A significant earthquake shook residents of Assam and close by states on Wednesday. Aftershocks proceed to rattle the area.

By Akash Kharita, Indian Institute of Know-how, Roorkee, Temblor science writing extern

Quotation: Kharita, A., 2021, Sturdy earthquake jolts Assam, Temblor, http://doi.org/10.32858/temblor.171

A powerful earthquake rocked the northeastern Indian state of Assam on Wednesday morning at 7:51 am native time. The Nationwide Centre for Seismology (NCS), India’s company chargeable for monitoring earthquakes by way of a regional community of seismic stations, decided that the magnitude-6.4 occasion struck 4.8 miles (7.7 kilometers) northwest of the city of Dhekiajuli and 26.7 miles (43 kilometers) west of Tezpur, a metropolis on the banks of the Brahmaputra River. The NCS studies that the quake ruptured at a depth of 10.5 miles (17 kilometers). America Geological Survey and European Mediterranean Seismological Centre each report that the occasion registered as a magnitude-6.0 earthquake. They calculated this magnitude utilizing teleseismic information collected from stations situated greater than roughly 620 miles (1000 kilometers) from the quake.

Earthquakes of magnitude-4.0 and bigger within the Assam and surrounding area (Jan. 1960-Mar. 2020), together with the April 28 mainshock. Credit score: Nationwide Centre for Seismology


Roughly 70 aftershocks, with magnitudes starting from 2.3 to 4.9, have been detected by NCS inside a day after the mainshock. These quakes seem to align alongside a roughly northwest-southeast development close to the mainshock. Aftershocks are frequent following giant earthquakes, and develop into much less frequent with time.

The chief minister of Assam, Sarbananda Sonowal, urged individuals to remain alert. Prime Minister Narendra Modi additionally assured these within the space that India’s central authorities would offer all the assistance it might.

As of now, there aren’t any rapid studies of lack of life, although a number of movies present extreme infrastructure harm within the metropolis Guwahati, the city of Tezpur and state of Bengal.

A quake within the Himalayan foothills

The state of Assam lies within the northeastern arm of India, sitting south of the mountainous nation of Bhutan and the rugged Indian state of Arunachal Pradesh. Nonetheless, Assam feels the consequences of convergence between the Indian and Eurasian tectonic plates that drives up the mountains. The southernmost Himalayan thrust fault — the kind of fault that successively constructed the Himalayan vary by shoving Eurasia over India — defines the boundary between the 2 plates. The floor expression of this boundary, interchangeably referred to as the Himalayan Frontal Thrust or the Fundamental Frontal Thrust, runs by way of the northern a part of Assam and has hosted a number of main earthquakes prior to now, whereas comparatively minor faults splice the state at totally different orientations.

The Shillong Plateau, which lies south of Assam, complicates the area in comparison with different components of the Himalayan vary. Based on Byron Adams, a Royal Society Dorothy Hodgkin Fellow on the College of Bristol, “It’s doable that the Indian plate behind the Shillong Plateau, which incorporates Assam, has low flexural power, so it doesn’t bend simply beneath the load of both the overriding Eurasian plate or sediment coming from the rivers draining the vary.” As a substitute, this a part of the Indian plate breaks, with quite a few faults slicing by way of the area at irregular angles, he says. “There could also be extra heterogeneities within the Indian crust within the east, in order the plate tries to subduct, it breaks as a substitute of bending, creating extra northwest-trending faults between the Himalayan vary and the Shillong Plateau.”

Due to the quite a few lively faults crisscrossing the area, the NCS classifies this space inside its zone of highest seismic hazard (Zone 5).

Map of India color-coded by seismic hazard level
Earthquake hazard zones in India. Credit score: PlaneMad/Wikimedia


The Kopili Fault

The mainshock possible struck close to the Kopili Fault, in accordance with a preliminary evaluation by the NCS. This fault traits in a northwest-southeast path, indicating it’s a “minor” fault, although it’s nonetheless able to substantial seismic exercise. Scientists imagine this fault hosted the 1869 magnitude-7.5 Chachar earthquake and the 1947 magnitude-7.3 Hajoi earthquake (Sutar et al., 2017). This fault could have performed a task in a 1941 magnitude-6.5 occasion (Kayal et. al., 2010).

Based on Supriyo Mitra, a professor on the Indian Institute of Science Schooling and Analysis Kolkata Seismological Observatory, the Kopili fault zone strikes with right-lateral strike-slip movement, which means that the northeastern aspect of the fault strikes to the southeast alongside a really steep fault floor. For reference, this is similar sense of movement alongside the famed San Andreas Fault. Because the Shillong Plateau strikes northward, says Mitra, that motion could also be accommodated by the Kopili Fault zone.

Adams factors out that the focal mechanism produced by america Geological Survey signifies a considerable thrust part, together with right-lateral motion. “In the event you put some right-lateral shear on these northwest-trending faults between the Himalayan vary and the Shillong Plateau, you would possibly count on one thing like what simply occurred in Assam.”

Discover an up to date earthquake hazard zone map right here


Kayal, J. R., Sergei S. Arefiev, Saurabh Baruah, Ruben Tatevossian, Naba Gogoi, Manichandra Sanoujam, J. L. Gautam, Devajit Hazarika, and Dipak Borah. “The 2009 Bhutan and Assam felt earthquakes (Mw 6.3 and 5.1) on the Kopili fault within the northeast Himalaya area.” Geomatics, Pure Hazards and Danger 1, no. 3 (2010): 273-281.

Nationwide Centre for Seimology Report on twenty eighth April 2021 Earthquake (M 6.4), Sonitpur, Assam, https://seismo.gov.in/websites/default/recordsdata/pressrelease/Assam_EQ_Report_28Apr2021.pdf

Sutar, A. Ok., Verma, M., Pandey, A. P., Bansal, B. Ok., Prasad, P. R., Rao, P. R., & Sharma, B. (2017). Evaluation of most earthquake potential of the Kopili fault zone in northeast India and powerful floor movement simulation. Journal of Asian Earth Sciences, 147, 439-451.

Additional Studying

Clark, M. Ok., & Bilham, R. (2008). Miocene rise of the Shillong Plateau and the start of the tip for the Jap Himalaya. Earth and Planetary Science Letters, 269(3-4), 337-351.

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