Tsunami Information, Tsunami Facts

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Tsunami

• The devastating impact of seismic sea waves

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Tsunami (harbor wave)
Seismic sea waves (NOT tidal waves)
Caused by processes that abruptly move large
volumes of ocean water
earthquake
submarine volcanic eruption
coastal/submarine landslide or rockfall
extraterrestrial impact
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How do EQ cause tsunami?
http//geology.com/articles/tsunami-geology.shtml
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Tsunami causes

• Unless there is an underwater landslide,
  strike-slip EQ WILL NOT cause tsunami
• Most tsunami generated by subduction zones

• Chile,
• Alaska,
• Japan,
• Cascadia,
• Philippines,
• New Zealand

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Tsunami wavelength

• Long wavelengths (over 100 km)
• Periods longer than 1 hour

316,800 ft 60 miles
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Tsunami wave speed

• Travel at high speeds 400 to 500 mph (200
  yards/sec)

• Alaska to CA 4 to 7 hrs
• Alaska to Hawaii 4 to 6 hrs
• Chile to Hawaii 14 to 15 hrs
• Chile to Japan 22 to 33 hrs

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Tsunami wave speed

• v speed
• g acceleration of gravity (9.8 m/sec2)
• d depth of ocean (m)
• deeper water means higher speed
• For d 4,600 m, v 763 km/hr
  (speed of jet plane)

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What happens when tsunami gets near shore?

• Tsunami slows down (shallower water)
• Example d 100 m, v 113 km/hr
• Wave gets taller
• ? gets shorter, T gets shorter

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Tsunami nears shore

• As wave gets into shallow water bottom of wave
  drags along ocean floor
• Top of wave still moving fast can cause cresting
  of wave, and breaking onto shore

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Tsunami run-up

• Run-up measurement of height of water onshore
  observed above a reference sea level
• Generally dont get big gigantic wave
• Water comes as a fast moving rise in tide that
  rapidly moves inland
• NOT JUST ONE WAVEmultiple waves coming in about
  ½ hour or so apart
• See tsunami wave simulator http//www.seed.slb.com
  /en/scictr/watch/living_planet/tsunami_wave/index.
  htm

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Energy in tsunami

• Loss of energy in a wave is inversely
  proportional to ?
• Since ? very long, little energy lost
• Waves can travel great distances and still be
  very distructive

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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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Tsunami
How could you evaluate the level of risk due to
tsunami?
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Possible tsunami run-up zones
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Seiche
standing wave

• Standing wave in an enclosed or partially
  enclosed body of water (similar to sloshing in a
  bath tub)

• Generated by wind or seismic activity
• Often swimming pools experience a seiche during EQ

http//earthquake.usgs.gov/learning/glossary.php?t
ermseiche
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Seiche potential in Lake Tahoe

• Scientists at UNR have determinsed the seiches
  have occurred on Lake Tahoe in the past
• Low probability on average once every 2000 to
  3000 years
• Good page on seiches w/great animation
  http//earthguide.ucsd.edu/earthguide/diagrams/wav
  es/swf/wave_seiche.html