Salt Lake, Rajarhat–New Town and densely populated areas in the south eastern fringes along EM Bypass that are witnessing a real-estate boom, along with other pockets spread across the city such as Kasba, Park Street, BBD Bag are floating on a bed of slurry and could just sink in case a major earthquake strikes Kolkata.
A four-year study by experts from IIT Kharagpur has revealed that Kolkata stands on soft alluvium-kind soil.
The study, which kicked off in July 2011, with funds from union ministry of earth sciences ended in June 2015. The results are presently being compiled along with the maps before it is handed over to both the state and union government in November this year.
This layer of sand, clay, silt and rotten vegetation extends up to a depth of 7.5 km from the surface. In between there are ground water aquifers, many of which are very shallow and are located within one to seven metres from the surface.
On the east there is the East Kolkata Wetlands and on the west flows the Hooghly. All these provide the water source that will contribute to the unstable nature of the region.
“A major earthquake measuring more than 6.5 on the Richter scale can lead to a phenomenon called liquefaction – a process by which soft soil mixes with water and temporarily loses its strength. The gluey mass acts like a fluid and buildings simply sink in it. This effect can be caused by earthquake shaking,” Sankar Kumar Nath, an IIT Kharagpur professor who led the study, told HT.
The situation is somewhat like wiggling your toes in the wet sand near the water on a sea beach.It would give you a feeling as if the ground beneath your feet is moving away.
Liquefaction is, however, not new to Kolkata. In 1934, an earthquake measuring 8.1 on the Richter scale with its epicenter somewhere near Bihar-Nepal border, triggered liquefaction in several parts of Kolkata. Several areas, including Metiabruz, Ballygunge, Rabindra Sadan, Sovabazar suffered major damage as has been found in ‘Memoirs of the Geological Survey of India’.
The team took soil samples from across 350 locations by digging bores covering an area of 435 sq km spanning over Salt Lake, Kolkata and Howrah. Water tables depths were also studied.
“We found that if liquefaction takes place it would be triggered in at least three layers. The most dangerous would be the one which would occur within 4 – 10 metres below the ground. This would cause the maximum damage to structures on the ground,” said Nath.
Earlier the entire country was categorized into four earthquake zones – II, III, IV and V. Cities under zone II were less vulnerable while those under V were prone to high-intensity earthquakes. While Mumbai, Chennai and Kolkata were categorized as a zone III cities Delhi came under zone IV and Srinagar and Guwahati under zone V.
“But now the ministry of earth sciences have kicked off a project where 42 cities would further divided into micro-zones as per their earthquake vulnerability. Kolkata has been divided into four zones. Some areas such as Kestopur, Salt Lake, Nicco Park comes under zone V and are highly hazardous areas, places around Sarsuna and Joka are least vulnerable,” Nath, who also heads the national steering committee for preparing microzonation map for the cities, told HT.