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Natural DisastersTsunami – The Great Wave

Aerial View of Japan Tsunami

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Alaska Tsunami 1946

•Aleutian Island, Alaska (1946)– 7.3M; traveled 659 km/hr, slowed to 47 km/hr in

Hilo– 135’ wave destroyed lighthouse, killed 5 crew

members; No warning sent to Hilo

Scotch Cap Lighthouse

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Alaska, 1946

• Hilo, Hawaii– Arrived 4.5 hrs. later– 150 deaths, 90 in Hilo

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What is a Tsunami?

• Abnormally long wavelength wave produced by sudden displacement of water in response to sudden fault movement

• Also called “seismic sea waves”

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Tsunami Characteristics

Wavelength

Wave Height

Frequency or Period

Velocity

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Wavelength

•Distance between two identical points of two successive waves

– Crest-to-crest or trough-to-trough•Normal ocean waves have wavelength ~100m•Tsunami is extremely long of ~500 km

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Wave Height

•Distance between crest and trough•Tsunami are merely 1 m height in the deep ocean

and may go undetected by boats and ships•As it approaches land (shallower water), the wave

slows and wave height may increase up to 30 m

Wave Height

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Period

•Amount of time for one full wavelength to pass a stationary point

•A normal wave’s period is ~5-20 seconds•A tsunami’s period is ~10 min. to 2 hours

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Velocity

•Speed of wave measured in distance per unit time; V = wavelength/period

•Normal waves travel ~90 km/hr (55 mph)•Tsunami waves travel up to ~890 km/hr (550 mph)

– Faster than a jet

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Tsunami Characteristics Summarized

• In the deep ocean, tsunami are almost imperceptible with 1-m height waves

•As wave approaches land (shallower water):– Velocity decreases– Wavelength decreases– Wave height increases – Energy stays the same

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Crest or Trough

• If crest approaches shore first, a large wave• If trough hits shore first, water recedes (drawdown)

– Followed immediately by crest

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What Causes a Tsunami?

• Anything that displaces a large volume of water suddenly

Meteor Impacts

Landslides

Volcanoes

Earthquakes

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Asteroid-Generated Tsunami

• Probability of a 1 km asteroid colliding with Earth is 1 in every 1,000,000 years

• Chances are extremely small, but event would be catastrophic

• Responsible for dinosaur extinction 65 million years ago

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Volcano-Generated Tsunami

• Explosions, pyroclastic flows, landslides and debris avalanches are produced by volcanic eruptions

• Large volume of water is rapidly displaced

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Volcano-Generated Tsunami

• Krakatau (1883)– 2/3 of island blown away– Generated 3 tsunami– >36,000 killed

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Volcano-Generated Tsunami• Canary Island (potential)

– 100,000-year recurrence interval– May reach east coast of N. American in 6-7

hours• May not be enough time to evacuate large cities

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Earthquake-Generated Tsunami

• Usually associated with normal or reverse fault movement– Water is displaced suddenly

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Earthquake-Generated Tsunami

• Chile, South America (1960)– 9.5M, largest EQ ever recorded– 3rd wave 11 m (30’) high; 1 hour period– 909 died; 834 missing– Tsunami Warning System in place

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Lessons from Chile

• Hawaii– 61 died

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Chile, 1960

• Japan– 181 deaths

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Landslide-Generated Tsunami

• As large mass of land falls into ocean, a huge volume of water is displaced

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Landslide-Generated Tsubnami

• Lituya Bay, Alaska (1958)– 150-m high (1700’) wave– Stripped vegetation– Ancient tree trim shows

previous occurrence

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Indonesia Tsunami, 2004

• Indonesia (2004)– 9.0M EQ; 8 minute duration– 15 m offset on thrust fault for 1,200 km

length– >283,000 deaths; >10,000 still missing

BeforeAfter

Banda Aceh

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Tsunami Dangers

Drowning

Severe abrasion by dragging

Thrown against solid objects

Carried out to sea in outgoing wave

Hit by debrisHouse, cars, trees, rocks

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Tsunami Hazard Mitigation

• Land Use Zoning

– Build to elevations above flood potential

– Structures engineered to resist erosion and scour

– Streets and buildings built perpendicular to shore

– vegetation

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The Pacific Tsunami Warning System

Two Steps1. Tsunami Watch

– Issued when an earthquake > 7.0M is detected in the Pacific Ocean

2. Tsunami Warning– Significant Tsunami is identified

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Tsunami Prediction

• Pressure sensor on ocean floor detects changes in wave height

• Transmit signal via satellite

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Surviving a Tsunami

1. If you feel an EQ when near the coast, get to high ground

2. Do not return to shore after initial wave3. Never go to the shore to watch a tsunami4. An unexpected rise or fall of sea level may

indicate an impending tsunami