Estimation of site response functions for the central seismic gap of Himalaya, India

In the analysis of seismic hazards of a particular region, the site response functions play a significant role. Site response functions for the central seismic gap become more important as the estimated possibility of manifestation of the earthquake (M ≥ 8) is 31% in a time window of fifty years (Khattri in Proc Indian Academy Sci Earth Planetary Sci 108(2): 87–92). This emphasizes the importance of analysis of seismic hazards in this region. The site effect or response functions have been assessed for the central seismic gap region in the present study using 87 recorded local events recorded at 50 sites located in the central seismic gap region of Himalaya. Among 50 stations, 9 stations are located in Higher Himalaya, 30 stations are situated in Lesser Himalaya, 06 stations are present in Sub-Himalaya, and the remaining 05 stations are situated in Indo-Gangetic plain. Here, the horizontal-to-vertical spectral ratio method has been used for evaluating the site response functions. The site response functions have been evaluated at different frequencies related to various types of structures. The estimated mean site response function value at the principal frequency (predominant) is found to be 6.2 for Higher Himalaya, 6.5 for Lesser Himalaya, 7.03 for Sub-Himalaya, and 9.6 for the stations located in the Indo-Gangetic plain. Further, the estimated site response functions and the station’s broad geology have been correlated with each other. With the help of estimated site response functions at various frequencies corresponding to various storey structures, the seismic hazard in the central seismic gap area has been discussed. The calculated site response functions are further useful for different studies like for simulation in strong ground motions and evaluating the earthquake source parameters.