About 2,500 years ago, a massive earthquake struck the Indian subcontinent, causing the Ganges to change course. The 2,575 km long river flows through northern India and Bangladesh to the Bay of Bengal. Within days or weeks of the catastrophe, the exact course changed to a new channel about 45 km away. It happened near the delta, about 200 km from the Bay of Bengal.
Another change of course
Rivers often change course due to repeated flooding or sediment build-up over the years. It often happens in deltas, but never before due to earthquakes.
Normally, sediments pile up. As the riverbed rises, the water changes direction. It is not a rapid process. But every now and then, within a few days or weeks, you get a rapid shift in the course of a river, called an avulsion.
What happened to the Ganges was a uniquely large avulsion. “I don’t think we’ve ever seen one this big anywhere,” said study co-author Michael Steckler.
The previously undocumented quake measured 7.5 to 8 on the Richter scale and was so powerful that, despite its center being 180 km away, it diverted a significant portion of the main channel.
Liz Chamberlain, the study’s lead author, and her colleagues were studying satellite images of the Ganges when they spotted a two-kilometer-wide crescent 45 kilometers from the river. Upon closer inspection, the crescent turned out to be a depression in the land, stretching for 100 kilometers and running almost parallel to the present-day river. The team believes it was the original headwaters of the Ganges.
Although the old channel is no longer a river, it still floods regularly. On the side of the old channel, the researchers saw vertical streaks of light-colored sand, known as seismites. Strong shaking from earthquakes put pressure on deep layers of sand and pushed them up through the mud, like sand volcanoes, creating streaks 40 cm wide.
Is there another one coming?
The ancient riverbed has many of these stripes, suggesting that they all formed at the same time. Analysis of the sediments confirms that they are 2,500 years old.
There are currently no recorded earthquakes in the area.
Such an earthquake could happen again. Recent studies show that a huge, hidden fault line under the Ganges Delta could trigger a megaquake that would affect 140 million people.
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