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Tsunami
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  1. Tsunami The devastating impact of seismic sea waves

  2. 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

  3. How do EQ cause tsunami? http://geology.com/articles/tsunami-geology.shtml

  4. 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

  5. Tsunami wavelength • Long wavelengths (over 100 km) • Periods longer than 1 hour 316,800 ft = 60 miles

  6. 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

  7. Tsunami wave speed v = speed~ • 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)

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. Detecting a tsunami • Pressure recorder on bottom of ocean • Buoy to communicate readings via satellite • Tsunami Warning Centers issue warning

  14. 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

  15. Tsunami Warning • When warning is issued – low lying areas are evacuated • U.S. Coast guard issues warnings over marine frequencies • Some places have sirens

  16. Tsunami How could you evaluate the level of risk due to tsunami?

  17. Possible tsunami “run-up” zones

  18. Seiche standing wave • 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

  19. 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/diagrams/waves/swf/wave_seiche.html