Tsunamis Travel Fast But Not At Infinite Speed | Briefing Document

introduction

In the context of the 2004 Sumatra tsunami, this briefing document gives information on what is a tsunami, why do they occur, what are the results (wave size, speed, distance travelled) and the effects on humanity and the environment. As well as an wide-ranging investigation on tsunamis, this briefing document at abelard.org provides illustrations and diagrams, together with numerous links to specialised websites.

[Tsunami means “harbour wave” in Japanese.]

how fast does a tsunami travel - the 2010 chilean event The 27th February, 2010 Chilean main event, at 06:34:17, was originally assessed at 8.8 on the Richter scale. This much milder tsunami event, relative to the Sumatrian ’quake-generated tsunami, took about 12 hours to reach Hawaii and about 13 to 14 hours to reach New Zealand. It was detected in Japan after a little over 24 hours, a distance of about 17,000 km or over 10,000 miles. So the tsunami appears to have been propagated at approximately 700 kph or or 400 mph; that is, about as fast as an airliner.

a tsunami strikes

This is probably the main source tracking the Sumatra tsunami. An animation of the first few hours only, so far. Bangladesh appears to have escaped lightly, despite having 17 million people living less than one metre above sea level. Another animation of the first three hours. These show weaker waves going North and South, than the waves going East and West. This is probably why much of India and Bangladesh has been little affected.

Seismic diagram of the Sumatran quake of 26 December 2004. Image credit: UNESCO

terrestrial consequences of the sumatran undersea quake

• “The shockwave shortened the period of our planet's rotation by some three microseconds. [...] [NASA] estimate that Earth now tilts by an extra 2.5 centimetres in the wake of the jolt.” [Quoted from nature.com]
• “The earth is regularly deformed by the daily passage of the sun and moon, raising a tide in the earth of about 20cm, and the vibrations we saw on Boxing Day shortly after the earthquake off the Indonesian coast were of a similar magnitude on our instruments.” — A fortnight after, “equipment was showing much of the planet was still ringing like a bell.[...] "The main signal we now see is a steady oscillation of a few parts in 10 billion of normal gravity, which corresponds to about a millimetre of vertical motion of the earth," ” [Quoted from Herald Sun]

A tsunami animation of the Pacific region [2.4mb .mov file].

There is a good write-up from the overseers of the US Tsunami Service, NOAA and Tsunamis.

“The waves radiate outward in all directions from the disturbance and can propagate across entire ocean basins. For example, in 1960 an earthquake in Chile caused a tsunami that swept across the Pacific to Japan. Tsunami waves are distinguished from ordinary ocean waves [1] by their great length between peaks, often exceeding 100 miles in the deep ocean, and by the long amount of time between these peaks, ranging from five minutes to an hour. The speed at which tsunamis travel depends on the ocean depth. A tsunami can exceed 500 mph in the deep ocean but slows to 20 or 30 mph in the shallow water near land. In less than 24 hours, a tsunami can cross the entire Pacific Ocean.

“In the deep ocean, a tsunami is barely noticeable and will only cause a small and slow rising and falling of the sea surface as it passes. Only as it approaches land does a tsunami become a hazard. As the tsunami approaches land and shallow water, the waves slow down and become compressed, causing them to grow in height. In the best of cases, the tsunami comes onshore like a quickly rising tide and causes a gentle flooding of low-lying coastal areas.”

(See also radar satellites capture tsunami wave images.)

Sea and tsunami defences, such as walls and levées, can turn out to be very inadequqte because the preceeding earthquake can cause substancial subsidence, thus lowering the actual height of defences.

The behaviour of a tsunami wave when caused by subduction [as happened with Sumatra]. With diagrams.

Mt Fuji seen from the sea, Hokusai, woodcut, 1834

When a wave hits the shore, it tends to act like a motorway pile-up to varying degrees. This is called the ‘run up’. A typical beach has a run up of 3; that is, a 5 foot wave will rise to 15 feet on beaching (three times the starting height of the wave).

This will, of course, depend on the beach slope. Naturally, a steep slope causes greater run up than a gentle beach. Even greater run ups would be expected in an inlet like a river or delta. Regions with run ups of approximately 40 are known.

Various types of tsunami give different patterns of waves formation and propagation, this is an area of study that is rapidly developing and involves computer modelling, real-world and experimental data. Study of the Shoemaker-Levy comet, which crashed into Jupiter in 1994, has resulted in increased confidence in computer models.

A common convenient, if grim, measure of the energy is by megatons of TNT, or numbers of Hiroshima-type bombs. Hiroshima was rated at about 15,000 tons of TNT.

sumatra tsunami present and past

• “Many who escaped death in what was possibly the deadliest tsunami in more than 200 years now face hunger and disease. The United Nations mobilized what it called the biggest relief operation in its history [...] ”