How Do Tsunamis Form? Why a Small Ripple at Sea Becomes a Wall of Water

On a beach, a tsunami often begins in silence. The sea draws back, exposing the seabed far beyond the usual low tide. Fish flop on the sand. A few minutes later, a wall of water arrives that does not break and retreat like a normal wave, but keeps coming, flooding streets and fields. How does the ocean produce such a force?

Not a normal wave

Ordinary waves are made by wind blowing over the sea surface. They move only the top few metres of water and have wavelengths of tens of metres. A tsunami is different: it moves the entire depth of the ocean, from the surface to the seabed, and its wavelength can be hundreds of kilometres. That is why it carries so much energy.

How a tsunami starts

Most tsunamis are caused by large earthquakes under the sea, especially at subduction zones, where one tectonic plate slides beneath another. The plates can lock together for centuries, building up strain. When they finally slip, the seafloor can jump up or down by several metres over an area hundreds of kilometres long. The water above is lifted or dropped with it, and as it tries to settle back, waves race away in all directions.

Not every undersea earthquake makes a tsunami. It usually needs to be strong, typically above about magnitude 7, shallow, and to move the seafloor vertically.

Fast and invisible at sea

In the deep ocean, a tsunami travels at astonishing speed, around 800 kilometres per hour, as fast as a passenger jet. It can cross the entire Pacific Ocean in less than a day. Yet in deep water its height is often less than a metre, spread over a huge wavelength. Ships at sea may not notice it pass beneath them.

Growing at the coast

As the tsunami approaches the coast and the water becomes shallower, it slows down. The front of the wave slows first, while the water behind keeps coming, so the water piles up and the wave grows taller, sometimes 10 metres or more. Bays and narrow inlets can funnel it even higher.

Often the first thing people see is the sea pulling back, when the trough of the wave arrives first. This is a crucial natural warning: the crest is coming, sometimes within minutes.

What to know about tsunamis

• It is a series of waves. A tsunami can last for hours, and the first wave is often not the largest. Never return to the shore after the first wave.
• Other causes: undersea landslides, volcanic eruptions and, rarely, asteroid impacts can also create tsunamis.
• 2004: a magnitude 9.1 earthquake off Sumatra caused the deadliest tsunami in recorded history, killing about 230,000 people in 14 countries around the Indian Ocean.
• 2011: the magnitude 9.0 Tōhoku earthquake in Japan caused waves that flooded large areas of coast and led to the Fukushima nuclear accident.
• Turkey: the Aegean and eastern Mediterranean have experienced tsunamis throughout history. In October 2020, the magnitude 7.0 Samos earthquake sent a small tsunami into the streets of Seferihisar in İzmir.

Staying safe

If you are on the coast and feel a strong earthquake, or see the sea suddenly retreat, move immediately to high ground or well inland. Do not wait for an official warning, and do not go to look at the exposed seabed. Today, warning centres use seismometers, deep-ocean pressure sensors and tide gauges to detect tsunamis and alert coastal areas, but natural signs can arrive faster than any alarm.

In short

• A tsunami is a series of waves that moves the entire depth of the ocean.
• Most are caused by undersea earthquakes that suddenly move the seafloor.
• At sea they travel at about 800 km/h but are often under a metre high.
• Near the coast they slow down and grow, sometimes over 10 metres high.
• A sudden retreat of the sea is a warning sign: head for high ground at once.
• Tsunamis also occur in the Aegean, as in Seferihisar in 2020.