Large parts of Kochi city are susceptible to amplification of seismic waves generated by distant earthquakes like the powerful one that struck the Himalayan region on Tuesday, geoscience experts say.
“Local geological conditions play a key role in the ground motion triggered by a distant earthquake,” says John Mathai, Scientist, National Centre for Earth Science Studies (NCESS). “Each site has a particular resonance frequency at which ground motion gets amplified. This highlights the importance of micro-zonation in assessing the earthquake risk of a particular site,” he said.
A seismic zonation study conducted by scientists of the Indian Institute of Technology (IIT), Roorkee, Benaras Hindu University, and NCESS had revealed that the area in and around the coastal belt and backwater region of Kochi, characterised by soft soil and sediments, was prone to amplified ground vibrations caused by transmission of the seismic waves. The study by H.N. Singh, D. Shanker, and others analysed ambient noise records to assess site-specific hazards due to ground motion in Kochi.
It found that amplification of ground motion was very high in the western parts of the city comprising the coastal belt, backwaters, and islands including Vypeen, Vallarpadam, Willingdon Island, Kadamakudi, Chittoor, Fort Kochi, Aroor, Vaduthala, Pannangad, and Turtibhagam. The zone covering approximately 34 per cent of the city is characterised by loose alluvial deposits above shallow hard rock. The scientists warn that buildings in this zone are highly prone to resonance when the frequency of earthquake-induced ground motion matches the natural frequency of the structures.
Pointing out that prolonged shaking caused by ground motion amplification could result in severe damage to buildings, the authors stress the need for strict enforcement of building codes in the zone. As far as possible, construction of high-rise buildings is to be avoided, the paper says. “Unlike the coastal areas, the hinterlands are marked by deep hard rock and hard charnockites and laterite soil which are not as susceptible to site amplification,” says Mr. Mathai.
