  | |
| UTC time | 1950-08-15 14:09:34 |
|---|---|
| ISC event | 895681 |
| USGS-ANSS | ComCat |
| Local date | August 15, 1950 (1950-08-15) |
| Local time | 19:39:34 IST |
| Magnitude | Mw 8.7[1] |
| Depth | 15 km (9.3 mi)[1] |
| Epicenter | 28°22′N96°27′E / 28.36°N 96.45°E /28.36; 96.45[1] |
| Fault | Main Himalayan Thrust |
| Type | Oblique-slip[2] |
| Areas affected | Assam, India Tibet, China |
| Max. intensity | MMI XI (Extreme)[3][4] |
| Casualties | 4,800 |
The1950 Assam–Tibet earthquake,[5] also known as theAssam earthquake,[5] occurred on 15 August and had amoment magnitude of 8.7. The epicentre was located in theMishmi Hills. It is one of thestrongest earthquakes ever recorded on land.
Occurring on a Tuesday at 7:39 PMIndian Standard Time, theearthquake was destructive in bothAssam (India) andTibet (China), and approximately 4,800 people were killed. The earthquake is notable as being the largest recorded quake caused bycontinental collision rather thansubduction, and is also notable for the loud noises produced by the quake and reported throughout the region.
Northeastern India and southern Tibet lie in the frontal part of the zone ofcollision between the northward movingIndian plate andEurasia. The thrust belt that marks this plate boundary zone is formed of a series of sub-parallel thrust faults. The thrust faults in this part of the Himalayas show a rapid lateral change in orientation from SW-NE trending to NW-SE trending in the area of the Namche–Barwasyntaxis. To the west the main active part of the thrust belt is theHimalayan (or Main) Frontal Thrust, which joins theMain Himalayan Thrust at depth. To the east the active frontal structure is theMishmi Thrust, which connects to the south with theNaga Thrust and the left-lateralstrike-slipSagaing Fault that together bound theBurma Microplate.[6]
In an attempt to further uncover the seismic history ofNortheast India, field studies were conducted by scientists with theNational Geophysical Research Institute andInstitute of Physics, Bhubaneswar. The study discovered signs ofsoil liquefaction includingsills andsand volcanoes inside at least twelvetrenches inalluvial fans and on theBurhi Dihing River Valley that were formed by pastseismic activity.Radiocarbon dating identified the deposits at roughly 500 years old, which would correspond with a recorded earthquake in 1548.[7]
The earthquake had an epicenter in theMishmi Hills to the northeast of the surface trace of the Mishmi Thrust Fault. Magnitudes calculated for the earthquake range from Mw 8.6–8.8, making it the largest onshore earthquake ever recorded and the largest not associated withsubduction.[6] It was the sixth largest earthquake of the 20th century.[8]
Aftershocks were numerous; many of them were of magnitude 6 and over and well enough recorded at distant stations for reasonably goodepicentre location. The aftershock epicenters covered a very large area over theAbor and Mishmi Hills of about 30,000 km2, which matches well with the area of maximum shaking as determined from the location of landslides from satellite imagery. This is interpreted to define the rupture area.[6]
Evidence forsurface rupture was discovered at multiple locations along the trace of the Abor Hills section of the Main Frontal Thrust (MFT) and along the Mishmi Thrust (MT). The total observed surface rupture length along both faults is ≥ 200 km.[6]
Publishedfocal mechanisms for this earthquake are contradictory with both thrust and strike-slip mechanisms supported. It has been proposed that the earthquake ruptured both the northwest-dipping MFT and the northeast-dipping MT. This combined source, being oblique thrust in nature, provides a good fit with observations of seismic arrivals. The average slip values on MT and MFT are 17 ± 1 m and 11 ± 1 m respectively, with an amount as high as 34 ± 2.5 m implied to have happened underneathWakro in the former fault. Together with the calculated rupture area of 330 × 90 km and a reasonable estimate for the crustal rigidity, this gives a moment magnitude of 8.7.[6]
The 1950 Assam–Tibet earthquake had devastating effects on both Assam and Tibet. In Assam, 1,526 fatalities were recorded[9] and another 3,300 were reported in Tibet[10] for a total of approximately 4,800 deaths.
Alterations of relief were brought about by many rock falls in the Mishmi Hills and surrounding forested regions. In the Abor Hills, 70 villages were destroyed with 156 casualties due to landslides. Landslides blocked the tributaries of theBrahmaputra. In theDibang Valley, alandslide lake burst without causing damage, but another atSubansiri River opened after an interval of 8 days and the wave, 7 m (23 ft) high, submerged several villages and killed 532 people.[citation needed]
The shock was more damaging in Assam, in terms of property loss, than theearthquake of 1897. In addition to the extreme shaking, there were floods when the rivers rose high after the earthquake bringing down sand, mud, trees, and all kinds of debris. Pilots flying over the meizoseismal area reported great changes in topography. This was largely due to enormouslandslides, some of which were photographed.
In Tibet,Heinrich Harrer reported strong shaking inLhasa and loud cracking noises from the earth.[11] Aftershocks were felt in Lhasa for days. In Rima, Tibet (modern-dayZayü Town),Frank Kingdon-Ward, noted violent shaking, extensive slides, and the rise of the streams.[12] Helen Myers Morse, an American missionary living in Putao, northernBurma at the time, wrote letters home describing the main shake, the numerous aftershocks, and of the noise coming out of the earth.[13]
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 belief 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.[14]
An article inScience, published in response to the2001 Bhuj earthquake, calculated that 70 percent of theHimalayas could experience an extremely powerful earthquake. The prediction came from research of the historical records from the area as well as the presumption that since the 1950 Medog earthquake enough slippage has taken place for a large earthquake to occur.[15] In 2015, the Himalayas were hit by a7.8-magnitude earthquake with an epicenter further west in Nepal.
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