Is chennai prone to earthquakes

Answer 1

CHENNAI: For a city that has one of the most vulnerable Coastlines in the country, here is more cause for worry: a major part of Chennai is seismically prone and is a `high hazard' earthquake area, according to a recent study by Anna University.

Carried out by the university's Centre for Disaster Mitigation and Management (CDMM), it calls for "seismic micro zoning" of the entire city. Also, new buildings should incorporate appropriate foundation designs and earthquake-resistant features.

Zonal classification

At present, Chennai, Coimbatore and Salem fall under Zone III of the Bureau of Indian Standards classification. As per the CDMM's microzonation profile, parts of the city such as areas in and around Adyar, Guindy, Vadapalani, Nungambakkam and Vyasarpadi may well fall within Zone IV (high to very high seismic risk).

"Chennai has been found to have loose soil till 15 metres of depth. Though the city is prone to quakes of the intensity of 5.8 on the Richter scale, the loose soil leads to seismic amplification in the range of 6.3 to 7.2 on the Richter scale," says S. Rajarathnam, Director, CDMM, who, along with another scientist, G.P. Ganapathy, conducted the study. The study analyses seismicity in areas with loose soil of varying depths, ranging from zero to three metres, three to six metres, six to nine metres, nine to twelve metres and twelve to fifteen metres.

At a depth of three metres, high seismic hazard areas were found distributed in the north and southwest and the mouth of the Adyar in east Chennai. The remaining areas were prone to moderate seismic hazard.

At a depth of six metres, parts of the Taramani-Tiruvanmiyur stretch and Indira Nagar fell under the `high seismic hazard' category. In central Chennai, `high hazard' areas were found in parts of R.A. Puram, Teynampet, Mylapore, Alwarpet, the Kodambakkam-Vadapalani belt, Chetpet, Egmore and the northeast of Thousand Lights. In the northwest, Anna Nagar West and Anna Nagar East were hazard prone areas. North Chennai was found to have the largest area of very high to high hazard prone areas in Vyasarpadi, Basin Bridge, Kondithope and George Town.

At nine meters depth, the high seismic hazard areas became linear and narrowed down to small areas in south Chennai, and the largest chunk was in the flood plain area. Hazard prone areas lay in Teynampet, Alwarpet, Mylapore, Gopalapuram, Royapettah and Thousand Lights.

At twelve-metre depth, high hazard areas were distributed in the central and western parts of the Cooum and Adyar flood plains. At fifteen metres, high hazard areas were in parts of West Mambalam, Nandanam, Mylapore, Saligramam and Alwarpet.

"The hazard can be nullified if foundations for new buildings penetrate the column of loose soil and rest on the hard rock surface below. Normally, multi-storeyed buildings are on safer ground as the foundation goes to fifteen feet. If medium-level buildings too have an appropriately deeper foundation, there is no cause for worry," the scientists say.