Chapter 3 of "Earthquake and Tsunami":

What is a Tsunami?

Table of Contents   3.1. Difference between tsunami and storm waves 3.2. How a tsunami starts - in principle 3.3. How a tsunami destroys - in principle 3.4. How the tsunami of 26 December 2004 started

3.1. Difference between tsunami and storm waves

A wave is not "water moving"; it is energy passing through water, agitating the water molecules and then leaving the same molecules behind (except when a wave breaks on a coast when "water is indeed moved", briefly onto land). The energy of waves in the drawings below is symbolised by a circle or an oval.

Wind-created waves do not reach deep below the surface although in severe storms they can reach heights of 7 m or more ( "freak waves" can go much higher, they are very rare and their origin is not well understood). A few meters below a surface whipped by a raging storm, all is quiet.

Tsunami waves are quite different. On seas with a depth of 4000 m or more, the tsunami's height on the sea surface may be less than 0.8 m and hardly noticeable. Indeed, almost all of the tsunami's energy is below the sea surface and sailors may not even notice a major tsunami wave passing under their ship.

in shallow seas and very close to the epicentre, tsunami waves have been known to reach around 40 m in height. On Haiwaii traces of ancient prehistoric tsunamis have been found that indicate a height of over 300 m. The troughs The size of the troughs between the waves depends on the way the waves were formed but they can can be as much as 160 km widee with a depth of only a few meters. The pictures below are not to scale as regards the height-to-width of tsunami waves!

3.2. How the tsunami starts - in principle

1. Before the earthquake: The Indo-Australian has been sliding under the continental Eurasian plate for millions of years. The point where the Indo-Australian plate vanished below is Eurasian plate is the Sunda trench at at around 4,000 m depth. The movement of the two plates against each other averages at around 60-70 mm per year over the milennia but is not continuous. Tensions build up when no movement takes place over long periods and this tension is eventually released in an earthquake. 2. During the earthquake The accumulated tensions are released suddenly. In the event of 26 December 2004, the Indo-Australian plate slipped around 20 m further below the Eurasian plate which in turn was lifted by about 5 m. Underwater landslides caused by the quake made a major contribution to the formation of the tsunami. The two large-scale movements set in motion an enormous amount of sea water. The colossal energy released by the adjustment of the two plates has been likened to the simultaneous explosion of 32,000 A-bombs of Hiroshima size. The waves generated by the earth movements 4,000 m below sea level spread mostly towards the west and northwest. 3. Tsunami waves travelling in deep water: The giant waves are barely noticeable on the surface as long as they travel in seas more than 4,000 m deep. The trough between wave crests can be as much as 160 km with the crests less than 1 m. As tsunamis are generated at the bottom of the sea, they are large-wavelength waves in the open sea, travelling with speeds of 700-800 km/hwith minimum loss of energy. 4. Tsunami waves hitting land: In shallow water, the tsunami waves slow down, rise from the normal sea surface and crowd together while becoming ever larger (see following graphics). Before hitting land, often (but not always) there is a "false ebb" when the sea temporarily withdraws before the giant waves arrive.

 

3.3. How the tsunami destroys - in principle

The depth and topography of the sea determines the form a tsunami wave takes and how it propagates. The way a tsunami breaks when reaching a coast is very difficult to predict. It depends on the topography that the wave has passed over on the high seas and on the form, steepness, height and form of the coastline hit. Within a few kilometers the hight of a tsunami wave can vary from lesss than a meter to 20 or more meters.   Light blue: normal sea level Dark blue line: normal water surface Dark blue: tsunami wave reaching higher than normal sea level brown: land

 

A wave is not "water moving"; it is energy passing through water, agitating the water molecules and then leaving the same molecules behind (except when a wave breaks on a coast when water is indeed moved briefly onto land). The drawing illustrates the wave energy of a tsunami wave in the form of an oval. If the oval does not touch sea bottom, the wave will barely rise above the normal sea level (shown light blue). The shallower the sea is that the tsunami wave is passing through, the more the wave rises above the normal sea level (i.e. the wave amplitude increases), the more it slows down and the closer the waves crowd together (i.e. the higher the energy density becomes). Light blue: normal sea level Dark blue: tsunami wave reaching higher than normal sea level brown: land

3.4. How the tsunami of 26 December 2004 started

Underwater landslides caused by submarine earthquakes are a major (perhaps usually the only) contributory factor to the formationof a tsunami wave. One enormous landslide off the west coast of Sumatra caused by the the quake of 26 Dec 2004 was discovered by the sonar of the British Navy's "HMS Scott" and is shown on the left. The view is to the south. (Picture courtesy Royal Navy).

