What is a tsunami? Long waves from a sudden push on the ocean, and why they hit the coast

The short answer: a wave train from a sudden push

NOAA defines a tsunami as a series of extremely long waves caused by a large and sudden displacement of the ocean. The most common cause by far is an earthquake under or near the sea floor: when the floor rises or drops, the water above it moves too, and as the water moves up and down to regain its balance, the tsunami radiates out in all directions. Of the events in the Global Historical Tsunami Database, 88 per cent were caused by earthquakes or by landslides that earthquakes set off.

Not every earthquake does it. Most tsunamis come from earthquakes of magnitude over 7.0 that occur under or very near the ocean, usually at subduction zones, where one plate dives under another, and less than 100 km below the surface. An earthquake generally has to exceed magnitude 8.0 to produce a dangerous tsunami on distant shores. About 10 to 15 per cent of damaging tsunamis come from strike-slip earthquakes, where the ground moves sideways, and these usually affect only coasts near the source.

Other things can also push the sea. Underwater or coastal landslides can send waves ashore within minutes. Volcanic activity did it at Krakatoa in 1883, when waves up to 135 feet (about 41 m) killed more than 34,000 people. Fast-moving storm systems can make small “meteotsunamis”, and a large asteroid or comet hitting the ocean could do it too. Calling a tsunami a “tidal wave” is wrong, because tides come from the pull of the Sun and Moon and have nothing to do with it.

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How fast and how high is a tsunami?

A tsunami’s speed depends only on the depth of the water, not on how far it has come: speed is the square root of gravity times depth. In 4,000 m of water that is the square root of 9.8 × 4,000, about 200 m/s or 700 km/h; NOAA gives about 475 mph, roughly 765 km/h, for 15,000 feet. In the deep ocean tsunamis can move faster than 800 km/h, like a jet plane, and cross an entire ocean in less than a day. From the Aleutian Islands to Hawaii takes about five hours.

In deep water the wave is barely noticeable: the wavelength, the distance between crests, may be hundreds of kilometres, and the height rarely more than about a metre, so ships at sea do not notice a tsunami passing beneath them. Wind waves have wavelengths of 90 to 180 metres and periods of 5 to 20 seconds; a tsunami has a period of 5 minutes to 2 hours and moves the whole column of water from the surface to the sea floor, not just the surface.

Near land the water gets shallow, the front slows to 30 to 50 km/h, the wavelength shrinks and the height grows. Most tsunamis are less than 3 m high when they strike, but in extreme cases they can exceed 30 m. A big one can flood low-lying land more than 1.6 km inland. On the coast it rarely looks like a towering breaking wave. NOAA says it may look like a fast-rising flood or a wall of water, and sometimes the sea first draws back so far that the sea floor, reefs and fish are exposed, like a very low tide.

Great tsunamis in numbers

On 26 December 2004 a magnitude 9.1 earthquake off northern Sumatra generated the deadliest tsunami in history. Impacts were felt in 15 countries in Southeast and South Asia and East and South Africa, with about 230,000 deaths and 1.7 million people displaced. On the Sumatran coast the waves reached up to 167 feet (about 51 m) and travelled up to 3 miles (about 5 km) inland. NOAA says that the losses were due in part to the lack of an official tsunami warning system in the Indian Ocean at the time. The photograph at the top of this guide shows that tsunami reaching Ao Nang, Thailand.

On 11 March 2011 a magnitude 9.1 earthquake off Honshu, Japan, produced a tsunami that reached 128 feet (about 39 m) high and almost 5 miles (about 8 km) inland. It displaced more than 500,000 people, and most of the 18,457 deaths were due to the tsunami. Outside Japan there was very little loss of life thanks to warnings and evacuations, though there was more than $91 million of damage in the United States and one death.

Where do they happen? Of 754 confirmed events between 1900 and 2015, about 78 per cent were in the Pacific Ocean, around the geologically active Ring of Fire, and about 21 per cent of all tsunamis since 1900 were generated off Japan. Tsunamis that cause damage or deaths near their source occur about twice a year; those that cause damage on distant shores, more than 1,000 km away, about twice a decade. There is no tsunami season: one can happen at any time of year and in any weather.

How tsunamis are detected, and the natural warning signs

NOAA runs two tsunami warning centres staffed around the clock. The first sign of a possible tsunami is usually an earthquake, and seismic waves travel about 100 times faster than tsunamis, so the centres know the location, depth and magnitude of the quake before the water arrives. They then check water-level data, above all from DART systems: a pressure recorder anchored on the ocean floor that senses the passing wave, linked by an acoustic signal to a surface buoy that relays it by satellite. In 2016 the United States had 39 such systems in the Pacific and Atlantic oceans and the Caribbean Sea.

The difficulty is near sources. A local tsunami may reach the coast in less than an hour, and one from a landslide can arrive within minutes, so there may not be time for a detailed official forecast. Ready.gov therefore tells coastal residents to learn the natural signs: an earthquake, a loud roar from the ocean, and unusual ocean behaviour such as a sudden rise or wall of water or sudden draining that exposes the sea floor.

The official advice is short. If an earthquake happens in a tsunami area, protect yourself first: Drop, Cover and Hold On. When the shaking stops, if there are natural signs or an official warning, move at once to high ground as far inland as possible, and do not wait for a siren or an order. Ready.gov suggests shelters at least 100 feet (30 m) above sea level or at least one mile (1.6 km) inland, and notes that evacuation routes are often marked with a wave and an arrow pointing to higher ground. Listen to local authorities and do not go back until they say it is safe, because the waves keep coming, and avoid floodwater, which can hide debris and electrical hazards.

How the VJOURNAL SMART BOOK shows it

The chapter “Earthquakes” in the free SMART BOOK “Everything about everything”, Volume II, on VITON13 SCHOOL ends with the moment “A wave that crosses an ocean”. There you meet the square-root rule from this guide, with the worked example of an ocean 4,000 metres deep, and the official advice to move to high ground at once if the ground shakes strongly near the coast.

The earlier moments of the chapter show what lifts the sea floor: a living 3D picture of two plates and a fault that locks and slips, and a sandbox where you pick a magnitude and compare it with the scale. A three-question quiz and sources you can open close the chapter.

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• SMART BOOK chapter: Earthquakes ↗

In everyday language

Push down on one end of a bathtub full of water and a long, low swell runs along the tub. A tsunami is the same thing on a planet scale: the sea floor lifts, the whole column of water above it is shoved, and a train of long, low waves sets off in every direction. Out at sea it is gentle; near the shore, where the water is shallow, the same energy is squeezed into a taller, slower wall.

Related lessons

• What causes earthquakes? Moving plates, locked faults and the snap that shakes the ground ↗