Tsunamis

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Tsunamis
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What is a tsunami ?

• A tsunami is a very long ocean wave generated by
  sudden displacement of the sea floor or of the
  oceanic mass
• The displacement of an equivalent volume of water
  generates the tsunami

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Terminology

• The term tsunami is a Japanese word meaning
  harbour wave
• It was so named because the wave is harmless
  until it enters a harbour
• It is frequently called a tidal wave, but it
  has nothing to do with tides

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Hazards and risks of tsunamis

• Tsunamis can hit with little or no warning
• 4,000 people have been killed between 1990 and
  2000
• The most prone areas are those associated with
  earthquakes and volcanoes (mainly subduction
  zones)

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1990-2000
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26 December 2004 ¼ million
fatalities
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Locally-generated tsunamis

• The subduction zone of Cascadia has potential for
  very large offshore quakes (M ? 8)
• There is a great danger of locally-generated
  tsunamis here, since they travel so fast
• Many large cities are found on the coast

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Structure of a wave

• Wavelength, ?, can exceed 200 km
• normal ocean waves have wavelengths of about 100
  m
• trough peak wave height, h amplitude

From Murck et al. (1996)
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Velocities in deep water

• Tsunamis travel very quickly relative to normal
  ocean waves
• This is particularly the case in open water,
  where velocities increase with water depth
• Velocities can reach 1,000 km/hr in open ocean
  (normal ocean wave 90 km/hr)
• Thus, velocities are about 10 times higher for
  tsunamis

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Shallow water

• In shallow water, the tsunami waves pile up
• As a result, velocities and wavelengths
  decrease...
• but at the same time, amplitudes can increase
  enormously...

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Amplitudes

• In deep water, wave amplitudes are generally less
  than 1 meter
• but in shallow water, amplitudes can reach 40
  meters or more above normal sea level

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Arrival of a tsunami on a coast

• The wave will break when its height exceeds one
  seventh (1/7) of its wavelength
• so some very long waves actually may not break
• initially, there may be a rise or fall (drawdown)
  in sea level (which may attract people, to their
  great misfortune)

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Long wavelengths and the coast

• Due to its long wavelength, it may take a long
  time for a tsunami wave to crest
• The wave then may remain high for several minutes
• And it may take a while (hours) for the crests of
  successive waves to reach the shoreso dont go
  surfing !

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Wave runup - complicated
An example of wave focussing at Krakatau, 1883

• This depends on several factors
• water depth
• sea floor profile
• shape of coastline (focussing of energy, tsunamis
  travelling up rivers

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Causes of tsunamis - all involve displacement of
water

• Earthquakes
• Volcanic activity
• Landslides
• Meteorite impacts

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Earthquakes

• Mainly vertical crustal movements
• so strike-slip faults perhaps less hazardous
• ...although these too can trigger mass movements
  such as landslides

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Damage due to tsunami

• Waves often full of debris (trees, cars, pieces
  of wood etc.)
• As the wave recedes, the debris drags more stuff
  with it
• Can recede as much as a km out to see, leaving
  shoreline empty with flopping fish, boats, etc.
  on the bottom

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Detecting a tsunami

• Pressure recorder on bottom of ocean
• Buoy to communicate readings via satellite
• Tsunami Warning Centers issue warning

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Tsunami Warning Centers

• Hawaii and Alaska
• When EQ considered capable of generating tsunami,
  send warning with estimated arrival time

• Once tsunami hits somewhere, tsunami watch
  established to monitor tide gauges and ocean buoys

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Tsunami Warning

• When warning is issued low lying areas are
  evacuated
• U.S. Coast guard issues warnings over marine
  frequencies
• Some places have sirens

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Response to tsunami

• Requires good emergency planning and preparation
• an educated and trained public
• which has access to information
• so the dissemination of this info needs to be
  efficient and reliable

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Personal mitigation

• Run (dont walk) to higher ground
• Tell your family and friends
• Never go to the beach to watch tsunamis

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The End