A new GPS study of Bangladesh, India, and Myanmar (Burma) has found startling evidence that the northeastern corner of the Indian subcontinent is actively colliding with Asia, potentially posing a major earthquake risk to one of the world’s most densely populated regions.

The years-long analysis is the first to incorporate GPS data from Bangladeshi tracking stations. It is now the latest volley in a long-running academic debate over if and how the geologically complex region is seismically active.

If the new study’s models are correct, the region—home to more than 140 million people—could be sitting atop an active megathrust fault, the same kind of geologic feature that caused the catastrophic magnitude 9.0 earthquake in Japan in 2011.

What’s more, the models suggest that the fault is stuck and has been accumulating stress for more than 400 years, since before the Mughals made the Bangladeshi city of Dhaka the regional capital in the early 1600s.

That means an area more than 124 miles (200 kilometers) wide may be spring-loaded with significant levels of tectonic strain, the researchers warn today in Nature Geoscience. If the entire fault were to give way at once, the team estimates that it could spawn earthquakes up to magnitude 9.0, causing vast devastation in a region underprepared for seismic catastrophes.

However, researchers do not know if and when the fault will give way.

“Whether this region actually will slip in one single earthquake, nobody can say yes or no,” says Vineet Gahalaut, a geologist at India’s National Geophysical Research Institute and an expert on the region’s seismicity who wasn’t involved with the study. “We don’t have enough data to prove or disprove this.”

Area under significant earthquake risk Lower bound model Upper bound model Population density, 2014 1,001-5,000 people (per sq. km.) Over 5,000 people INDIA BANGLADESH INDIA Mizo Hills Dhaka MYANMAR (BURMA) Kolkata (Calcutta) Chittagong Chin Hills Bay of Bengal 50 mi 50 km LAUREN C. TIERNEY, NG STAFF SOURCEs: Michael S. Steckler And Others, NATURE GEOSCIENCE, 2016; LANDSCAN 2014 NEPAL Guwahati Saidpur INDIA Imphal BANGLADESH Rajshahi INDIA Mizo Hills Dhaka Khulna Haora MYANMAR (BURMA) Kolkata (Calcutta) Chittagong Chin Hills Area under significant earthquake risk Lower bound model Bay of Bengal Upper bound model Population density, 2014 1,001-5,000 people (per sq. km.) 50 mi Over 5,000 people 50 km LAUREN C. TIERNEY, NG STAFF SOURCEs: Michael S. Steckler And Others, NATURE GEOSCIENCE, 2016; LANDSCAN 2014

Locked and Loaded

For more than 40 million years, the Indian subcontinent has been crashing slowly into Asia in a geological pileup that created and continues to feed the Himalaya.

As the mountains erode, sediment washes into the Ganges and Brahmaputra, among the world’s largest rivers, and flows into the Bay of Bengal at a current rate of a billion tons a year.

Over millions of years, piled-up sediment has extended the continental margin near Bangladesh by about 250 miles (400 kilometers). Some parts of the region’s crust are caked with more than 12 miles (19 kilometers) of the stuff.

The region’s rich sedimentary deposits, key to its fertility, make it even more challenging for researchers to determine the threat facing Dhaka—a megacity with more than 14 million people in its metropolitan area—and its surroundings.

“There are thick sediments everywhere … and that kind of area is covering a lot of geology,” says Ashraf Uddin, an Auburn University sedimentary geologist who wasn’t involved with the study. “We don’t get to see it, [and] we don’t get to study it very well.”

From 2003 to 2014, researchers led by geologist Michael Steckler of Columbia University’s Lamont-Doherty Earth Observatory installed and carefully monitored 26 GPS tracking stations across Bangladesh, to see how the country’s western half was moving relative to the Indian Peninsula.

When they combined their data with previous GPS studies done in northeast India and Myanmar, they determined that overall, Myanmar’s Shan Plateau is moving southwest at a rate of nearly 2 inches (51 millimeters) a year relative to the Indian Peninsula.

After subtracting out the known movements of the region’s faults—a tricky task—the researchers were left with an extra 0.51 to 0.67 inches (13 to 17 millimeters) of annual motion, consistent with the Indian tectonic plate actively slipping underneath the Eurasian plate. What’s more, the nature of movement across the region suggested that the Indian plate was stuck, locked to the underside of northwest Myanmar’s mountains.

“It’s very dangerous to us,” says study co-author Syed Humayun Akhter, a seismologist at Bangladesh’s University of Dhaka. “More elastic energy is being accumulated.”

Shake Up

While the region’s sediments take up some of the strain along the newly proposed fault, they’re not especially stable, particularly around the rapidly developed eastern outskirts of Dhaka. If a major earthquake strikes, the sediments could even amplify the seismic waves.

“Dhaka’s basically like building a city on a bowl of Jell-O,” says Steckler.

Meanwhile, building codes in the rapidly growing city have long gone ignored, and the public remains unsure of what to do during an earthquake.

The lack of preparedness has already posed threats: When a magnitude 7.8 earthquake struck Nepal, to the northwest of Bangladesh, in April 2015, tremors caused buildings to tilt and reportedly led to the deaths of at least three Bangladeshis as a result of panic attacks, cardiac arrest, and stampedes.

“We are preparing … but not to the level mark,” says Akhter. “The government and NGOs are trying to educate the people, but it’s slow.”