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Lessons / Restless Earth

Restless Earth · 4 min · 4 steps

Why some earthquakes make a tsunami

Plays here

Goal. Explain how a tsunami starts, how it travels and what to do near the coast.

Steps

  1. Predict

    Which earthquake is most likely to cause a large tsunami?

    • A deep earthquake under a continent
    • A large, shallow earthquake under the sea at a subduction zone
    • Any earthquake of magnitude 5
    Show the answer

    A large, shallow earthquake under the sea at a subduction zone. A large, shallow earthquake under the sea at a subduction zone, which can lift or drop the seafloor by metres.

    A tsunami starts when the seafloor suddenly moves up or down and lifts the whole column of water above it. That happens mostly in big, shallow earthquakes where one plate dives under another, along trenches like these.

  2. On 11 March 2011 a magnitude 9.0 to 9.1 earthquake off north-east Japan raised the seafloor by several metres. The tsunami ran up to about 40 m above sea level on parts of the coast, and more than 18,000 people died or were never found.

  3. In the deep ocean a tsunami travels as fast as a jet plane, about 700 to 800 km/h, as a wave less than a metre high that ships barely notice. Near the coast it slows down and piles up into a fast-rising flood.

  4. Taiwan faces the Ryukyu Trench to the north-east and the Manila Trench to the south-west, so tsunamis matter here too. If you feel strong or long shaking near the coast, or see the sea suddenly pull back, go to high ground at once without waiting for a warning.

The common mistake

Myth

A tsunami is one giant breaking wave, like a surfing wave.

Fact. A tsunami is a series of very long waves, often more than 100 km from crest to crest. It arrives as a fast-rising flood that keeps coming for many minutes, and the first wave is often not the largest.

Check yourself

Question 1

What starts a tsunami?

  • Wind blowing over the sea
  • The seafloor suddenly moving and lifting the water above it
  • The Moon's tides
Show the answer

The seafloor suddenly moving and lifting the water above it. An undersea earthquake, landslide or eruption that moves the seafloor pushes up the whole column of water.

Question 2

How fast can a tsunami cross the deep ocean?

  • About 10 km/h
  • About 700 to 800 km/h
  • Faster than sound
Show the answer

About 700 to 800 km/h. In water 4 km deep a tsunami travels at about 700 km/h, as fast as a jet plane.

Question 3

You are on a beach and feel a strong earthquake. What should you do?

  • Wait for an official warning
  • Go to high ground at once
  • Walk out to see if the sea pulls back
Show the answer

Go to high ground at once. Natural warnings come first: strong shaking or a sudden drop in the sea means go to high ground immediately.

Words in this lesson

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Curriculum

Taiwan 12-year curriculum (108 課綱), natural science
EMd-Vc-4
US Next Generation Science Standards
MS-ESS3-2
England national curriculum
GCSE Geography: tectonic hazards