Powerful ancient earthquake likely rerouted the Ganga 2,500 years ago — and there are risks of another such event in future

A powerful earthquake that struck roughly 2,500 years ago may be responsible for the current course of Ganga river, according to a new study. 

Researchers believe the temblor, estimated at a magnitude of 7 or 8, caused the river to abruptly abandon its previous channel in what is now Bangladesh and carve a new path, stated the paper published in journal Nature Communications.

“We show that earthquake-driven avulsion of large river networks is another real, yet largely unrecognised threat to seismically active lowlands,” the paper read, adding that such cascading hazards could be devastating in the Bengal basin. If a similar magnitude earthquake strikes this region, large-scale flooding may occur.

Rerouting, also known as avulsions, occurs when rivers experience high sediment loads, which fill up river beds and force them to seek new channels during floods. 

Human activities also have an impact — engineering solutions can constrain waterways and increase flood risk. In 2008, Kosi river, a large tributary of the Ganga in India, shifted course by over 120 kilometres during a seasonal flood. This displaced 3 million people and claimed more than 250 lives. 

Unlike these events, an avulsion triggered by an earthquake can occur more or less instantaneously.

“It was not previously confirmed that earthquakes could drive avulsion in deltas, especially for an immense river like the Ganga,” lead author Liz Chamberlain, an assistant professor at the Netherlands’ Wageningen University, said in a statement.

However, these impacts have been difficult to study. A recent satellite-based synthesis of channel avulsions discovered no statistical correlation with earthquakes during the period covered by instrumental data. 

However, millennial-scale reconstructions indicate that earthquakes had an impact on the geometries of inland river channels in the New Madrid seismic zone in the United States. Similarly, earthquakes in India’s Rann of Kutch have resulted in widespread flooding due to fault-induced water diversions. 

While evidence suggests that river channels have been displaced near ruptured faults (fractures between two blocks of rock), the impact of seismicity on more distant lowland channels is unknown or poorly documented.

With the help of satellite imagery, the team identified the likely former main channel of the river, flowing 100 km south of the Bangladesh capital of Dhaka.

A classic sign of a landscape disrupted by an earthquake: A vein of sand that has been pushed up through darker-coloured sediments. Photo: Liz Chamberlain

It is now a low-lying area about 1.5 km wide and runs intermittently parallel for some 100 km to the current river course. The older channel is now filled with mud and frequently floods. The area is under rice cultivation.

The team visited the site, where they spotted a freshly dug excavation for a pond that had not yet been filled with water. They also observed distinct vertical dikes of light-coloured sand cutting up through horizontal layers of mud — a well-known feature created by earthquakes.

These were called seismites — sediments deformed by seismic shaking — and measured 30 or 40 centimetres wide, cutting up through 3 or 4 meters of mud.

When the team performed chemical analyses of sand grains and particles of mud, they observed that the eruptions and the abandonment, along with the infilling of the channel, occurred about 2,500 years ago. They also noted that a big earthquake triggered an avulsion. 

“As a tectonically active delta with big dynamic rivers, Bangladesh is a spectacular place to study the interaction of tectonic events and river channel change. We knew there had been earthquakes in the delta in the past and we knew that the river channel had moved, but we did not expect to find sedimentary deposits that coupled the two processes,” Chamberlain told Down To Earth.

Such events could occur in the presence of the ‘right ingredients’, such as the monsoon season, when rivers have a lot of power, when the ground is temporarily sunken by the weight of water and when sand is likely to slump or erode.

A 2016 study suggested that an earthquake of comparable magnitude could strike the region again, affecting some 140 million people.  “Large earthquakes impact large areas and can have long-lasting economic, social and political effects,” Syed Humayun Akhter, vice-chancellor of Bangladesh Open University and a coauthor, explained in a statement.

The Ganga is not alone. Other sedimentary basins sitting on tectonically active settings worldwide like Chao Phraya (Thailand), Colorado (US), Copper (Alaska), Fraser (Canada), Indus, China’s Yangtze and Yellow rivers, are likely at risk due to seismic activities, the study highlighted.