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Place 04 · The Ridge and the Trench · Chapter 4 of 7
When the Seafloor Snaps
When a subduction zone lets go, the seafloor jumps and lifts the whole ocean above it, and that bump of water crosses the Pacific at the speed of a jet.
About 15 minutes · 1 video · 1 3D model · 2 quick activities
Start here
The wave you cannot see
Out in the open ocean a tsunami is rarely more than about a meter tall, and a ship would barely notice it passing underneath. But its waves can be hundreds of kilometers long, so even 4,000 m of water counts as shallow to it, and it races along at more than 700 km an hour. Only when it reaches the coast does it slow down, pile up and pour inland.
Watch this
See it move
TED-Ed — How tsunamis work. Under four minutes of animated physics on why a wave nobody notices at sea turns into a flood that does not stop at the coast.
While you watch, look for
- What sets off most tsunamis
- Why a tsunami grows taller as it comes into shallow water
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The snap
A megathrust lets go
At a subduction zone the two plates lock together while the sinking plate keeps moving, bending the edge of the top plate like a springboard. When the fault finally slips, the seafloor springs up or drops by meters within minutes, lifting or dropping the entire column of water above it. These megathrust earthquakes are the biggest on Earth, including the largest ever recorded: magnitude 9.5 off Chile in 1960, where the Nazca Plate dives under South America.
The race
Shallow water, everywhere
Because a tsunami’s wavelength is far longer than the ocean is deep, it behaves as a shallow-water wave, and its speed depends only on the depth: speed ≈ √(g × depth). Over 4,000 m of water that is about 200 m a second, more than 700 km an hour. In 1960 the Chilean tsunami took about 15 hours to reach Hawaii, where it killed 61 people in Hilo, and it went on to kill 139 in Japan.
The arrival
Slow down, stand up
Near the coast the water gets shallow, so the wave slows, shortens and grows tall, arriving not as a curling surf wave but as a fast, rising flood that keeps coming. Out at sea, NOAA’s DART stations, pressure sensors on the deep seafloor that report through buoys at the surface, spot passing tsunamis so warnings can go out. But near the source there may be no time: if the ground shakes hard or the sea suddenly pulls back, go to high ground or inland right away.
Photos: Jan Schwebel, Brett Sayles, Max J / Pexels
Play with it
Watch one day of earthquakes in 3D
This 3D model plots every recorded earthquake around Japan on March 11, 2011, and animates them hour by hour, with the tilted surface of the subduction zone drawn in. Press play, spin it to look from the side, and find the magnitude 9.1 main shock at 05:46 GMT; then watch the aftershocks swarm along the plate boundary for the rest of the day.
3D model: “Tōhoku earthquake – 2011” by Loïc Norgeot on Sketchfab (CC BY), using USGS ANSS earthquake catalog, NOAA ETOPO1 relief and USGS Slab2 data.
Two questions to answer while you are in there
- Seen from the side, do the earthquakes stay at the surface, or do they follow the tilted surface of the sinking plate down into the Earth?
- The aftershocks kept coming all day. Judging by how far they spread along the coast, was the part of the fault that slipped small or enormous?
Try it yourself
Check yourself
Try it
Spot the Real Warning Signs
Click every word that is a natural tsunami warning sign or something a tsunami really does. Leave the myths alone.
Try it · interactive
Tsunami Racer
Set the ocean depth and watch a tsunami’s speed follow √(g × depth); then race one across the Pacific from Chile or Japan and watch it slow down and stand up tall as it reaches shallow water near the coast.
Open the Tsunami Racer in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Tsunami
Long, fast wave set off by seafloor movement
Megathrust earthquake
Huge quake on a subduction zone fault
Shallow-water wave
Wave whose speed depends only on depth
DART buoy
Deep-sea sensor station that detects passing tsunamis
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: When the Seafloor Snaps
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 10.4 — Tsunamis — LibreTexts · 6 minutes
- Webb 4.8 — Earthquakes and Plate Tectonics — LibreTexts · 10 minutes
- NOAA Tsunami Warning Centers — Tsunami Frequently Asked Questions — NOAA · browse the questions · 10 minutes
Photo credits (Pexels): Ray Bilcliff, Rafael Rodrigues, Jan Schwebel, Brett Sayles, Max J.