Home » Oceanography » What Causes a Tsunami?

Tsunami Geology - What Causes a Tsunami?

What causes a tsunami?... A tsunami is a large ocean wave that is caused by sudden motion on the ocean floor. This sudden motion could be an earthquake, a powerful volcanic eruption, or an underwater landslide. The impact of a large meteorite could also cause a tsunami. Tsunamis travel across the open ocean at great speeds and build into large deadly waves in the shallow water of a shoreline.

Subduction Zones are Potential Tsunami Locations

Most tsunamis are caused by earthquakes generated in a subduction zone, an area where an oceanic plate is being forced down into the mantle by plate tectonic forces. The friction between the subducting plate and the overriding plate is enormous. This friction prevents a slow and steady rate of subduction and instead the two plates become "stuck".

Accumulated Seismic Energy

As the stuck plate continues to descend into the mantle the motion causes a slow distortion of the overriding plage. The result is an accumulation of energy very similar to the energy stored in a compressed spring. Energy can accumulate in the overriding plate over a long period of time - decades or even centuries.

Earthquake Causes Tsunami

Energy accumulates in the overriding plate until it exceeds the frictional forces between the two stuck plates. When this happens, the overriding plate snaps back into an unrestrained position. This sudden motion is the cause of the tsunami - because it gives an enormous shove to the overlying water. At the same time, inland areas of the overriding plate are suddenly lowered.

Tsunami Races Away From the Epicenter

The moving wave begins travelling out from where the earthquake has occurred. Some of the water travels out and across the ocean basin, and, at the same time, water rushes landward to flood the recently lowered shoreline.

Tsunamis Travel Rapidly Across Ocean Basis

Tsunamis travel swiftly across the open ocean. The map below shows how a tsunami produced by an earthquake along the coast of Chile in 1960 traveled across the Pacific Ocean, reaching Hawaii in about 15 hours and Japan in less than 24 hours.

Tsunami "Wave Train"

Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. The chart below is a tidal gauge record from Onagawa, Japan beginning at the time of the 1960 Chile earthquake. Time is plotted along the horizontal axis and water level is plotted on the vertical axis. Note the normal rise and fall of the ocean surface, caused by tides, during the early part of this record. Then recorded are a few waves a little larger than normal followed by several much larger waves. In many tsunami events the shoreline is pounded by repeated large waves.


Find it on

More from

Hand Lens
Hand Lens A 10-power folding magnifier in a metal case. A frequently used lab and field tool.
Gemstones of the World
Gemstones of the World - A book by Robert Schumann. Over one million copies sold.
gem photos
100+ Gems - Photos of over 100 beautiful gems ranging from the popular to the obscure.
Shale: The rock that is quickly transforming the energy industry.
Sliding Rocks
Sliding Rocks Mystery: What causes these rocks to slide across a Death Valley playa?
Ammolite is a fossil and a gemstone. It is shell material from fossil ammonites.
Difficult Rocks
Difficult Rocks - Elementary students find lots of rocks that you will not be able to identify.
Plate Tectoncs
Zoom in on Plate Boundaries: See the details of plate tectonics in satellite view.

Tsunami generation images by USGS.


Tsunami geology

Earthquake starts tsunami

Tsunami waves spread

Tsunami travel time map

Tsunami wave train
All images shown above are by USGS.

Diamonds Don't Form From Coal
Largest Oil Spills Map
San Andreas Fault
Igneous, Metamorphic, and Sedimentary Rocks
The Only Diamond Mine in the USA
Marcellus Shale
What is Geology?
Articles About Volcanoes

© 2005-2016 All Rights Reserved.
Images, code, and content on this website are property of and are protected by copyright law. does not grant permission for any use, republication, or redistribution.
Images, code and content owned by others are marked on the pages where they appear.