Estimation of site response functions for the Kumaun-Garhwal region of Himalaya, India

The Kumaun-Garhwal region of Himalaya falls in the central seismic gap of Himalaya where the probability of occurrence of the great earthquake has been estimated as 31% in a 50-year time window. This underlines the importance of evaluation of seismic hazard in this region. One-dimensional (1D) site response functions provide one-dimensional predictions in a 3D environment and play an important role in assessing the seismic hazard of a region. In the present study, 1D site response functions have been estimated for the Kumaun-Garhwal region using 56 local earthquakes recorded at 14 sites. Ten sites, out of the 14 sites, are situated in Garhwal Himalaya, 3 sites are located in adjacent Kumaun Himalaya and 01 in Indo-Gangetic plain. The widely used horizontal to vertical spectral ratio technique has been adopted here for this purpose. The seismic spectral amplification levels have been obtained at the predominant frequency as well as at five different frequencies corresponding to different types of buildings. The average site spectral amplification levels at the predominant frequency for the sites in higher Himalaya, lesser Himalaya, and in outer Himalaya are found to be 3.94, 4.43, and 5.98 respectively. These estimated values are correlated with rock/soil types in these three sub-divisions of Himalaya. The only site (Deoband) situated in the Indo-Gangetic plain gives the site spectral amplification level of 7.17 at predominant frequency. The seismic hazard to different story buildings in the region based on the estimated site spectral amplification levels is discussed. The computed site spectral amplification functions are useful for estimating the earthquake source parameters and for the simulating techniques for earthquake strong ground motions.