The causes of tsunamis
The causes of tsunamis
Tsunami
•Tsunamis are secondary hazards
caused number by a number of primary
Example
hazards such as landslides,
earthquakes and even asteroid strikes.
•The most common cause of major
Max.
wave
height
Krakatoa eruption, 1883
35 m
Okushiri, Japan, 1993
11 m
Indian Ocean, 2004
15 m
translates as ‘harbour wave’
Samoa, 2009
14 m
•Tsunami are sometimes called tidal
Sendai, Japan, 2011
38 m
tsunamis are sub-marine
earthquakes (earthquakes under the
sea-bed)
•The word ‘tsunami’ is Japanese, and
waves, but in fact have nothing to do
with tides.
The causes of tsunamis
Tsunami triggers
•
Tsunamis occur when a force displaces a large volume of water
•
If you throw a stone in a pond, you create a mini version of concentric
tsunami waves
Earthquake
Volcanic
collapse
Landslide
Asteroid strike
The causes of tsunamis
Tsunamigenic earthquakes
•Not all earthquakes that occur under the sea
or ocean generate tsunamis
•Those that do are called ‘tsunamigenic’
meaning tsunami-causing
•To generate a tsunami, the earthquake has
to cause a vertical displacement of the sea
bed (see animation)
•This in turn displaces water upward, which
generates a tsunami at the ocean surface
•Horizontal displacements of the sea bed
(strike-slip faults) do not generate tsunami
The causes of tsunamis
Tsunami waves in open oceans
•
In deep ocean water, tsunami waves
cannot be seen with the naked eye
•
This is because the wave heights
are under 1 m, and often only a few
tens of centimetres
•
Tsunami waves have wave lengths
of 200 km crest to crest
•
Tsunami waves travel at speeds of
up to 800 km an hour
•
As the animation shows, a tsunami
originating in Hawaii takes
10-11 hours to reach Australia
Tsunami travel time.
Each segment = 1 hour
The causes of tsunamis
Shoaling waters
•Only at the coast do tsunamis reveal their true character.
•This is because ocean’s shallow as the coast is neared – this is called shoaling.
•As water depth decreases, friction between the tsunami wave and the sea bed
slows the wave down.
•As the wave slows, wavelength dramatically decreases but wave height
dramatically increases.
•This change produces a series of huge waves metres high:
The causes of tsunamis
More a flood than a wave
•
When tsunamis hit land the effect is much
more like a flood than a wave
•
The volume of water, and long
wavelength, mean that water is pushed
onshore continually and does not drain
away like the backwash of a normal wave
•
The dramatic footage of the 2011 Sendai
tsunami shows this very clearly (click on
the link)
•
Most tsunamis are not just one wave, but
several over a period of many minutes –
this is sometimes called a tsunami wavetrain
One of several videos
posted on YouTube by
Russia Today, showing the
2011 Japanese tsunami
coming ashore:
The causes of tsunamis
Run-up height
•Very large tsunami events such as the 2011 Japanese tsunami or the 2004
Indian Ocean tsunami can have run-up heights of 30 m or more
• This means a high-water level 30 m above average sea level
•If land at the coast is relatively flat, a tsunami can penetrate hundreds of
metres inland.
•In the case of the 2011 Japanese tsunami, some waves reached nearly 10 km
inland from the coast