An earthquake changed Padma's course 2,500 years ago. Can it happen again?
Several rivers in Bangladesh changed course due to strong earthquakes in the past. A recent study indicates that the ongoing collision of the Indian, Eurasian, and Burmese tectonic plates may lead to similar extreme events in the future
Can a river abandon its current channel and carve a new path due to seismic activities such as an earthquake?
This is one question geologists around the world have pondered for decades, and they have finally come to a concrete conclusion, with a groundbreaking study published in the Nature Communications journal on 17 June.
The study is titled "Cascading hazards of a major Bengal basin earthquake and abrupt avulsion of the Ganges River."
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The study provides a strong argument about the potential impact on Bangladesh, known for its large, dynamic river systems and related flooding challenges, if it experiences similar earthquakes in the future due to the ongoing collision of the Indian, Eurasian, and Burmese tectonic plates.
To begin with, the study shows that a powerful earthquake, estimated to be of magnitude 7 or 8, which struck around 2,500 years ago, may be responsible for the current course of the Ganga.
The Ganga originates in the Himalayas and flows for some 1,600 miles, eventually merging with other major rivers, including the Brahmaputra and the Meghna. These rivers create a complex network of waterways that flow into the Bay of Bengal, covering regions of Bangladesh and India.
The combination of a major river being close to ready to avulse, and a major earthquake nearby, is clearly a rare event, as both are very infrequent. That is why finding that this occurred 2,500 years ago and can be documented is important.
Collectively, they constitute the world's second-largest river system in terms of discharge, surpassed only by the Amazon.
Today, the Ganga, known as the Padma in Bangladesh, is situated about 50 kilometres south of Dhaka. However, using satellite imagery, the research team found evidence of its former course approximately 100 kilometres from the city.
It is now a low-lying area about 1.5 kilometres wide that intermittently runs parallel to the current river course for around 100 kilometres. The old channel is filled with mud, frequently floods, and is used for rice cultivation.
The study's co-author Dr Michael Steckler, Geophysicist at Lamont-Doherty Earth Observatory, part of the Columbia Climate School, shared that geologists have long been concerned that a large earthquake could change the course of rivers in Bangladesh. This study provides evidence that this has indeed happened at least once in the past.
According to him, the Ganga-Padma is currently situated near the western edge of the Indo-Burma Subduction Zone. This suggests that there is a possibility that an earthquake could uplift the river channel, leading to another avulsion.
"The combination of a major river being close to ready to avulse, and a major earthquake nearby, is clearly a rare event, as both are very infrequent. That is why finding that this occurred 2,500 years ago and can be documented is important," Dr Steckler told The Business Standard.
Lead author Dr Liz Chamberlain, Assistant Professor at the Wageningen University in the Netherlands, weighed in on the fact that even though the chance of a recurrence of a linked earthquake and river-path movement is low, if such an event were to occur today, it would be "an extreme event" for Bangladesh.
Bangladesh is a low earthquake-prone country but very vulnerable to major earthquakes. The geotectonic set-up of the country, which is located along two of the planet's active plate boundaries, suggests high probabilities of damaging future earthquakes and the possibility of rarer but extraordinarily large earthquakes that can cause damage far from their epicentres.
"Our study shows that in addition to damaging shaking of buildings and infrastructure, a large earthquake could bring the risk of river flooding in Bangladesh," she said.
Dr Syed Humayun Akhter, Vice Chancellor of Bangladesh Open University and another co-author of the study, also underscored that the findings of this study have profound implications.
He pointed out that the earthquake in 1787 caused the Brahmaputra River to change its course to become the Jamuna River. Similarly, the Meghna River, which previously flowed along the foothills of Lalmai, shifted 20 to 40 kilometres westward due to a major earthquake that occurred approximately 1,000-1,200 years ago.
According to him, Bangladesh is a low earthquake-prone country but very vulnerable to major earthquakes. The geotectonic set-up of the country, which is located along two of the planet's active plate boundaries, suggests high probabilities of damaging future earthquakes and the possibility of rarer but extraordinarily large earthquakes that can cause damage far from their epicentres.
Geological records show that large earthquakes have previously ravaged the country and the neighbouring region several times in the historic and prehistoric past, leaving signatures in the sediments, such as liquefaction, sediment eruptions from sub-surface high-pressure geologic horizons, changes in landform, and river avulsion.
For instance, the earthquake on 2 April 1762, on the Arakan coast, led to eruptions of mud volcanoes in Sitakunda Hill about 300 km north of the epicentre. The eruption brought up large tabular blocks of crystalline limestone from deep-seated formations.
"The earthquake was so violent and destructive that it generated a tsunami and agitated water bodies of jhils and rivers in Dhaka, causing the loss of 500 people, as boats capsized and houses washed away," said Dr Akhter, also a former Professor of Geology at Dhaka University.
He further noted that the country's position adjacent to the very active Himalayan front in the north and Burma deformation front in the east, exposes it to strong shaking from a variety of earthquake sources that can produce tremors of magnitude 8 or greater.
Notably, Dr Akhter was also co-author of another study in 2016 which found evidence of increasing strain in Bangladesh, where two tectonic plates underlie the world's largest river delta.
The threat is a Subduction Zone, where one section of earth's crust, or tectonic plate, is slowly thrusting under another.
Earth's largest earthquakes, such as the 2004 Indian Ocean tsunami and the 2011 Tohoku quake in Japan, have occurred along such zones. However, all known Subduction Zones were under the ocean, but this one is entirely under land, thus significantly multiplying the threat level.
"The effects of strong shaking from such an earthquake would directly affect much of the country, including densely populated Dhaka. Large earthquakes occur less frequently than serious floods, but they can affect much larger areas and have long-lasting economic, social, and political effects," Dr Akhter explained.
Hence, Dr Steven Goodbred, Chair of Earth and Environmental Sciences at Vanderbilt University, and another co-author of the study, suggested that the study could help Bangladeshi planners and policy makers to "make better, science-informed decisions about infrastructure design and placement in the future."
