Shear Wave Velocity Investigation for Ten Representative Sites of National Capital Territory, New Delhi, India

1. Introduction

Rapidly growing population exceeding 16 million in National Capital Territory (NCT), New Delhi is subjected to significant seismic risk. Delhi, the capital city and its surroundings have experienced several earthquakes in the historical past (Kafi Khan, 1874; Iyengar, 2000). During the recent times the most significant earthquake was August 1960 (M 6.0) having its epicenter with in NCT, New Delhi (Srivastava & Somayajulu, 1966). The city also experiences ground vibrations due to Himalayan earthquake. The 1905 Kangra and 1991 Uttarkashi earthquakes cause damage of intensity VI, V respectively in NCT region. So the seismic status of Delhi, in terms of historical seismicity and intensity experienced in recent times indicates the influence of soft soil deposits. Further, Bhuj earthquake of 2001 and Kashmir earthquake of 2005 has well demonstrated that the unconsolidated materials of young sedimentary basins can have a profound effect on the spatial distribution of earthquake ground motion amplification, resulting in variation in the severity of damage to buildings, transportation corridors and other lifeline infrastructures. This had also been experienced during the earthquakes of Mexico City in 1985, San Francisco in 1989, Los Angles in 1995 (Kramer, 1996; Ansal, 2004). Most of the NCT region is also on either river/lake deposits or thick sedimentary deposits except ridge area (comprises quartzite bed rocks). The shear wave of near surface material has long been considered as a key parameter in thick sedimentary deposits (Borcherdt, 1994) and characterized as a best indicator of stiffness (Bullen, 1963; Aki & Richard, 1980). Most of the building codes are based primarily on average shear wave velocity in the upper 30 m soil profile (Kramer, 1996; Street et al., 2001; Gosar et al., 2008). The shear wave velocities have been computed for ten representative sites in NCT Delhi region to validate 1^st level of microzonation map which discretizes the NCT into nine units based on quaternary-Holocene litho-fill content and other geotechnical data collected from different contributing institutions (Figure 1, Joshi et al., 2006; Shukla et al., 2007). So, the derived Vs profiles will be helpful to some extent to for future planning of extensive shear wave velocity investigations.

Figure 1.

Location shown as triangles on 1^st level seismic microzonation map of NCT Delhi based on parameters a) geological condition map b) bed rock and subsurface configuration of ridges and basins c) geomorphological and relief d) ground water conditions and e) frequency and amplifications based on ambient noise Nakamura technique after Joshi et al. (2006). Joshi et al 2006 defined the hazard level of each unit 1) Weathered rock zone: moderate hazard due weathering induced pronounced ground response 2) chhatarpur basin: high hazard due to basin effect 3) Central Delhi: Moderate hazard due to basin effect 4) Newer Alluvium proximal to Yamuna river: high hazard due to liquefaction potential 5) zone of basin margin effect west of Delhi ridge: high hazard 6) South Nazafgarh: Moderate hazard due to high amplification and liquefaction 7) Northwest Delhi plains: Moderate hazard due to thick soft sediments 8) Layer of impedance contrast at shallow depth: moderate hazard shown in light yellow colour 9), Ridge ambience of exposed rock: low hazard shown in grey colour