Swellnet Dispatch Tsunami: Approaching the Speed of Sound

In: Swellnet Dispatch by Craig Brokensha 3 Comments Thu 12th Apr '12

Tags: tsunami , wavelength , period , sumatra , japan , speed , tidal wave , banda aceh , Craig Brokensha

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Most people identify tsunamis with destructive walls of water surging across low lying islands. This is often the case even when a tsunami is only a metre or two in height.

So how does a tsunami differ from the surfable waves we see across our beaches day to day?

The main factor is the period and speed of the tsunami. We know that long period groundswells are much stronger than short period windwells, and that they refract more easily into protected locations. Tsunamis can exhibit periods - the time between successive wave crests - of more than ten times that of a typical groundswell, commonly minutes to sometimes even hours.

Last night's series of major earthquakes off the West Sumatran coast (8.6 and 8.2 on the Richter scale) generated several small tsunamis; their small amplitude owing to a horizontal rather than vertical movement of the ocean floor. Nevertheless, wave periods of 18 minutes were recorded at monitoring stations on Cocos Island and Trincomalee in Sri Lanka.

With such large periods tsunamis are regarded as shallow water waves due to their wavelength - the distance between successive waves crests - being greater than the depth of the water it's moving through.

This means that there's a large amount of energy stored beneath the ocean to depths of thousands of metres, and once this energy starts to reach shallower depths such as an island it starts to pile up and surge.

Because a tsunami is a shallow water wave, a simple equation can be used to calculate its speed and hence arrival time. The travel time is calculated by taking the square root of gravity (9.8m/s^2) multiplied the average water depth. Using last night's tsunami as an example, the water depth off of Sumatra is approximately 5,000 metres, which means that the travel time is approximately 220 metres per second. That's an impressive 797 kilometres per hour, with the speed of sound coming in at 1,224 kilometres per hour.

This means that the island of Simeulue - closest to the epicentre - had only thirty three minutes notice to evacuate following the earthquake. The arrival of the tsunami was estimated at Cocos Island just two hours after the earthquake; while the Maldives had a little more time to spare: two hours and forty minutes.

So while a tsunami with a height of one metre doesn't sound threatening, its forward speed of 797 kilometres per hour can cause catastrophic damage as the surge moves inland for many minutes behind the initial impact.

Below is a small selection of locations and the time it would take for the tsunami to be felt relative to the earthquake off Sumatra.

Simeulue - 33 minutes
Sri Lanka - 1:42 minutes
Maldives - 2:38 minutes
Perth - 5:38 minutes
Durban - 9:23 minutes 

// CRAIG BROKENSHA 

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