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