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Tsunamis. What is a tsunami ? zA tsunami is a very long ocean wave generated by sudden displacement of the sea floor or of the oceanic mass zThe displacement.

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Presentation on theme: "Tsunamis. What is a tsunami ? zA tsunami is a very long ocean wave generated by sudden displacement of the sea floor or of the oceanic mass zThe displacement."— Presentation transcript:

1 Tsunamis

2 What is a tsunami ? zA tsunami is a very long ocean wave generated by sudden displacement of the sea floor or of the oceanic mass zThe displacement of an equivalent volume of water generates the tsunami

3 Terminology zThe term “tsunami” is a Japanese word meaning “harbour wave” zIt was so named because the wave is harmless until it enters a harbour zIt is frequently called a “tidal wave”, but it has nothing to do with tides

4 Hazards and risks of tsunamis zTsunamis can hit with little or no warning z4,000 people have been killed between 1990 and 2000 zThe most prone areas are those associated with earthquakes and volcanoes (mainly subduction zones)

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6 26 December 2004: ¼ million fatalities

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8 Locally-generated tsunamis zThe subduction zone of Cascadia has potential for very large offshore quakes (M  8) zThere is a great danger of locally-generated tsunamis here, since they travel so fast zMany large cities are found on the coast

9 Structure of a wave zWavelength,, can exceed 200 km znormal ocean waves have wavelengths of about 100 m ztrough; peak; wave height, h; amplitude From Murck et al. (1996)

10 Velocities and energies zVelocity = x (water depth) ½ zwhere water depth is in meters and velocity is in meters/second (1 m/s = 3.6 km/hr) zWave energy  h 2 (approximately)

11 Velocities in deep water zTsunamis travel very quickly relative to normal ocean waves zThis is particularly the case in open water, where velocities increase with water depth zVelocities can reach 1,000 km/hr in open ocean (normal ocean wave: ~90 km/hr) zThus, velocities are about 10 times higher for tsunamis

12 Shallow water zIn shallow water, the tsunami waves pile up zAs a result, velocities and wavelengths decrease... z…but at the same time, amplitudes can increase enormously...

13 Amplitudes zIn deep water, wave amplitudes are generally less than 1 meter… z…but in shallow water, amplitudes can reach 40 meters or more above normal sea level

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15 Arrival of a tsunami on a coast zThe wave will break when its height exceeds ~one seventh (1/7) of its wavelength… z…so some very long waves actually may not break zinitially, there may be a rise or fall (drawdown) in sea level (which may attract people, to their great misfortune)

16 Long wavelengths and the coast zDue to its long wavelength, it may take a long time for a tsunami wave to crest zThe wave then may remain high for several minutes zAnd it may take a while (hours) for the crests of successive waves to reach the shore…so don’t go surfing !

17 Wave runup - complicated zThis depends on several factors: zwater depth zsea floor profile zshape of coastline (focussing of energy, tsunamis travelling up rivers An example of wave focussing at Krakatau, 1883

18 Causes of tsunamis - all involve displacement of water zEarthquakes zVolcanic activity zLandslides zMeteorite impacts

19 Earthquakes zMainly vertical crustal movements… z…so strike-slip faults perhaps less hazardous… z...although these too can trigger mass movements such as landslides

20 Types of faults

21 Earthquakes zIn general, the larger the quake, the larger the tsunami…but not a perfect correlation zSome anomalously large tsunamis generated from small quakes… z...energy released at longer periods than can be registered on normal seismometers ?

22 Shallow quakes zQuake energy  seismic moment = slip x fault area x rigidity of rocks zFor a given quake magnitude, if displacement is large, then rigidity may be low zThis may indicate that the shallow parts of subduction zones are frictionally weak (unconsolidated sediments, fractures, fluids, etc.)

23 Submarine landslides zAnother contributing factor to large tsunamis may be submarine landslides: z-generated by shaking associated with the earthquake z-cause additional displacement of water, thus a larger and more complicated tsunami event

24 Subduction association zTsunamis typically are associated with earthquakes generated at subduction zones zRupture of sea floor surface zSediment slumps into subduction trench

25 Volcanic activity zDisplacement of rock zSubmarine caldera collapse (e.g., along faults) (Krakatau 1883) zEntrance of pyroclastic flows into water (Krakatau 1883) zSubaerial lateral collapse, generating debris avalanches which enter water (Unzen 1792)

26 Landslides zLandslides often are generated by quakes or volcanoes zalso occur on subduction trench slopes (steep) zalso can occur in enclosed bodies of water (lakes, bays, reservoirs, etc.) (rockfalls, slumps of unconsolidated material, etc.)

27 Landslides zEnormous submarine landslides can occur on the flanks of ocean islands (e.g., Hawaii, Canaries) zThe wave washup can approach 400 meters in some cases

28 Canary Islands

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30 Meteorite impacts zToo terrible to contemplate !!! zHundreds to thousands of meters in height ? zTerminal Cretaceous event zRead and find out !

