Are earthquake risks for structures in the Himalayas and northeastern states overestimated? Scientists, civil engineers and representatives from several government departments are expected to hold talks and report to the Cabinet Secretariat in the coming weeks.
This comes after the Bureau of Indian Standards (BIS) “withdrawn” earlier this month a set of updated criteria that construction projects should incorporate to prevent collapse in the event of an earthquake.
The withdrawal was prompted — as The Hindu reported on March 7 — by a Cabinet Secretariat order that said the new standards had “substantially affected … ongoing and future infrastructure projects including metro rail projects” and that “a holistic and comprehensive review of the revised IS 1893 has been undertaken taking into account the perspectives of all stakeholders”.
However, ten years of studies – commissioned and approved by the government – and involving scientists from India’s most renowned institutions with expertise in geology, seismology, geotechnical engineering and civil engineering are unequivocal: the potential damage to all structures, whether residential, bridges, industrial structures or reservoirs, dams, power plants in the Himalayan states, is much higher than existing building risk assessments.
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 binding guidelines to be followed by engineers and architects for buildings and infrastructure to survive seismic activity.
India’s seismic zoning maps delineate the country into four zones (Zone 2II, III 3, IV4 and V5). Zone 2 II is the quietest part of India’s seismic landscape. The ground beneath you may shake noticeably during your lifetime, but the forces involved are relatively modest. The 2016 version of India’s seismic zoning map – the one that was supposed to be replaced by the now-withdrawn 2025 avatar – assigns a design acceleration of 0.10g in Zone II, meaning engineers expect the lateral force on the building to be no more than about 10% of the downward gravitational pull in the region of the strongest earthquake.
Earthquake-prone areas in India have been reduced with each subsequent revision, due to increased awareness of the threat of earthquakes. | 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, i.e. three and a half times higher than for zone II. With these forces, the earth rocks sideways with a force equal to one-third of gravity. Unsecured furniture topples over, people can’t stand without holding onto something, and buildings experience forces that can bend steel columns, crack concrete, and cause floors to stack on top of each other if not properly designed. This is the zone assigned to areas along the Himalayan front and parts of northeastern India—regions lying directly on or adjacent to one of the most active tectonic boundaries on Earth, where the Indian plate meets the Eurasian plate and where magnitude 8 earthquakes have occurred within living memory. These proportions—10%, 36%—are usually expressed as 0.10 g and 0.36 g in seismic zoning are called “Peak Ground Acceleration” (PGA) values.
At a distance of, say, 20 km from the fault – close enough to be in the strong shock zone, but not directly on the fault – 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 more energy, over the same distance would typically produce PGA values in the range of 0.40 to 0.80 g. At very small distances, say 10 km, a magnitude 7 can produce PGA values in excess of 1.0 g. So the ratio is roughly a factor of 2.5 to 3.5 in PGA for a one unit increase in magnitude over the same distance.
Too conservative
Before the BIS withdrawal, there was a 2024 paper in the peer-reviewed Indian Journal of Earth System Sciences that radically changed the way India attributed earthquake risk in building design since the 1960s. It devised a new method for estimating earthquake hazard risk and adapted it to how the rest of the world calculates it, inevitably – and ultimately controversially – raising the PGA values that underpin current building regulations.
This was not a purely academic study: it was the result of a project commissioned by the National Disaster Management Authority (NDMA) in 2019 from IIT-Madras and BIS to bring India at par with the rest of the world and create a “probabilistic risk assessment map” for India. This was preceded by two other projects, one from 2007–11 (funded by NDMA) and another from 2013–17 funded by the Ministry of Science and Technology, both of which aimed to move earthquake hazard assessment in India towards a probabilistic framework.
The NDMA accepted this study, after which the results were published by BIS in IS 1893 (Part 1) as an update to India’s draft 2025 Earthquake Hazard and Zone Map – which has since been withdrawn.
The authors of the paper are scientists from some of India’s most prestigious institutions, including IITs Bombay and Madras, the Atomic Energy Regulatory Board and the Geological Survey of India.
“These PGA values are not derived from any quantitative assessment of earthquake hazard and are abysmally low, especially for higher earthquake zones. For example, areas of the Himalayan plate boundary and northeastern India with the potential to produce earthquakes exceeding magnitude 8.0 are covered by earthquake zones IV and V with design PGA values of 187g, 0.324, respectively. The plateau earthquake in northeastern India has reportedly resulted in to PGA values greater than 1.0 g,” their study states.
