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The tsunami phenomenon

Saturday 1 January 2005, by RAMACHANDRAN*Rajesh

Stealth, speed and enormous power of destruction.

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REUTERS
A video footage shows the tsunami rising in Penang , Malaysia.

THE series of massive ocean waves - indeed speeding walls of water - that caused devastating inundation in the southeastern coastline of India and the eastern coastline of Sri Lanka are known in geological parlance as tsunami, a Japanese word meaning "harbour wave". A tsunami moves silently but rapidly across the ocean and rises unexpectedly as destructive high waves along shallow coastal waters, causing widespread devastation over land along the coastline.

Tsunamis are relatively rare in the Indian Ocean region but not unprecedented. Most tsunamis occur in the Pacific Ocean and 86 per cent of them are the products of undersea earthquakes around the Pacific Rim, where collisions of tectonic plates result in what are known as subduction zones. During the 1990s, as many as 82 tsunamis were reported worldwide, a rate stated to be much higher than the historical average of 57 a decade. The last 10 major tsunamis have consumed more than 4,000 lives. But the present one, which started out from the Sumatra coast in Indonesia and affected countries in the Indian Ocean region, including India, Sri Lanka and Thailand, has claimed more lives than the last 10 put together.

Though often referred to as "tidal waves", a tsunami has nothing in common with normal wind-driven sea waves and tides. Breezes blowing across the ocean cause waves of short wavelengths - the crest-to-crest distance - on the sea surface. These waves cause currents that are mainly confined to a shallow oceanic layer beneath which one has relatively calm water. Strong winds may be able to generate even 30-metre-high waves in the open ocean but even these do not move the deep waters. Tides, which occur all over the earth twice a day, do produce currents that reach the ocean bottom - just as tsunamis do - but these too are of shorter wavelengths as compared to tsunamis.

Tsunamis are not generated by the gravitational pull of the moon or the sun. These are produced impulsively by an undersea earthquake, and rarely by volcanic eruptions, meteorite impacts or underwater landslides. Indeed, the deadliest tsunami in recorded history followed the eruption and virtual obliteration of Indonesia’s Krakatoa volcano in 1883, which killed an estimated 36,000 people, most of them owing to the tsunamis that resulted. Simply put, terrestrial earthquakes are associated with ground shaking that is a result of elastic waves travelling through the solid earth. Tsunamis, on the other hand, are caused by submarine earthquakes that set off waves with long wavelengths in water and the most destructive tsunamis are caused by subduction zone earthquakes.

A subduction zone is where two of the earth’s rigid tectonic plates are converging towards one another (roughly at few centrimetres per year), and one plate, usually composed of heavier oceanic material, dives beneath the other generally lighter plate of continental material. At the boundary where the two rub against each other, the lower one drags and flexes the top one slightly downward. When the flexing exceeds the frictional strength of the inter-plate contact, the upper plate rebounds to its original position causing sea-floor displacement much like the swimming spring-board. This happens so quickly that the sea surface assumes the shape of the sea-floor displacement.

The potential energy of displacement is converted into the kinetic energy of horizontal motion. This disturbance propagates outward as a tsunami. And the wave height will at best be a couple of metres. Unlike a tidal wave, a tsunami extends deep down into the ocean waters. That is, a tsunami crest is just the very tip of a very vast mass of water in motion. Within several minutes of the quake, the initial tsunami will split into one that travels out to the deep ocean (distant tsunami) and another that travels towards the nearby coast (local tsunami). The height above the mean sea level (MSL) of the two oppositely travelling tsunamis is about half that of the original tsunami.

The speed at which both travel varies as the square root of the water depth. Therefore, deep ocean tsunamis travel faster than local tsunamis. In the deep ocean, this wave travels at speeds of 500-1,000 km/hr. That is, the slope of the wave - which extends hundreds of kilometres - is so gentle, that even ships travelling on top of a tsunami wave will not feel it. Because the momentum of the tsunami is so great, it can travel great distances with little loss of energy. The 1990 Chilean tsunami had enough force to travel for 22 hours across thousands of kilometres to kill people in Japan.

