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The 2011 Sikkim earthquake (M 6.9) of the Himalaya has shown that a moderate size earthquake may also cause severe damage to the region. Simulated accelerograms using a modified hybrid technique have been generated at spatially distributed 702 points for a moderate size (M 6.9) earthquake in the Sikkim region of Himalaya to evaluate the seismic hazard in the region. The peak ground acceleration (PGA) values and response spectra have been derived from the simulated accelerograms. The synthetic PGA values, corresponding modified mercalli (MM) intensity values and response spectral acceleration values have been contoured to present the scenario hazard maps of the region. A PGA value of 400 cm/s2 has been estimated in the region near the epicentre of the earthquake. The response spectral acceleration values for five periods corresponding to the natural period (T) of single-storey buildings, double-storey buildings, 3–4 storey buildings, tall buildings and high-rise buildings have been obtained. The maximum values of response spectral accelerations estimated in the centre of meizoseismal zone are 1200 cm/s2 (T = 0.1 s), 1800 cm/s2 (T = 0.2 s), 900 cm/s2 (T = 0.5 s), 800 cm/s2 (T = 1.0 s) and 80 cm/s2 (T = 6 s). The decay curves show that PGA values in the west direction are lower as compared to the values in other directions at similar distances. Also, it has been observed that the PGA and spectral acceleration values decay slowly in the west direction compared to other directions. The scenario hazard maps present here are expected to be useful for mitigating seismic hazards from the region as these maps indicate the damage potential due to future moderate size earthquakes in the region. Article Highlights: The application of a modified hybrid technique has been presented to evaluate the seismic hazard based on simulating accelerograms for a moderate size earthquake in the Sikkim region of Himalaya. The synthetic accelerograms have been generated at 702 points distributed spatially on a grid. The scenario hazard maps showing the spatial distribution of PGA values, MM intensity values and response spectral acceleration values at different periods. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]