Are Risk to structures How high are the earthquakes expected in the Himalayas and north-eastern states? Scientists, structural engineers, representatives from several government ministries are expected to deliberate and report to the Cabinet Secretariat in the coming weeks.
This comes after the Bureau of Indian Standards (BIS) earlier this month ‘withdrew’ a set of updated norms that construction projects must follow to avoid collapse if hit by earthquakes.
A return signal was given – e.g. The Hindu Reported on 7 March – By an order of the Cabinet Secretariat, which stated that the new standards would “materially impact… ongoing and future infrastructure projects including metro rail projects” and “a holistic and comprehensive review of the revised IS 1893 should be conducted taking into account the viewpoint of all stakeholders.”
However, a decade of studies – commissioned and approved by the government – and involving scientists from India’s most prestigious institutions specializing in geology, seismology, geotechnical engineering and structural engineering, are clear: the potential damage to all structures, whether residential buildings, bridges, industrial structures or water tanks, dams, power plants in the Himalayan states, far exceeds current risk assessments, which in turn are factored into existing building codes across the country.
four zones
IS 1893 ‘Criteria for Earthquake-Resistant Design of Structures’ is a five-part document published by the Bureau of Indian Standards (BIS) that provides mandatory guidelines that engineers and architects must follow to ensure that buildings and infrastructure can survive seismic activity.
Seismic zone maps of India delineate the country into four zones (Zones 2II, III 3, IV4 and V5). Zone 2 II is the quietest part of India’s seismic landscape. The ground beneath you may move significantly during your lifetime, but the forces involved are relatively minor. The 2016 version of India’s seismic zoning map – which was to be replaced by the now withdrawn 2025 incarnation – provides a design acceleration of 0.10 g in Zone II, meaning engineers expect the lateral force on a building during the strongest possible earthquake in the region to be no more than about 10% of gravity’s downward pull.
Due to increasing perception of earthquake risk, India’s earthquake zones were reduced with each subsequent revision. | Photo Credit: Journal of Earth System Science, Vol. 133, article no. 158, (2024)
Zone V is a fundamentally different proposition. The design acceleration is at least 0.36 g or three and a half times that of Zone II. At those forces, the ground is bending sideways with a force equal to one third of gravity. Unsecured furniture falls, people can’t stand without holding on to something, and buildings experience forces that can bend steel columns, shatter concrete, and cause floors to collapse on each other if not appropriately designed. This is the area assigned to the Himalayan front regions and parts of northeastern India – areas located directly on or adjacent to one of the most active tectonic plate boundaries on Earth, where the Indian Plate is subducting into the Eurasian Plate and where a magnitude 8 earthquake has occurred within living memory. These proportions – 10%, 36% – are usually expressed as 0.10 g and 0.36 g and are called ‘peak ground acceleration’ (PGA) values in seismic zoning language.
At a distance of, say, 20 km from the fault – close enough to be in the zone of strong shaking, but not directly at the rupture – a magnitude 6 earthquake on the rock could produce PGA values in the range of 0.15–0.30 g used. A magnitude 7 earthquake, which releases about 32 times as much energy over the same distance, would typically produce PGA values in the range of 0.40 to 0.80 g. At very close distances, say 10 km, magnitude 7 can produce PGA values greater than 1.0 g. So, the ratio for a one-unit increase in magnitude over the same distance is approximately a factor of 2.5 to 3.5 in PGA.
very conservative
2024 paper was in peer-review before BIS withdrawal Indian Journal of Earth System Sciences This fundamentally reversed how India had accounted for earthquake risk in building design since the 1960s. It proposed a new methodology for estimating earthquake hazard risk and brought it in line with calculations from the rest of the world and, essentially – and ultimately controversially – increased PGA values under current building codes.
This was not a purely academic study: it was the result of a project launched in 2019 by the National Disaster Management Authority (NDMA), IIT-Madras and BIS, which aimed to bring India up to speed with the rest of the world and create a ‘potential hazard risk assessment map’ for India. It was preceded by two other projects, one in 2007–11 (funded by NDMA) and the other in 2013–17 funded by the Department of Science and Technology, both aimed at moving India’s earthquake hazard assessment towards a probabilistic framework.
The study was accepted by NDMA, following which its results were published by BIS as the 2025 update of the design earthquake hazard and zoning map of India in IS 1893 (Part 1) – which was later withdrawn.
The paper’s authors include scientists from some of India’s most prestigious institutions, including the IITs of Bombay and Madras, the Atomic Energy Regulatory Board and the Geological Survey of India.
“These PGA values have not been derived on the basis of any quantitative earthquake hazard assessment and are extremely low, especially for high seismicity zones. For example, the Himalayan plate boundary and areas of Northeast India that have the potential to generate earthquakes greater than magnitude 8.0 fall under Earthquake Zones IV and V with design PGA values of 0.24 and 0.36 g, respectively, while the Great Shillong Plateau earthquake of 1897 in North-East India resulted in PGA values are reported to be higher than 1.0 g,” reported in their study.
