What causes a tsunami
A tsunami is a series of ocean waves with very long wavelengths (typically hundreds of kilometres) caused by large-scale disturbances of the ocean, such as:
- earthquakes
- landslide
- volcanic eruptions
- explosions
- meteorites.
Tsunami is a Japanese word with the English translation 'harbour wave'. In the past, tsunamis have been referred to as 'tidal waves' or 'seismic sea waves'. The term 'tidal wave' is misleading – even though a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the gravitational influences of the moon, sun, and planets.
The term 'seismic sea wave' is also misleading. 'Seismic' implies an earthquake-related generation mechanism, but a tsunami can also be caused by a non-seismic event, such as a landslide or meteorite impact.
Tsunamis are also often confused with storm surges, even though they are quite different phenomena. A storm surge is a rapid rise in coastal sea-level caused by a significant meteorological event and are often associated with tropical cyclones or deep low pressure systems. Learn more about storm surge on our Tides and sea level page.
How often tsunamis occur in Australia
Australia is surrounded to the northwest and east by some 8,000 km of active tectonic plate boundaries capable of generating tsunamis that could reach our coastline within 2–4 hours. One-third of earthquakes worldwide occur along these boundaries.
Tsunamis are recorded in Australia about once every 2 years. Most recorded tsunamis were small and did not cause flooding to land. But even small tsunamis can be dangerous to swimmers and mariners. We detect and issue warnings for these events, as they can bring:
- strong currents
- dangerous rips and waves.
Tsunamis capable of causing coastal flooding to Australia are rare but still possible.
How fast tsunamis can move
Tsunamis can have wavelengths ranging from 10–500 km and wave periods of up to an hour. As a result of their long wavelengths, tsunamis act as shallow-water waves, which move at speed dependent on the water depth.
In the deep ocean, for example, the typical depth is around 4000 m. At this depth a tsunami could travel at around 200 m a second or more than 700 km per hour. By comparison, in water 20 metres deep a tsunami travels around 14 m a second (50 km per hour).
For tsunamis generated by underwater earthquakes, the wave height is determined by how much the seafloor is displaced. The distance and time between waves are also determined by the size and shape of the underwater disturbance.
As well as travelling at high speeds, tsunamis can also travel large distances with limited energy losses. As the tsunami propagates across the ocean, the wave crests can undergo refraction (bending), which is caused by segments of the wave moving at different speeds as the water depth along the wave crest varies.
What happens when a tsunami approaches land
A tsunami travels at a speed related to the water depth, so as it moves from the deep ocean to shallower water it slows. Its energy flux, which is dependent on both its wave speed and height, remains nearly constant so that as the tsunami's speed slows, its height grows. This is called shoaling and is why a tsunami may be unnoticeable at sea but may grow several metres or more closer to the coast.
Tsunamis begin to lose energy as they rush onshore, but still reach the coast with tremendous amounts of energy. Depending on whether the tsunami's crest or trough reaches the shore first, it may appear as a rapidly rising or falling tide. Local water depth may also cause it to appear as a series of waves.
Tsunamis have great erosion potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. They can also crush homes and other coastal structures, and cause flooding hundreds of metres inland.
Tsunamis may reach a maximum vertical height onshore above sea level, often called a run-up height, of tens of metres.
Learn more about tsunamis at UNESCO IOC Tsunami Glossary and Geoscience Australia – Tsunami.