Tsunami

A tsunami is a series of big waves in the ocean or a large lake caused when a lot of water is moved suddenly. This can happen during earthquakes, volcanic eruptions, or underwater explosions. These events push the water, creating waves that are very different from normal ocean waves made by wind or tides caused by the Moon and the Sun.

Tsunamis are not like regular waves you might see at the beach. Instead, the water level can rise very quickly, looking like a fast-rising tide. These waves usually come in a group, with time gaps ranging from minutes to hours between each wave. The height of the waves can be very high—sometimes tens of meters—causing huge damage along coastlines.

One of the most terrible tsunamis happened in the Indian Ocean in 2004. It was one of the deadliest natural disasters in history, affecting 14 countries and causing the loss of many lives. Scientists have known about tsunamis for a long time, but they are still learning more about why some big underwater earthquakes cause tsunamis and others do not. This research helps people understand how to keep ourselves safer when a tsunami happens.

Terminology

Tsunami

The word "tsunami" comes from the Japanese language, where it means 'harbour wave'. In English, people often drop the first sound of the word, but it is still widely used to describe these big waves in the water.

Tidal wave

Sometimes people call tsunamis "tidal waves". This is because they can look like very high tides moving onto land. But tsunamis and tides are caused by very different things. Tides are caused by the pull of the moon and sun, while tsunamis happen when a large amount of water is moved suddenly, like during an earthquake. Scientists prefer the term "tsunami" because it is more accurate.

Seismic sea wave

Another name for tsunamis is "seismic sea wave" because they are often caused by earthquakes under the water. However, tsunamis can also be caused by other events, such as underwater landslides, volcanic eruptions, or even meteorites hitting the ocean.

Other terms

When waves are caused by landslides falling into water, they are sometimes called "landslide-triggered tsunamis" or other special names to show they are not caused by earthquakes. If a tsunami happens without any nearby earthquake, it may be called an "orphan tsunami". This can occur when an earthquake far away causes a tsunami, especially in the past when people could not easily share news across long distances.

History

See also: List of tsunamis

Japan has the longest recorded history of tsunamis.

As early as 426 BC the Greek historian Thucydides wrote in his book History of the Peloponnesian War about why tsunamis happen. He was the first to suggest that earthquakes under the ocean cause these big waves. The oldest known tsunami happened in 479 BC at a place called Potidaea in Greece. It was likely caused by an earthquake and may have helped protect the town from being attacked.

The Roman historian Ammianus Marcellinus described what happens during a tsunami after a big tsunami hit Alexandria in 365 AD. Tsunamis can also happen in the Mediterranean Sea and parts of Europe. Some important past tsunamis include the 1755 Lisbon earthquake and tsunami, the 1783 Calabrian earthquakes, and the 1908 Messina earthquake. The 1908 tsunami in Sicily and Calabria was one of the deadliest in modern Europe.

The 2004 Indian Ocean earthquake and tsunami was the worst of its kind in modern times, affecting many lives. The Sumatran area often experiences tsunamis because of frequent earthquakes near the coast.

Causes

Tsunamis happen when a big amount of water in the ocean or a large lake gets moved suddenly. This can be caused by events like earthquakes, landslides, volcanic eruptions, or even glaciers breaking off. These events push a lot of water out of its normal place, creating big waves.

Earthquakes are the most common cause of tsunamis. When the sea floor moves quickly during an earthquake, it can push the water above it up or down. This makes the water move in big waves. Some earthquakes under the sea can create tsunamis that travel across the ocean. Landslides under the water can also cause tsunamis by moving large amounts of mud or rock into the water. Volcanic eruptions can sometimes cause tsunamis too, either by landslides from the volcano or by explosions under the water.

Characteristics

Tsunamis are big waves caused by events like earthquakes, landslides, volcanic eruptions, glacier breaking, and space rocks hitting the water. These waves can hurt places by pushing a wall of water onto land very fast and by pulling a lot of water and debris away from the shore, even if the waves don’t look very big.

Normal ocean waves from wind are small, about 100 meters apart and only 2 meters high. But tsunamis in deep ocean water are much bigger, stretching up to 200 kilometers from one wave top to the next. They move very fast—over 800 kilometers per hour—but because they are so big, the water only moves up and down a little bit. This makes them hard to notice far out at sea. When a tsunami gets close to shore, the water gets shallower, so the wave slows down but gets much taller, sometimes taking minutes to reach its full height.

