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Assam EQ 2021

A Note on the Largest Earthquake to have Shaken Assam in past 70 Years

[This brief report is compiled by Prof. Hemant B Kaushik, IIT Guwahati with inputs from the Internet and media coverage of the event.]

A powerful earthquake of magnitude 6.4 (Data from National Center for Seismology) shook Assam for about 30 seconds in the morning of 28 April 2021. The timing of the earthquake (7:51 am IST) was such that most of the people were indoors as schools were closed due to COVID-19 pandemic and people were getting ready for attending their professional duties. The depth of the focus was 17 km and the epicenter was located at 26.69 N and 92.36 E in the Sonitpur district of Assam, at an aerial distance of about 11 km from the nearest town of Dhekiajuli, and 75 km from the state capital Dispur (Guwahati). According to the USGS the magnitude of the earthquake was 6.0 and the epicenter was reported at almost similar location (26.782 N and 92.436 E) with a deeper focal depth of 34 km. 13 aftershocks of magnitude 3.0 or higher struck the region during the next three days. People were reportedly frightened during the 3 aftershocks of magnitude 4.0 and above as far as in Guwahati. Though the region is prone to earthquakes and occurrence of earthquakes is quite frequent here, people were terrified with the power of the earthquake and the aftershocks. Most of the people came out of their homes during the shaking and waited outside for several minutes as couple of aftershocks hit immediately.

The shaking was felt in not only the neighboring states but also in Bangladesh, Bhutan, China, and Myanmar. The maximum Modified Mercalli Intensity of the shaking was reported as VI to VII by PAGER (V in the state capital). No deaths due to structural failure is reported, however, two elderly persons lost their lives during the earthquake shaking due to heart attacks. Parts of Bhairavkund hills in Udalguri, which is quite close to the epicenter, collapsed after the massive earthquake. There are no reports of any major structural damage, though significant nonstructural damage has occurred in large number of buildings. Most of this damage is related to cracks in partition masonry walls, failure of false ceilings, and collapse of boundary walls. One building in Nagaon (about 40 km from the epicenter) tilted and leaned over the adjacent building. Huge ground cracks have been reported from the epicentral and adjoining areas. In some areas near the epicenter liquefaction was also prominently observed. Several important Government and private buildings in Guwahati suffered significant nonstructural damages, and were partially closed for inspection. Damage to water pipelines resulted in flooding of rooms in some hotels and apartment buildings. As has always been the case, there are no reports of any damage to the vernacular Assam-type houses in the entire region. The actual economic loss from the earthquake will be huge even though there were no major structural damage, primarily due to partial closure of the facilities for inspection and renovation.

The earthquake rang a bell again, but likely to be quickly forgotten in view of the ongoing COVID-19 pandemic! The strong shaking again reminded the people of necessity of strong adherence to the design codes for earthquake resistant infrastructure. Fortunately, the biggest earthquake to have hit the region in 70 years did not result in major structural damage, and possibility of a bigger disaster, that could have occurred due to the combined effects of the earthquake and the COVID-19 pandemic, was avoided. The moderate-size earthquake should be regarded as a warning to the Government and the other stakeholders who must hasten their act to strengthen vulnerable buildings and start following the Indian Standards in design and construction of civil infrastructure. Most of the nonstructural damage that has occurred, even in small houses, are in reinforced concrete buildings. We can remind ourselves that small family dwellings need not be constructed in reinforced concrete. There are other proven housing typologies, for example, Assam-type houses and Confined masonry houses, that may be adopted for smaller buildings at least. It is time to learn the lesson from the past earthquakes that have struck other parts of the country resulting in huge number of casualties and economic losses, and improve the construction scenario for reducing the seismic risk in the already vulnerable region.

 


Figure showing epicenters of the main shock and aftershocks, and collapse of parts of Bhairavkund hills in Udalguri, Assam during the strong shaking

 


Tilting of a five-storey building during the earthquake in Nagaon, Assam. Diagonal shear cracking and subsequent collapse of masonry infill wall and out-of-plane collapse of masonry parapet walls

 


Failure of false ceiling and claddings in Guwahati

 


Ground cracking near the epicentral region

 


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