The
tsunami that struck eastern peninsular coast of our country in December
2004 attacked certain parts of a state with vengeance while did not
cause much harm on other parts. This raised queries in the minds of
geologists and oceanographers why did tsunami made such distinction?
What were the reasons that slowed down the ferocity of devastating
Tsunami? Today we will try to find out what kind of studies have been
carried out to seek answers to these questions and what was the outcome.
N.K.
Thakur and A. Pradeep Kumar of the National Geophysical Research
Institute (NGRI), Hyderabad, report that oblique sub-duction of the
Indo-Australian plate under the Burmese plate in northern Sumatra
and in the Andaman-Nicobar region in a thrust motion generated a
massive tsunami which traversed all around to bring about unexpected
misery and calamity. It was found that due to variable amplitudes
the onslaught of the waves on the shores varied from place to place.
This happens because tsunami waves are generated as a result of the
'shaking' in the abyss of the sea caused by an earthquake or
volcanic eruption. Tsunami wave heights vary due to shoaling
(becoming shallow gradually) and the topography of the sea floor.
They selected the eastern coastline which faced the brunt of the
tsunami. Indian eastern coastline is about 3200 km. The physiography
of the coast has the ups and downs of the depth and width of the
shelf. The east coast has a unique feature, the Bengal Fan or the
area where the Ganga and Brahmaputra Rivers dispose their load of
sediments. The Bengal Fan is an area of continental rise. The
present day Eastern coast was once upon a time part of the
Antarctica landmass. It broke off from their and the Indian landmass
drifted northwards till it collided with the Asian plate. The coast
therefore, has retained many of the original features as they
existed at the time off of breaking apart. The width of the
continental shelf on the eastern margin is not uniform. It is wider
on the northern side, while on the southern side the shelf is steep
as revealed by the bathymetry. The region between India and Sri
Lanka is shallow. Off the eastern coast sea is monotonous in the
abyssal depths.
Tsunami had come as a rude shock, because till then the geological
fraternity of India was rather lax on that front. Fortunately, the
Survey Of India had placed tidal gauges along the coast long ago.
These came handy to measure the amplitude of tsunami waves. Data was
collected by the scientists of the National Institute of
Oceanography immediately after the disaster and later it was
analysed by Thakur and his colleague. They found that the amplitude
was maximum at Paradeep (>3m), then at Chennai and Visakhapatnam
(2m). It seems that Nagapattinam bore the maximum damage; the tidal
gauge stationed there was damaged and had become un-operational.
Along the western coast the tsunami could generate only low tides.
Once a tsunami attacks a shore it leaves marks on houses and also
leaves behind the debris it transports inland from the ocean. These
form significant evidences to work out the tsunami run-up survey,
which in turn indicates the velocity and also the release of energy
by the tsunami. It also helps preparing plans for settling people
away from the area submerged by tsunami waves. Such marks indicating
the inundation area are quite perishable. Therefore speed in
studying them is vital. NGRI team carried out a quick survey
immediately after the tsunami and found highest marks at
Nagapattinam (5 m), Karaikal (4.4 m), Pondicherry (2.6 m) and
Chennai (2.8 m).
While the NGRI team was studying the Tamilnadu coast, N.P. Kurien
and his colleagues from the Centre for earth Science Studies,
Thiruvananthapuram, had taken up similar studies on the Kerala coast
on the western margin. They reported maximum inundation and damage
in the vicinity of Kayamkulam. But this part of the coast was under
a high tide at that time, therefore the damage was more. The
northern part of Kerala coast was under low tide, hence it suffered
minimum damage.
Computer based high resolution GEBCO and Etopo2 bathymetry data sets
were used by Thakur and his colleagues to work out the sea floor
relief and topography. Such methods help is creating a 3D picture of
the under water terrain. Our continental margins in the peninsular
region are considered passive margins. Steep valleys in the ocean
near such margins are normally not present. To the surprise of the
investigators there are steep valleys in the vicinity of
Nagapattinam and Chennai. At Nagapattinam bathymetry shows steep
slope on the east and shallow relief of the sea floor on the south
near Sri Lanka.
While studying the bathymetric gradients, Thakur and his co-workers
found stppe gradients in the vicinity of Nagapattinam and Chennai.
As compared to these other stations show a rough sea floor as
indicated by a number of highs and lows. However, towards north the
magnitude of gradient is markedly reduced.
We know that the eastern Gondwanaland broke away from Africa and
Antarctica some 120 million years ago. This was followed by seafloor
spreading and evolution of the oceanic lithospheres and underlying
the present Bay of Bengal. The present day geomorphic
characteristics of the shelf slope have much to do with the
phenomenon of breaking away. The present configuration is partly due
to the break away history and partly the gift of sediments derived
from the Himalayas. As the floating landmass collided with the
Eurasian plate some 54 million years ago the result was the birth of
the highest mountain range. The high slopes of which produced plenty
of debris to be carried by the rivers to the ocean. In addition the
Ice ages too left their tell tale marks on the continental shelf.
Broadening of the shelf indicates the extent to which the margin was
exposed to ice age, says Thakur.
The pattern of the continental shelf on the eastern margin is not
uniform; it narrows down near Nellore and Cuddalore and broadens in
the north. The slope and gradient of the bathymetry are steep near
these two places and also at Chennai. From the bathymetry thus
generated Thakur and his colleagues conclude that places where the
gradient is steep become the places from where the heights of the
waves start building up rapidly.
Nagapattinam and Chennai were the places where maximum damage to
property took place. Incidentally these are the places where shape
and bathymetry might have influenced the built up of the waves to
attain extra heights and cause more damage says Thakur. Out of these
places Nagapattinam shows maximum changes in shape of the
continental shelf and bathymetry. Cuddalore does not show
significant change in bathymetry, there the shape of the Bay of the
coast creek-like feature is supposed to have helped the waves to
rise high and spread more landwards.
Studies carried out by Kurien and his co-workers in Kerala also
confirm the phenomenon that local geomorphology does affect the
impact of tsunami waves. They found that the sea walls constructed
along the Kerala coast were not of much use. Well built sea walls
did help save the population from the wrath of tsunami at
Thanagassery and Neendakara but at most other places they failed.
Edavanakkad for example is a place where despite being well built
the sea wall gave way with huge boulders torn away and thrown
inland, adding to the misery of people. Considering the varied
geomorphology at the coasts Kurien and his colleagues recommend
strict compliance of CRZ rules at the coast to save lives from
future devastation.
Infact what applies for Kerala coast applies for the entire coastal
belt of India. CRZ rules are blatantly flouted and may god forbid if
a tsunami strikes the coastal metro like Mumbai it would be
impossible to save lives. Yet a detailed bathymetry would certainly
help the planners to chalk out strategies and also help in the
identification of most hazardous locations.
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The
tsunami that struck eastern peninsular coast of our country in December
2004 attacked certain parts of a state with vengeance while did not
cause much harm on other parts. This raised queries in the minds of
geologists and oceanographers why did tsunami made such distinction?
What were the reasons that slowed down the ferocity of devastating
Tsunami? Today we will try to find out what kind of studies have been
carried out to seek answers to these questions and what was the outcome.
