The Physics of Tsunamis

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The Physics of Tsunamis

• Wayne M. Lawton
• Department of Mathematics
• National University of Singapore
• 2 Science Drive 2
• Singapore 117543

Email matwml_at_nus.edu.sg Tel [phone]
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Tsunamis in the Media
movie Impact U.S. President claims tsunami
speed 2sound is his claim real or
exaggerated ? what about this tsunami photo from
Phuket ?
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Hollywood Hype
tsunami speed
Physical Facts
Pacific Ocean 1-7 km http//www.es.flinders.edu.a
u/mattom/regoc/text/8topo.html Atlantic
Ocean 1-7 km http//www.es.flinders.edu.au/matto
m/regoc/text/14topo.html Indian Ocean 1-7
km http//www.es.flinders.edu.au/mattom/regoc/tex
t/11topo.html
356 lt speed (4 km) 713km/hour lt 943
speed of sound in air (0 C) 1192 km / hour
ocean depth needs to be 7 km 6.4 44.8 km
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Internet Assertion
Be sure and open the picture for a shock of your
life to see what the tsunami looked like just
before hitting Puket, Thailand. This picture is
not a fake. It appears to have been taken from a
hi-rise building window in downtown Phuket
Thailand. The power of nature is hard to
comprehend, especially the destructiveness of
water. We have all seen the pictures on TV but
I don't think any of us really understand how big
or how bad this wave was. This may be the most
impressive picture I've seen. It gives me a
better understanding of how 150,000 people
perished in this disaster. If you look at this
picture that was taken right before the wave hit,
it will send a chill down your spine. Just look
at the top of the wave compared to the top of the
building it is about to hit.
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Urban Legends
Origins   This is another photograph being
circulated as a picture of the tsunami that
struck Indian Ocean shorelines in Asia on 26
December 2004, this one purportedly taken from
atop a high-rise building in Phuket, Thailand.
This looks to be another case of any image
depicting large waves being grabbed and passed
around as a "real photograph" of the December
2004 tsunami. The image clearly doesn't fit its
accompanying description because It looks like
something other than a photograph, such as a
still frame from a movie or a composite image.
The waves that struck the coast of Thailand after
the December 2004 Indian Ocean earthquake were
about 4 meters in height, but the ones pictured
here look several stories tall. The skyline
depicted doesn't appear resemble the city of
Phuket. The traffic flow shown isn't right for
Thailand, where motorists drive on the left-hand
side of the road. As the operator of the World
City Photo Archive helpfully informed us, this
image is actually a manipulated depiction of the
skyline of Antofagasta, Chile
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Scientific Approach to Tsunamis
while lacking the entertainment appeal, provides
understanding that has proven both necessary and
useful for the prediction and control of natural
events
Why are earthquakes the major source of tsunamis
? Why do many earthquakes not cause tsunamis
? What is the surface geometry of tsunamis
? What is the water movement in tsunamis ? How is
energy transported in tsunamis ? How, after a
powerful earthquake, can a tsunami be detected
and how can its propagation be predicted ?
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Tsunamis
are physically generated by events that result
in the rapid and coherent (in the same
direction) motion of a massive amount of sea water
which of the following energy sources can
physically generate tsunamis ? sun heating,
whales, impacts from extraterrestrial objects
(meteorites, comets, asteroids), thermonuclear
explosions, storms, landslides, earthquakes,
tidal forces, evil spirits, volcanoes
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Energy of an Earthquake
http//www.seismo.unr.edu/ftp/pub/louie/class/100/
magnitude.html
log E(joules) 4.8 1.5Richter-Scale-Magnitude
1 J (joule) kg meter2 / second2 watt
second
One Ton (2000 lbs) TNT 4.148x109 J
Richter TNT for Seismic Example Magnitude Energy
Yield (approximate) -1.5 6 ounces Breaking a
rock on a lab table 1.0 30 pounds Large Blast at
a Construction Site 2.0 1 ton Large Quarry or
Mine Blast 4.0 1,000 tons Small Nuclear Weapon
4.5 5,100 tons Average Tornado (total energy)
5.0 32,000 tons 6.0 1 million tons Double
Spring Flat, NV Quake, 1994 7.0 32 million tons
Hyogo-Ken Nanbu, Japan Quake, 1995 Largest
Therm. Weapon 7.5 160 million tons Landers, CA
Quake, 1992 8.0 1 billion tons San Francisco, CA
Quake, 1906 8.5 5 billion tons Anchorage, AK
Quake, 1964 (THIS IS 2 X 1019 j) 9.0 32 billion
tons Chilean Quake, 1960 12.0 160 trillion tons
(Fault Earth in half through center, OR Earth's
daily solar energy)
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Physical Properties of Water
Water is incompressible, viscous, massive
which of these properties explains the following ?

