An earthquake changed the course of the Ganges River. Could it happen again? – State of the planet

A new study shows that a major earthquake that occurred 2,500 years ago caused one of the largest rivers on Earth to suddenly change its course. The previously undocumented earthquake changed the course of the main channel of the Ganges River in what is now densely populated Bangladesh, which remains vulnerable to large earthquakes. The study has just been published in the journal Nature Communications.

Scientists have documented many changes in the river's course, called fluctuations, including some in response to earthquakes. However, “I don't think we've seen this large anywhere before,” said study co-author Michael Stickler, a geophysicist at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School. He said it could have easily overwhelmed anyone and anything in the wrong place at the wrong time.

“It has not been previously confirmed that earthquakes can lead to fluctuations in a delta, especially for a massive river like the Ganges,” said lead author Liz Chamberlain, an assistant professor at Wageningen University in the Netherlands.

The lowlands of Bangladesh are in many places an elaborate mixture of land and water that sometimes changes places. (Photo by Steve Goodbread)

The Ganges originates in the Himalayas and flows for approximately 1,600 miles, eventually combining with other major rivers including the Brahmaputra and Meghna to form a maze of waterways that empty into a vast area of ​​the Bay of Bengal stretching across Bangladesh and India. Together they form the second largest river system in the world as measured by discharge. (Amazon is first.)

Like other rivers that pass through the great delta, the Ganges periodically undergoes minor or major changes in its course without any assistance from earthquakes. Sediments washed upstream settle and accumulate in the channel, until the river bed eventually grows higher than the surrounding flood plain. At some point, it breaks through the water and begins to build a new path for itself. But this generally does not happen all at once, it may take successive floods over the course of years or decades. On the other hand, earthquake-related volatility can occur more or less instantly, Stickler said.

Through satellite images, the authors of the new study discovered what they say is most likely the river's former main channel, about 100 kilometers south of the Bangladeshi capital, Dhaka. This is a low-lying area about 1.5 km wide that can be found intermittently for about 100 km parallel to the current river course. It is filled with clay, often floods, and is used mainly for rice cultivation.

A classic sign of a landscape disrupted by an earthquake: a strip of sand pushed through dark-colored sediment. (Photo by Liz Chamberlain)

Chamberlain and other researchers were exploring this area in 2018 when they found newly excavated fossils of a pond that was not yet filled with water. On one side, they spotted distinct vertical dykes of light-colored sand cutting through horizontal layers of clay. This is a well-known feature created by earthquakes: in such areas of water, constant shaking can compress buried sand layers and inject them upward through the overlying clay. The result: actual sand volcanoes, which can erupt on the surface. They are called “earthquakes” here, and they were 30 or 40 cm wide, cutting through 3 or 4 meters of clay.

Further investigation showed that the earthquakes were directed in a systematic pattern, suggesting that they were all generated at the same time. Chemical analyzes of sand grains and clay particles showed that volcanic eruptions and channel abandonment and filling occurred about 2,500 years ago. Furthermore, there was a similar site about 85 kilometers downstream in the ancient channel that filled with clay at the same time. Authors' conclusion: This was a large, sudden overturn caused by an estimated magnitude 7 or 8 earthquake.

They say the quake could have had one of two possible origins. One is the subduction zone to the south and east, where a huge plate of oceanic crust is thrusting beneath Bangladesh, Myanmar and northeastern India. Or it may have come from giant rift faults at the foot of the Himalayas to the north, which are rising as the Indian subcontinent slowly collides with the rest of Asia. A 2016 study led by Stickler showed that these areas are now experiencing stress and can produce earthquakes similar to those that occurred 2,500 years ago. The last wave of this size occurred in 1762, triggering a deadly tsunami that traveled up the river to Dhaka. Another probably occurred around 1140 AD.

A 2016 study estimates that a repeat of such an earthquake could affect 140 million people. “Large earthquakes affect large areas and can have long-term economic, social and political impacts,” said Syed Humayun Akhtar, vice chancellor of Bangladesh Open University and co-author of the two studies.

The Ganges is not the only river facing such dangers. Other areas located in a tectonically active delta include the Yellow River in China; The Irrawaddy in Myanmar; the Klamath, San Joaquin, and Santa Clara rivers, which flow off the west coast of the United States; And the Jordan River, which extends on the borders of Syria, Jordan, the Palestinian West Bank, and Israel.

Other authors on the new study are at the University of Cologne in Germany. Dhaka University; Bangladesh Professional University; Noakhali University of Science and Technology, Bangladesh; and the University of Salzburg, Austria. The research was funded by the US National Science Foundation.

More: Michael Stickler discusses geohazards in Bangladesh on Planet on the Move.

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