Tsunami Formation 4 Panel — free printable diagram

How a tsunami forms, step by step

A tsunami is a series of waves caused by a large, sudden displacement of water — most often an undersea earthquake that jolts the seafloor vertically. The diagram breaks that into stages: the seafloor moves, the whole water column above it is displaced, the wave crosses the open ocean almost unnoticed, and it rears up only as it reaches shallow water near the coast.

Most common cause

Undersea earthquake at a plate boundary

Other causes

Underwater landslides, volcanic eruptions, meteorite impacts

Speed in deep ocean

Up to about 800 km/h (500 mph)

Height in deep ocean

Often less than 1 metre

Wavelength

Can exceed 100 km

Arrives as

A series of waves, not a single one

• The trigger has to move water vertically. An earthquake that shifts the seafloor sideways displaces very little water, which is why not every undersea earthquake produces a tsunami.
• Unlike a wind-driven wave, which disturbs only the surface, a tsunami moves the entire depth of water from seabed to surface. That is where its energy comes from.
• In deep water the wave is extremely long and very low, so a ship at sea may pass over one without noticing it.
• Near the coast the shallow seabed slows the front of the wave while the water behind it piles up. The wave shortens and grows dramatically in height — the process called shoaling.
• The sea sometimes draws sharply back from the shore before the wave arrives, exposing the seabed. That drawback is a natural warning sign and means the water will return.
• A tsunami arrives as a wave train over hours, and the first wave is often not the largest — a point worth drawing out when students study coastal hazard planning.