Drawback

All waves have high points and low points, like the top and bottom of a wave. For a tsunami, either the high point or the low point might reach the shore first. If the high point arrives first, a big wave or sudden flood might happen right away. But if the low point arrives first, something called a "drawback" can happen. This means the water pulls back far from the shore, showing areas that are usually under water. This can happen for hundreds of metres away from the land.

A harmful tsunami usually has a wave cycle of about twelve minutes. In the drawback phase, the water pulls back, and areas far below sea level can become visible after about three minutes. Then, over the next six minutes, the low point of the wave turns into a high point, which might flood the coast and cause damage. After another six minutes, the wave turns from a high point back to a low point, and the flood waters pull back again in a second drawback. People and things might get carried out to sea. This pattern can repeat with more waves.

Scales of intensity and magnitude

Scientists have created different ways to measure how strong tsunamis are, similar to how they measure earthquakes. One way is to look at how high the water rises along the coast. For example, if the water rises about 2.8 meters, it gets a certain number on the scale. If it rises higher, like 5.5 meters, it gets a higher number.

Another way scientists measure tsunamis is by looking at how much energy the tsunami has. They use special formulas to calculate this, which helps them compare different tsunamis to each other.

Tsunami heights

Tsunamis can be described by how tall the waves are and how far they reach onto land.

• Amplitude, Wave Height, or Tsunami Height: This is how tall the tsunami wave is compared to the normal sea level at the time it happens.
• Run-up Height, or Inundation Height: This is how high the tsunami reaches above sea level on land. The maximum run-up height is the highest point the water reaches.
• Flow Depth: This is how tall the tsunami wave is above the ground, no matter where the ground is compared to sea level.
• (Maximum) Water Level: This is the highest point above sea level that can be seen from marks left by the water. It is different from run-up height because it may not always be at the edge of the flooded area.

Warnings and predictions

See also: Tsunami warning system

If you see the water pull back from the shore, it can be a sign that a big wave is coming. If this happens, it is important to run to higher ground right away or go to the top floors of nearby buildings to stay safe.

In 2004, a ten-year-old girl named Tilly Smith from Surrey, England, was on a beach in Phuket, Thailand. She had just learned about tsunamis in school and told her family that a tsunami might be coming. Her family warned others, and this helped save many lives.

Tsunamis cannot be predicted exactly, even when we know about an earthquake. Scientists study each earthquake to decide whether to issue a warning. There are some signs that can help, and special systems can give warnings quickly after an earthquake happens. These systems use sensors on the ocean floor to watch for changes in water pressure.

Areas at risk of tsunamis often have warning systems to alert people before a wave reaches land. For example, on the west coast of the United States, signs show where to go to escape. In Japan, people are taught about earthquakes and tsunamis, and there are sirens along the coast to give warnings.

The Pacific Tsunami Warning System is based in Honolulu, Hawaiʻi. It watches for earthquakes in the Pacific Ocean. Not every earthquake causes a tsunami, but computers help experts decide the risk after each one.

Computers can help predict when a tsunami will arrive, usually just a few minutes before it gets there. Sensors on the ocean floor send information right away. Using this information and maps of the ocean and coast, experts can guess how big the wave will be. Countries around the Pacific work together in the Tsunami Warning System and practice what to do.

On the United States west coast, warnings are also sent on TV and radio by the National Weather Service using the Emergency Alert System.

Possible animal reaction

Further information: Infrasound § Animal reaction

Some scientists think certain animals might sense big earthquakes or tsunamis before they happen. If true, watching animals could give an early warning. However, this idea is not proven. There are stories about animals moving to higher ground during past events, but many animals were not saved. Some people went to the shore to see what was happening and were caught by the waves.

Mitigation

See also: Seawall

In places where tsunamis happen often, special building designs are used to make buildings stronger against tsunami damage. Japan, where people first started studying tsunamis after a big disaster in 1896, has built many tall walls—up to 12 metres high—to protect towns near the water. Some places have also built big floodgates and channels to guide the water away. But sometimes, tsunamis can jump over these walls.

For example, during the 2011 tsunami in Japan, the waves were so big that they went over the walls at the Fukushima nuclear plant, causing serious problems. In another area called Iwate Prefecture, there were 25 kilometres of tsunami walls, but more than half of them fell down during the same tsunami, and many buildings were damaged.

In 1993, a tsunami hit a town called Okushiri, Hokkaidō, just minutes after an earthquake. The waves were as tall as a 10-storey building! Even though the town had a wall all around it, the waves went right over the wall and destroyed many wooden buildings. The wall may have slowed the waves a little, but it couldn’t stop the damage.