GUWAHATI, April 30 – The earthquake on August 15, 1950, had caused extensive damage to the Upper Assam areas and Arunachal Pradesh, besides the Tibet region of China. According to the US Geological Survey (USGS), that quake was more damaging in Assam than that of the 1897, in terms of property loss. It had led to a damage of $ 25 million or above. Its epicentre was located near Rima in Tibet.
However, there are differences of opinion on its magnitude. The USGS has stated that the magnitude of that earthquake was recalculated and determined it to be of 8.6, while the India Meteorological Department (IMD) had stated that its magnitude was 8.5.
The USGS, which has consulted various published accounts and documents to prepare its report on that earthquake (Historic Earthquakes: Assam-Tibet 1950 August 15), said that the 1950 earthquake had taken a toll of 780 human lives in Tibet and between 1,526 and 1,530 lives in Assam. Since the casualty figures of Tibet and Subansiri Valley in Assam are not entered exactly in the list of total casualty figure, it is feared that the actual casualty figure due to that earthquake could be much higher than what had been published.
It has said that 2,000 homes, temples and mosques were destroyed under the impact of that earthquake and the Brahmaputra Basin in NE India was the worst affected area.
“At least 780 people (were) killed and many buildings collapsed in the Nyingchi-Qamdo-Zhamo (Rima, Zayu) area of eastern Tibet. Sand blows, ground cracks and large landslides occurred in the area. In the Medog area, the village of Yedong slid into the Yarlung Zangbo (Brahmaputra) River and was washed away. The quake was felt at Lhasa and in Sichuan and Yunnan Provinces of China. Severe damage also occurred in the Sivasagar-Sadiya area of Assam and in the surrounding hills.
“About 70 villages were destroyed in the Abor Hills, mostly by landslides. Large landslides blocked the Subansiri River. This natural dam broke 8 days later, creating a wave 7 m (23 ft) high which inundated several villages and killed 536 people. The quake was felt as far away as Calcutta. Seiches were observed in many lakes and fjords of Norway and in at least three reservoirs in England,” said the USGS.
The term ‘seismic seiche’ was coined by Anders Kvale in 1955 to describe oscillations of lake levels in Norway and England caused by the 1950 Assam earthquake, the USGS said.
Its intensity scale was found to be X (intensity scale 10) at Sadiya, Passighat, Doom Dooma, Dibrugarh, North Lakhimpur and Sivasagar; IX at Digboi and Golaghat; VIII at Tezpur, Gauhati (Guwahati) and Shillong; VI at Dhaka, Calcutta, Dhubri, Darjeeling and Imphal. Its macro-seismic area was 1,794,000 sq km, of which 49,700 sq km suffered great damage.
The USGS said that there were many incidents of rock-fall and destruction of forest areas in the Mishimi Hills. In the Arbor Hills, 70 villages were destroyed with 156 casualties due to landslides. Dykes blocked the tributaries of the Brahmaputra. That of the Dibang valley broke without causing any damage.
To the effects of shaking were added those of flood. The rivers rose high after the earthquake, bringing down sand, mud, trees and all kinds of debris. Pilots flying over the meizoseismal area (the area of maximum damage) reported great changes in topography; this was largely due to enormous slides, some of which were photographed.
The only available on-the-spot account is that of F Kingdon-Ward, a botanical explorer who was at Rima on that fateful day. He confirmed violent shaking at Rima, extensive slides and the rise of stream waters. Aftershocks were numerous; many of them were of magnitude 6 and over. One of the more westerly aftershocks, a few days later, was felt more extensively in Assam than the main shock.
Kingdon-Ward heard heavy explosive sounds near the epicentre of the earthquake following the shock, coming apparently from high in the air. Those sounds were heard at many points in India and Burma, at distances of over 750 miles.