If Delhi or Mumbai witness an earthquake of a magnitude of over six on the Richter scale, the damage will be much more immense. While in Mumbai over 34 lakh structures are vulnerable to a quake, in Delhi, this number goes over 33 lakh.

In a nationwide study conducted by the Building Material and Technology Promotion Council, more than 250 million buildings across the country are vulnerable to the an earthquake, with the capital cities topping the list as they have the maximum number of hazardous structures.

The BMC along with IIT-Bombay has conducted a similar study for the city. "We found that the high-rise buildings and old structures are more vulnerable. The slums, because of their low height and easy exits, are likely to face lesser damage," said professor Ravi Sinha, earthquake engineering expert at IIT-B.

In Delhi, the areas around the Yamuna bed have been marked has highly vulnerable. Several buildings that are over a hundred years old too have been declared hazardous. "With the increase in the Floor Area Ratio, the number of storeys in the buildings have increased, but no efforts have been made to strengthen the buildings," said Sampurnananda Mahapatra, senior earthquake scientist.

While there is no foolproof and accurate mechanism of predicting a quake's timing, but seismologist are of the view that an earthquake of a similar intensity could revisit the Himalayas and could have an effect on the Indian plains.

"The stress released from the earthquake can travel up to five hundred kilometers. Going by that logic, there is a possibility of an earthquake along the Indian Himalayas," explained Dr BK Rastogi of the Institute of Seismological Research, Gujarat. The Kangra region in Himachal Pradesh had witnessed an earthquake of magnitude of over seven richter in 1905. "An earthquake is due in the region," added Rastogi.

The National Disaster Management Authority that has compiled the vulnerability atlas wants the states to make builders and engineers accountable for poor-quality construction. "Currently there is no check or penalty clause to bind the builder. The bill to make the real estate developers accountable has been introduced in the Parliament in 2007, we will push for it to be passed at the earliest," said a senior NDMA functionary.

Mumbai, located along the seismic zone 4, is approaching an earthquake of close to six magnitude. The Koyna Dam in Satara, in the same zone, had received an earthquake of 6.5 magnitude in 1967.

How frequent is a big quake?Data collected by over 300 years also suggest that an earthquake of eight on the Richter scale repeats itself every 100 years, an earthquake of seven every 50 years, and a quake of six can surface every 30-40 years.Gujarat's 2001 earthquake had witnessed a shock of 7.7 magnitude in 1905. The quake repeated itself after 96 years.