Tsunamis

A tsunami is a giant ocean wave (or series of waves) caused by a sudden disturbance under the sea, usually a large underwater earthquake, but sometimes a volcanic eruption, an underwater landslide, or even an asteroid impact. Tsunami waves are different from normal wind-driven waves: they travel across whole oceans at the speed of a jet aircraft (around 800 km/h), can build to over 30 metres tall as they reach shallow coasts, and can cause catastrophic damage hundreds or thousands of kilometres from where they started. The word "tsunami" is Japanese, meaning "harbour wave".

• Word originJapanese tsu + nami"Harbour wave"
• Travel speed in deep oceanApprox. 800 km/hAbout as fast as a jet airliner
• Speed in shallow waterApprox. 50 km/hBut waves grow much taller
• WavelengthUp to 500 kmCrests far apart in deep water
• Deadliest in recent history2004 Indian OceanApprox. 230,000 deaths
• Tallest wave heightApprox. 524 mLituya Bay, Alaska, 1958 (mega-tsunami)

How tsunamis form

Most tsunamis are caused by underwater earthquakes at subduction zones: places where one tectonic plate dives under another. When the locked rocks of the two plates suddenly slip, the seabed can lift or drop by metres in seconds. The huge volume of water above the seabed is suddenly pushed up or pulled down, creating a wave that spreads out in all directions like ripples in a pond.

Less commonly, tsunamis can also be caused by:

• Underwater landslides: huge masses of sediment slumping down ocean slopes.
• Volcanic eruptions: especially when the cone of an island volcano collapses into the sea.
• Asteroid impacts: extremely rare but catastrophic. The asteroid 66 million years ago that killed the dinosaurs is thought to have caused tsunamis hundreds of metres tall.

Why tsunamis are so different from normal waves

Ordinary waves are made by wind blowing over the surface of the water. They are short (only a few hundred metres between crests) and shallow (only the top 100 metres or so of water is actually moving).

A tsunami is the entire water column moving, from the surface all the way down to the seabed. In deep ocean, tsunami waves are very long (up to 500 km between crests) but only about a metre tall: ships at sea may not even notice one passing underneath. As the wave reaches shallow water near the coast, friction with the seabed slows it down. But the water behind keeps coming, piling up on the front of the wave. The wave gets taller and steeper, sometimes reaching tens of metres before it crashes onto the shore.

The 2004 Indian Ocean tsunami

On Boxing Day, 26 December 2004, a magnitude 9.1 earthquake off the coast of Sumatra in Indonesia triggered one of the deadliest natural disasters in modern history. The seabed lifted by up to 20 metres along a fault rupture 1,300 km long. The resulting tsunami spread across the entire Indian Ocean at the speed of a jet airliner.

Within hours, giant waves had struck the coasts of Indonesia, Thailand, India, Sri Lanka, Maldives, Somalia and several other countries. Some waves reached 30 metres in height. Approximately 230,000 people died across 14 countries: the largest death toll from any natural disaster in over 100 years. Many of those killed were tourists on holiday in Phuket and other beach resorts. The 2004 tsunami changed the world's approach to ocean disaster warning, leading to the creation of the Indian Ocean Tsunami Warning System.

Fact The tallest tsunami wave ever recorded happened in Lituya Bay, Alaska, in July 1958. A magnitude 7.8 earthquake triggered a massive rockfall into the narrow bay. The displaced water surged up the opposite shore to a height of 524 metres: taller than the Empire State Building. Three people in fishing boats survived by being lifted over a forest of trees and dropped back into the bay. Two others were killed. The event is called a mega-tsunami because the wave was so much taller than ordinary tsunamis.

Tsunami warning systems

Since the 2004 disaster, most ocean countries are connected to international tsunami warning systems that detect underwater earthquakes and predict tsunamis within minutes. The systems use:

• A global network of seismometers that detect undersea earthquakes.
• DART buoys (Deep-ocean Assessment and Reporting of Tsunamis): special pressure sensors on the seabed that detect the passing tsunami wave overhead.
• Tide gauges at coastal sites that record arriving waves.

When a tsunami is detected, warnings are broadcast via radio, TV, mobile phone alerts and tsunami sirens on beaches. People in low-lying coastal areas are told to evacuate to higher ground immediately. Even a few minutes of warning can save many lives.

Did you know? One of the warning signs of an approaching tsunami is the sea suddenly retreating a long way from the beach, exposing the seabed. This happens because the trough of the wave often arrives first. Anyone who sees this should run inland immediately. A 10-year-old British girl, Tilly Smith, recognised this sign on a Thai beach in 2004 from a school geography lesson and warned around 100 tourists to flee, saving their lives. She had learned about it only two weeks earlier.

Deeper dive: the Cascadia subduction zone and the next big one

One of the biggest tsunami threats today is the Cascadia subduction zone: a 1,000 km fault running off the coast of the US Pacific Northwest (Washington, Oregon and northern California). Cascadia is where the Juan de Fuca Plate dives under the North American Plate.

For a long time scientists thought Cascadia was inactive. But in the 1980s, evidence started to emerge from buried forests of dead cedar trees in coastal estuaries, all killed by sudden saltwater flooding at the same time. By comparing tree rings, scientists pinned the event to January 1700. They then found Japanese records of a mysterious unexplained tsunami in January 1700, which arrived without a felt earthquake nearby. The two pieces of evidence pointed to a magnitude 9 earthquake on Cascadia, and a tsunami that crossed the entire Pacific to hit Japan 10 hours later.

Subsequent geological surveys of buried tsunami deposits showed that Cascadia has produced a great earthquake every approximately 200 to 800 years (average 500). The most recent was 1700. So Cascadia is overdue, although "overdue" in geological terms could mean tomorrow, in 50 years, or in 500 years.

If Cascadia ruptures fully, it could produce a magnitude 9 earthquake plus a giant tsunami. Coastal Oregon and Washington could see waves over 10 metres tall arriving within 15 to 30 minutes. The region has spent recent decades preparing: building tsunami evacuation routes, vertical evacuation towers, and improved warning systems. But the timeline for the next "Big One" remains genuinely unknown.

For more, see what is an earthquake and tectonic plates.