75 years since Assam quake, Himalayas prep for big hydro tasks
On August 15, 1950, India was celebrating its Independence Day. The temper was upbeat all through the nation. Simply as celebrations have been winding down for the day, an excellent catastrophe struck. At round 7:30 pm, an earthquake of magnitude 8.6 — the strongest ever recorded on land — jolted the nation’s Northeast and a few neighbouring areas past the border.
In accordance with some stories, the earth shook for about 4 to eight minutes. Mountains stumbled and buildings fell, inflicting widespread loss of life and destruction. An English botanist and explorer named Frank Kingdon-Ward was tenting at Rima (Zayu) on the day. He reported later: “I used to be seated writing my diary close to the doorway to our tent. All of a sudden, after the faintest tremor, there got here an appalling noise, and the earth started to shake violently. … We have been instantly thrown to the bottom. The lantern too was knocked over and went out immediately.”
The earthquake was felt over an space of three million sq. km throughout India, Myanmar, and Bangladesh, Tibet, and South China. It wrecked houses, farms, and railway tracks, bridges, and different utilities. A discipline investigation report described how “rail tracks have been torn up and twisted into snake-like patterns”, in a vivid demonstration of how the land and constructions deform in response to shear waves. On the Indian aspect alone, greater than 1,500 folks misplaced their lives, and 50,000 to 1,00,000 cattle have been killed. The quake was felt as far afield as Lhasa and Sichuan, and in Yunnan province in China. Within the Medog space of Japanese Tibet, Yedong village slid into the Yarlung Zangbo river, and greater than 4,000 casualties have been reported from Tibet. Extreme injury was wrought within the Sibsagar-Sadiya space of Assam as properly.
Extra destruction was to observe within the days after the earthquake. Many hills had been sheared by the earthquake. Rocky particles falling into the valleys under blocked rivers — earlier than giving means days later, producing flash floods downstream that killed a whole lot of individuals dwelling on the rivers’ banks. On September 9, two weeks after the earthquake, the then Prime Minister Jawaharlal Nehru mentioned in a nationwide broadcast on All India Radio: “The Brahmaputra was blocked up for some time, after which broke via, they got here down with a rush and a roar, a excessive wall of water sweeping down and flooding massive areas and washing away villages and fields and gardens. … The stays of villages, animals, together with cattle and elephants, and enormous portions of timber floated down these raging waters…”.
Revisiting the earthquake
The supply of the Nice Assam Earthquake lay 40 km west of Rima (Zayu), the village close to the India-Tibet border within the Mishmi hills the place Kingdon-Ward was camped. The quake occurred alongside the boundary the place the Indian and Eurasian Plates collided, close to the jap terminus of the Himalayas, at a depth of 15 km. The rupture prolonged from the Mishmi thrust of the Japanese Himalayas to the Himalayan Frontal Thrust of Arunachal Pradesh, finishing a curvilinear movement across the mountainous bend (see picture above).
Because it occurred, the shaking additionally occurred at a time when seismographic networks have been increasing worldwide, offering nice impetus to earthquake monitoring and the following improvement of the idea of plate tectonics. It was additionally the time instrumental monitoring of earthquakes had began in India, with the India Meteorological Division (IMD) organising its first seismological observatory at Alipore in Kolkata district in 1898.
Immediately, we perceive the Nice Assam Earthquake occurred on a plate boundary shaped by the collision of continental plates, like these alongside different components of the Himalayan plate boundary. Nonetheless, it’s differentiated by the truth that its supply was within the Japanese Himalaya, which is tectonically very advanced. GPS knowledge point out that whereas Indian and Eurasian continental plates are converging at about 20 mm/12 months on common throughout the Himalayas, within the Japanese Himalayas it ranges from 10 mm to 38 mm/12 months.
This variation may very properly be attributable to difficulties in capturing tectonic complexity and variations in knowledge high quality. Whereas the principle arcuate Himalaya plate boundary resulted from the collision of the Indian and Eurasian plates, the Sunda plate can also be concerned within the northeastern Himalaya, creating a fancy construction.
As a result of plate rotation, main structural parts additionally take sharp turns and the regional strike shifts from the overall NE-SW course to the NW-SE course post-collision, forming what geologists name the Japanese Himalayan Syntaxis (EHS). That is the place the Nice Assam Earthquake seemingly occurred.
Not like different Himalayan earthquakes, which have exhibited a thrusting mechanism — the place one block of the fault thrusts over the opposite — the Assam earthquake displayed a part of strike-slip movement, with each blocks sliding previous one another alongside the fault. This instructed it was related with the EHS trending within the NW-SE course.
Fashions additionally point out a thrusting part, seemingly ensuing from the propagation of earthquake faulting in direction of the west, the place thrust tectonics are predominant. The potential for a number of faults being linked to the Assam earthquake can also be supported by the distribution of revised aftershock places, unfold over a large zone east of the principle shock epicentre.
Certainly, most researchers consider the earthquake could have began off on the Syntaxial bend whereas additionally activating Himalayan thrust faults to the west. Historic paperwork from the Ahom interval (1228-1826) reveal the Northeast India area skilled earthquakes in 1548, 1596, and 1697 AD, all of unsure magnitudes. Geological research have additionally revealed a serious medieval earthquake between 1262 and 1635 AD.
Quakes of the long run
Wanting again, the Nice Assam Earthquake delivered to scientists a major lesson on the damaging potential of main earthquakes within the Himalayas. Can the same earthquake strike Northeast India once more? Because the Indian plate continues to maneuver ahead, earthquakes are an integral a part of its future. But our present information doesn’t permit us to foretell when, the place or how sturdy the following quake shall be. For now, we solely know the Central Himalayas are essentially the most doubtlessly energetic phase and will generate a 1950-type earthquake in future.
The seismic occasion additionally highlighted that we’re extra susceptible at this time, primarily as a result of exponential progress of the constructed setting and concrete areas, leaving the panorama to look very totally different from the best way it did 75 years in the past. As we plan for extra developmental actions and undertake heavy infrastructural tasks, together with massive dams, on this tectonically fragile space, we should replicate on the pictures from the 1950 quake.
Lastly, the occasion additionally proved the Himalayan tectonic segments alongside the 2500-km-long stretch are absolutely able to producing earthquakes of magnitude 8.6 or greater. As each China and India put together to construct massive hydroelectric tasks within the Japanese Himalayan bend, probably the most seismically susceptible components of the majestic vary, it’s clear the highway forward shall be lengthy and difficult.
Kusala Rajendran is a former professor on the Centre for Earth Sciences, Indian Institute of Science, Bengaluru. C.P. Rajendran is an adjunct professor on the Nationwide Institute of Superior Sciences, Bengaluru.
Printed – August 14, 2025 05:30 am IST
