Natural Hazards and Disasters
Chapter 5
Tsunami
Tsunami is a “harbor wave”
• Waves rise highest where focused in bays or
harbors
Tsunami have many causes
• Sudden displacement of large volume of water
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Earthquake
Volcanic eruptions
landslides or rockfalls
volcano flank collapses
asteroid impacts
San Francisco
Tsunami have two waves
Panel 1--Initiation: Near the source of
submarine earthquakes, the seafloor is
"permanently" uplifted and downdropped, pushing the entire water column
up and down. Pushing water above mean
sea level then drives horizontal
propagation of the tsunami wave.
Panel 2--Split: Initial tsunami split into a
tsunami that travels out to the deep ocean
(distant tsunami) and another tsunami that
travels towards the nearby coast (local
tsunami).
GLG110 Fall 2002 Arrowsmith
http://walrus.wr.usgs.gov/tsunami/basics.html
Life of a Tsunami—cont’d
Panel 3--Amplification: As the local
tsunami travels over the continental slope,
the amplitude increases and the
wavelength decreases. This results in
steepening of the leading wave--an
important control of wave runup at the
coast (next panel).
Panel 4--Runup: Most tsunamis do not
result in giant breaking waves. Rather,
they come in much like very strong and
very fast tides (i.e., a rapid, local rise in
sea level). Much of the damage inflicted
by tsunamis is caused by strong currents
and floating debris. After runup, part of the
tsunami energy is reflected back to the
open ocean.
GLG110 Fall 2002 Arrowsmith
Tsunami speed and height
• Initial wave is 1 meter tall but moves 500 mph
• Wave at shoreline is 10 m tall but moves 30 mph
• Waves heights range 8-40 feet (but can be 100 ft)
• Wave speed = square root of g * height
Tsunami speed and height
Tsunami speed and height
Earthquake-Generated Tsunami
Tsunami from Volcano Flank Collapse
Tsunami Movement
• Waves described by wavelength, height, period
• Tsunami in open ocean:
– Small wave heights
– Average wavelength about 360 kilometers
– Period may be half an hour
Tsunami Movement
• Typical waves:
– Driven by wind
– Particles travel in circular
motion that fades downward
– Velocity depends on
wavelength
– ‘Touch bottom’ at depths less
than half wavelength
• Tsunami waves:
– Enormously long wavelength
– ‘Touch bottom’ everywhere in
ocean
– Velocity depends on water
depth
– Slow down dramatically in
shallower water
Tsunami Movement
• Tsunami waves in open
ocean:
– Travel as fast as 870
kilometers per hour
– May slow to 150-300 km/hr
on continental shelf
• Tsunami waves in
shallower water:
– Front of wave slows first but
rear keeps coming
– Wave becomes much shorter
– Volume of wave remains
same
– Height must rise dramatically,
up to six times height in open
ocean
Tsunami travels rapidly.
Tsunami Warnings
Tsunami Warnings
Tsunami Warnings
Case In Point: Lituya Bay
Case In Point
Thailand Earthquake
• 2 hours later, giant wave surged
• More than 5000 people died in that area
In minutes 230,000 people lost their lives, and over 430,000 homes
were reduced to rubble
Over 100,000 fishing boats were destroyed. The
livelihoods of over 5 million people were thrown into jeopardy
More than 2,174 miles of roads destroyed. $10
billion in damages in barely 24 hours.
Tsunami Experiments
What to do in a Tsunami
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Be aware after large local earthquake
Obey Tsunami warnings
The first wave is not the biggest
Elevation is key: hills, upper floors, trees
Move inland, even if it is flat
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Serie…
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