 

3.5. How the tsunami of 26 December 2004 spread

The configuration in 3 dimensions of the newly-born tsunami, approx. 15 minutes after its creation. The view is towards north-west. Note that the highest waves (around 37 m) were around the unfortunate city of Banda Aceh at the northern tip of Sumatra. For more pictures and an animated 17 MB movie of the expanding tsunami, see: http://walrus.wr.usgs.gov/tsunami/sumatraEQ/SumatraNW1pic.html

 

The red lines show the tsunami front at half-hourly intervals. The background shows the sea depth contours and is from Manfred Leier, World Atlas of the Oceans, 2000, Firefly Books, Buffalo NY, USA. From the epicenter of the primary earthquake (1), the tsunami spreads out at, initially, around 700 km/h and with a starting height of 37 m. By the time the wavefront has reached Banda Aceh (3), its height has already been reduced to "only" 12 m, when reaching the island of Sabang (4) it is 6 m and when reaching the coast near Sigli (5) east of Bandar Aceh it is 5 m in height. The estimates of the tunami's early height are based on the extent and distance from the sea of damage caused. After clearing the northern tip of Sumatra, the tsunami does not lose significant hight anymore but instead slows down in the shallower waters. When the Tsunami reaches the Thai island of Phuket (6) it still produces heights of 4.5 m (Karon beach) and 5.5 m (Patong beach). Because the power of the tsunami was deflected by Simeulue island (2) immediately to the south of the epicentre and by the west coast of Sumatra, much of the wave energy was deflected to the west and the north. The short time delay between initial and deflected wave energy also caused two tsunamis to move closely together northwards to the Nicobar (7) and Andaman (8) islands. Coastlines and islands with significant loss of life and serious coastal damage are coloured red.   For photographs of the damage caused by the tsunami, see Pictures

The map below shows just how far and how quickly the tsunami travelled and how far. Human casualties were reported from East African beaches 7-8 hours after the main earthquake near Sumatra. The height of the tsunami had reduced to around 0.5 m by the time it reached the African coast. This may not sound much but if someone is taken completely by surprise, even such a relatively small wave can still be deadly.

The red lines show the progress of the tsunami across the Indian Ocean in half-hourly intervals. It is noticeable that the speed of the tsunami's advance is not significantly slowed down as long as the wave moves in deep waters.

1. Sri Lanka 2. Andaman islands 3. Nicobar islands 4. Simeulue island 5. Nias island 6. Mentawai islands 7. Maldive islands 8. Seychelle islands

The wave hight in meters (m) at any time during the tsunami (modelled)
(adapted and simplified from Quirin Schiermeier, "On the Trail of Destruction", Nature 433:350-353, 27th January 2005.

Epicenter 35 to 12 m 12 to 4 m 4 to 2 m 2 to 1 m 1 to 0.6 m 0.6 to 0.4 m 0.4 to 0.2 m 0.2 to 0.1 m not significantly affected 1 m = 3.28 ft.

The tsunami that that was caused by the Sumatra earthquake of 26th December 2004 did not just unsettle the Indian Ocean - its effects were felt world-wide, in the Indian, the Atlantic as well as in the Pacific Oceans. It has now also been proven that tsunami waves travelling wide distances orient themselves largely along oceanic mountain ranges (drawing below dapted from Titov V., Rabinovich A. B., Mofjeld H.O., Thomson R.E., Gonzalez F.I. 2005, Science, 23 September 2005, 309:2045-2048). As the authors note, the wave amplitudes, directionality, and global propagation pattern of the 2004 Sumatra tsunami were primarily determined by the orientation and intensity of the off-shore seismic line source and subsequently by the trapping effect of the mid-ocean ridge topographic waveguides.

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Last changed 16 August 2008