Seismic microzonation of Chandigarh city, northwest Himalaya, India using geophysical (active and passive techniques) along with geotechnical methods

Chandigarh city, northwest Himalaya, India is under high seismic risk due to underlain lithology and its location in the seismically active belt of Himalaya. The Ambient Noise Measurements (HVSR) from 200 sites and shear wave profiles obtained using Multiple Simulations with one Receiver (MSoR) and downhole tests were used to assess near surface material properties in Chandigarh city. The HVSR results reveal fundamental frequency variation from 0.84 to 1.09 Hz and site amplification in the order of 2 to 3.5. The joint-fit-inversion modelling of the HVSR curve with the dispersion curve helped derive geological stratum depth (160–200 m) having Vs >760 m/s. The Multi-channel Analysis of Surface Waves (MASW) provided high-resolution data to enable us to understand lateral and vertical variation up to 30 m depth and the presence of surface dissolution features underneath the city. Both active and passive geophysical methods help to develop a shear wave velocity (Vs) map of Chandigarh city and characterise the city under soil class ‘D' (180–360 m/s). The frequency map and amplification maps were also plotted using HVSR analysis. The geotechnical and geophysical data helped to generate a subsoil model of the study area and identify the main litho-types above the bedrock. The site amplification and response functions were computed for each site incorporating layer thickness, their modulus curves, shear wave velocity of each layer and soil type by simulating with strong motion earthquake. The study will help understand the pattern of damage that can be expected from future Himalayan earthquakes.