An initial model of seismic microzonation of Sikkim Himalaya through thematic mapping and GIS integration of geological and strong motion features

Seismic microzonation and hazard mapping was undertaken in the Sikkim Himalaya with local site conditions and strong ground motion attributes incorporated into a geographic information system. A strong motion network in Sikkim consisting of 9 digital accelerographs recorded more than 100 events during 1998-2002, of which 72 events are selected with signal-to-noise ratios ≥3 for the estimation of site response (SR), peak ground acceleration (PGA) and resonance frequency (RF) at all stations. With these data and inputs from IRS-1C LISS III digital data, topo-sheets, geographical boundary of the State of Sikkim, surface geological maps, soil taxonomy map at 1:50,000 scale and seismic refraction profiles, the seismological and geological thematic maps, namely, SR, PGA, RF, lithology, soil class, slope, drainage, and landslide layers were generated. The geological and seismological layers are assigned normalized weights and feature ranks following a pair-wise comparison hierarchical approach and later integrated through GIS to create the microzonation map of the region. The overall SR, PGA and resonance frequency show an increasing trend in a NW-SE direction, peaking at Singtam in the lesser Himalaya. Six major hazard zones are demarcated with different percentages of probability index values in the geological, seismological hazard and microzonation maps. The maximum risk is attached to a probability greater than 78% in the Singtam and adjoining area. These maps offer generally better spatial representation of seismic hazards including site-specific analysis as a first level microzonation attempt.