Site Classification of the Strong Motion Stations of Uttarakhand, India, Based on the Model Horizontal to Vertical Spectral Ratio

In order to monitor the ongoing seismicity in different parts of the active regions of India, 300 state of the art digital strong motion accelerographs are installed. The accelerograms from the recording stations are available in PESMOS website. The recording stations are classified based on the physical description of local geology and not based on in-situ field tests. Thus, there exist an ambiguity in the assessment of local site conditions for these recording stations. Unless the site class (SC) is known, recorded ground motions cannot be used with confidence. Several research efforts have been directed towards the use of horizontal to vertical spectral ratio (HVSR) technique as a tool to access the local site condition. There is consensus among the scientific community regarding the peak frequency (fpeak) obtained from the HVSR method to identify the soil fundamental frequency. In this study, theoretical HVSR curves for 12 recording stations located in the Tarai region of Uttarakhand are generated using a Matlab tool called Model HVSR to obtain the fpeak and peak amplification (Apeak). Further, these values of fpeak and Apeak for all the 12 recording stations are estimated using the HVSR based on strong ground motion records. Results obtained from both the above methods show 1: 1 correspondence. The values of fpeak obtained for 12 stations range from 1 Hz to 5 Hz based on model HVSR and from 1.1 Hz to 3.3 Hz using the HVSR based on strong ground motion records. These findings are also matching with the existing field studies. Present analyses show a clear difference in the SCs suggested by PESMOS and the one obtained from the present study. Once correct information on SC is known, recorded ground motion at each station can be used with confidence.