Tsunamis are grouped by what displaces the water, and there are four main types: tectonic tsunamis, set off by undersea earthquakes, the cause behind the deadliest tsunami on record; volcanic tsunamis, from eruptions, collapsing calderas and pyroclastic flows hitting the sea; landslide tsunamis, from rock falls into bays or the collapse of underwater slopes, which produce the tallest waves of all; and meteotsunamis, pushed up by sudden jumps in air pressure under storms. Rarer causes include meteorite impacts and explosions, such as the 1917 Halifax Explosion, which raised an 18-metre wave in the harbour at Halifax, Nova Scotia. About 80% of tsunamis occur in the Pacific Ocean. Whatever the cause, the physics afterwards is the same, and each type is easiest to understand through the place where it struck hardest.
The word itself is Japanese, 津波, meaning “harbour wave”. In the open ocean a tsunami is barely visible: its wavelength can reach 200 km, against about 100 m for an ordinary wind wave, but its height may be only about a metre, and it travels at well over 800 km/h, roughly the speed of a jet airliner. As it reaches shallow water near a coast, the front of the wave slows to below 80 km/h while the water behind keeps arriving, so the wave grows tall, a process called shoaling. If the trough arrives first, the sea can draw back hundreds of metres from the shore, exposing the seabed, before the crest floods in. That drawback can be the only natural warning.
Banda Aceh 2004 and Sendai 2011: Tectonic Tsunamis From Megathrust Faults
Banda Aceh, Sumatra
Tectonic tsunamis start at subduction zones, where one plate slides beneath another along a huge, gently sloping fault called a megathrust. The plates lock together and strain builds for centuries; when the fault slips, the seafloor above it jumps up or down by several metres and lifts the entire column of ocean above it. On 26 December 2004 at 07:58 local time, the northern section of the Sunda megathrust ruptured off the west coast of Aceh, in northern Sumatra. The earthquake had a magnitude of about 9.2 to 9.3, and the fault broke over 1,200 to 1,300 km, the longest rupture ever observed, with faulting that lasted at least ten minutes.
Northern Sumatra was hit within minutes. In Aceh, waves came ashore at about 24 m along long stretches of coastline and up to 30 m in places. Splay faults, secondary “pop-up” faults branching off the main rupture, raised narrow strips of seafloor in seconds and added to the height of the waves that destroyed the town of Lhoknga. The same wave reached Sri Lanka and the east coast of India 90 minutes to two hours later, and Thailand about two hours later, although Thailand was closer, because the wave slowed in the shallow Andaman Sea. It was still measurable 16 hours later at Struisbaai in South Africa, about 8,500 km away. An estimated 227,898 people were killed in 14 countries, making it the deadliest tsunami in recorded history.
Sendai and Miyako, Japan
Japan shows how little time a local tectonic tsunami allows. On 11 March 2011 at 14:46 local time, a magnitude 9.0–9.1 megathrust earthquake struck 72 km east of the Oshika Peninsula in the Tōhoku region. It was the most powerful earthquake ever recorded in Japan. The tsunami travelled at about 700 km/h in the Sendai area and pushed up to 10 km inland, and residents of Sendai had only eight to ten minutes of warning. In Miyako, in Iwate Prefecture, the waves may have reached 40.5 m. Official figures released in 2021 put the toll at 19,759 dead and 2,553 missing, and the tsunami caused the meltdowns at the Fukushima Daiichi nuclear power plant. The lesson of both events is the same: a megathrust earthquake can move enough water to cross an ocean, but the deadliest waves fall on the coast nearest the fault. The plate boundaries most likely to produce such events ring the Pacific, as described in our guide to tsunami zones in the Pacific.
Krakatoa 1883, Anak Krakatau 2018 and Hunga Tonga 2022: Three Ways a Volcano Moves the Sea
Volcanic tsunamis have more than one mechanism, and the Sunda Strait between Java and Sumatra has seen two of them from the same volcano. On 27 August 1883, Krakatoa produced a series of enormous explosions; the third, at 10:02 a.m., was heard 3,110 km away in Perth, Western Australia. Most of the island collapsed into a caldera. Volcanologists think the tsunamis were not caused by the blasts themselves but by huge pyroclastic flows, avalanches of hot gas and rock from collapsing eruption columns, that poured several cubic kilometres of material into the sea and displaced an equal volume of water. The town of Merak on Java was destroyed by a wave 46 m high, waves reached 42 m on the west coast of Java and 24 m on the south coast of Sumatra, and at least 36,417 deaths are attributed to the eruption and its tsunamis.
