Studies on Site Specific Responses and its relationship with the Geological Formations in Kochi City of Kerala State, India

The site response studies have been undertaken by measuring records of ambient noise vibrations in Kochi city of Kerala state, India with the objective of defining the site effects as this forms the most important input associated with the local geological conditions and constitute an important part of any seismic hazard assessment and microzonation map. The resonance frequency and the ground motion amplification thus estimated show very interesting features in their spatial variability and have specific range of resonance frequency associated with different geological formations from soft soils to compact rocky sites. The results suggest that the resonance frequency varies significantly within short distances in and around Kochi city. The detailed seismic microzonation map prepared for Kochi city contains five classes of resonance frequency (<1.1, 1.1-3.0, 3.1-5.0, 5.1-10.0 and >10 Hz) and three classes of site amplification (<5.1. 5.1-10.0, >10.0). The resonance frequency is spatially distributed in three NW-SE trending segments parallel to the coast which differ appreciably in their site response characteristics. The lowest resonance frequency values (≤ 1.0 Hz) coupled with generally high site amplification were observed in coastal and backwater areas covered with younger alluvial deposits, and high resonance frequency values (> 5 Hz) to charnockites and laterites in the hinterlands. It is demonstrated that site specific natural frequency can be used very successfully to delineate geological formations in a region provided closely spaced ambient noise data are available.