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The science behind the catastrophe: Why Sri Lanka should be relatively safe for quite some time
Power of nature
By Professor Ananda Gunatilaka
It is amazing that an earthquake occurring 1,500 km away in Indonesia could cause a tsunami mega-disaster in Sri Lanka. Such is the power of nature. Earthquakes are typical in regions where two tectonic plates interact. Here the north-east moving sea floor of the Indian Plate was thrust under the adjacent and overriding Burma Micro-Plate (Fig. 1). In the process, over 1,000 km of the Earth's crust and mantle just 30 km beneath the sea floor snapped, moving the Burma Plate 13 metres over the Indian Plate.

The thrusting rapidly raised the sea floor, generating tsunami that devastated coastlines of the southern Bay of Bengal, Thailand, the Andaman and Nicobar Islands and Myanmar (Fig. 2). Sumatra (including Aceh) where the death toll stands at more than 100,000, had no chance at all, being right on the margins of the two plates. It is invariably these high magnitude megathrust earthquakes at convergent plate margins that generate tsunami.

About 95% of all seismic activity in the Earth is concentrated in the Pacific Ocean region, where tsunami affects 26 countries. The Indian Ocean was always considered a "tsunami backwater", but prone to heavy cyclones.

The tsunami
A tsunami becomes visible only in the shallower waters of coastlines, harbours and bays, where the friction and drag against the sea floor causes the fast moving waves to bunch up and gain height as devastating walls of water. Sri Lanka with its very narrow continental shelf and wide coastal plain was particularly prone to damage as the unexpected tsunami released its energy suddenly. In wide and shallow shelves, the waves would have gradually dissipated much of the energy before reaching the shoreline.

It is a misconception to believe that all tsunami are caused by earthquakes. There is a type of tsunami that is unrelated to seismic activity and earthquakes. This tsunami is triigered by massive submarine sediment slides (or undersea avalanches) generated within continental slopes due to slope and sediment instability, massive landslides or avalanches that end in the ocean, large meteorite impacts and violent volcanic eruptions. The huge sediment or lava masses in motion will displace a massive volume of water to generate destructive tsunami.

The tsunami generated by the stupendous explosion of Krakatoa volcano in the straits between Java and Sumatra (in 1883) reached 40 metres in height and drowned 36,000 people on nearby coasts. Its effect was felt in Sri Lanka within hours of the eruption. The Ceylon Observer of August 27, 1883 reported the event and only one death.

Not every submarine earthquake will generate tsunami. Just two weeks ago an earthquake of 8.1 magnitude occurred between Tasmania and Antarctica, but no tsunami was caused. The distance between the earthquake epicenter and Sri Lanka is 1500 km (see Fig. 2). The quake occurred at 6.59 a.m. (Sri Lanka time); at 8.31 a.m. the Amparai coast was struck by the tsunami and the rest of the coastline within the hour. Wave speeds must have reached 1000 km/hour. With some minutes taken for seismic analysis by the recording centres and issuance of a warning (by whom? and to whom?) to the surrounding countries, about 60 to 70 minutes would be available for evasive action.

Even if we picked up the tsunami warnings, it still could not have mattered very much as Sri Lanka does not have a proven Civil Defence Programme to evacuate people in disaster situations. To tell them that the sea was going to flood the land within the hour would have surely been taken as a hoax! To move over three million people in the coastal belt to safety (in the night!) would have been logistically impossible. No need to look for scapegoats!

Further, there was no history of tsunami in this part of the Indian Ocean and hence the need to educate the public had not arisen. The stage was set for a surreal disaster – more than 30,000 dead and rising, a million people displaced and drastic damage to property and infrastructure facilities. The immense power of flowing water was depicted in dramatic fashion. When the first wave hit Kalmunai at 8.31 a.m, Aceh-Sumatra was already devastated and the water had receded. It took just 20-25 minutes for the wave to reach the Sumatran coast.

The initial energy release of the Sumatra quake is estimated at approximately 2x1019 m.ergs (which is equivalent to total energy used in the USA for one year). Much of the energy of the tsunami is conserved during its travel in deep water and is then suddenly released on reaching shallow water, causing enormous coastal destruction. Most waves do not exactly break on the shore as a large wall of water, but rather continues inland as a fast stream of high water (like a tidal bore) swallowing everything on its path.

Such a situation may have occurred in the Yala sanctuary and Mullaitivu area, where the sea water column reportedly extended up to several km inland. In many areas of Sri Lanka, the walls of water were up to 10 metres high when they lashed against the shoreline. Just four or five waves were enough to cause all the agony we have seen.

Prognosis
The Indian Ocean seafloor will continue to be consumed by the Earth's mantle along the Sunda Trench. It will take a very long time (geologic time-scales) to build up sufficient stress in the same region to cause another very high magnitude tsunami generating earthquake. Seismologists can now predict where some earthquakes are likely to occur, but not when exactly.

However, another 25 years of research will most likely solve the latter problem too. Global stress distribution maps are already available for many plate margins. Subducting plate margins (called active margins) are present only on the eastern Indian Ocean side, where the potential tsunami danger to Sri Lanka lies. A seismic gap is a region where large amounts of stress have accumulated over a long period without causing an earthquake. Stress maps indicate that prior to the quake of 26/12; a long seismic gap existed along the Sunda Trench, except for a small region in the middle part of the trench where a previous quake had released some amount of stress.

