Posted on 12 March 2011 at 08:59h

We are all shocked by the news of the tsunami in Japan and are thoughts are with the families who have lost their loved ones and homes. Many of us teaching in international schools have students from the Pacific region which brings the suffering even more closer to home. After such a tragedy students will often want to understand what exactly happened so it's worth reading up on tsunamis so you are able to answer questions in class. I have done a bit of research and have found some interesting facts.

• When the tsunami is first formed by the disturbance of the sea floor it can have a wavelength of several hundred km and a speed of up to 1000kmhr^-1.
• Even though the water is not shallow the long wavelength makes the waves behave like shallow water waves with a velocity = √gh (same as in the waves in a bucket practical).
• The energy loss as the wave spreads out across the ocean is related to the wavelength so the tsunami loses little energy as it spreads out, this is why there were tsunami warning as far away as Peru.
• The energy per unit surface area is related to the square of the amplitude.
• As the tsunami enters shallow water they slow down and their wavelength gets less. The energy in the wave must be conserved but the energy per unit surface area increases which results in an increase in amplitude, this is called shoaling and can be demonstrated in this applet.

The University of washington website has some useful information and animations

And some interesting maps from magic seaweed showing the predicted spread of energy and displacement.