A powerful earthquake diverted the course of the Ganges 2,500 years ago, leaving behind “sand volcanoes”

A powerful earthquake diverted the course of the Ganges 2,500 years ago, leaving behind “sand volcanoes”
A powerful earthquake diverted the course of the Ganges 2,500 years ago, leaving behind “sand volcanoes”

Considered sacred by more than a billion inhabitants of the Indian world, and not only Hindus (Géoconfluences, 2023), the Ganges is especially talked about for the massive pollution of its waters by industrial discharges, pesticides and plastic. Which does not prevent believers from bathing in it, or even drinking it, to wash away their sins.

A new study today lifts the veil on the history of this river, 2,500 kilometers long between the Himalayan glacier where it has its source and the delta that bears its name. A much more tumultuous past than we thought, because it was marked by a powerful earthquake which deviated its course some 2,500 years ago (Nature Communications, June 17, 2024).

“Anyone in the wrong place at the wrong time could have been flooded.”underlines Mickael Steckler, co-author of the study, geophysicist at the Columbia Climate School at Columbia University, United States (press release).

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According to this work, this would be a striking example “d’avulsion”, that is to say a sudden change in the course of a river. If earthquakes are sometimes the cause, the case of the Ganges would nevertheless prove to be singular. “It had not been confirmed until now that earthquakes could cause avulsion within deltas, in particular for such an immense river”notes Professor Elizabeth “Liz” Chamberlain of the Dutch University of Wageningen, first author of the study.

An earthquake of magnitude 7 to 8

The team embarked on the trail of avulsion after discovering in 2018, in the ground dug for a future pond about a hundred kilometers south of the capital of Bangladesh, the presence of “sand volcanoes”, that is to say, vertical cords of light sand crossing horizontal layers of darker mud.

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Like other types of seismites (deformations in sediments), these “sand volcanoes” are very clear indicators of the occurrence of an earthquake. In submerged areas, strong tremors can in fact put buried layers of sand under pressure and inject them upwards. Those at the sites studied measure 30 to 40 cm wide, cutting through 3 to 4 m of mud.

By carrying out chemical analyzes on both sand grains and mud particles, the authors demonstrated that the sand “eruptions” – as well as the change in the river bed – had probably taken place around 2 years ago. 500 years. A similar site discovered 85 km downstream then allowed them to estimate the extent of the avulsion, and therefore the approximate magnitude of the earthquake: between 7 and 8.

On satellite images, the ancient bed of the Ganges now corresponds to a low-lying area about 1.5 kilometers wide, found intermittently over a hundred kilometers, more or less parallel to the current course of the river. Filled with mud, it is frequently flooded and is mainly used for growing rice.

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140 million people threatened?

What if the Ganges moved again? The hypothesis is not excluded. A previous study from 2016 shows that the region is currently experiencing geological tensions, capable of triggering earthquakes comparable to the one that occurred 2,500 years ago – or more recent events, such as the earthquake of 1762, at the origin of a deadly tsunami which had traveled up the river.

Today, the occurrence of such an earthquake could affect 140 million people, the authors calculated. “Large earthquakes affect large areas and can have lasting economic, social and political effects”warns Syed Humayun Akhter, vice-chancellor of the Bangladesh Open University, who contributed to both studies.

Elsewhere in the world, other rivers are nestled within tectonically active deltas: the Yellow River in China, the Irrawaddy in Myanmar, the Klamath, San Joaquin and Santa Clara rivers, which flow off the coast of the west coast of the United States, and the Jordan River, which crosses the borders of Syria, Jordan, the West Bank and Israel, the statement lists.

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