India jolted by shifting tectonic plates
INDIA: Tremors from the large magnitude earthquake which
struck south-eastern Iran were felt in parts of north and north-western
India because it was a deep earthquake, and not a shallow one, with the
hypocentre estimated to be about 82 km. (Hypocentre is the position
below the surface of the earth where the built-up strain energy in the
rocks is first released and the fault begins to rupture.
It is directly below the epicentre). In a quick estimation of the
parameters, the USGS earlier put the hypocentre at only 15.2 km deep but
later corrected it to 82 km.
At lower levels, shockwaves can travel long distances, points out D.
Sri Nagesh of the National Geophysical research Institute, Hyderabad.
The fact that the ground in the northern part of the country is
dominated by alluvial silt deposited by the mighty Himalayan rivers
further helps the shockwaves travel easily.
Dr. Nagesh, however, clarified that the mild tremors felt in Odisha
and Assam earlier in the day were not linked to this event and they were
The fact that it is a deep-seated quake points to its being caused
not due to a surface fault but due to a faulting in the subduction zone.
Subduction zones occur when oceanic crust of one tectonic plate dives
under the oceanic crust or continental crust of another and there is a
build-up of strain energy in the plates.
The region has complex tectonics with the Indian plate subducting
obliquely under the Eurasian plate in particular, points out Dr. Nagesh.
It is thus prone to large quakes and the 2005 Muzaffarabad temblor and
the 1935 Quetta earthquake were examples, he says.
No fewer than four major tectonic plates (Arabia, Eurasia, India and
Africa) and minor smaller tectonic blocks such as the Oman plate are
converging and the compression that is taking place is responsible for
seismicity and tectonics in the region, says Dr. Nagesh. While the
Arabian plate is converging in the north-northeastern direction on the
Makran coast of Iran and Pakistan at about 37 mm/yr with respect to the
Eurasian plate, the Indian plate is converging northwards at about the
The subducted Arabian plate is known to be seismically active to
depths of about 160 km, according to the USGS.
“The frequency of moderate and large earthquakes within the subducted
Arabian plate is not high compared to similar events in some other
subducted plates worldwide,” the USGS has said. But, it adds, several
quakes have occurred in the region of the present Iranian event over the
past 40 years.
In 1983, there was a magnitude 6.7 shock 50 km to the south and in
January 2011, a 7.2 magnitude temblor occurred approximately 200 km to
the east, in a similar tectonic environment.
- THE HINDU