Site-specific ground motion simulation and seismic response analysis for microzonation of Nanded City, India

Abstract

We study strong ground motion characteristics and ground response analysis of different sites in Nanded City, India. Synthetic strong ground motion records are obtained for two scenario earthquakes of magnitude M 5.2 and M 6.8 in southern Peninsular India using the finite fault model. Shear-wave velocity averaged over top 30 m of the soil is obtained from multichannel analysis of surface wave survey at 60 sites at depths of 20–50 m below the ground surface. The sites in different parts of the city are characterized by peak ground acceleration (PGA) at the surface and bedrock level, 5 % damped response spectra at the ground surface and the amplification factor defined as the ratio of ground motion at the surface to that at the bedrock. The area east to the lineament is characterized by low V S30 values (<200 m/s), probably associated with Godavari River basin sediments with alluvial deposits, composed of brown clay with intercalated bands of sand and gravel. A relatively high PGA value at the surface of about 0.24 g for the M 5.2 target event and hence high ground motion amplification indicative of major damage are observed at sites E-NE to the lineament in the area that are associated with the top soil column consisting of unconsolidated river basin sediments with very low V S30 values (<200 m/s). The amplification factor for the city ranges from 3 to 8 for the highest target event of magnitude M 6.8. There is a strong lateral variation in PGA values at the surface for the part of the city lying to the east of the lineament and parallel to its trend, where a relatively high amplification factor of 6–8 is found at a dominant frequency of 1.5–2.5 Hz, which can be considered as a significant zone of amplification and hence areas with high seismic hazard. The site-specific response spectra show that the peak of spectral acceleration for different sites occurs around 3 Hz. There is a large spatial variation of spectral acceleration at any given frequency for the city. For example, site nos. 9 and 50 are characterized by a peak in spectral acceleration of 0.55 and 0.9 g, respectively, at 3 Hz frequency.