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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".
Image by USGS
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.
Image by USGS
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.
Image by USGS
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.
Image by USGS
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.
Image by USGS
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.
Image by USGS
The material above describes how tsunamis are generated
and how they travel rapidly across an ocean basin. For more
detailed information on this topic the following websites
are recommended.
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