SEISMIC MICROZONATION OF CHANDIGARH CITY

Abstract

The rapid growth of the world's population over the past few decades has led to a concentration of people, buildings and infrastructure in urban areas. The importance of estimating the seismic ground motion in these areas, for earthquake risk mitigation is well recognized. One of the important ways for mitigation of the adverse effects of the earthquakes is the proper assessment of seismic hazard associated with the region and seismic microzonation of the regions. Seismic microzonation is the process of estimating response of soil layers under earthquake excitation and thus the variation of ground motion characteristic on the ground surface. Microzonation should provides general guidelines for the types of new structures that are most suited to an area and it should also provide information on the relative damage potential for the existing structures in a region. It includes a multi-disciplinary approach with major contributions from geology, seismology, geotechnical and structural engineering. Chandigarh, presently the Union Territory (UT) and capital city of Punjab and Haryana, located at about 250 kms North of Delhi at the foot hills of Siwaliks, is a great centre of attraction in the recent times. The population of Chandigarh city based on recent census is around one million; As per Seismic Zoning Map of India (IS 1893, 2002) the city falls in Zone IV. Its surrounding regions have experienced earthquakes of magnitude 6 and above since historical times. The area is also vulnerable to possible future large earthquakes in Central Himalayas. Number of important structures of this populated city could be prone to damage due to an earthquake of considerable magnitude. Thus, Seismic microzonation of the cities like Chandigarh, situated in earthquake prone area becomes important, so that the planners can make use of it for better land use planning and safe development. In the present study, Seismic Microzonation of Chandigarh city has been proposed. The analysis has been carried out to fmd out the shear wave velocity at various depths in different bore holes of Chandigarh region. Shear wave velocity is computed on the basis of the uncorrected N-values. The empirical correlation between Vs and uncorrected N-value proposed by Hanumantharao and Ramana (2008) has been used to calculate the shear wave velocity. Various seismotectonic features around the Chandigarh region were identified, whole surrounding region was divided in six zones and seismicity data of each zone were studied. PGA at bedrock level on a dense grid was computed using Boore and Atkinson (2008) attenuation relationship, considering all the tectonic features. It was found that the Peak Ground Acceleration values at bedrock level are almost uniform in the entire Chandigarh city. Spectrum compatible time history for return period of 100, 225, 475, 2500 and 10000 year were generated at bedrock. Soil profile for different sectors are made with the help of borehole data, then this soil profiles and generated spectrum compatible time history for different return periods were used to generate ground motion at surface level. The Peak Ground Acceleration and the spectral acceleration values for time period of 0.1, 0.5, and 1 sec at surface level were computed by using Pro-SHAKE. A predominant frequency at different sites and corresponding amplification factor has also been estimated by using Pro-SHAKE. After calculating above mentioned parameters, the Seismic Microzonation maps have been prepared using Arc-GIS software. iv