 Microzonation is a process that involves incorporation of geologic, seismologic and geotechnical concerns into economically, sociologically and politically justifiable and defensible land-use planning for earthquake effects so that architects and engineers can site and design structures that will be less susceptible to damage during major earthquakes. This exercise is similar to the macro level hazard evaluation requiring more rigorous inputs about site specific geological conditions, ground response to earthquake motions and their effects on the safety of constructions taking into consideration the design aspects of buildings, ground conditions which would enhance the earthquake effects like the liquefaction of soils, the ground water conditions and the static and dynamic characteristics of foundation or of the stability of slopes in the hilly terrain. To be useful, microzonation should provide general guidelines for the types of new structures that are most suited to an area. It should also provide information on the relative damage potential of the existing structures in a region. It follows, therefore, that if the principles of microzonation are correctly and judiciously applied, they could be useful in establishing criteria for land-use planning and a strategy for the formulation of a systematic and informed decision-making process, for the siting and development of new communities in areas that are made hazardous by nature.
The Sikkim Himalaya which is part of the northeast India is seismically one of the six most active regions of the world being placed in zone V with PGA > 400gal. In this Atlas an attempt has been made to put forth a composite investigation towards: 1) prediction of Maximum Credible Earthquake (MCE) in the region from GSHAP consideration, 2) seismotectonic study from the strong motion seismometry recorded by the nine station Sikkim Strong Motion Array (SSMA) and its correlation with the findings of micro-earthquake survey, 3) to work out a power law relationship of QS with frequency to represent the overall attenuation of seismic wave energy in the region from the events with focal depth less than 35 km and thus defining the path effect precisely, 4) detailed analysis of Site Response (SR) at all the recording stations by receiver function analysis (HVSR) and generalized inversion (GINV) techniques, their comparison and the dependency of SR on the source azimuthal variation, 5) simulation of source spectra and hence the spectral acceleration at various sites for different magnitude so that the source parameterization could be carried out, new empirical relations developed between various source parameters, namely, M0 & MW and fc & MW for further simulation of spectral acceleration for greater magnitude earthquakes representing MCE of the region for a scenario due to a Brune omega-squared circular-crack source model on MBT, and 6) to develop regional as well as site specific local spectral attenuation laws at different geometrically central frequencies in the low, moderate and high frequency bands. This combined analysis is a maiden effort to address the seismic hazard potential of the Sikkim Himalaya.
The seismic ground motion hazard is mapped in the Sikkim Himalaya with local and regional site conditions incorporated through Geographic Information System (GIS). The geomophological themes with inputs from IRS-1C LISS III digital data, topo-sheets, geographical boundary of the State of Sikkim, surface geological maps, soil taxonomy map in 1:50,000 scale and seismic refraction profiles, are overlaid, united and integrated to form the base site condition coverage of the region. The seismological themes, namely, Site Response (SR), Peak Ground Acceleration (PGA), and Predominant Frequency (PF) are assigned normalized weights and feature ranks following a pair-wise comparison hierarchical approach and later integrated to evolve the seismic hazard map. The overall SR, PGA and PF show an increasing trend in the NW-SE direction peaking at Singtam in the lesser Himalaya. For the geometrically central frequency 2.5 Hz the probabilistic Seismic Microzonation map identifies six major zones with the hazard index ranging between 0.0 - 0.1, 0.1 - 0.2, 0.2 - 0.3, 0.3 - 0.4, 0.4 - 0.5, and greater than 0.5.The maximum risk is attached to the seismic hazard index greater than 0.5. at Singtam and its neighborhood, where PGA values range between 800 - 850 gal. At the geometrically central frequency 7.5 Hz six major zones are mapped, with the hazard index range between 0.0 - 0.1, 0.1 - 0.2, 0.2 - 0.3, 0.3 - 0.4, 0.4 - 0.5, and greater than 0.5.The maximum risk is attached to the seismic hazard index greater than 0.5 at Mangan and its immediate neighborhood, where PGA values range between 700 - 800 gal. The probabilistic Seismic Microzonation map at the geometrically central frequency 15 Hz identifies five major zones with the hazard index range between 0.0 - 0.1, 0.1 - 0.2, 0.2 - 0.3, 0.3 - 0.4, 0.4 - 0.5. The maximum risk is attached to the seismic hazard index between 0.4 - 0.5 in the region encompassing Singtam, Gangtok and Mangan, where PGA values range between 800 - 850 gal. |
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