• when the container is turned the water
• does not immediately turn with it, but
  eventually will start to turn as long as the
  container is turning

2. the water will fall when the bottom is not held
3. when the container is raised the water is
raised raises the same distance
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Energy Transmission
Earthquakes generate tsunamis when they cause
large water displacements, this requires that
they cause large movements of earth in direction
normal (perpendicular) to the earth-ocean
interface
which of the motions can generate tsunamis?
ocean
ocean
ocean
earth
earth
earth
earth
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Slow Rising of Ocean Floor
We consider a large area of ocean floor that
raises up
air
ocean
earth
if it moves very slowly it transmits energy from
the earthquake (seismic energy) into potential
energy of the water, equivalent to lifting a
certain volume of, water up to the sea level.
the potential energy remains as potential energy
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Fast Rising of Ocean Floor
air
ocean
earth
if it moves fast it transmits more energy from
the earthquake (seismic energy) into more
potential energy of the water, equivalent to
lifting a certain volume of water up to a height
above sea level
why does the earth do more work when it rises
fast ?
does the lifted water stay above the sea level ?
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Falling of Lifted Water
As gravity works to restore equilibrium to
the incompressible water the falling water lifts
water n its sides and that water falls in a
cascade
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Real Picture
wavelength
of course water near the edge is rounded, after
several fall-rise cycles the picture (from far
right side looks more like) at the bottom
direction of propagation
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Surface Geometry
wave height
graph of
direction of propagation
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Wave Velocity
depends on both wavelength and water depth
http//hyperphysics.phy-astr.gsu.edu/hbase/waves/w
atwav2.html
deep water - surface (wind generated) waves
shallow water - tsunami waves
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Surface Wave Parameters on the Deep Ocean
Amplitude maximum H(x), y 0 sea level
Period time one cycle of wave to go by
http//observe.arc.nasa.gov/nasa/exhibits/tsunami/
tsun_physics.html
Wind Waves A 10ft
300 ft
10-20mph
316800 ft
450-650mph
Tsunamis A 1.6ft
convert these parameter values into civilised
units !
what is ratio of periods of tsunamis to wind
waves ?
why are tsunamis hard to detect ON the deep ocean
?
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Water Movement in Tsunamis
Wind Waves water particles move in near circles,
whose diameters decrease rapidly with depth
Tsunamis water particles move in elongated
ellipses whose lengths decrease linearly to zero
at the bottom
http//electron4.phys.utk.edu/141/dec8/December20
8.htm
http//www.coastal.udel.edu/faculty/rad/linearplot
.html
http//mmu.jcu.edu.au/coastal/wavemaker.html
would tsunamis be easy to detect by scuba divers ?
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Energy Transport in Tsunamis
is through kinetic energy of horizontal water
motion in deep ocean, and in contrast to wind
waves
little viscosity loss since velocity changes
slowly
no dispersion since v does not depend on
wavelength
as wave approaches land amplitude increases to A
gt 100 ft, lambda 5000-10000ft, v 30-200mph
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Tsunami Detection and Prediction
how can tsunamis be detected in deep oceans ?
once detected how can their future propagation
be predicted rapidly to issue effective warnings
?
http//serc.carleton.edu/NAGTWorkshops/visualizati
on/collections/tsunami.html
http//wcatwc.arh.noaa.gov/