The island that grew in the caldera, Anak Krakatau (“child of Krakatoa”), produced a second type. At about 21:38 on 22 December 2018, a large part of its south-western flank collapsed into the caldera, and the landslide sent a tsunami into the coasts of Banten and Lampung, including the beach resort of Anyer. There was no earthquake to trigger warnings, it was dark, and it was the holiday season: 426 people died. Indonesia had a warning system for earthquake tsunamis but none for volcanic ones, and the disaster prompted it and other countries to build volcanic activity into their tsunami alerts. The longer history of the volcano is told in our complete guide to Krakatoa.
The third mechanism came from Tonga. On 15 January 2022 the submarine volcano Hunga Tonga–Hunga Haʻapai, 65 km north of Tongatapu, erupted with a force rated at least VEI 5, sending its plume to a peak of 58 km. One study estimated that the explosion displaced 6.6 km³ of seawater. Waves up to 20 m high struck Tonga, and run-up reached 45 m on the uninhabited island of Tofua. But the eruption also launched pressure waves through the atmosphere that circled the planet, and these coupled with the ocean as they passed, raising extra tsunamis far from the volcano, something forecast systems built for earthquakes had not allowed for. Two people drowned in Peru, on the other side of the Pacific, when 2 m waves reached the coast.
Lituya Bay 1958 and the Storegga Slide: Landslide Tsunamis, Narrow and Vast
Lituya Bay, Alaska
A landslide tsunami starts when a large mass of rock, ice or sediment drops suddenly into water. Because the energy is released in a small area, these waves can be very tall close to the source, then fade quickly with distance. The record belongs to Lituya Bay, a T-shaped fjord about 11 km long and 3 km wide on the Gulf of Alaska, which sits on the Fairweather Fault. On the night of 9 July 1958, a magnitude 7.8 to 8.3 earthquake on that fault shook about 30 million cubic metres of rock, some 90 million tonnes, off the cliffs into Gilbert Inlet at the head of the bay. The impact was heard 80 km away. The water surged up the opposite slope and stripped trees to a height of 524 m, higher than any other tsunami on record.
Howard Ulrich and his seven-year-old son were anchored inside the bay on their boat, the Edrie, and watched the wave form; it reached them two to three minutes later, carried them toward the southern shore and back, and they survived. Five people were killed in all. The bay had produced large waves before: about 120 m in 1854, 60 m in 1899 and 150 m in 1936.
Storegga, Norwegian Sea
Underwater landslides can be far larger. Around 6200 BCE, the edge of Norway’s continental shelf about 100 km north-west of the Møre coast failed in three underwater slides, the Storegga (“Great Edge”) Slides; carbon dating puts the latest at about 6225 to 6170 BCE. About 290 km of shelf gave way and some 3,500 km³ of debris slid down into the Norwegian Sea, among the largest known submarine landslides. The resulting tsunami crossed the North Atlantic; in Scotland, its sand layer has been found in Montrose Basin and the Firth of Forth up to 29 km inland and 4 m above today’s high tide. One proposed trigger is an earthquake that destabilised methane clathrate, an ice-like solid in the seabed sediments that expands about 164 times in volume when it turns to gas. It has been estimated that the wave may have killed up to a quarter of Britain’s Mesolithic population.
Nagasaki Bay and Lake Michigan: Meteotsunamis Made by Weather
The fourth type has no earthquake, volcano or landslide behind it. A meteotsunami forms when a fast-moving storm, squall line or front changes air pressure abruptly over the water, pressing the surface down in one place and letting it rise in another. On its own the effect is small, but resonance can amplify it, and the size of the wave depends heavily on the speed and severity of the storm front. Meteotsunamis lack the energy of a large seismic tsunami and stay local, but they can look so similar that the two are hard to tell apart. Only about 3% of historical tsunami events from 2000 BC to 2014 are known to have had a meteorological origin, although many may have been recorded as something else.
They are frequent enough in some places to have local names: abiki in Japan, rissaga in Catalan-speaking parts of the western Mediterranean, šćiga in Croatia. Long, narrow bays whose natural sloshing period matches the incoming waves are the most at risk. In Nagasaki Bay on 31 March 1979, a meteotsunami reached 5 m and killed three people. The Great Lakes are surprisingly exposed: NOAA estimated in 2019 that they see about 126 meteotsunamis a year on average, against about 25 on the US East Coast. In 1929 a 6 m wave swept ten people to their deaths at Grand Haven, Michigan, and in 1954 a 3 m wave washed people off the piers of the Chicago waterfront, drowning seven.
Seen side by side, the four types differ mainly in reach. A megathrust tsunami like Aceh’s can do damage on the far side of an ocean, as it did in Somalia; a landslide wave like Lituya Bay’s can climb 524 m up a fjord wall yet fade quickly with distance; volcanic waves fall somewhere between; and a meteotsunami stays inside one bay or lake. The physics of shoaling and drawback that links them all is set out step by step in our explainer on how tsunamis form.