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How Tsunamis Form
Seafloor displacement and shoaling waves
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A tsunami is a series of powerful ocean waves caused by a sudden disturbance that moves a large amount of water. Most tsunamis form when an underwater earthquake lifts or drops part of the seafloor, but volcanic eruptions, landslides, and meteor impacts can also trigger them. In the deep ocean, a tsunami may be only a small rise in sea level, so ships may barely notice it. The danger grows near shore, where the wave can rise, flood inland, and cause severe damage.
When the seafloor moves suddenly, it pushes the water above it upward or downward, creating waves that spread outward in all directions. In deep water, tsunami waves can travel about 500 mph, with very long wavelengths and low heights. As the waves enter shallow water, friction with the seafloor slows them, their wavelength shortens, and their height increases. This process, called shoaling, can create a high surge of water that runs up the coast and floods low areas.
Key Facts
- Tsunamis usually form when an underwater earthquake suddenly displaces the seafloor and the water above it.
- In deep ocean water, tsunami speed can be about 500 mph, similar to the speed of a jet airplane.
- Wave speed in shallow water can be estimated by v = sqrt(gd), where g is gravity and d is water depth.
- As water depth decreases, tsunami speed decreases, wavelength shortens, and wave height increases.
- A tsunami is a wave train, meaning several waves may arrive over minutes to hours, not just one wave.
- Run-up is the maximum vertical height that tsunami water reaches above normal sea level on land.
Vocabulary
- Tsunami
- A series of long ocean waves caused by the sudden displacement of a large volume of water.
- Seafloor fault
- A break in Earth’s crust under the ocean where blocks of rock can move during an earthquake.
- Wavelength
- The distance from one wave crest to the next wave crest.
- Shoaling
- The process in which a wave slows down and grows taller as it moves into shallower water.
- Run-up
- The farthest vertical rise of tsunami water above normal sea level as it moves onto land.
Common Mistakes to Avoid
- Calling a tsunami a tidal wave is wrong because tsunamis are not caused by tides, which come mainly from the gravity of the Moon and Sun.
- Assuming a tsunami is always tall in the deep ocean is wrong because deep-ocean tsunamis often have low wave heights but very long wavelengths and high speeds.
- Thinking the first wave is always the largest is wrong because later waves in a tsunami wave train can be larger and more destructive.
- Ignoring a sudden ocean drawback is wrong because water pulling far back from shore can be a natural warning that a tsunami wave may soon arrive.
Practice Questions
- 1 A tsunami travels 500 miles across the deep ocean at 500 mph. How long does it take to reach the coast?
- 2 Use v = sqrt(gd) with g = 9.8 m/s^2 to estimate the speed of a tsunami in water 100 m deep. Give your answer in m/s.
- 3 Explain why a tsunami that is barely noticeable in deep water can become dangerous when it reaches a shallow coastal shelf.