How Do Tsunamis Form? From Seafloor Movement to Coastal Waves

Many tsunamis begin at tectonic plate boundaries where one plate is being forced beneath another. Stress can build along the fault until the rocks suddenly slip. If that movement pushes part of the seafloor upward or downward, the water above it is displaced too. Gravity then pulls the disturbed water back toward equilibrium, creating waves that spread away from the source.

The key to how tsunamis form is vertical displacement. A powerful earthquake does not automatically produce a large tsunami. The location, depth, fault movement, and amount of seafloor displacement all matter. Earthquakes that mainly move rocks sideways may cause much less water displacement than events that lift or drop a broad section of the ocean floor.

In the deep ocean, tsunami waves can travel very quickly while remaining surprisingly low in height. Their wavelengths can be enormous, so a ship far offshore may barely notice one passing. The wave is moving a huge quantity of energy and water, but that energy is spread through a very long wave.

As the tsunami enters shallower coastal water, its speed decreases. The water cannot move in the same way as it did in the deep ocean, so the wave becomes shorter and its height can increase. This process helps explain why the origin of a tsunami is only half the story. Coastal shape and water depth strongly influence what happens when the waves arrive.

Underwater landslides can suddenly push water aside and generate tsunami waves. Volcanic eruptions, collapsing volcanic slopes, and rare impacts from large objects can also displace enough water to create a tsunami. These events differ in cause, but the basic mechanism is similar: a large amount of water is rapidly disturbed.

When learning how tsunamis form, do not imagine one ordinary breaking wave moving across an ocean. A tsunami usually arrives as a series of waves, and the first wave may not be the largest. Water can also withdraw unusually far from shore before some arrivals. Natural warning signs and official alerts matter because dangerous currents and flooding can continue after an initial wave has passed.

The Indian Ocean tsunami of 26 December 2004 came from a magnitude 9.1 earthquake off Sumatra that lifted a stretch of seafloor around 1,300 kilometres long. Waves reached Thailand in about two hours and East Africa in seven, and roughly 230,000 people died, most of them with no warning at all, because the Indian Ocean had no detection network. The 2011 Tohoku tsunami struck Japan, which had the best warning system in the world, yet waves that reached 40 metres in places still killed around 18,000, largely because the first alert underestimated the earthquake's size and sea walls built for smaller events gave a false sense of safety.

Modern systems use seafloor pressure sensors that detect the passing wave in deep water and relay it by satellite buoy, giving minutes to hours of notice. The lesson repeated in both disasters is that a warning is only useful if people already know to move to high ground the moment the ground shakes or the sea withdraws.

Tsunamis are so long compared with the depth of the ocean that they behave as shallow-water waves, and their speed depends almost entirely on depth. A good approximation is the square root of gravity multiplied by water depth. In open ocean around 4,000 metres deep, that gives the square root of 9.8 times 4,000, close to 200 metres per second, or roughly 700 kilometres per hour, the cruising speed of a jet airliner. This simple formula is why warning centres can predict arrival times across a whole ocean within minutes of an earthquake.

Put a depth of 10 metres into the same formula and the speed drops to about 10 metres per second, around 36 kilometres per hour. That is much slower, but still faster than anyone can run, so outrunning a wave on a beach is not a realistic plan. The long wavelength also explains a common misconception. A tsunami is often called a tidal wave, but it has nothing to do with tides. Because each wave can take many minutes to pass, it often arrives like a sea level that keeps rising, rather than a single curling breaker.

Geologists find ancient tsunamis by digging. A large wave sweeping inland leaves a distinctive sheet of sand, often containing marine shells and microscopic sea creatures, on top of peat, soil or lake mud that formed on land. Layers like these along the coasts of Scotland, Norway and the Shetland Islands record the Storegga slides, huge underwater landslides off Norway that sent a tsunami across the North Sea roughly 8,000 years ago. Dating such layers tells scientists how often a coastline has been struck, far beyond the span of written history.

The most famous detective case links two continents. Japanese records describe a tsunami in January 1700 that arrived without any earthquake being felt in Japan, a so-called orphan tsunami. On the Pacific coast of North America, scientists found drowned forests of dead trees, killed when the land dropped during a great earthquake, and tree rings showed they died between the growing seasons of 1699 and 1700. Wave travel times matched the Japanese records, pinning the source to the Cascadia subduction zone on 26 January 1700, an earthquake of around magnitude 9.

Most warning systems start with earthquakes, because seismometers detect them within minutes. A tsunami caused by a volcano can slip past that net. On the evening of 22 December 2018, part of the flank of Anak Krakatau, a volcano in the Sunda Strait of Indonesia, collapsed into the sea during an eruption. There was no large earthquake to trigger an alert, and waves struck the coasts of Java and Sumatra within about half an hour, killing more than 400 people, including members of a crowd at a beachside concert. The disaster prompted new monitoring of volcanoes and sea levels in the strait.

The eruption of Hunga Tonga-Hunga Ha'apai in the South Pacific in January 2022 revealed another mechanism. The blast sent a pressure wave through the atmosphere that circled the globe several times and was recorded by weather stations everywhere. As it passed over the oceans, it pushed on the water and set off small sea-level surges, some arriving far from Tonga hours earlier than an ordinary tsunami could have travelled. Waves reached as far as Peru, where two people drowned. Barometers had recorded a similar pressure wave after Krakatoa erupted in 1883, but the Tonga event was measured in far more detail.

Tsunamis begin when an event such as an undersea earthquake suddenly displaces a large volume of water. The resulting long waves travel across the ocean, slow in shallow water, and can rise dramatically near coasts. The size of the hazard depends on both the source and the coastline.