He argues that acceleration prices in India are very conservative by international standards.
“The design acceleration is considered to be two times (or more) in similar areas across the world compared to India’s current field map,” said a multi-author study led by researchers at IIT-Madras. Therefore, there is a need to revise India’s current earthquake-resistant design code based on an efficient quantitative technique such as hazard analysis, but it included several other authors from other institutions and organizations.
Government House in Shillong before and after the earthquake of 1897. | Photo Credit: Public Domain
barely a century old
In previous assessments, an area was assigned Zone IV or Zone V classification only retrospectively, that is, after it had experienced a significant earthquake. Nearby areas, which were often more susceptible, were considered to be at lower risk, even though evidence had built up over the decades that they were often areas of suppressed stress that had not been released.
The current structure also does not take into account local soil conditions, which can amplify waves radiating from the center, thereby increasing pressure on the building. It also overlooks a network of 168 monitoring stations, most of them in the Himalayas, which also relay data on small earthquakes of magnitude 2 and 3 and their associated energy waves from India’s neighbourhood, including Afghanistan and Xinjiang.
“The Earth is a dynamic system. Every 50 years or so, there is about a 10% change in PGA values. Age affects the human body, and so does the Earth,” said OP Mishra, former director of the National Center for Seismology (NCS), India’s official repository of earthquake data. “In a probabilistic assessment you take into account a variety of factors that can influence shocks and that serves as a reference point for builders… India has stated its commitment to be disaster resistant by 2047.”
Dr. Mishra was part of the BIS committee that revised the code.
The historical record of major earthquakes in India is extremely short relative to their recurrence intervals. The authors cite four major Himalayan earthquakes since the late 1800s – Shillong in 1897, Kangra in 1905, Bihar-Nepal in 1934, and Assam-Tibet in 1950. But these four events occurred on different segments of the plate boundary. According to the authors’ estimates, any individual segment may experience a major earthquake only once every 250 to 500 years. India’s instrumental seismic record is barely a century old, and the historical record, though long, is poor and incomplete. A 500-year recurrence event has a reasonable probability of occurring during the 50-year design life of a building (about 10%), but may not occur at all during the observation period available for a given fault segment.
There is no system to keep its accounts in the traditional approach.
‘Costs will increase’
More than 79% of India’s population lives under moderate to severe earthquake risk with approximately 57% of its land. By 2046, the urban population is projected to exceed the rural population, the study underlines the need for updated acceleration values.
The new map specifies PGA values of 0.15 g, 0.3025 g, 0.4535 g, 0.605 g and 0.75 g for Zones II to VI, almost doubling the threat estimates in the higher zones and bringing them closer to internationally comparable zones. These are the values that have troubled agencies like the Delhi Metro Rail Corporation and the National Dam Safety Authority.
The new map also introduces a fifth zone, Zone VI, for areas where PGA is estimated at more than 0.6 grams. It captures the most seismically active areas – parts of the Himalayan plate boundary and northeastern India – which were previously lumped together in Zone V at an inadequate 0.36 g. It is based on a far richer dataset and methodology than any predecessor: an earthquake catalog of 69,519 events spanning from 2600 BC to December 2019.
Raw area map of India. | Photo Credit: Journal of Earth System Science, Vol. 133, article no. 158, (2024)
“The estimates are scientific. However a figure like 0.75 grams represents the worst case scenario. Usually the practice is to halve the PGA value as representative of the potential and then, depending on the severity of the formation, the formation is planned,” said a government department official, who declined to be on record.
“Recent evidence shows that 95% of those who die in earthquakes are those who live in 1-3-storey houses that are inadequately designed,” said another scientist who was involved in the exercise, but also declined to be identified. “The 0.75 factor is also a low number. Although there are ongoing discussions about what adjustments can be made to the design, it is quite clear that the risks we have now calculated are close to reality.” Pakistan and Nepal use values closer to 0.75 grams, this person said, and the United States and Japan routinely calculate values of 1 gram or more, depending on the area at stake.
An NDMA official, who was aware of the developments surrounding the zoning map, said that the results of the scientific assessment were the result of “very pure science”. He said that although the NDMA had indeed considered the report, the difficulty that emerged was that the recommended acceleration values were “too high” and would put considerable pressure on government public expenditure.
“On the one hand our position is that a large portion does not even follow the existing (2016) code. Then with these stringent figures, the cost of steel, cement will increase manifold – where there is money to build four schools or clinics in a village, only one will be built. Ultimately, this does not give freedom to builders to make their own assessments and calculate the PGA risks. It became too much of a theoretical exercise.”
NDMA did not respond to requests for official comment.
jacob.koshy@thehindu.co.in
