Natural Hazards and Disasters Chapter 5 Tsunami

Tsunami is a “harbor wave”• Waves rise highest where focused in bays or

harbors

Tsunami have many causes• Sudden displacement of large volume of water

– Earthquake– Volcanic eruptions– landslides or rockfalls– volcano flank collapses– asteroid impacts

San Francisco

GLG110 Fall 2002 Arrowsmith

Tsunami have two waves

Panel 1--Initiation: Near the source of submarine earthquakes, the seafloor is "permanently" uplifted and down-dropped, pushing the entire water column up and down. Pushing water above mean sea level then drives horizontal propagation of the tsunami wave.Panel 2--Split: Initial tsunami split into a tsunami that travels out to the deep ocean (distant tsunami) and another tsunami that travels towards the nearby coast (local tsunami).

GLG110 Fall 2002 Arrowsmith

Life of a Tsunami—cont’dhttp://walrus.wr.usgs.gov/tsunami/basics.html

Panel 3--Amplification: As the local tsunami travels over the continental slope, the amplitude increases and the wavelength decreases. This results in steepening of the leading wave--an important control of wave runup at the coast (next panel).

Panel 4--Runup: Most tsunamis do not result in giant breaking waves. Rather, they come in much like very strong and very fast tides (i.e., a rapid, local rise in sea level). Much of the damage inflicted by tsunamis is caused by strong currents and floating debris. After runup, part of the tsunami energy is reflected back to the open ocean.

Tsunami speed and height• Initial wave is 1 meter tall but moves 500 mph• Wave at shoreline is 10 m tall but moves 30 mph

• Waves heights range 8-40 feet (but can be 100 ft)

• Wave speed = square root of g * height

Tsunami speed and height

Tsunami speed and height

Earthquake-Generated Tsunami

Tsunami from Volcano Flank Collapse

Tsunami Movement• Waves described by wavelength, height, period• Tsunami in open ocean:

– Small wave heights– Average wavelength about 360 kilometers– Period may be half an hour

Tsunami Movement• Typical waves:

– Driven by wind– Particles travel in circular

motion that fades downward– Velocity depends on

wavelength– ‘Touch bottom’ at depths less

than half wavelength• Tsunami waves:

– Enormously long wavelength– ‘Touch bottom’ everywhere in

ocean– Velocity depends on water

depth– Slow down dramatically in

shallower water

Tsunami Movement• Tsunami waves in open

ocean:– Travel as fast as 870

kilometers per hour– May slow to 150-300 km/hr

on continental shelf• Tsunami waves in

shallower water:– Front of wave slows first but

rear keeps coming– Wave becomes much shorter– Volume of wave remains

same– Height must rise dramatically,

up to six times height in open ocean

Tsunami travels rapidly.

Tsunami Warnings

Tsunami Warnings

Tsunami Warnings

Case In Point: Lituya Bay

Case In Point

Thailand Earthquake• 2 hours later, giant wave surged• More than 5000 people died in that area

In minutes 230,000 people lost their lives, and over 430,000 homes were reduced to rubble

Over 100,000 fishing boats were destroyed. Thelivelihoods of over 5 million people were thrown into jeopardy

More than 2,174 miles of roads destroyed. $10 billion in damages in barely 24 hours.

Tsunami Experiments

What to do in a Tsunami• Be aware after large local earthquake• Obey Tsunami warnings• The first wave is not the biggest• Elevation is key: hills, upper floors, trees• Move inland, even if it is flat

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