Bay of Bengal risks tsunami too
The northern tip of the Bay of Bengal, which is home to more than 60 million people, is at risk from tsunami-generating earthquakes, an Australian seismologist says.
Cummins has also called for further investigations, including a network of GPS observations and geological studies to confirm his hypothesis.
Cummins says it is widely accepted that there is a high risk of another giant earthquake just off the coast of central Sumatra.
This is to the east of the 2004 Boxing Day earthquake that devastated coastal communities across South and Southeast Asia, killing more than 200,000 people.
But he says there is currently little concern about the subduction zone to the north in the Bay of Bengal, along the coast of Burma.
This area includes Bangladesh's second-largest city of Chittagong, with an estimated population of more than 6 million.
It also includes the Ganges-Bramaputra delta, which has more than 60 million people, or about 1% of the world's population, living within 10 metres of sea level.
Cummins says previous interpretations of the tectonics in the northern Bay of Bengal suggest large tsunami-generating earthquakes are unlikely.
This is because there is no active subduction, when one tectonic plate slides beneath another, along the Burmese coast.
And if some subduction did occur, it would result in earthquakes rupturing a relatively short (300 kilometre) length of plate boundary.
But Cummins says Indian geologists interpret the active deformation along the Chittagong-Tripura fold belt, along the coast near Chittagong, to mean the coast is changing in response to active subduction.
They believe the associated fault, or at least how it is expressed at the earth's surface, lies off the Burmese coast.
If true, Cummins says, this would mean a much longer section, about 900 kilometres, of fault is underwater. He says this, coupled with the direction the plates are moving in, make the fault capable of generating a tsunami.
"The plates are pushing against each other, rather than moving side-by-side as would be the case for a fault directed due north, and so the thrust earthquakes that can cause a tsunami are more likely to occur," he says.
Cummins says recent marine seismic surveys support this interpretation.
And his analysis of historical accounts of the 1762 Arakan earthquake show that rupture extended 700 kilometres along the Arakan coast, from just west of the Irrawaddy Delta to the northern tip of the Bay of Bengal.
On the radar
The scientist says the point of his paper is to put the northern tip of the Bay of Bengal on the tsunami hazard radar.
He says the region is home to huge urban agglomerations, such as Kolkata and Dhaka, with a large proportion of their populations living in homes that are "unlikely to be seismically resilient".
"Even if only a small fraction of this population is vulnerable to a giant tsunamigenic earthquake along the Arakan subduction zone, it seems likely that the number of lives at risk may be over a million," he says.
He says a dense network of GPS observations should be established along Burma's Arakan coast, to determine whether "the ground is moving in a way consistent with a locked subduction zone that is building up stress towards another earthquake".
"Geological investigations should be able to unearth traces of the subsidence near Chittagong, and perhaps of a tsunami if there was one," he says, "and may also be able to extend the record to prehistoric events that would allow us to estimate how often these events occur."