A Comprehensive Site Response and Site Classification of the Garhwal-Kumaun Himalaya, Central Seismic Gap (CSG), India

We attempted a comprehensive site response study of Garhwal-Kumaun Himalaya in the Central seismic Gap (CSG) using the data collected from 97 strong motion, microtremor and broadband stations. Site response parameters such as predominant frequency (f(peak)), site amplification (A(max)) and average shear-wave velocity up to a depth of 30 m (V-S30) were estimated by adopting the Nakamura technique. Spatial maps of these parameters were prepared to show the distribution of site response functions across the region. The estimated parameters show a good correlation with site geology. Decreasing order of sediment thickness from foothills to Lesser Himalaya was observed in the region. The estimated V-S30 values were correlated with global V-S30 values as well as available field measurements and 1D velocity inversion results, it shows a good agreement. The sites were classified as per the NEHRP site classification scheme based on the V-S30 value. From this study, we inferred that the V-S30 values estimated from the Nakamura technique can be used as an alternate proxy for determining the average shear wave velocity up to a depth of 30 m in the absence of substantial field measurements due to challenging terrain conditions. Such results of site response analysis may be used for preliminary earthquake hazard assessment in the region.