Tsunami Wave Lab: Fast in the Deep, Tall at the Shore
Use the shallow-water wave rule, speed = √(g × depth), to see that a tsunami crosses the deep ocean at jet speed and then slows and grows over the shelf, and to read the travel time as warning time.
Before you try
What if the ocean were the same depth everywhere? Would a tsunami still grow at the coast?
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What happened?
Move a control to see what changes and why.
Shallow-water speed and Green’s law on a smooth seabed; real run-up also depends on the coast’s shape, and heights are drawn far taller than reality.
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Try these ideas
- Move the earthquake from 100 km to 1,000 km away and compare warning times.
- Halve the deep-ocean depth. Does the wave get faster or slower?
- Try the 2004 preset and check the travel time against the real two hours.
Why it works
An undersea earthquake can lift a huge patch of seabed and the water above it by a metre or so. That ripple is a tsunami. Because its wavelength is hundreds of kilometres, it behaves like a shallow-water wave even over 4 km of ocean, and its speed depends only on depth: about 700 km/h in the deep Indian Ocean. Ships at sea barely notice it.
Nearer the coast the water shallows, the wave slows to a few tens of kilometres an hour, and its energy piles up so the crest grows many metres tall. Often the sea first pulls back from the beach as the trough arrives, a warning sign to run inland. The 2004 tsunami took about two hours to reach Tamil Nadu from Sumatra; deep-ocean buoys now give warnings. In this lab you set the depth, shelf, distance and wave, send the wave, and read speed, height and warning time.
Check your understanding
Question 1 of 4
A tsunami’s speed in the open ocean depends on
Why does a tsunami grow taller near the coast?
The sea drawing back from the beach is
A tsunami 1,000 km away over 4 km deep water arrives in roughly
Explain it in your own words
What changed? Why did it happen? Where might you notice this in everyday life?
For parents & teachers
- Look up how the Indian Tsunami Early Warning Centre works.
- Ask: why did the 2004 tsunami surprise so many people?
- Compare a tsunami with a wind wave at the beach: what is different?