One Dimensional Ground Response Analysis and Identification of Liquefiable Strata of Guwahati City

Abstract

According to seismologist the Himalayan region between the epicenter of the 1897 great Assam earthquake and the 1950 Assam earthquake is due for a large earthquake of magnitude greater than 8. Guwahati city lies at about 100–200 km from this zone. This has highlighted the need to better understand the vulnerability of land to seismic hazards caused by future earthquakes in Guwahati city. The effect of large modifications in seismic waves that occur due to variation in soil properties near the surface of the earth is of great importance in geotechnical earthquake engineering. The response of a structure depends on its regional seismicity, source mechanism, geology and local soil conditions. This work is based on the one dimensional ground response analysis (GRA) using the equivalent linear technique conducted for one hundred twenty (120) locations in Guwahati city. The input motion of 2011 Sikkim earthquake (Mw = 6.9) having bedrock PGA of 0.152 g at 30 m depth has been used for the purpose. This motion is selected because it belongs to the same tectonic regime. The results of variation of PGA with depth, strain and maximum stress ratio distribution over depth are obtained for all boreholes. It has been observed that the peak ground acceleration (PGA) of the input motion gets amplified at ground surface. The surface PGA was found to vary from a maximum value of 0.62 g at Kalapahar (in Paltanbazar area) to a minimum value of 0.15 g near Bar Sajai Ghoramara (in Beltola area) in Guwahati city. The spectral accelerations at various locations have been compared with the spectral acceleration of rocky or hard soil sites (IS: 1893–2002) for the bedrock 0.152 g PGA input motion. Deterministic assessment of SPT based liquefaction potential was next carried out. The surface PGA was used to determine the cyclic stress ratio in each location. Liquefaction susceptibility as determined is presented as a GIS based map showing zones of levels of risk of liquefaction. This work has been mainly carried out to initiate such studies in this seismic prone region and to address the problems caused due to seismic hazards.