On August 15, 1950, India was celebrating its Independence Day. The temper was upbeat all through the nation. Just as celebrations had been winding down for the day, a nice 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.
According to some experiences, the earth shook for about 4 to eight minutes. Mountains stumbled and buildings fell, inflicting widespread dying and destruction. An English botanist and explorer named Frank Kingdon-Ward was tenting at Rima (Zayu) on the day. He reported later: “I was seated writing my diary near the entrance to our tent. Suddenly, after the faintest tremor, there came an appalling noise, and the earth began to shake violently. … We were immediately thrown to the ground. The lantern too was knocked over and went out instantly.”
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 subject investigation report described how “rail tracks were 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 facet alone, greater than 1,500 folks misplaced their lives, and 50,000 to 1,00,000 cattle had been killed. The quake was felt as far afield as Lhasa and Sichuan, and in Yunnan province in China. In the Medog space of Eastern Tibet, Yedong village slid into the Yarlung Zangbo river, and greater than 4,000 casualties had been reported from Tibet. Severe harm was wrought within the Sibsagar-Sadiya space of Assam as effectively.
More destruction was to comply with within the days after the earthquake. Many hills had been sheared by the earthquake. Rocky particles falling into the valleys beneath blocked rivers — earlier than giving manner days later, producing flash floods downstream that killed tons of of individuals dwelling on the rivers’ banks. On September 9, two weeks after the earthquake, the then Prime Minister Jawaharlal Nehru stated in a nationwide broadcast on All India Radio: “The Brahmaputra was blocked up for a while, and then broke through, they came down with a rush and a roar, a high wall of water sweeping down and flooding large areas and washing away villages and fields and gardens. … The remains of villages, animals, including cattle and elephants, and large quantities of timber floated down these raging waters…”.
Revisiting the earthquake
The supply of the Great 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 japanese terminus of the Himalayas, at a depth of 15 km. The rupture prolonged from the Mishmi thrust of the Eastern Himalayas to the Himalayan Frontal Thrust of Arunachal Pradesh, finishing a curvilinear movement across the mountainous bend (see picture above).
As it occurred, the shaking additionally occurred at a time when seismographic networks had been increasing worldwide, offering nice impetus to earthquake monitoring and the next improvement of the idea of plate tectonics. It was additionally the time instrumental monitoring of earthquakes had began in India, with the India Meteorological Department (IMD) organising its first seismological observatory at Alipore in Kolkata district in 1898.
Today, we perceive the Great Assam Earthquake occurred on a plate boundary fashioned by the collision of continental plates, like these alongside different components of the Himalayan plate boundary. However, it’s differentiated by the truth that its supply was within the Eastern Himalaya, which is tectonically very advanced. GPS information point out that whereas Indian and Eurasian continental plates are converging at about 20 mm/yr on common throughout the Himalayas, within the Eastern Himalayas it ranges from 10 mm to 38 mm/yr.
This variation may very effectively be attributable to difficulties in capturing tectonic complexity and variations in information high quality. While the primary arcuate Himalaya plate boundary resulted from the collision of the Indian and Eurasian plates, the Sunda plate can be concerned within the northeastern Himalaya, creating a posh construction.
Due to plate rotation, main structural parts additionally take sharp turns and the regional strike shifts from the final NE-SW path to the NW-SE path post-collision, forming what geologists name the Eastern Himalayan Syntaxis (EHS). This is the place the Great Assam Earthquake probably occurred.
Unlike 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 path.
Models additionally point out a thrusting part, probably ensuing from the propagation of earthquake faulting in direction of the west, the place thrust tectonics are predominant. The chance of a number of faults being linked to the Assam earthquake can be supported by the distribution of revised aftershock areas, unfold over a large zone east of the primary shock epicentre.
Indeed, most researchers imagine the earthquake might have began off on the Syntaxial bend whereas additionally activating Himalayan thrust faults to the west. Historical 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 significant medieval earthquake between 1262 and 1635 AD.
Quakes of the longer term
Looking again, the Great 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? As the Indian plate continues to maneuver ahead, earthquakes are an integral a part of its future. Yet our present data doesn’t enable us to foretell when, the place or how robust the subsequent quake will likely be. For now, we solely know the Central Himalayas are probably the most doubtlessly lively phase and will generate a 1950-type earthquake in future.
The seismic occasion additionally highlighted that we’re extra weak in the present day, primarily because of the exponential progress of the constructed atmosphere and concrete areas, leaving the panorama to look very totally different from the way in which it did 75 years in the past. As we plan for extra developmental actions and undertake heavy infrastructural projects, together with large dams, on this tectonically fragile space, we should replicate on the photographs from the 1950 quake.
Finally, 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 larger. As each China and India put together to construct large hydroelectric projects within the Eastern Himalayan bend, some of the seismically weak components of the majestic vary, it’s clear the street forward will likely 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 National Institute of Advanced Sciences, Bengaluru.
