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🌊 How does a tsunami form?

Seafloor displacement launches long ocean waves that stay modest at sea and tower at the coast.

lessons

~10 min

to learn

🔬 Science

subject

Adults

level

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What you’ll learn

The seafloor moves firstDefine a tsunami as water-column displacement, usually from seafloor motion, not wind waves.Thrust quakes lift or drop the seafloor; landslides and volcanoes can too. The whole ocean depth is involved from the start.
Fast in deep water, tall near shoreExplain deep-ocean travel versus shoaling and coastal flooding.Open-ocean waves are long and fast, often low at the surface. Shallow water slows them and stacks height into inundating surges.
Minutes to hours of warningConnect seismic detection, ocean sensors, and local versus distant risk.Quakes outrun water waves. DART and gauges confirm the real train. Local coasts may have only minutes; distant coasts can have hours.

Questions this course answers

What usually starts a classic large tsunami?

Most major tsunamis begin when undersea earthquakes (especially thrusts) lift or drop the seafloor and displace the overlying ocean.

Order what happens as a tsunami approaches land

Shoaling converts a fast, long, often modest-height deep-ocean wave into a slower, taller coastal flood.

Why can a distant tsunami give hours of warning while a local one may give only minutes?

Across an ocean, detection and wave travel provide time. Near the rupture, the water wave can arrive before elaborate confirmation, so felt shaking and immediate high ground matter.

Grounded in trusted sources

NOAA / National Weather Service Tsunami Program educational materials
UNESCO-IOC tsunami awareness and deep-ocean DART overview pages
USGS earthquake and tsunami science summaries on subduction-zone sources
Pacific Tsunami Warning Center public explainers on travel times and coastal effects

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