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📘 How does a tsunami cross an ocean?

A tsunami begins when a large volume of seawater is disturbed suddenly, most often because an undersea earthquake lifts or drops a broad patch of seafloor. The displaced water column then spreads as a train of long waves, carrying energy th

5
lessons
~25 min
to learn
Adults
level
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What you’ll learn

  1. A seafloor move starts the journeyExplain how sudden seafloor displacement launches a tsunami wave train.A tsunami begins when a broad water column is displaced, usually by an undersea earthquake.
  2. Depth sets the travel speedUse the depth-speed relationship to explain how a tsunami crosses an ocean.For a long wave, deeper water permits faster propagation and bathymetry shapes the route.
  3. The wave changes near shoreDescribe shoaling, wavelength shortening, and local coastal amplification.Decreasing depth slows the wave, compresses it, and generally increases its height.
  4. How scientists track the crossingDescribe how models, bathymetry, satellites, and sensors track tsunami travel.Travel-time calculations are tested and refined with deep-ocean and coastal observations.
  5. One wave, many coastsConnect ocean-scale propagation to different local coastal impacts.The same wave train can produce different arrival effects because coastlines and seabeds reshape it.

Questions this course answers

What most directly starts a tsunami?

An abrupt seafloor movement can displace the overlying water column and launch long waves.

Match each feature to its role.

These distinctions identify what creates and sustains a tsunami.

What controls tsunami speed in the deep ocean?

For a long wave, c is approximately √(gH), so deeper water means faster travel.

Put the propagation sequence in order.

The wave is generated first, travels rapidly in deep water, then responds to decreasing depth.

What is shoaling?

Decreasing depth slows the wave and concentrates its energy into a shorter, higher disturbance.

Match the change to its cause or effect.

The sequence connects ocean depth to coastal impact.

Grounded in trusted sources

  • NOAA, Tsunami Propagation, https://prod-01-alb-www-noaa.woc.noaa.gov/jetstream/tsunamis/tsunami-propagation
  • NOAA NCEI, Introduction to Global Historical Tsunami Data, https://www.ncei.noaa.gov/products/natural-hazards/tsunamis-earthquakes-volcanoes/tsunamis/global-historical-data
  • NOAA NCEI, Tsunami Travel Time Maps, https://www.ncei.noaa.gov/index.php/products/natural-hazards/tsunamis-earthquakes-volcanoes/tsunamis/travel-time-maps
  • U.S. Geological Survey, Life of a Tsunami, https://www.usgs.gov/centers/pcmsc/life-tsunami
  • U.S. Geological Survey, Samoa Disaster Highlights Danger of Tsunamis, https://www.usgs.gov/centers/pcmsc/news/samoa-disaster-highlights-danger-tsunamis-generated-outer-rise-earthquakes
  • NOAA PMEL, Tsunami!, https://www.pmel.noaa.gov/pubs/outstand/gonz2088/gonz2088.shtml
  • Wikimedia Commons MediaWiki API, image metadata and thumbnails, https://commons.wikimedia.org/w/api.php

Every Wunder lesson is built from real, reputable sources — never invented.

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