Seismic site characterization using Vs30 and site amplification in Gandhinagar region, Gujarat, India

Gujarat is prone to earthquake hazard of different levels from moderate to high, assigned as zones II–V in the seismic zoning map of India. Many multistorey buildings collapsed in Ahmedabad city at a distance of 225 km from the location of the 2001 Bhuj earthquake. Gandhinagar falls in zone III where an intensity of VII or VIII from the regional large earthquakes or local earthquakes of magnitude 6 can be expected; which can damage single and multistorey buildings. Thus, there is a need for site characterization and seismic hazard mapping of the area. Shear-wave velocities were measured using the MASW technique at 63 sites in and around Gandhinagar. Based on Vs30 in most of Gandhinagar the soils have been classified as D-type (180–360 m/s) in accordance with the NEHRP provision, except the northern part of the city (sites 27, 51, 53 and 54), where Vs30 values larger than 360 m/s qualify the area as a NEHRP class C-type soil (360–760 m/s). However, nearly the whole of Gandhinagar soil has Vs > 250 m/s, that is considered to be strong. Further for the estimation of site amplification of the study area, we have used earthquake as well as microtremor records. Site amplification up to 4.4 was observed in the frequency range 0.4–2 Hz. The frequency range 3–10 Hz shows amplification up to 2.4 times at some sites. The frequency range 0.4–2.0 Hz falls in the natural frequency range of multistorey buildings with more than three floors. It is inferred from shear-wave velocity measurements and site amplification that multistorey buildings of more than three floors in this region require careful designing.