He claims that India’s acceleration figures are too conservative by international standards.
“Design acceleration in similar areas around the world is considered to be two times (or more) compared to that in the existing zoning map of India. Hence, there is a need to revise the current Indian earthquake-resistant design code based on a qualified quantitative technique such as risk analysis,” said the multi-authored study led by researchers from IIT-Madras, but including several other organizations from other institutes.
Government House in Shillong before and after the earthquake of 1897. | Photo credit: Public domain
Barely a century old
In previous assessments, a region was assigned a Zone IV or Zone V classification only retrospectively, that is, after it had experienced a significant earthquake. Surrounding areas, which were often equally susceptible, were thought to be at lower risk, although evidence has accumulated over decades that these are often areas of pent-up stress that has not been released.
The current framework also failed to take into account local soil conditions, which could amplify waves emanating from the centre, thereby increasing the forces to which the building was subjected. It also overlooks a network of 168 monitoring stations, most of them in the Himalayas, that transmit data on even small earthquakes of magnitude 2 and 3 and their associated energy waves from India’s neighborhood, including Afghanistan and Xinjiang.
“The Earth is a dynamic system. Every 50 years or so, PGA values change by about 10%. 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 earthquake data repository. “In the probabilistic assessment, you take into account various factors that could affect shocks and that serves as a reference point for builders… India has committed to be disaster-resilient by 2047.”
Dr. Mishra was part of the BIS committee to revise 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 19th century—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. Any single segment may experience a major earthquake only once every 250 to 500 years, the authors estimate. India’s instrumental seismic record is barely a century old, and the historical record, though longer, is patchy and incomplete. A 500-year recurring event has a reasonable probability of occurring during the 50-year design life of the building (roughly 10%), but may not have occurred at all during the available observation time for the failure segment.
The traditional approach has no mechanism to account for this.
“Costs will rise”
More than 79% of India’s population lives on approximately 57% of its land, under moderate to severe earthquake hazard. By 2046, the urban population is expected to exceed the rural population, with the study highlighting the need for updated acceleration figures.
The new map assigns Zones II to VI PGA values of 0.15g, 0.3025g, 0.4535g, 0.605g and 0.75g, which nearly doubles the hazard estimates in the higher zones and brings them closer to what comparable areas internationally are designed for. It is these figures that are likely to have rattled 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 estimates exceed 0.6 g. This captures the most seismically active areas—parts of the Himalayan plate boundary and northeast India—that were previously all lumped together in Zone V at a deficient 0.36 g. It is also based on a much richer data set and methodology than any predecessor: an earthquake catalog containing 69,519 events from 2600 BC to December 2019.
Raw zone map of India. | Photo credit: Journal of Earth System Science, Vol. 133, Article No. 158 (2024)
“Assessments are scientific. However, a number like 0.75g represents a worst-case scenario. The practice is usually to halve the PGA value as a representative of what is likely and then, depending on the criticality of the structure, plan the structure,” said a government department official who declined to be on the record.
“Recent evidence shows that 95% of those who die in an earthquake are those who live in 1-3 story houses that are not adequately designed,” said another scientist who participated in the exercise but also declined to be identified. “A factor of 0.75 is also a lower number. Although there is going to be a consultation on what design modifications can be made, it is very clear that the risks we have now calculated are closer to reality.” Pakistan and Nepal, the person added, use values close to 0.75 g, while the United States and Japan routinely calculate values of 1 g or more, depending on the area involved.
One NDMA official who was privy to the developments surrounding the zone map said the results of the scientific assessment were the result of “too much pure science”. He said that while the NDMA did consider the report, the issue that emerged was that the recommended acceleration values ”were too high” and would put a significant burden on the government’s public expenditure.
“On the one hand, we have a situation where a large part does not even comply with the existing (2016) regulations. Then with these strict numbers, the cost of steel and cement will increase many times – where there is money to build four schools or clinics in a village, only one will be built. Finally, this does not give the builders the freedom to do their own assessment and calculate the risks of PGA and theoretical exercises.”
The NDMA did not respond to requests for official comment.
jacob.koshy@thehindu.co.in
