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Earthquakes, caused by the shifting of the Earth’s tectonic plates, have the potential to change the face of the earth. Now, for the first time, scientists have evidence that earthquakes can reroute rivers: It happened to the Ganges 2,500 years ago.
The Ganges snakes its way from the Himalaya Mountains, and as it runs through India and Bangladesh, its waters meet with the Brahmaputra and Meghna rivers. By the time it empties into the Bay of Europe, it has formed the largest delta system in the world, carrying more water than any river complex on the planet except the Amazon and the Congo.
The Ganges Delta is a very exciting place to work because it has these large, 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 about 60 miles (100 kilometers) south of Dhaka, the capital of Bangladesh, with the help of satellite maps and digital elevation models that showed the valley-like channels through which the river flowed. a time.
“Rivers tend to move completely from one place to another in the landscape, and we call that a process called avulsion,” said Chamberlain, an assistant professor in the department of environmental sciences at Wageningen University & Research in the Netherlands.
Scientists looked for evidence of this slow, steady process of erosion in the Ganges Delta but found evidence of something far more dramatic in the past, hidden in grains of sand.
earthquakes cause ‘sand volcanoes’
After a day’s work, the researchers noticed a ditch on the side of the road while driving back to their hotel. “Because it was newly excavated, we could see the stratigraphy, or the layering of the sediments on its walls, so we got down there,” Chamberlain said.
The researchers noticed 10-foot-long (3 meter) vertical columns of light-colored sand among the darker mud: a hallmark of an earthquake-affected riverbed. “The way you get these is when shaking during an earthquake causes the sand and mud to move, and the mud moves differently than the sand,” Chamberlain said. “Mud is very sticky, and 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 on more volume. And if these grains of sand are confined by mud, pressure increases. “If that pressure is great enough, the sand can push up through the overlying mud and effectively create a sand volcano,” she said.
In a study in July 2016, Dr. Michael Steckler, co-author of the new paper and a geophysicist at the Columbia Climate School Lamont-Earth Observatory in New York, recreated the previous movements of tectonic plates – huge pieces of the Earth’s crust that move slowly. mantle — which accounts for earthquakes occurring in the Ganges Delta.
His models showed that the probable source of earthquakes in the region is more than 100 miles (160 kilometers) away from the sand volcanoes found by Chamberlain and her colleagues. Based on the large size of the sand volcano, the quake must have been at least a magnitude 7 or 8 — approaching the magnitude of the Great 1906 San Francisco earthquake.
Sediments reveal ancient secrets
To determine how long ago this massive earthquake struck, Chamberlain and her colleagues used a method called optically-induced luminescence. “It’s directly measuring grains of sand or mud and seeing when these sediment grains were last exposed to sunlight,” she said.
When sediment grains are buried, they are exposed to low levels of radiation, which they store as energy. Working in a dark room so that the sediment samples would not be exposed to light, Chamberlain and her colleagues used an instrument to measure how much radiation the grains were exposed to, indicating how long ago the sediments were deposited the earthquake. The researchers calculated that the sand volcanoes were formed 2,500 years ago.
About 50 miles (85 kilometers) away from the sand volcanoes, the scientists also found a large river channel filled with mud at about the same time. This result shows that the course of the river changed significantly 2,500 years ago. The proximity of these events in time and space suggests that this rerouting of the Ganges was caused by a massive earthquake 2,500 years ago.
“From an engineering standpoint, this is something we’re very concerned about, the stability of our waterways,” said Dr. Jonathan Stewart, a professor of civil and environmental engineering at UCLA who was not involved in the project. . He said the study was “useful from several perspectives,” providing more information about how often large earthquakes occur in Bangladesh and the areas that could be affected if a major one were to occur again.
monitoring vibrations today
If a similar earthquake were to occur 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 possibility of similar events in the future,” said study co-author Dr. Syed Humayun Akhter, professor of geology and Bangladesh Open University Vice-Private.
He noted that individual actions and government efforts, including investments in seismic monitoring and early warning systems, infrastructure improvements, and public outreach, are needed 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 dynamics,” Chamberlain said.
She said studies like this, “help us look into the past and see how the Earth works, or how it can work over time scales of 100,000 years or even longer. And that is very important to understand what we have today and what could happen in the future.”
Kate Golembiewski He is a Chicago-based freelance science writer covering zoology, thermodynamics and death. She is the host of the comedy talk show “A Scientist Walks Into a Bar.”
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