In India, Bangladesh and Myanmar, a Major Earthquake is Inevitable
A monster earthquake of possibly magnitude 8.2 to 9 is brewing
beneath Bangladesh, Myanmar and Northeast India, imperilling 140
million people within 99 km of its potential epicentre, according to a
new paper in the journal Nature Geoscience.
When
that might happen is unclear. It could be 500 years from now or it
could be tomorrow, but the researchers said an earthquake is inevitable.
The
62,159-sq-km area, along what is called the “Indo-Burmese arc”, which
runs through India, Bangladesh and Myanmar, is where the Indian tectonic
plate–a raft of the earth’s crust that bears the subcontinent–is diving
obliquely under the Sunda plate in Myanmar, at around 46 mm a year.
The
two plates have been stuck for 400 years, and a growing strain could
eventually liberate itself in a large earthquake, researchers said.
The evidence comes from the first detailed picture of the
region’s tectonic motion. A team of scientists from the US, Singapore
and Bangladesh analysed the combined data over 13 years from global
positioning system (GPS)–a network of satellites around the earth used
to locate positions, in this case plates that form the planet’s
crust–stations in Bangladesh, with additional data from India and
Myanmar.
This is the first regional GPS study that uses data
from across the entire plate-boundary region (locations where two
tectonic plates meet),” Michael Steckler, the lead author of the study
and a geophysicist at Columbia University’s Lamont-Doherty Earth
Observatory told IndiaSpend.
As many as 107 Indian towns and
cities lie in areas prone to earthquakes, with 60 percent of the
subcontinental landmass vulnerable, IndiaSpend reported in April 2016.
Steckler
said they have long guessed a megathrust fault–the interface between
tectonic plates–may be lurking underneath Bangladesh, but they did not
have the data or the model.
Now, using data from the GPS arrays,
which can track the crust’s movements accurate to within 2 mm each day,
“we can see the motion of the various blocks and across faults over the
years”.
Topographic map of the Ganges–Brahmaputra Delta and Indo-Burman
Foldbelt showing GPS velocities. Plate boundaries and major faults are
shown in black and grey, respectively. (Photo Courtesy: Indiaspend/Michael Steckler et al)
The region is unstable below the surface as much as on the
surface, with a daily dose of social, economic and security problems.
Fault lines crisscross the innards and surface. “The presence of a
locked megathrust plate boundary represents an underappreciated hazard
in one of the most densely populated regions of the world,” said the
paper.
The red line is the area of the fault, an area of about 24,000
square miles, which could jerk in the event of an earthquake. An
earthquake would likely start at the deep eastern end and grow to the
west. (Photo Courtesy: Indiaspend/Christopher Small and Michael Steckler)
India and Asia: A Nature of Great Tectonic Conflict
The newly identified subduction zone–where one tectonic plate
dives under the other–yet again illustrates the patchwork of faults in
South Asia. As India plows into Asia, some of the continent is wrapping
around the side of the Indian plate, causing considerable deformation in
Asia.
The Indian and Eurasian plates have been in conflict for
50 million years at this collision zone, with the Indian plate diving,
northward, under the Eurasian plate.
This produces the combination of mostly sideways and some convergence in this complicated zone
Michael Steckler
The world’s largest earthquakes occur at subduction zones,
when slabs of heavy ocean crust slide under the other plate below the
ocean floor or under neighbouring continental crusts.
In 2004,
the 9.3 magnitude Indian Ocean temblor, epicentred in the Indian Ocean
off the coast of Sumatra, triggering a tsunami that killed 227,898 people; in 2011, another 9 magnitude earthquake epicentred off the coast of Tohoku in Japan setting off a tsunami, killing 15,891 and triggering the Fukushima nuclear meltdown.
The Wobbly Effect of a 19-km-high Pile of Ganga-Brahmaputra Mud
However, this particular subduction zone in the Indo-Burma
region is right under our feet, so to say, and its nature has been a
bone of contention.
Since the motion between the plates is
angled, some researchers thought the two plates were sliding past each
other and that there was no significant coming-together between the
plates and hence no subduction, a more violent affair.
The latest
GPS evidence indicates motion consistent with subduction. “Furthermore,”
said Steckler, “the pattern of deformation indicates that the two
plates are locked–they are stuck together (the Indian plate is stuck to
the underside of mountains in Northeast India and Myanmar)–but will
eventually jump in a large earthquake.”
Another peculiarity of the
region is its sedimentary build-up. As the Himalayas erode, mud pours
into the Ganga and Brahmaputra and finally spills into the Bay of
Bengal.
This mud is being scraped off and folded up to form the
Indo-Burman Ranges, like a rug being pushed against a wall, as Steckler
put it, further destabilising the plates.
This mud also feeds the
plains of the Ganga and Brahmaputra basin, turning it one of the world’s
most fertile regions. The researchers estimated that sediment disgorged
from the rivers has moved the edge of the continent westward by about
400 km near Bangladesh.
With the mud piled up as thick as 19 vertical km, it is challenging for scientists to pinpoint the contours of a future temblor.
140 Million People Living on a Jelly-like Land
In effect, the region’s 140 million people are living on
jelly. In the event of an earthquake, Steckler said, the thick sediment
may reduce “high-frequency shaking” but will amplify “low-frequency
ground shaking”, which refers to violent shaking that can topple
buildings.
The present study focused on the eastern boundary
of the Indian plate, the Indo-Burman range, but tectonic disquiet grows
as well on the northern boundary, on the Main Himalayan Thrust as IndiaSpend reported here and here.
As IndiaSpendreported
in October 2005, there is a possibility of a more severe earthquake in
the Himalayas–although it isn’t certain when–than the 7.8-magnitude
temblor that devastated Nepal on April 25, 2015.
However, Steckler acknowledged that a study of this nature calls for more data collection and interpretation.
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