SPECTRAL ANALYSIS OF SURFACE WAVES AND AMBIENT NOISE FOR MICROZONATION OF DEHRADUN CITY

Abstract

Dehradun the capital of Uttaranchal state is habited in earthquake prone area in Himalayas, where a lot of development activities are taking place. Dehradun is situated in Doon valley bounded by the Himalayas and MBT in the north, Siwalik hills and HFT in south. Dehradun is situated in sedimentary basin where the presence of these thrusts i.e. MBT and HFT can cause a lot of damage. The city lies in the seismic zone IV and earthquake intensity up to X is expected. It has witnessed a lot of damage during the 1905 Kangra (M=8.5), 1991 Uttarkashi (M=7.0) and Chamoli (M=6.8) earthquake. Due to undulating topography and spatial variation of soil/rock, seismic microzonation for the city is required. The site characterization has been carried out by conducting Spectral Analysis of Surface Waves (SASW) tests at 11 different locations in the city. The latitude and longitude of the test sites is also measured by using the GPS system. The 24 channel engineering seismograph McSEIS-SX 1125M and geophones of 4.5 Hz has been used to acquire the data. The acquired data has been processed using Seisimager software for getting 1D/2D shear wave velocity profiles. These soil characteristics and spectrum compatible time history at bed rock has been used as input in ProSHAKE software for estimating the spectra, amplification characteristics in terms of fundamental frequency, peak ground acceleration (PGA) at the surface of the sites.