The last Sumatra earthquake released a vast amount of stress along a 1,200 km segment from the north of Andaman to near the Sunda Straits, leaving a gap in the easternmost part of the trench and oriented E-W (opposite northwestern Australia). Consequently, Sri Lanka should be relatively safe for quite some time from tsunami originating in the eastern Indian Ocean.

The oceanic region between Sri Lanka and East Africa is devoid of such converging plate margins and hence is of low potential for high magnitude earthquakes. There is thus no threat of tsunami from the western Indian Ocean as it is made up mainly of Mid-Ocean Ridges and divergent or separating plate boundaries (called passive margins). The Oman-Makran trench boundary is not a threat to Sri Lanka, as any tsunami originating there would be directed towards the nearby mountainous and steep Oman shoreline, which would smother the wave in no time.

Sri Lanka itself has never been the focus for an earthquake. The low magnitude tremors felt over the last few years probably originated offshore to the SW of the island or near to Chagos-Laccadiv Ridge, and are part of the normal seismicity of ridge axes. Any changes here will be on geologic time scales (millions of years). I am confident that any immediate danger has subsided and our present geological knowledge does not indicate the possibility of another tsunami for many years to come. Geologists are not prophets! They can only interpret the data based on current thinking of deep-seated geological processes in the Ocean.

The December 26 tsunami is now the greatest ever recorded anywhere in terms of the loss of life. In the last 250 years, there have been only three other tsunamis that caused comparable loss of life (from both the quake and the tsunami). They were the famous Lisbon, Portugal tsunami (1755): 8.7M and 70,000 dead; Messina, Italy (1908) 7.2M and 70- to 100,000 dead: and Flores Island, Indonesia (1992): 7.5M and 25,000 dead.

The largest quake ever recorded (9.5M) and accompanying tsunami (1960 in Chile), resulted in a total loss of life of less than 5000. It travelled a distance of 17,000 km in 22 hours before it struck Honshu in Japan, when 200 people perished. Basically, there had been four catastrophic tsunami in 250 years and the December 26 tsunami was the first in the Indian Ocean.

The 1883 tsunami that was felt in Sri Lanka was a non-entity. In 1970 and 1992 respectively, 330,000 and 210,000 people reportedly perished in the famous cyclones of the Bay of Bengal. In terms of probabilities, I am more concerned with frequent, destructive cyclones than a future rare tsunami affecting Sri Lanka. On the 26th morning, the National Geophysical Research Institute in Hyderabad, India (NGRI) picked up the earthquake and located its epicentre. At 8 a.m. its news bulletin indicated no threat from the quake or a tsunami!

The aftermath
The main aid donor nations will ensure that the entire region will have a state of the art early warning system in the near future. What can be learnt from this unique event? Firstly, we use our common sense and not panic.

Tsunamis sometimes signal their arrival by a precursory rise and fall of coastal water. This occurred in Sri Lanka. Tide gauges should pick this up. This natural warning should be heeded. Move away to high ground as soon as possible. The animals of Yala did.
If you live close to the coast and feel a strong tremor or have heard in the news of a strong earthquake having occurred in the Indian Ocean or in a nearby coastal region, be prepared to leave everything and move to higher ground without panicking.
People have lost their lives by going down to the beach to watch a tsunami. The first tsunami waves after hitting the coasts, almost immediately roll back with great force and undertow. A vast area of sea bottom (up to 300 metres according to witnesses) is suddenly exposed with fish and shells strewn all over. People run down to collect. The wave instantly returns with even greater force and it is all too late to run back. Don't make this mistake.
Tsunami can go up river channels and estuaries causing floods. If you live on river banks, quickly move away to high ground.
= Tsunami are continuously tracked by weather centres across the world after an earthquake. A linkage should be established with such centres through the internet or phone lines and the progress of the impending disaster should be conveyed to responsible authorities here. If correctly done, we should have at least an hour to take evasive action and save lives. The Meteorology Department is an obvious focal point for this.
= Local emergency organizations should be set up along the coastal towns to warn the people. How are these organizations to be informed of the coming wave?
= Completely prohibit by law any new constructions within at least 100 metres (or better 300 m as the current law stipulates) of the coastline. If anybody violates the law, prosecute them severely. This should come as a Presidential Decree. It would also decrease the coastal pollution and release much required space for leisure activity.
= It appears that the less affected coastal areas from destruction were mostly mangrove belts, which acted as a protective barrier for the waves, but not always. Preservation of such belts is vital.
= A massive reconstruction effort is sure to follow. This would require vast volumes of fine aggregate (sand) for concrete and masonry, which is in severe short supply even now. Use this opportunity to shift over to sea sand for construction. The massive cleaning up operation and better construction standards will also beautify our coastline.
= The need for civil defence training to handle impending disasters is required, without which any early warning system is superfluous.

Tsunami is a word unknown to most people in Sri Lanka. Now it will be a new word in our vocabulary. It is a geological phenomenon of awesome magnitude and devastating power, which has humbled us all.
(The writer was Professor of Geology at the University of Kuwait in the Arabian Gulf and Professor and Chair of Earth Sciences at the University of Oman. He was also a visiting research professor at the Institute of Marine Sciences, University of California, Santa Barbara. Presently retired, he lives in Sri Lanka.)

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