A fresh roadside pit revealed a dramatic shift in the Ganges. Scientists say they’ve traced the cause

By Kate Golembiewski, CNN

(CNN) — Earthquakes, caused by the shifting of Earth’s tectonic plates, have the potential to transform the face of the world. Now, for the first time, scientists have evidence that earthquakes can reroute rivers: It happened to the Ganges River 2,500 years ago.

The Ganges snakes its way from the Himalaya Mountains, and as it flows through India and Bangladesh, its waters combine with the Brahmaputra and Meghna rivers. By the time it empties into the Bay of Bengal, it has formed the world’s largest delta system, transporting more water than any river complex on the planet except for the Amazon and the Congo.

The Ganges Delta is “a really exciting place to work because it has these big, dynamic river channels,” said Dr. Elizabeth Chamberlain, lead author of a new study published June 17 in the journal Nature Communications.

She and her colleagues surveyed the delta region roughly 60 miles (100 kilometers) south of Dhaka, the capital of Bangladesh, aided by satellite maps and digital elevation models that showed the valley-like channels where the river once flowed.

“It’s normal for rivers to move completely from one place to another in the landscape, and we refer to that as a process called avulsion,” said Chamberlain, an assistant professor in the environmental sciences division at Wageningen University & Research in the Netherlands.

The scientists looked for evidence of this slow, steady avulsion process in the Ganges Delta but found evidence of something much more dramatic in its ancient past, hidden in grains of sand.

Earthquakes cause ‘sand volcanoes’

After a day of work, the researchers noticed a pit at the side of the road while driving back to their hotel. “Because it was freshly dug, we could see the stratigraphy, or layering of the sediments on its walls, and so we hopped down in it,” Chamberlain said.

The researchers noticed 10-foot-long (3-meter) vertical columns of light-colored sand amid the darker mud: a hallmark of a riverbed affected by an earthquake. “The way you get these is, the shaking during an earthquake causes the sand and the mud to move, and the mud moves differently than the sand,” Chamberlain said. “Mud is very sticky, and it tends to stick together, and the sand grains move more as individual pieces, especially when they’re in water.”

When the sand grains move around during an earthquake, they take up more volume. And if these sand grains are constricted by mud, pressure builds up. “If that pressure is great enough, the sand can push up through the mud that’s lying on top of them and cause, effectively, a sand volcano,” she said.

In a July 2016 study, Dr. Michael Steckler, a coauthor of the new paper and a geophysicist at the Columbia Climate School Lamont-Earth Observatory in New York, had previously reconstructed the tectonic plate movements — gigantic slowly moving pieces of Earth’s crust and uppermost mantle — that account for earthquakes experienced in the Ganges Delta.

His models showed that the likely source of earthquakes in the region is more than 100 miles (160 kilometers) away from the sand volcanoes that Chamberlain and her colleagues found. Based on the large size of the sand volcanoes, the quake must have been at least a 7 or an 8 magnitude — approaching the size of the Great 1906 San Francisco earthquake.

Sediment reveals ancient secrets

To determine how long ago this massive earthquake hit, Chamberlain and her colleagues used a method called optically stimulated luminescence. “It’s directly measuring sand or mud grains and looking at when these sediment grains were last exposed to sunlight,” she said.

When grains of sediment are buried, they get exposed to low levels of radiation, which they store as energy. Working in a darkroom so as not to contaminate the samples of sediment with light, Chamberlain and her colleagues used a tool to measure how much radiation the grains were exposed to, thus revealing how long ago the sediments were buried by the earthquake. The researchers calculated that the sand volcanoes formed 2,500 years ago.

About 50 miles (85 kilometers) away from the sand volcanoes, the scientists also found a large river channel that filled with mud at roughly the same time. This finding indicates that 2,500 years ago, the course of the river dramatically changed. The proximity of these events in both time and space suggests that a massive earthquake 2,500 years ago is the cause of this rerouting of the Ganges.

“From an engineering point of view, this is something that we do worry a lot about, the stability of our waterways,” said Dr. Jonathan Stewart, a professor of civil and environmental engineering at UCLA who was not involved with the project. He said that the study was “useful from several perspectives,” providing more information about how frequently large earthquakes occur in Bangladesh and which areas might be affected if a major one happens again.

Monitoring quakes today

If a similar earthquake happened in the Ganges Delta today, more than 140 million people in the area could be affected. “These findings can inform earthquake mitigation and preparedness efforts by highlighting the potential for similar events in the future,” said study coauthor Dr. Syed Humayun Akhter, a professor of geology and vice provost of the Bangladesh Open University.

He noted that both individual actions and government efforts, including investments in seismic monitoring and early warning systems, infrastructure improvements, and public outreach, are necessary to keep people safe if another earthquake strikes.

“These kinds of extreme events don’t happen very often, so we don’t always have a modern record of everything that happens with Earth’s dynamics,” Chamberlain said.

Studies like this, she added, “help us peer into the past and see how Earth operates, or can operate over 100,000-year or even longer time scales. And that’s really important for understanding what we have at the surface today and what might happen in the future.”

Kate Golembiewski is a freelance science writer based in Chicago who geeks out about zoology, thermodynamics and death. She hosts the comedy talk show “A Scientist Walks Into a Bar.”

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