Estimation of local site effects using microtremor testing in Vijayawada city, India

The microtremor method is very useful in urban areas to estimate local site effects for microzonation purposes. As seismic waves propagate from bedrock to the ground, their characteristics are site specific due to the heterogeneity of substrata. Source, path and site effects are prime parameters that affect ground response. Many destructive earthquakes that occurred in the past (e.g. Bhuj 2001, Bingol 2003, Kashmir 2005, Haiti 2010, Tohoku 2011, Van 2011 and Yunnan 2012) clearly illustrate the effects of local soils on damage severity and pattern. The Indian sub-continent has more than 400 major faults that influence seismic activity and India has experienced several devastating earthquakes (Assam 1897 (M  =  8·7), Kangra 1905 (M  =  8·6), Bihar-Nepal 1934 (M  =  8·4), Assam-Tibet 1950 (M  =  8·7), Latur 1993 (M  =  6·4), Chamoli 1999 (M  =  6·8), Bhuj 2001 (M  =  7·6) and Sikkim 2011 (M  =  6·9)). The state of Andhra Pradesh is located in the central part of peninsular India (zones II and III according to Indian seismic code) and has a record of earthquakes along the coast. This study considered Vijayawada (zone III), the third largest city in the state. Microtremor surveys were carried out at 75 different locations in the Vijayawada urban area and analysis was carried out using the Nakamura technique. Dynamic characterisation was carried out by considering the shape of the response curve, horizontal to vertical (H/V) amplitude, predominant frequency and the characteristic soil profile at all the test sites. Based on this detailed analysis, a classification is proposed and a predominant frequency map of the study area was developed. It was found that areas in the north eastern and south eastern parts of the city, with silty and clayey sand formations, have comparatively high predominant frequencies (≥ 4 Hz). The northern and western parts of the city, with high silty clay and silty sand, are characterised by moderate frequency values (2–4 Hz). Low (< 2 Hz) values of predominant frequencies were observed at a few locations. The H/V amplitudes are high (2–3) in the eastern and western regions.