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, strikeslip EQ WILL NOT cause tsunami
 Most tsunami generated by subduction zones
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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)
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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
Tsunami wave speed
v = speed ~ gd
 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)
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 distructive
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
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
Tsunami
How could you evaluate the level of risk
due to tsunami?
Possible tsunami
“run-up” zones
standing wave
Seiche
 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?term=seiche
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/dia
grams/waves/swf/wave_seiche.html