Does Bangladesh threaten a mega earthquake?

Newly discovered subduction zone has been building tension for at least 400 years

Under Bangladesh researchers have discovered a previously unrecognized subduction zone - and this is under strong tension. © NASA
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Danger in the Underground: The densely populated Bangladesh could sit on a ticking time bomb. Because the country lies in the middle of a previously unrecognized subduction zone - an interface of the earth's crust, which has been hooked for at least 400 years. As a result, there builds enormous tension, which could be discharged in a quake of magnitude 8 or even 9, the researchers warn in the journal "Nature Geoscience". Millions of people could be in danger.

Bangladesh is one of the most densely populated countries in the world - literally built on sand. Because a large part of the country lies on a kilometer-thick layer of mud, the large rivers Ganges and Brahmaputra have deposited in this delta. For the inhabitants of this shallow alluvium this means that they are exposed to floods caused by storms, heavy rainfall and rising sea levels almost defenseless.

But it gets worse, as Michael Steckler from Columbia University in New York and his colleagues found out. For their study, they had evaluated ten years of data from a seismic network set up in 2003 in Bangladesh. Their results show that the subsurface below this region is much more unstable than previously thought.

Active subduction zone

Bangladesh lies in an area where the Indian continental plate encounters the Burmese plate - so much was already known. Previously it was believed that this plate boundary is rather flat and strong earthquakes are therefore likely to be expected in India and Pakistan. But the new data show a different, more threatening picture. Thus, under Bangladesh, there is a real, deep-reaching subduction zone.

As geologists have discovered, east Bangladesh is moving relatively fast towards Myanmar along this subduction zone - at least 46 millimeters per year. Part of the resulting tensions is derived from the previously known superficial dislocations in Myanmar and India. But the rest - about 17 millimeters per year - pushes the Indian plate under Bangladesh in the depth. display

The potential earthquake zone (red line) covers a good 60, 000 square kilometers and an area where 140 million people live. Small Chris Small / Lamont-Doherty Earth Observatory

Blocked for at least 400 years

The problem with this: In an area of ​​250 by 250 kilometers, this movement is blocked by entangled rock, as the researchers found out. "Some of us suspected this danger, but we did not have the data or a model, " Steckler explains. "Now we have both and can estimate the size of this zone."

As the researchers report, the tension built up in this blockade has not been discharged in a strong earthquake for at least 400 years. So much energy has accumulated in the subsurface - enough to let the subsoil at the plate boundary be pushed forward by around 5.50 meters at one go. If the blockade lasts longer, then it could even be up to 30 meters offset, so Steckler and his colleagues.

Preparations for a mega earthquake

If the voltage of the blockade zone discharges, this would trigger a strong earthquake of at least the magnitude 8.2, possibly even 9. Bangladesh could face an earthquake of the strongest known category. "We do not know when it will happen, it can be tomorrow or in 500 years, " says Steckler. "But we definitely see that one is imminent."

If the subduction zone under Bangladesh experiences a massive earthquake, researchers estimate that more than 140 million people could be affected. The megacity Dhaka with its 15 million inhabitants is also in the danger zone. In addition: The unstable alluvial land under a large part of the country could be lost to earthquakes and buildings, roads and other sink.

The metropolis of Dhaka is in the potential earthquake area. PratyashaSaha / CC-by-sa 4.0

Catastrophic consequences

The consequences of such an earthquake would be catastrophic: "Bangladesh is overcrowded everywhere, " says Syed Humayun Akhter from the University of Dhaka. "In addition, all gas fields, heavy industry sites and power plants are in the earthquake zone and are likely to be destroyed. In Dhaka, such a quake would cause an unimaginable catastrophe and could even lead to the city having to be abandoned afterwards. "

In addition to the direct destruction by the earth, such a quake could cause severe flooding. Because historical data prove that the labile alluvial land in the delta is easily deformed during earthquakes and thereby the rivers overflow their banks and look for a new bed, as the researchers report. In view of the enormous increase in population density and infrastructure since then, even a moderately moderate earthquake could cause devastating destruction in this region.

To better assess the risk and potential timing of a future earthquake, the researchers now want to install another 70 seismometers. They will also monitor the subsoil in neighboring Myanmar and provide more data on the subduction zone reaching into this region. "We know so little about their geometry and how far it goes, " says James Ni of New Mexico State University. (Nature Geoscience, 2016; doi: 10.1038 / ngeo2760)

(Lamont-Doherty Earth Observatory, Columbia University, 13.07.2016 - NPO)