Bathymetry Helps to Fathom Tsunami Power

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. 


More by :  V. K. Joshi (Bijji)

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