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What is a tsunami?
Tsunami is the Japanese term meaning wave ('nami') in a harbour ('tsu'). It is a series of travelling waves of extremely long length and period, usually generated by disturbances associated with earthquakes occurring below or near the ocean floor.
A tsunami is not a single wave or a tide. It is a series of waves that can cross entire ocean basins and, on reaching shallow coastal water, cause dangerous flooding and powerful currents that may continue for hours.
The term “tsunami” is internationally used because it covers waves generated by earthquakes as well as other sudden disturbances, including volcanic eruptions and submarine landslides.
What causes a tsunami?
This is the highest tsunami wave ever recorded
Beneath the ocean floor, tectonic plates move slowly against one another. When stress between them is released in a large undersea earthquake, the seabed may suddenly rise or fall, displacing the water above it.
That displaced water moves outward in all directions as a tsunami: a series of extremely long waves that can travel across an entire ocean.
Most tsunamis are caused by large, shallow undersea earthquakes. Less commonly, underwater or coastal landslides, volcanic eruptions, and other rare disturbances can also trigger them
What makes a tsunami more likely or more dangerous?
Tsunami potential is greatest for large, shallow undersea earthquakes that cause substantial vertical movement of the seabed. The larger the area of seafloor displaced, the more water is set in motion.
A tsunami’s impact also depends on the coast it reaches. Reefs, bays, river entrances, undersea features, and the slope of the shore can focus or reduce its energy, meaning nearby communities may experience very different levels of flooding.
Near the source, a tsunami can be especially dangerous because waves may arrive within minutes, leaving little time for official warnings and evacuation
How tsunamis travel
A tsunami is not like a wind wave. It moves the whole column of water, from the seabed to the surface.
In the deep ocean it can travel as fast as a jet plane while rising less than a metre, so ships at sea barely notice it.
A tsunami can cross an ocean in hours, which is why distant coasts can be warned in time.
As it reaches shallow coastal water, a tsunami slows, its waves grow higher, and its currents become more dangerous.
This video chronicles the tsunami warning chain for a M9.5 earthquake off Northern Chile as Pacific Tsunami Warning Center (PTWC) staff quickly analyze seismic and sea level data, forecast tsunami wave heights, and disseminate their threat assessments to country National Tsunami Warning Centers around the Pacific.
Measuring tsunamis at the coast
A tsunami is described by what it does at the coast, not only by the earthquake that caused it. Four measures matter most.
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Water height
How far the sea rises above normal level, recorded by tide gauges and ocean sensors.
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Run-up
The highest vertical elevation that seawater reaches on land, measured above sea level.
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Inundation distance
How far inland the water travels.
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Arrival time
When the first tsunami wave reaches a location – minutes after a nearby source, or many hours for a distant one.
How we know it is coming
Tsunami warning centres combine global seismic networks, deep-ocean DART pressure sensors, and coastal tide gauges to detect possible tsunamis and issue alerts. Because seismic waves travel much faster than tsunamis, early earthquake information can provide the first indication of a possible threat.
Natural signs also matter especially for local tsunamis, which may arrive within minutes. If you feel strong or long shaking near the coast, see the sea suddenly rise or recede, or hear a loud roar from the ocean, move immediately to higher ground or inland; do not wait for an official warning.
Japan operates one of the world's most advanced tsunami warning systems. Its Meteorological Agency issues three levels of alert about three minutes after an earthquake: Tsunami Advisory, Tsunami Warning and Major Tsunami Warning.
The most extreme alerts use the word "huge" to signal that people must evacuate at once.
Offshore, a seafloor network called S-net, built after 2011 by NIED, has 150 observatories on the Japan Trench. It has been in full operation since April 2017.
Key steps for tsunami preparedness
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Early warning
Detect and alert fast. Reliable systems and clear messages save lives.
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Healthy ecosystems
Protect mangroves, reefs, and dunes.
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Resilient infrastructure
Build smarter. Strong, elevated, and well-planned structures reduce damage.
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Community preparedness
Know the signs, plan evacuation routes, and practice regularly.
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Regional cooperation
Share data, warnings, and expertise across borders.
The Tsunami Ready Programme
The Tsunami Ready Recognition Programme is led by UNESCO-IOC. It is a voluntary, performance-based recognition initiative that promotes comprehensive tsunami preparedness at the community level. It encourages coastal communities to adopt mitigation measures that align with a standardized set of indicators. The programme contributes significantly to achieving the goal of the Sendai Framework of reducing disaster losses by promoting resilience-building measures that protect lives, livelihoods, and economies from the devastating impacts of tsunamis from all sources.
While being recognized as Tsunami Ready does not guarantee immunity from tsunami impacts, it signifies that a community has taken essential steps to minimize potential losses and protect its residents.
Play and learn to stop disasters
We are not powerless against tsunami. Instead, tsunami preparedness acts as both a public good and a sound economic strategy, through protecting lives and development gains. Use the Stop Disasters game this World Tsunami Awareness Day to see firsthand the actions needed to protect lives and development gains. In the tsunami hazard scenario, use the given time and budget to best protect your coastline, and discover the power we have to be tsunami ready.
Test your strategy in the Stop Disasters tsunami scenario: plan, protect, and prepare a resilient village before disaster strikes!
Latest tsunami additions in the Knowledge Base
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