What causes tsunami?

A tsunami is a series of waves travelling across the ocean due to a sudden displacement of a large body of water. This displacement can be caused by events such as undersea earthquakes, undersea landslides, land sliding into the ocean, volcanic eruptions or even asteroid impact.

Undersea earthquakes

Over 80% of tsunami in the Pacific Ocean are thought to have been caused by undersea earthquakes. Australia is surrounded by 8,000 kilometres of active tectonic plate boundaries and most earthquakes and volcanic eruptions occur where these plates meet. These boundaries are called subduction zones.

tectonic plates around Australia The red lines on this map indicate the closest tectonic plate boundaries to Australia.
map showing earthquake activity around Australia The red and pink dots in this image represent past earthquakes. They are located along the tectonic plate boundaries.


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Undersea landslides and land sliding into the sea

Undersea landslides and land sliding into the sea may cause localised tsunami. Undersea landslides occur when a large amount of sediment is dislodged from the seafloor, displacing a water column and potentially generating tsunami. Land sliding into the sea, usually caused by an earthquake, may also cause destructive local tsunami.

Lituya Bay Alaska after 1958 tsunami A landslide into Lituya Bay, Alaska, in 1958 caused a localised tsunami. The bare areas around the lake side in this image indicate where the trees were stripped away by the tsunami.


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Volcanic eruptions

Though less common, volcanic eruptions in or near the ocean have potential to cause tsunami. These occur in several ways:

  • destructive collapses of coastal, inland and underwater volcanoes which result in massive landslides
  • pyroclastic flows, which are dense mixtures of hot blocks, pumice, ash and gas, plunging down volcanic slopes into the ocean pushing water outwards or;
  • a volcano collapsing after an eruption, causing overlying water to drop suddenly.
map showing volcanic activity around Australia Australasian active volcanoes in the last 10,000 years.


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Asteroid impact

An asteroid impact into the ocean could cause tsunami. Fortunately, this should be rare and there are no documented accounts of an asteroid having generated tsunami.

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