As the tsunamis (both local and distant) approach the shallow coastal waters, their wavelength decreases and the amplitude increases several fold. As the waves hit against the slope of the coastline, the long waves pile on one another and the wavelength is reduced while the amplitude increases. As the waves travel over the near-shore region, a tsunami `run-up’ occurs. Run-up is a measure of the height of water observed onshore above MSL. Tsunamis do not result in breaking waves like the normal surf waves on a beach. They come in like very powerful and fast local rises in sea level and travel much farther inland than normal waves. Much of the damage inflicted by tsunamis is on account of strong currents and floating debris.

After run-up, part of the tsunami energy is reflected back to the open ocean. In addition, a tsunami can generate a particular type of waves called edge waves, which travel back and forth, parallel to the shore. The geometry of the seafloor warping near the coast has a significant influence on this. These effects result in repetitive arrivals of the tsunami waves at a particular point on the coast rather than a single wave. Because of the complicated behaviour of the phenomenon of the waves near the coast, the first run-up of a tsunami is often not the largest, emphasising the importance of not returning to the beach for several hours after a tsunami hits. In certain cases, the sea can seem to draw a breath and empty the coast. This is almost immediately followed by a wall of water inundating the coast.

The December 26 tsunami that hit South-East and South Asia is clearly the biggest ever, in terms of the earthquake that triggered it as well as the extent of destruction it caused, in recent history. This, according to the United States Geological Survey (USGS), is the fourth largest earthquake in the world since 1900 and the largest since the 1964 earthquake in Prince William Sound, Alaska. The causative sea disturbance was an earthquake of magnitude above 8.5 on the Richter scale - the most recent value given by the USGS is 9.0 - whose epicentre was off the west coast of northern Sumatra (3.3° N, 95.78° E) and at 10 km depth. The quake occurred at 0059 hours Coordinated Universal Time (UTC - same as Greenwich Mean Time or GMT). The location is 250 km south-southeast from Banda Aceh, Sumatra, 1,260 km south-southwest of Bangkok and 1,605 km northwest of Jakarta.

The eastern Indian coast in Tamil Nadu is about 2,000 km from the epicentre. The wave appears to have hit Cuddalore first, barely one and a half hours after the event. That makes the tsunami that hit the Indian coast to be an extremely fast one, with a speed of about 900 km/hr. The first wave to hit Chennai, according to the Surveyor General of India, was at 0840 hours Indian Standard Time (IST); Machilipattam was struck at 1000 hrs. The Survey of India (SoI) maintains tidal gauges along the eastern coast. There are only three of them located in the affected region: Chennai, Nagapattinam and Tuticorin. Apparently, all three have been destroyed in the disaster. Therefore, only rough estimates of the run-up is available.

The ports, however, maintain what are known as tide poles with markings on them. The Chennai Port recorded a tidal wave height of 4.1 metres, while the Ennore Port Trust recorded 3.5 m. The normal maximum tolerance that these ports are designed to handle is about a metre of tide. These are, of course, rough parameters, and the SoI is trying to determine the exact values.

The subduction that caused the earthquake appears to have been a massive one. The "shallow thrust-type" earthquake, according to the USGS, occurred at the interface between the Indian and Burmese tectonic plates. In this region, the Burmese plate is characterised by significant strain partitioning owing to the oblique convergence of the Indian and Australian plates to the west and the Sunda and Eurasian plates to the east. Off the west coast of northern Sumatra, the Indian plate moves in a northeastward direction at about 5 cm a year relative to the Burmese plate. According to USGS, preliminary locations of larger aftershocks show that approximately 1,000 km of plate boundary slipped as a result of the main Sumatra earthquake. Aftershocks are distributed along much of the shallow plate boundary between Northern Sumatra (3° N) to near Andaman Island (14° N). Indeed, Andaman has experienced a series of aftershocks, most of them of magnitude 5.5-6.5 and one as high as 7.3 on December 27 at 0421 hr UTC.

The Andaman region itself is seismically very active. The earliest recorded tsunamis on the eastern Indian coast are, in fact, because of subduction events off the Car Nicobar island. The first was on December 31, 1881, which had a magnitude of at least 7.5. A more recent tsunami event is that of June 26, 1941, which had a magnitude of at least 8.5. Both caused fairly severe damage to masonry and other structures on the coast. The number of deaths that these may have caused is not known.

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