Multi criteria study for seismic hazard assessment of UNESCO world heritage Ahmedabad City, Gujarat, Western India

Ahmedabad, the most populous city of Gujarat, assigned zone III in the seismic zone map of India (BIS 2002), has experienced moderate earthquakes in the past. Several high-rise buildings were collapsed or severely damaged in the city during 2001 Bhuj earthquake (Mw 7.6), which was 240 km from the city. Keeping this in view, micro-level seismic hazard assessment in the city is carried out using geotechnical, geological, and geophysical inputs, which may help in designing buildings and other civil engineering structures and will reduce the probability of loss of life and property in this region. A total of 23 boreholes (11 boreholes of 80 m, 7 boreholes of 40 m and 5 boreholes of 35 m) were drilled at the different locations in the city. To estimate the shear-wave velocity, we have employed direct and indirect methods. PS logging is carried out in 11 boreholes, and shallow geophysical investigation (multi-channel analysis of surface waves, MASW) is carried out at 54 sites. The field and laboratory tests on soil samples, geophysical investigations, and seismotectonic information enabled us to estimate soil overburden thickness, shear-wave velocity, factor of safety against liquefaction, and site response in terms of amplification factor. The peak ground acceleration was estimated at engineering bed rock level (Vs 760 m/s) by PSHA. All this information is used in preparing an integrated seismic hazard (SH) map of the Ahmedabad City using analytical hierarchal process. The seismic hazard map is characterized into three broad categories: low, moderate, and high. The western part of the Ahmedabad shows the highest hazard. The northern and the eastern parts show moderate seismic hazard. It is observed that the presence of sand and flood plain deposits along the Sabarmati river increases the hazard. The study has also highlighted that the presence of a paleochannel increases the overall hazard, which is clearly visible in integrated hazard map.