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Tsunami Basics

Tsunami Basics. "Know your enemy and know yourself and you can fight a hundred battles without disaster." Sun Tzu Chinese general and author, b.500 BC. Some “Known” Large Tsunami Events: Prehistoric (100 – 200 ka) - Hawaii Landslides 1600 BC Santorini Explosion

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Tsunami Basics

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  1. Tsunami Basics Page 1

  2. "Know your enemy and know yourself and you can fight a hundred battles without disaster." • Sun TzuChinese general and author, b.500 BC Page 2

  3. Page 3

  4. Some “Known” Large Tsunami Events: Prehistoric (100 – 200 ka) - Hawaii Landslides 1600 BC Santorini Explosion 1491 Mahuika Crater impact, New Zealand (est Mw > 9.0) 1700 Cascadia (est Mw > 9.0) 1755 Lisbon, Portugal ( Mw = 8.5, 90,000 dead) 1883 Krakatau (est 36,000 dead) 1896 Sanriku, Japan (Ms = 7.0, est 22,000 dead) 1960 Chile (Mw = 9.5, 500-2300 dead) 1964 Alaska (Mw = 9.2, 122 dead) 2004 Indonesia (Mw = 9.0, > 225,000 dead) See http://www.msu.edu/~fujita/earthquake/bigquake.html For historic earthquake data Page 4

  5. Outline: • Terminology • Waves in the Open Ocean • Exercise – Wave travel times • Tsunami Generation Mechanisms Page 5

  6. Terminology - : Amplitude Schematic of water wave Page 6

  7. Terminology: Deep Water Wave? > 1/20? Deep water waves are usually driven by surface shear forces. The energy goes into the water from the surface (like with wind) Page 7

  8. Terminology: Shallow Water Wave? < 1/20? Shallow water waves have energy distributed thoughout the water depth. They “feel” the bottom of the ocean. Page 8

  9. Other things that we often talk about with waves. • Long wave? • Small Amplitude Wave? • Answers to these and other questions about the waves we are studying lead us to decide on whether to use: • Linear shallow water wave theory • Non-linear shallow water wave theory • Finite amplitude wave theory • And other options… Page 9

  10. How do tsunamis differ from “day at the beach” waves? Order of magnitude estimates: Wind Waves Tsunamis Wavelength 10-1000 m 100 km Period 10 – 100 sec 10 min Ocean Amplitude 1-100 m 1 m Shore Amplitude 1-10 m 1-10 m * Water motion More near surface Entire column *except very near field events (100 m) Page 10

  11. A word about energy • The energy carried by a wave must come from somewhere. For example in wind waves it is the wind that transfers energy to the water by pushing on the water surface. • In terms of mechanical energy there are two main components: • Potential Energy: Raising a body of water up above its original level • And • Kinetic Energy: The energy of the moving water • If we keep in mind that all waves take energy to create, then the issue of tsunamis becomes somewhat more straightforward… Page 11

  12. A little exercise to amaze your friends: Calculate the propagation time (travel time) of a tsunami using only a calculator and a map. Recall: Tsunamis behave like shallow water waves. Their speed is governed by water depth. In fact, a good first order estimate is: c Page 12

  13. Krakatau - 1883 • Let’s estimate how long it took the wave to travel from Indonesia to Madagascar http://www.pbs.org/wgbh/nova/teachers/activities/3208_tsunami.html http://www.timeanddate.com/worldclock/distance.html • http://www.ngdc.noaa.gov/seg/hazard/tsu_travel_time.shtml Page 13

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  15. Now you try…. http://nctr.pmel.noaa.gov/education/science/docs/Alaska_travel_times.pdf http://nctr.pmel.noaa.gov/education/science/docs/Alaska_Travel_times.xls http://www.timeanddate.com/worldclock/distance.html Page 15

  16. Causes of Tsunamis: • Any impulse that causes large scale displacement of the sea surface. • Seismic Rupture • Landslide • Undersea Eruption or Explosion • Meteor Impact Page 16

  17. More on energy • Some estimates put the seismic energy released during the Sumatra 2004 tsunami at 500 million tons of TNT – or about 33,000 times the energy released by the bomb at Hiroshima. • Where did all that energy go? • Deforming a large part of the earth’s crust • Seismic waves (earthquake) • Tsunami waves - raising up (potential energy) and accelerating (kinetic energy) the water Page 17

  18. Magnitude 9.0 OFF THE WEST COAST OF NORTHERN SUMATRA Sunday, December 26, 2004 at 00:58:53 UTC Page 18

  19. Surface Projection of Slip Distribution Courtesy of Chen Ji, Caltech Page 19

  20. Projected Surface Displacements (by Chen Ji, Caltech) Page 20

  21. Tsunami Generation: • Efficiency of tsunami generation by crustal deformation: • Controlled by the speed at which the rupture propagates down the fault • Earthquakes with slow rupture velocities are the most efficient tsunami generators, “tsunami earthquakes”. • This capacity for tsunami generation is commonly characterized by a Tsunami Magnitude, Mt. Page 21

  22. Recall: Basic Concept of Seismology Seismic moment - measure of work done by earthquake. Mo = m A D where m = shear modulus of rock A = rupture area D = average displacement over rupture area Moment magnitude log Mo Mw = ------------ - 10.7 1.5 Page 22

  23. Tsunami Magnitude: • One formula for tsunami magnitude is: • Mt = log H2 + log X + 5.55 • Mt: Tsunami magnitude • H2: Maximum crest-to-trough amplitude on tide gage record in meters • X: Distance from epicenter to station along the shortest oceanic path in km • (Ref. Abe, K., Phys. Earth Planet. Inter., 27, 194-205, 1981) Page 23

  24. Tsunami Magnitude: • Some tsunami events with Mt = 9.0 and greater: • 1837 Valdivia, Chile 9.3 • 1841 Kamchatka 9.0 • 1868 Arica, Chile 9.0 • 1877 Iquique, Chile 9.0 • 1946 Aleutians 9.3 • 1952 Kamchatka 9.0 • 1957 Aleutians 9.0 • 1960 Chile 9.4 • 1964 Alaska 9.1 • 2004 Sumatra, Indonesia 9.0 • Ref: (Abe, K., J. Geophys. Res., 84, 1561-1568, 1979): Page 24

  25. Landslide generation of tsunamis • So if we understand that a tsunami is generated by an impulse that causes large scale displacement of the sea surface then it is easy to see that landslides can also generate tsunamis. Skagway, Alaska 1994 Page 25

  26. Skagway, Alaska 1994 Possible lanslide generated tsunami Page 26

  27. Example of landslide source computation Page 27

  28. What about Volcanoes? • So if we understand that a tsunami starts with a large scale displacement of the sea surface then it is easy to see that volcanoes can also generate tsunamis. The surface displacement can be caused by lava flows, sea floor motion or even the release of a large amount of gas. • http://www.volcanolive.com/tsunami10.html Page 28

  29. And what about Meteors? So if we understand that a tsunami is generated by an impulse that causes large scale displacement of the sea surface then it is easy to see that meteor/asteroid impacts can also generate tsunamis. Page 29

  30. Phases of a Tsunami Event: • Generation • Propagation • Shoaling • Inundation Page 30

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