Seismic Ground Response Analysis of Some Typical Sites of Guwahati City

Introduction

An earthquake causes rupture along a fault plane and the generated seismic waves propagate through underlying bedrock and reach the earth’s surface. The amplification and de-amplification of ground motion from the bedrock to the surface depend on various stratification or geological parameters. For design of any earthquake resistant structure, selection of design ground motion play an important role. The design ground motion at bedrock level depends on various parameters like: faulting mechanism and its seismicity, local geology, surface topography, type of bed rock and dynamic properties of the soil medium through which the seismic waves travel. Ground response analyses (GRA) need to be performed for obtaining ground motions at surface level for a chosen bed rock motion. Ground response analysis can be performed by using one dimensional, two dimensional and three dimensional methods (Kramer, 1996). Further GRA also helps in development of design response spectra; to evaluate dynamic stress and strain for liquefaction hazard evaluation and to determine the earthquake induced forces that can lead to instability of various structures.

Several studies were reported in literature on seismic ground response analyses for Indian cities. Ranjan (2005) carried out seismic response analysis of Dehradun city and reported that the range of spectral acceleration was 0.06 g – 0.37 g at frequency range 1 Hz –10 Hz. Pallav et al. (2010) simulated 1869 Cachar earthquake rock level ground motion at Imphal city (PGA lies between 0.1 g – 0.16 g) and also estimated amplification factors at ground level which were in the order of 1.5 – 2.0. Govindaraju and Bhattacharya (2011) carried out site-specific earthquake response study for hazard assessment in Kolkata city. They observed that the amplification of ground motion were in the range of 4.46–4.82 with the fundamental period ranging from 0.81 s to 1.17 s and the maximum spectral accelerations were in the range of 0.78 g –0.95 g. Phanikanth et al. (2011) performed equivalent linear ground response analyses for Mumbai city.

Estimation of ground motion amplification due to the local soil sites is a complex problem to the designers and more important for fast growing cities like Guwahati in India. Northeast India is one of the most seismically active regions of the world. As per Seismic hazards zonation map by the Bureau of Indian Standards (IS 1893), Guwahati city comes under zone V (i.e., very severe zone) with a zone factor 0.36. Guwahati city had experienced many major earthquakes in the past. Among them the 1897 Assam earthquake and 1950 Assam earthquake are the most historical disasters. Ambraseys and Bilham (2003) reported the earthquake MSK intensity (Medvedev–Sponheuer–Karnik) of 8 for Guwahati city during the great 1897Assam intraplate earthquake which also exhibited widespread liquefaction. Reddy et al. (2009) carried out extensive field investigations in the meizo-seismal area of the great Assam 1950 earthquake to explore the paleo-seismic history of the North-East region through documentation of liquefaction features. They reported that the intensity of the 1950 Assam Earthquake lies between IX – X.