As many as 140 million people in Bangladesh may be at risk from a huge earthquake as pressure builds beneath the surface of one of the world's most densely populated nations, US and Bangladeshi scientists say.
Sediment flows from the Ganges and Brahmaputra rivers have layered parts of the country with as much as 20 kilometres of sand and mud, masking until now the extension of the same fault line that triggered the 2004 Sumatra tsunami and killed 230,000 people.
"The fault is entirely in the sub-surface," said Michael Steckler, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory and lead author of a paper published this week in Nature Geoscience. "It's been suspected but we haven't had the data to prove that [the pressure] is actually been building up."
Data collected since 2004 by Professor Steckler's team has found that a juncture between major tectonic plates in the region is locked and loading up with stress.
Measurements found convergence of tectonic plates at the rate of 13-17 mm per year "on an active, shallowly dipping and locked megathrust fault", the paper said.
That build-up has the potential to trigger major shifts in land and also an earthquake with a magnitude of between 8.2 and 9.
"We can see the strain building up, we can see the motion of the plates but we can't estimate when something might happen," Professor Steckler told Fairfax Media, adding that the build-up had been going on for at least 400 years and perhaps as long as 2000.
"It would certainly be one of the largest [recorded quakes]," he said. "I suspect it might be at the lower end [of the 8.2-9 scale], but I can't rule out a really large one."
(See chart below showing the Indian Plate colliding with the Eurasian Plate that has created the Himalayas and triggered major recorded earthquakes. The Burma or Sunda Plate is also shifting westwards into Bangladesh.)
Scientists can't tell when the next major quake will strike nor where the rupture in the fault will be the most severe. However, Bangladeshi's location mostly on the river delta of the giant Ganges-Brahmaputra Delta makes the bulk of its 160 million population particularly vulnerable, should a large one occur.
Professor Steckler said a 250 kilometre by 250 kilometre region - including the capital, Dhaka, a megacity with a population of 15 million - is of significant risk.
This area (see chart below) is home to about 140 million people, and sits atop deep sediment that could liquefy in many places in the advent of a big quake. Whole buildings and infrastructure could sink into the sand and soil.
Buildings might "not even break, but literally fall over because the soil loses solidity", he said.
Dhaka's streets, which already experience traffic that "can be really terrible in normal times", could quickly become impassable. "Getting emergency services, foods and water to people is going to be a nightmare," Professor Steckler said.
Syed Humayun Akhter, from Dhaka University, said the capital could become "totally a dead city ... it will be really devastating".
"All the natural gas fields, heavy industries and electric power plants are located close to potential earthquakes, and they are likely to be destroyed," Dr Akhter said. "In Dhaka, the catastrophic picture will be beyond our imagination, and could even lead to abandonment of the city."
The warning extends not just to Bangladesh but also to Aizawl, the capital of India's Mizoram state to the north-east, which could be subject to landslides in a major quake, Professor Steckler said.
Another risk is that the rivers themselves could change course, adding to the misery of those affected by the quake.
"If the ground moves by 10 metres, it's entirely possible to get one to two metres of uplift [of land]," Professor Steckler said. "That can be enough to really rearrange the rivers" - as has happened in the past.
Authorities can reduce the future impact of a big quake by tightening building codes - only introduced in 1993 - and enforcing them, he said.
