An ancient earthquake changed the course of the Ganges River

As rivers chew their way through the landscape, they naturally meander—especially in their relatively flat deltas, where sediments can accumulate and divert the river one way or another (SN: 4/1/14). Course changes can unfold gradually over time ranging from years to centuries, says Elizabeth Chamberlain, a geophysicist at Wageningen University and Research in the Netherlands. But the jump in the channel caused by an earthquake can happen within weeks or days, she says.

After shifting a river channel, the old waterway can gradually fill with sediment. However, evidence of the old channel usually remains, Chamberlain says. While looking at satellite images of the Ganges delta, she and her colleagues noticed a slight depression that formed a crescent shape about 45 kilometers from the actual Ganges. That depression was nearly 2 kilometers wide and stretched for tens of kilometers. At one time, it had probably been a major channel of the Ganges, the team surmised.

While doing fieldwork nearby in 2018, the team decided to personally check the depression, including taking samples for dating. Then, good luck struck.

While driving home, the researchers came across an open sinkhole; someone had dug up the ground for a pond that they planned to fill with water the next day. On one side of the pit, the researchers noticed distinctive, mostly vertical bands of light-colored sand embedded within darker horizontal layers of mud—a type of deposit known as seismites.

These features are, in essence, the frozen-in-time remains of ancient sand volcanoes that formed when seismic waves from a distant earthquake exerted pressure on an underground layer of watery sand. That pressurized sludge is blasted up through the upper layers of the silt mud.

Sometimes referred to as “sand shocks,” these features are very difficult to explain but for an earthquake, says John Shaw, a sedimentologist at the University of Arkansas in Fayetteville, who was not involved in the new study. “They just don’t happen for no reason.”

Based on the width of the sand shocks, the depth of the overlying sediments, and the distance to the nearest major fault zone (more than 180 kilometers), the earthquake that produced the seismites probably ranged between magnitude 7 and 8, Chamberlain and her colleagues report June 17 in Nature Communications.

Analyzes of sand grains in the seismites suggest the tremor must have occurred about 2,500 years ago, although there are no written records of this ancient earthquake, Chamberlain says. Because the lower-level—and therefore older—mud that accumulated in the channel was deposited at the same time as the seismites formed, the team linked the earthquake to the change in the river’s path.

Flooding from a river shift caused by a similar earthquake today could threaten as many as 170 million people—a number equal to about half the population of the United States—who live in an area the size of Illinois, India, and Bangladesh. , says Chamberlain. In this delta region, much of the infrastructure is built on mounds of sediment dredged up by rivers and piled up to provide some elevation above the nearby flood plains, making it extremely vulnerable if and when a round occurs. another swing.

Such a risk only adds to the threats of flooded deltas that come with rising sea levels due to climate change (SN: 3/10/22). It’s a big concern, says Shaw: “How do you anticipate and prepare for events you haven’t experienced?”