Bangalore: Tall buildings in Delhi will come crashing down if a strong earthquake occurs in the northwest region of the Himalayas, warns a research seismologist who had predicted the Sumatran quake that caused the deadly tsunami in 2004.
Buildings taller than 17 metres in the nation`s capital are vulnerable even though the city is more than 300 km away from the Uttarakhand-Himachal region where scientists expect the next high magnitude earthquake, Pune-based Arun Bapat said.
Bapat, formerly head of the earthquake engineering department at the Central Water and Power Research Station, says his warning is based on a careful analysis of damage caused by the 7.9 magnitude Gujarat earthquake that occurred Jan 26, 2001, with its epicentre near Bhuj.
"Maximum destruction from an earthquake is normally confined to an area of 20 to 30 km radius from the epicentre," Bapat said.
"However, in the case of the Bhuj quake, extensive damage was caused in Ahmedabad, which is about 320 km from Bhuj." While tall buildings in Ahmedabad collapsed, the damage was minimal to buildings that had only two or three floors, he says.
The "distance effect" - where the damage is felt far away from the epicentre -- is characteristic of "Rayleigh waves" produced during an earthquake, explains Bapat.
He said there are about 100 tall buildings in the Delhi municipal area and an equal number in the nearby areas of Noida, Gurgaon, Faridabad and Ghaziabad, all of which need strengthening to protect against Rayleigh waves.
Unlike the "P" and "S" waves that travel through the body of the earth and cause damage close to the epicentre, the Rayleigh waves roll along the surface of the earth just like waves on the ocean and cause damage at a distance -- typically between 150 to 550 km from the epicentre, he says.
The damage due to Rayleigh waves occur at a distance because the "amplitude" or strength of these waves is higher far away from the epicentre than closer to it, says Bapat.
A situation similar to what happened in Ahmedabad during the Bhuj earthquake will be repeated in Delhi if an earthquake of magnitude 7.5 or more occurs in Himachal or Uttarakhand, he says.
"Rayleigh waves from such an earthquake would definitely cause heavy damage to tall structures in Delhi and the entire National Capital Region (NCR)," he said.
According to Bapat, despite the Bhuj earthquake - that wreaked havoc in distant Ahmedabad -- the Bureau of Indian Standards (BIS) has not revised the seismic code for tall structures to take into account the likely effect of Rayleigh waves.
The BIS code only provides guidelines for design and construction of buildings to protect them from the adverse effects of "S" waves but not Rayleigh waves, he said.
According to Bapat, during the 8.1 magnitude Mexican earthquake on Sep 19, 1985, Mexico City suffered extensive damage although the epicentre of this earthquake was located at a distance of about 530 km on the Pacific coast.
Again the 8.0 magnitude earthquake witnessed by Pakistan Oct 8, 2005, destroyed the tall buildings in Islamabad although the epicentre of this quake was about 150 km from the Pakistani capital.
These examples from the recent past in addition to the experience of Delhi`s Qutab Minar during the powerful earthquake in the Himalayas Sep 1, 1803 -- when two upper floors of the 72-metre tall structure were dislodged -- are enough to give an idea of the "distance effect" on tall structures, Bapat said.
Mexico revised its seismic code after the 1985 earthquake damage and many countries including the United States, China and Japan have taken steps to protect the tall structures from possible damage due to Rayleigh waves, Bapat said.
"But the BIS is yet to initiate any action about revision of the seismic code in India," he said. "If no action is taken immediately, it is quite possible that the scenarios at Mexico City and Ahmedabad may be repeated in the NCR of Delhi."
Not only Delhi but all cities located at a vulnerable distance from potential epicentres of large magnitude earthquakes should make suitable provisions in the seismic codes, he said.
Other vulnerable cities which could see damage to tall structures from large magnitude earthquakes in northeast India are Kolkata as well as Dhaka and Chittagong in Bangladesh, says Bapat.
"Lahore and Islamabad in Pakistan could suffer from large magnitude earthquakes in the Himalayas and Hindukush while Mumbai and Karachi could possibly suffer damage due to a tsunami produced by an earthquake in the Makran coast."