The sleeping dragon

When most people think of seismically-active regions, Bangladesh is not ordinarily near the top of the list.

Cyclones, sure.  They come roaring up the Bay of Bengal with a horrifying regularity, and most of the country is low enough in elevation that the storms barely even slow down.  The worst was the 1970 Bhola cyclone, which still holds a record as the deadliest storm in recorded history.  The official death toll was five hundred thousand, but is likely higher than that, mostly people who lived in the lowlands near the city of Chittagong.

Unfortunately for the citizens of Bangladesh, though, they're also at high risk for earthquakes -- something that has only been recognized recently.  A 2021 study led by Muhammad Qumrul Hassan of the University of Dhaka found that the region is right on top of the junction of three different tectonic plates, the Eurasian Plate, the Indian Plate, and the small Burma Plate ("small" here means geographic area, not capacity for damage -- the devastating 2004 earthquake and tsunami was caused by a slippage of the Burma Plate relative to the Indian Plate).  But the compression and twisting of the land near the junction has created enough stresses that the entire country is crisscrossed with faults, most notably the Dauki Fault and the Haflong Thrust (which crosses into the Indian states of Meghalaya and Assam to the north).

The whole thing is exceedingly complex, and still poorly understood.  Imagine laying a sheet of pie crust on a table, and you and two friends each stand around it and push, pull, or twist it from the edge.  The sheet will wrinkle, tear, and hump up in places, but exactly where those deformations will end up isn't easily predictable because it depends on where there was weakness in the dough before you started messing with it.  This is the situation with the chunk of the Earth's crust that underlies Bangladesh.  Add to that the fact that the region is poor, and much of it is jungle- or swamp-covered and pretty inaccessible to study, and you have a picture of the extent to which we don't understand the situation.

However -- alarmingly -- a 2016 study found that the entire region has been building up stress for at least four hundred years, meaning when the some piece of fault slips, it's likely to be catastrophic.

The whole topic comes up because of a rather terrifying discovery that was the subject of a paper this week in Nature Communications.  Geoscientists Elizabeth Chamberlain (of Wageningen University). Michael Steckler (of Columbia Univeristy), and colleagues were studying a puzzling historical shift in the channel of the Ganges River, and quite by accident -- it was in an area some locals were digging in to create a pond -- they saw the unmistakable signs of seismites.  These are features in rock layers created by massive earthquakes, in this case a column of sand that had erupted through pre-existing strata during a colossal temblor.  Upon analysis, they found that the river had changed course because of a massive earthquake about 2,500 years ago.

Imagine an event big enough to shift the path of a river that size.

A change in the course of a river is called an avulsion, and it normally takes decades or centuries.  (It's an avulsion of the Mississippi River that the levee system in southern Louisiana is attempting to prevent -- something I wrote about a couple of weeks ago.)  Seismic avulsions are much less common, but when they happen it's sudden and spectacular.  The only other one I've ever heard of is the shift in the Mississippi caused by the 1812 New Madrid earthquake, which dropped the land so much it cut off a meander and created Reelfoot Lake.

The seismic record in Bangladesh indicates that they're dangerously at risk for another earthquake -- and because of the complexity and our lack of comprehension of the fault system underlying the country, the geologists aren't certain where is likeliest to rupture.  There's a sleeping dragon underneath one of the poorest countries in Asia -- and we're only beginning to understand when and how it might suddenly awaken.

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