frequency less than 1 Hz, while higher frequencies could be observed in some northwestern and southern parts of the city. Finally, it is shown that, the ambient noise measurements were highly useful for evaluating the site effect and estimating important seismic parameters (fundamental frequency and shear wave velocity).