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.
Understanding How Tsunamis Form
The crucial part of a tsunami source is vertical movement. An earthquake can release huge energy without making a major tsunami if the fault mostly slides sideways. A strong tsunami is more likely when a section of seafloor suddenly rises or sinks over a large area.
The water surface is pushed out of its usual level. Gravity then pulls the raised water downward and pulls nearby water toward the lowered area.
This starts a broad moving disturbance. Unlike a wind wave, which mainly affects water near the surface, a tsunami moves water through much of the full ocean depth.
A tsunami carries energy across a very wide stretch of water. Its crest can be far from the next crest, sometimes hundreds of kilometres away. This is why the period between arriving waves can be many minutes.
In deep water, the relationship between depth and speed is especially important. Wave speed is approximately the square root of gravity times water depth. Deeper water allows the disturbance to move faster.
As the front of the wave reaches a continental shelf or a bay, it slows before the water behind it does. Water begins to pile up, changing the wave shape and increasing its height.
The coastline can greatly change the outcome. A narrow bay, river mouth, or harbour can focus incoming water into a smaller space. Sloping shores may produce a fast-rising flood that travels far inland.
Steep coasts can produce violent currents and rapid changes in water level. Hills, reefs, offshore islands, and the shape of the seafloor can direct wave energy toward one community while reducing it at another nearby place.
The highest run-up does not always occur at the first point where the tsunami reaches land. Local geography matters as much as the size of the original earthquake.
Tsunamis are dangerous because the moving water behaves more like a powerful flood than a single breaking surf wave. Water can carry cars, boats, trees, building parts, and other debris. When the flow reverses, it can drag objects and people back toward the sea.
The first arrival may be a rise in water, a sudden retreat of the shoreline, or a strong current. A retreating sea is a natural warning sign, but it does not happen in every event. People near a coast who feel a long or strong earthquake should move to high ground or inland without waiting to inspect the shore.
Scientists use earthquake records, ocean pressure sensors, tide gauges, and computer models to estimate whether a tsunami has formed and where it may travel. These tools improve warnings, but they cannot predict every local effect exactly. Students should pay attention to the difference between wave height offshore, coastal water level, and run-up on land.
They describe different parts of the same event. It is equally important to remember that later waves can be larger than earlier ones. Official safety instructions remain important until authorities announce that the danger has passed.
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.