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Tsunami
The devastating impact
of seismic sea waves
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
How do EQ cause tsunami?
http://geology.com/articles/tsunami-geology.shtml
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
Tsunami wavelength
Long wavelengths (over 100 km)
Periods longer than 1 hour
316,800 ft = 60 miles
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
What happens when tsunami
gets near shore?
Tsunami slows down (shallower water)
Example: d = 100 m, v = 113 km/hr
Wave gets taller
λ gets shorter, T gets shorter
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
Tsunami run-up
Run-up = measurement of height of water
onshore observed above a reference sea level
Generally don’t get big gigantic wave
Water comes as a fast moving rise in tide that
rapidly moves inland
NOT JUST ONE WAVE…multiple 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
Energy in tsunami
Loss of energy in a wave is inversely
proportional to λ
Since λ very long, little energy lost
Waves can travel great distances and
still be very destructive
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 sea,
leaving shoreline empty with flopping
fish, boats, etc. on the bottom
Detecting a tsunami
Pressure recorder on bottom of ocean
Buoy to communicate readings via
satellite
Tsunami Warning Centers issue
warning
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
Tsunami Warning
When warning is issued – low lying areas are evacuated
U.S. Coast guard issues warnings over marine frequencies
Some places have sirens