Topics Earth, weather and climate

How do tsunamis form?

Most tsunamis form when a large section of the seafloor suddenly moves up or down, usually during an undersea earthquake. That motion lifts or drops the whole column of water above it, and gravity pulls the displaced water back toward level. The result is not one wave but a series of them, racing outward in every direction.

What makes tsunamis strange is how they behave. In deep ocean, the waves can travel at the speed of a jet airliner yet be low enough that a ship might hardly notice them. As they reach shallow water near a coast, they slow down and bunch up, and the water piles higher. Often the first sign on shore is not a towering wall but a fast, powerful surge of rising water, sometimes preceded by the sea pulling back.

Earthquakes are not the only cause. Underwater landslides, volcanic eruptions and, very rarely, asteroid impacts can also push enough water to do it. An episode on this topic would walk through each trigger, explain why wave speed depends on depth, and look at how warning systems work. bre's hosts are AI, so they can make mistakes, but you can press Talk and ask them to slow down or explain any step again.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. What a tsunami actually isA tsunami is a series of long waves caused by a large, sudden displacement of water. It is not the same thing as a wind-driven wave or a tide.
  2. The seafloor lurches: subduction earthquakesMost big tsunamis come from places where one tectonic plate slides under another. When the stuck plates slip, the seafloor jumps and shoves the water above it.
  3. Other triggers: landslides and volcanoesCollapsing slopes, undersea slides and volcanic eruptions can also displace huge volumes of water. Asteroid impacts are possible in principle but extremely rare.
  4. Fast and low in the open oceanIn deep water these waves have very long wavelengths and travel at great speed, yet rise only a little above the surface. That is why ships at sea may barely notice them.
  5. Why the wave grows near shoreAs the water gets shallower, the wave slows and its energy is squeezed into less depth, so it rises. Coastline shape and seafloor slope change how bad it gets.
  6. Warning systems and what they can and cannot doSeismic sensors and ocean buoys help detect tsunamis and estimate where they are headed. Warning time depends on how far the source is from the coast.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Arlo. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Okay, I'll ask the obvious question. When people picture a tsunami, they picture a giant wave out at sea. Is that what's happening out there?

  2. ArloAI host

    No. Not even close.

  3. breAI host

    Right, so what is it? Because I've always imagined a wall of water crossing the ocean.

  4. ArloAI host

    In deep water it's long and low. Spread over many miles. A ship can pass over it and barely feel a lift. It's the speed that's scary, not the height.

  5. breAI host

    So where does it start? Something has to shove all that water.

  6. ArloAI host

    Usually the seafloor. Plates grind against each other, get stuck, then slip. The bottom jumps, and the water above it goes with it.

  7. breAI host

    So the whole column of water gets moved at once, not just the surface.

  8. ArloAI host

    Yep. Whole column. That's the difference from a normal wave.

Questions people also ask

What is the main cause of tsunamis?
Most tsunamis are caused by large undersea earthquakes, especially where one tectonic plate slides beneath another. The sudden shift of the seafloor displaces the water above it. Underwater landslides and volcanic eruptions can also cause them, though less often.
Why does the sea sometimes pull back before a tsunami?
If the trough of the first wave reaches the shore before the crest, the water appears to drain away. This does not always happen, so it is not a reliable warning on its own. Official alerts and natural signs like strong shaking matter more.
How fast do tsunamis travel?
In deep ocean, tsunamis can travel at several hundred miles per hour, roughly the speed of a jet airliner. Speed depends on water depth, so they slow down as they approach shallow coastal water, even as the wave grows taller.
Can a tsunami be predicted?
Scientists cannot predict when an earthquake will happen, so they cannot predict a tsunami in advance. They can detect one after the triggering event using seismic and ocean sensors, then estimate where and when waves will arrive. The hosts are AI and can make errors.

Related topics

More: all 300 topics, earth, weather and climate, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.