How to play Tsunami Race
- Choose a test and read the depth, the distance, or the speed on the card.
- Use v = √(g·h) with g = 9.8 m/s², 1 m/s = 3.6 km/h, t = d ÷ v, and h = v² ÷ g.
- Type your prediction and press Start. The wave crosses the sea, sped up.
- Compare the orange dot and dashed line with the blue wave and its line on the right.
Why depth sets a tsunami's speed
Wind waves are short, and only the top few metres of water move with them. A tsunami, raised by an earthquake that lifts the seabed, is different: its wavelength is hundreds of kilometres, far longer than the ocean is deep, so the whole depth of water sloshes along with it. Waves like this are called shallow-water waves, even in the middle of the ocean, and their speed depends only on the depth and gravity: v = √(g·h). Over 4000 m of water that is about 200 m/s, or 700 km/h, as fast as a jet. Out at sea a tsunami is often less than a metre high, and ships hardly notice it. As it reaches shallow water near the coast, it slows down: in 10 m of water it moves at about 10 m/s. The front slows first while the water behind is still fast, so the wave bunches up and grows many metres tall. Warning centres use v = √(g·h) and maps of the seabed to predict when a tsunami will arrive, often hours ahead.
A focused learning model
Each stretch of sea has one depth; the wave keeps its shape and loses no energy, and the slope to the coast is not modelled. g = 9.8 m/s². The depth and the wave's height are drawn much larger than the distances. Time runs faster than real time by the factor shown, so each run lasts 4 s; the clock shows the real time in minutes. A run ends as the middle of the wave reaches the far side. Answers within 2% are right.