A groundbreaking study has revealed that a major earthquake 2,500 years ago dramatically altered the course of one of the world’s largest rivers, the Ganges. This previously undocumented earthquake diverted the main channel of the Ganges in what is now Bangladesh, a region still susceptible to significant seismic activity. The study, recently published in the journal Nature Communications, sheds light on the potential for future catastrophic changes in river courses due to seismic events.

Historically, many river course changes, known as avulsions, have been documented, some of which were triggered by earthquakes. However, the scale of the Ganges avulsion is unprecedented. I don’t think we’ve ever seen one this large anywhere, said Michael Steckler, a geophysicist at the Lamont-Doherty Earth Observatory, part of Columbia University’s Climate School. Such an event could have easily inundated anyone or anything in the wrong place at the wrong time, Steckler noted.

Lead author Liz Chamberlain, an assistant professor at Wageningen University in the Netherlands, remarked, It had not been previously confirmed that earthquakes could cause avulsions in deltas, especially for a river as immense as the Ganges.

The Ganges originates in the Himalayas and flows approximately 2,600 kilometers, eventually merging with other major rivers, including the Brahmaputra and the Meghna. Together, these rivers form a complex network of waterways that empty into a broad expanse of the Bay of Bengal, spanning Bangladesh and India. This network constitutes the world’s second-largest river system by discharge, second only to the Amazon.

Another satellite view of the Ganges delta.
Another satellite view of the Ganges delta. Credit: NASA / Public domain / Wikimedia Commons

Like other rivers traversing large deltas, the Ganges periodically experiences course changes without the aid of earthquakes. Sediments carried downstream settle and accumulate in the river channel, eventually causing the riverbed to subtly rise above the surrounding floodplain. At some point, the water breaks through and begins carving a new path. But this usually doesn’t happen all at once; it can take successive floods over years or decades. An earthquake-related avulsion, on the other hand, can happen more or less instantly, Steckler explained.

Satellite imagery used by the researchers revealed what they believe to be the ancient main channel of the river, located about 100 kilometers south of Bangladesh’s capital, Dhaka. This low-lying area, approximately 1.5 kilometers wide, extends intermittently for about 100 kilometers, roughly parallel to the current river course. The area, filled with mud, frequently floods and is mainly used for rice cultivation.

In 2018, Chamberlain and other researchers discovered distinctive vertical dikes of light-colored sand cutting through horizontal layers of mud in a freshly dug pond. This feature, known to be created by earthquakes, results from sustained shaking that pressurizes buried sand layers, causing them to inject upwards through the overlying mud, forming sand volcanoes, or seismites. These seismites were found to be 30 to 40 centimeters wide, cutting through 3 to 4 meters of mud.

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

Further investigation showed that these seismites were oriented systematically, indicating they were created simultaneously. Chemical analyses of the sand grains and mud particles revealed that the eruptions and the abandonment and filling of the channel occurred about 2,500 years ago. Additionally, a similar site 85 kilometers downstream in the old channel filled with mud at the same time. The authors concluded that this was a sudden, large avulsion triggered by an earthquake estimated at magnitude 7 or 8.

The earthquake could have originated from one of two potential sources. One is a subduction zone to the south and east, where a massive oceanic crustal plate is pushing under Bangladesh, Myanmar, and northeastern India. Alternatively, it could have come from giant faults at the foot of the Himalayas to the north, which are rising due to the slow collision of the Indian subcontinent with the rest of Asia. A 2016 study led by Steckler indicates that these zones are currently accumulating stress and could produce earthquakes comparable to the one 2,500 years ago. The last such event occurred in 1762, generating a deadly tsunami that traveled upriver to Dhaka. Another might have occurred around 1140 AD.

The 2016 study estimates that a modern recurrence of such an earthquake could affect 140 million people. Large earthquakes impact extensive areas and can have lasting economic, social, and political effects, said Syed Humayun Akhter, vice-chancellor of Bangladesh Open University and co-author of both studies.

The Ganges is not the only river facing such risks. Other rivers in tectonically active deltas include China’s Yellow River, Myanmar’s Irrawaddy, the Klamath, San Joaquin, and Santa Clara rivers on the US West Coast, and the Jordan River, spanning the borders of Syria, Jordan, the Palestinian West Bank, and Israel.

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