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Natural Disasters

Natural Disasters. Tsunami – The Great Wave Aerial View of Japan Tsunami. 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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Natural Disasters

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  1. Natural Disasters Tsunami – The Great Wave Aerial View of Japan Tsunami

  2. 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

  3. Alaska, 1946 • Hilo, Hawaii • Arrived 4.5 hrs. later • 150 deaths, 90 in Hilo

  4. 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”

  5. Tsunami Characteristics Wavelength Wave Height Frequency or Period Velocity

  6. 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

  7. Wave Height 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

  8. 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

  9. 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

  10. 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

  11. Crest or Trough • If crest approaches shore first, a large wave • If trough hits shore first, water recedes (drawdown) • Followed immediately by crest

  12. What Causes a Tsunami? • Anything that displaces a large volume of water suddenly Meteor Impacts Landslides Volcanoes Earthquakes

  13. 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

  14. Volcano-Generated Tsunami • Explosions, pyroclastic flows, landslides and debris avalanches are produced by volcanic eruptions • Large volume of water is rapidly displaced

  15. Volcano-Generated Tsunami • Krakatau (1883) • 2/3 of island blown away • Generated 3 tsunami • >36,000 killed

  16. 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

  17. Earthquake-Generated Tsunami • Usually associated with normal or reverse fault movement • Water is displaced suddenly

  18. 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

  19. Lessons from Chile • Hawaii • 61 died

  20. Chile, 1960 • Japan • 181 deaths

  21. Landslide-Generated Tsunami • As large mass of land falls into ocean, a huge volume of water is displaced

  22. Landslide-Generated Tsubnami • Lituya Bay, Alaska (1958) • 150-m high (1700’) wave • Stripped vegetation • Ancient tree trim shows previous occurrence

  23. 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 Banda Aceh After Before

  24. Tsunami Dangers Drowning Severe abrasion by dragging Thrown against solid objects Carried out to sea in outgoing wave Hit by debris House, cars, trees, rocks

  25. 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

  26. The Pacific Tsunami Warning System Two Steps • Tsunami Watch • Issued when an earthquake > 7.0M is detected in the Pacific Ocean • Tsunami Warning • Significant Tsunami is identified

  27. Tsunami Prediction • Pressure sensor on ocean floor detects changes in wave height • Transmit signal via satellite

  28. Surviving a Tsunami • If you feel an EQ when near the coast, get to high ground • Do not return to shore after initial wave • Never go to the shore to watch a tsunami • An unexpected rise or fall of sea level may indicate an impending tsunami

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