Understanding Delhi’s vulnerability to quakes
85.5% of the houses in Delhi have moderate damage risk according to the Vulnerability Atlas of India (1997)
Delhi is located in the country’s Seismic Zone 4, which magnifies chances of earthquakes of magnitude 7.0 to 7.2 occurring here. The city suffered some light damage with six buildings reportedly developing cracks due to the magnitude 7.9 earthquake in Nepal last Saturday. A 6.9 tremor, taken to be an aftershock, also occurred in the NCR on Sunday a little after noon. Delhi has also felt the after affects of the earthquakes at Uttarkashi (1991) and Chamoli (1999) in recent years. Even the earthquake of magnitude 5.6 that hit the Bay of Bengal on May 21, 2014, was reported to have jolted Delhi and NCR among other places!
A low ridge of quartzite exists in the NNE-SSW direction in the an otherwise flat terrain of Delhi; this is considered to be an extension of the Aravalli mountain range that runs for several hundred kilometres south of Delhi. Geological features, called ‘faults’, which are rupture planes in rocks that have brought about displacements in them, are believed to be responsible for generating earthquakes if they are still ‘ active’. Iyengar and Ghosh (2004) have catalogued a number of such f aults around Delhi of which the Mahendragarh-Dehra Dun and the Moradabad Faults would appear to be important.
Earthquakes in the past around Delhi
A good number of earthquakes appear t o have occur red around Delhi from 1720. The Chamoli temblor of magnitude 6.5 with its epicentre about 280 km from Delhi had caused damage to several buildings in the Capital. Because of its proximity to the Himalayan Range, the city is prone to damage by large earthquakes in the Himalaya besides nearby shocks.
Safety of buildings in Delhi
It does not need to be overemphasised that it is the falling parts of buildings that account for all the casualties during an earthquake. Earthquakeresistant construction was known even in the 1930s as shown by Dr WD West (1937) of the Geological Survey of India who investigated the Quetta Earthquake of 1935 in undivided India. Ironically, however, “till date there is no legal framework to require that all constructions in Delhi must implement seismic code provisions” according to the National Disaster Management Authority, New Delhi (2014). This is a sorry state of affairs and conformity to standard building codes needs to be enforced by appropriate legislation since 85.5% of the houses in Delhi have moderate damage risk according to the Vulnerability Atlas of India (1997).
Safety of very tall highrises
Our structural engineers and architects keep on assuring us that they can erect all the highrise buildings on ‘raft foundations’ which will enable them to sway like ships “pitching and tossing during a storm but not sinking”. However, with due respect to all of them, a significant finding by Chris Rorres, a mathematician at the University of Pennsylvania, reported by Dana Mackenzie in the June 2005 issue of Discover, goes to show that the buildings whose heights were 1.8 times their base lengths could overturn during an earthquake and remain tilted permanently even after the earthquake vibrations passed off.
There are a good number of buildings in the NCR that are more than 30 storeys high – 155 to be precise. In Gurgaon and Noida there are 16 more than 40 storeys high and 6 over 50 storeys according to Liases Foras. It will be prudent to look at their seismic safety and also curb the tendency to go too vertical in construction.
People wait out side their office buildings after a massive earth quake in New Delhi, on Saturday, April 25