Preparing for a big earthquake

A boy walks past cracks in the asphalt left during the January 12 earthquake in Port-au-Prince, Haiti.

A boy walks past cracks in the asphalt left during the January 12 earthquake in Port-au-Prince, Haiti.   | Photo Credit: Rodrigo Abd


Haiti, where 2,30,000 people are thought to have perished in an earthquake in January, has heightened the concerns of our seismologists and engineers.

“Earthquakes don't kill people, buildings do.” The adage is very much on the minds of seismologists and engineers who worry about what might happen if India were rocked by a powerful quake. Haiti's experience, where 2,30,000 people are thought to have perished in an earthquake that hit the Caribbean island in January, has heightened those concerns.

The Haiti quake was more than twice as lethal as any previous magnitude 7 event, observed seismologist Roger Bilham of the University of Colorado in a report in Nature.

The death and injury that befell about 15 per cent of those living in and around the Haitian capital were the direct consequence of decades of unsupervised construction, when every possible mistake went unchecked. “In recent earthquakes, buildings have acted as weapons of mass destruction,” Dr. Bilham went on to point out. Such deaths could be reduced significantly if minimal construction guidelines were mandated in all cities, especially those with a history of previous earthquakes.

That advice undoubtedly applies to India, which is no stranger to earthquakes. Many strong earthquakes strike at the Himalayas, the vast mountain chain, extending 2,900 km, that was thrown up when the Indian plate slammed into Eurasia between 40 million years and 50 million years ago. The strains produced by the Indian plate continuing to grind steadily northwards get released periodically in the form of earthquakes.

Medieval quakes occurred in the central Himalaya with magnitudes close to 9, Dr. Bilham told this correspondent. One that took place in June 1505 destroyed Agra, a number of other cities and huge areas of southern Tibet and Nepal. It is not possible to say when such a powerful quake will recur. But magnitude 7 quakes are quite likely, he added.

The central and northeast Himalayas have been historically deficient in earthquakes compared to other parts of the mountain range, according to C.P. Rajendran of the Centre for Earth Sciences, Indian Institute of Science, Bangalore. Earthquakes of magnitude 7 or greater could reasonably be expected in these places in the foreseeable future.

Just south of the Himalayas are vast plains where many populous towns and cities have sprung up on alluvial soil. This soil amplifies the shaking of the ground that an earthquake in the mountain produces.

“It is a big problem in India,” said Dr. Bilham. The Brahmaputra, the whole Gangetic plain and the Punjab are the worst places to site large cities. There may be as many as 50 million people living in the cities.

A large earthquake in the central Himalaya in 1803 not only triggered landslides that smothered villages in the hills but also caused damage in places as far away as Delhi, noted Dr. Rajendran in a commentary published in Current Science. The total casualties might have been in their thousands. It would be prudent to calculate the risk in the region if such an earthquake were to occur today in the Himalayas.

Nor are places elsewhere in the country free from the risk. There are known and as yet unknown geological faults in other parts of the country too that could become active again with potentially disastrous consequences, Dr. Rajendran told The Hindu. The Bhuj quake in January 2001, for instance, wreaked havoc on a considerable area in Gujarat and claimed over 20,000 lives.

Buildings, bridges, power plants and other structures can be designed to withstand earthquakes. Over the years, the Bureau of Indian Standards (BIS) has issued quake engineering codes that lay down specific requirements for various structures. For this purpose, the country has been divided into four zones of differing seismic risk. However, the codes are often not mandatory or are poorly enforced.

“Are we going to wait for a massive disaster before we wake up?” asked Sudhir K. Jain, a leading earthquake engineer and Director of the newly-established Indian Institute of Technology, Gandhinagar, in a telephone interview.

The country was going through a major development phase wherein infrastructure was added at an unprecedented pace, he pointed out in a paper published in 2005. It was a great opportunity to ensure that all new structures met seismic requirements. Instead, a huge number of unsafe buildings were continuing to be built every day in different cities and towns.

On the Andaman and Nicobar islands, for instance, a region of high earthquake risk, various buildings, jetties and even an important bridge were put up without paying heed to the seismic codes. When a quake of 9.3 magnitude that produced the lethal Indian Ocean tsunami struck in December 2004, these structures were badly damaged, in some cases irreparably, Dr. Jain and others observed in another paper.

The biggest problem was that the earthquake engineering codes were simply not being followed, he said during the interview. “First, we need to agree that we will enforce the codes.” The Central and State governments as well as the local authorities must act to ensure that unsafe buildings do not continue to be put up, he said.

After the Bhuj earthquake, many municipal authorities began demanding that a structural engineer (and others such as architects and builders) certify that a building complied with seismic codes. “Unfortunately, such certificates are easy to procure, sometimes on payment of small money, and need not have any correlation with how a building is built,” he pointed out in a paper.

Many existing buildings were likely to be unsafe in the event of a quake, he said. There had to be a long-term strategy to survey and retrofit at least important buildings, such as hospitals, so that they would become less vulnerable, he added. This might take decades but the process must start immediately.

The number of houses in the country went up by 45 per cent between the censuses of 1991 and 2001, said Anand S. Arya, Emeritus Professor at the Department of Earthquake Engineering in IIT, Roorkee. The number of “kachcha” houses made of clay had remained practically the same while those made with burnt brick had increased The latter, depending on the mortar used, would be a little less vulnerable than clay houses. But as most builders of brick houses would not probably have followed the code for earthquake resistance, these too could be badly damaged. The Indian code applicable to houses balanced the need for safety with what people could afford, pointed out Dr. Arya, who chairs the BIS committee for earthquake engineering codes. Even in the zone with the highest seismic risk, following the code in the construction of a house would increase its cost by only about six per cent. On the other hand, retrofitting earthquake resistance features could cost twice as much.

Safety was not just an engineering issue but a social one as well, observed Dr. R.N. Iyengar, former director of the Central Building Research Institute at Roorkee and now head of the Centre for Disaster Mitigation at the Jain University in Bangalore. A shed need not be built to the same level of safety as a hospital.

India was too vast and varied a country to be adequately covered with just four seismic zones. The existing codes give specifications for structures based on the maximum earthquake intensity that could be expected in each zone. Such an approach could not take into account the spatial variation in ground motion within a zone during a quake.

The National Disaster Management Authority was in the process of creating a probabilistic hazard map for the whole country, said Dr. Iyengar, who heads the committee overseeing the process. The map would provide the probabilities of ground motion that could be expected in squares of about 625 sq km during an earthquake. Using such information, a hospital or a school could be designed for a higher level of safety than a shed.

“We should not be caught unawares when the next major earthquake occurs,” said Dr. Rajendran.

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Printable version | Jan 25, 2020 8:33:49 AM | https://www.thehindu.com/opinion/lead/Preparing-for-a-big-earthquake/article16627863.ece

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