Wednesday, January 26, 2011

Historic Earthquakes

Assam - Tibet
1950 August 15 14:09 UTC

Assam - Tibet

At least 780 people killed and many buildings collapsed in the Nyingchi-Qamdo-Zhamo (Rima, Zayu) area of eastern Tibet. Sandblows, 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, China. Severe damage (X) also occurred in the Sibsagar-Sadiya area of Assam, India 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 innundated several villages and killed 536 people. The quake was felt (VI) as far away as Calcutta. Seiches were observed in many lakes and fjords of Norway and in at least 3 reservoirs in England. Many sources call this the Assam-Tibet earthquake or even the Assam earthquake, even though nearly all place the epicenter in Tibet. Thus it is possible that the casualties for Tibet are not included in the total, as well as those from the Subansiri River flood. Furthermore, Gu et al. do not give casualty totals for Yedong or other areas of the most severe damage in Tibet. Therefore, the actual casualty toll may be much higher than the value given.

N.B: The magnitude for this earthquake has been recalculated since the following articles were written. Magnitude 8.6 is a better determination of the size of this earthquake.

Intensity scale X at Sadya, Passighat, Dum Duma, Dibrugarh, North Lakhimpur, and Sibsagar; IX at Digboi and Galaghat; VIII at Tezpur, Ganhati, and Shillong; VI at Daca, Calcutta Dhubri, Darjeeling, and Imphal. Macroseismic area 1,794,000 sq km, of which 49,700 sq km suffered great damage.

This great earthquake, destructive in Assam and Tibet, has a calculated magnitude of 8.6 and Strasbourg regards it as the most important since the introduction of seismological observing stations. Alterations of relief were brought about by many rock falls in the Mishmi Hills and destruction of forest areas. In the Arbor Hills 70 villages were destroyed with 156 casualties due to landslides. Dykes blocked the tributaries of the Brahmaputra; that in the Dibang valley broke without causing damage, but that at Subansiri opened after an intermal of 8 days and the wave, 7 metres high, submerged several villages and killed 532 persons.

From ISC Bulletin, The International Seismological Summary.

Two thousand homes, temples and mosques destroyed. Hardest hit is the Brahmaputra Basin in NE India

From Seismological Notes, Bulletin of the Seimological Society of America. "Seismological Notes" is a list of recent important earthquakes with short summaries included in each issue of the BSSA.

Strictly this was not an Indian earthquake; the epicenter was near Rima, in a region claimed by both China and Tibet. It is one of the few earthquakes to which the instrumentally determined magnitude, 8.7, is assigned. This shock was more damaging in Assam, in terms of property loss, than the earthquake of 1897. 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 flyng over the meizoseismal area 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. However, he had little opportunity for obeservations; he confirms violent shaking at Rima, extensive slides, and the rise of the streams, but his attention was perforce directed to the difficulties of getting out and back to India. Aftershocks were numerous; many of them were of mangitude 6 and over and well enough recorded at distant stations for reasonably good epicenter location. From such data Dr. Tandon, of the Indian seismological service, established an enormous geographical spread of this activity, from about 90 deg to 97 deg east longitude, with the epicenter of the great earthquake near the eastern margin. One of the more westerly aftershocks, a few days later, was felt more extensively in Assam than the main shock; this led certain journalists to the absurd conclusion that the later shock was 'bigger' and must be the greatest earthquake of all time! This is a typical example of the confusion between the essential concepts of magnitude and intensity. The extraordinary sounds heard by Kingdon-Ward and many others at the times of the main earthquake have been specially investigated. Seiches were observed as far away as Norway and England. (p. 63-64.)

Kingdon-Ward, near the epicenter of the great Tibet earthquake of 1950, heard heavy explosive sounds following the shock, coming apparently from high in the air. These sounds were heard at many points in India and Burma, to distances of over 750 miles. (p. 128.)

From Richter, Charles F., 1958, Elementary seismology: San Francisco, W.H. Freeman and Company, 768 p.

1950 Aug 15 14:09, 26.6 N 96.5E, maximum intensity XI, 1526 deaths, extreme damage [extreme = $25 million or more U.S. at the time of the earthquake]. India: Assam. India-China.

From Dunbar, Paula K., Lockridge, Patricia A., and Whiteside, Lowell S., 1992, Catalog of significant earthquakes 2150 B.C. - 1991 A.D.: U. S. Dept. of Commerce, National Oceanic and Atmospheric Administration.

The term seismic seiche was coined by Anders Kvale in 1955 to describe oscillations of lake levels in Norway and England caused by the earthquake of August 1950 in Assam, India. (p. 153.)

1950 August 15, India, Assam, 1526 deaths, magnitude 8.6, Surface faulting (p. 271.)

From Bolt, Bruce A., 1993, Earthquakes: New York, W.H. Freeman and Company, 331 p.

1950 Aug. 15, India, Assam, 1,530 deaths, magnitude 8.7 (p. 4.)

No comments:

Post a Comment