31 4 case histories zAlaska 1964 (earthquake-generated) zKrakatau 1883 (caldera-generated) zUnzen 1792 (landslide-generated) zGrand Banks 1929 (submarine landslide- generated

32 1964 Alaska quake and tsunami Prince William Sound

33 epicenter Old Valdez

34 1964 events z27 March 1964, 5:36 PM local time (early evening, people in their homes) zMagnitude 9.2 quake…largest ever recorded in North America…second largest ever zShaking lasted 4-5 minutes (to compare, the 1906 San Francisco event lasted seconds

35 Tectonic setting zSubduction in the Aleutian region results in very large quakes zBetween : z7 quakes with M  8 z60 quakes with M  7

36 Tsunami generation zIn this region, tsunamis are generated by two mechanisms: z1) large vertical movements of the sea floor along faults (local and distant tsunamis) z2) slumping of material, both underwater and from land to water, by ground shaking

37 Nature of the 1964 tsunami z106 people were killed by the wave, 114 people total (consider the small coastal population of the area) zThe extensive ground deformation caused by the quake triggered tsunamis

38 Destructive force of the wave zAvalanches and landslides were generated zSome of these generated locally damaging tsunamis zThe force of such a wave can be seen in this picture

39 Boat runups zCarried inland by tsunami waves, boats acted as battering rams, efficiently destroying buildings zHere is a beached boat at Seward after the events

40 Submarine sliding at Valdez, Seward, and Whittier zThese towns were built on unconsolidated sediments zSeismic shaking ruptured petroleum storage tanks in these towns, causing fires zThe shaking also initiated submarine landslides, causing tsunami waves

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42 Effects at Valdez zThe landslides carried burning oil out into the bays… z…while the tsunamis returned the burning oil to the harbours and townsites, exacerbating the fires Old and new Valdez Unconsolidated sediments

43 Wave runup zThis is Valdez Inlet after the main tsunami hit zHere the wave runup was the highest, reaching 67 meters zAt Kodiak, tsunami effects were made worse by tectonic subsidence (faulting) Wave runup

44 Valdez zIt took 2-3 minutes to generate the tsunami from the landslide z30 people died z$ 15 million US in damage

45 Distant effects zAs you can see, the wave affected the entire Pacific basin zThe tsunami was hugely destructive along the west coast of Canada and the US (but only 16 dead) Each colour band represents a 1-hour tsunami travel time increment

46 The eruption of Krakatau 1883 zKrakatau is a volcano located between Java and Sumatra zIt is mainly a submarine volcano, with its top sticking out of the water

47 Krakatau

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49 Caldera collapse zThe cataclysmic eruption occurred on August 1883 zA submarine caldera was formed zDisplacement of material during collapse generated a series of devastating tsunamis

50 Two views of the caldera margin on Rakata, one soon after the eruption and the other in 1979

51 This is Anak Krakatau, which emerged through the sea in It is within the caldera

52 Tsunami z36,000 people were killed by the tsunami along the coasts of Java and Sumatra zAt least 3 great waves occurred z165 coastal villages were destroyed by the waves zThe largest waves were recorded by tide gauges up to 7,000 km away on the Arabian Peninsula

53 Tsunami zCoral blocks up to 600 tons were carried inland… z…these served efficiently as natural battering rams zRunup heights reached 40 meters

54 Maximum runup heights in meters (from Simkin and Fiske, 1983) Telok Betong

55 buoy Shaded grey is submerged area red=boat yellow=buoy blue=hill Telok Betong From Simkin and Fiske (1983) Before... …and after hill

56 The District Hall in Telok Betong. The tsunami stopped just before this building, sparing the people cowering inside

57 The hill near Telok Betong. The lower part of the hill has been cleansed of its vegetation by the tsunami

58 Boat runup…the Berouw... zThis boat, named the Berouw, was carried 2.5 km inland at Telok Betong by the wave, which reached 24 m in height

59 …and inland emplacement of its mooring buoy zThis is the Berouw’s mooring buoy, also carried inland zIt is now a visually pleasing monument overlooking Telok Betong

60 Refraction diagram of the tsunami, showing transport times in minutes From Simkin and Fiske (1983) Krakatau

61 26 December 2004 earthquake and tsunami From Brumbaugh (1999) Magnitude

62 A warning to Indonesians: Kerry Sieh’s poster and efforts to educate people beforehand

63 Plate tectonics of the eastern Indian Ocean region Courtesy USGS

64 From Lay et al 2005, Science Tectonics and previous great earthquakes

65 From Lay et al 2005, Science Cumulative energy from global seismicity

66 From Liu et al 2005, Science Tsunami runups (blue) and maximum tsunami heights (black) in Sri Lanka

67 Global propagation of the 26 December 2004 tsunami based on a model by Titov et al 2005 in Science

68 Tsunami wave heights around the world (from Titov et al 2005 Science)

69 Unzen volcano, Japan: 1792 collapse of Mt. Mayuyama zIn addition to its recent lava dome and pyroclastic flow activity ( ), the volcano also has collapsed catastrophically in the past

70 Mt. Mayuyama scar islands Pyroclastic debris,

71 The 21 May 1792 collapse zA debris avalanche occurred from Mt. Mayuyama in 1792 about 1 month after lava stopped flowing from Fugen-dake (site of recent activity) zThe avalanche was triggered by two quakes Fugen-dake Mt. Mayuyama

72 Tsunamis zThe debris avalanche entered the Ariake Sea, generating a tsunami zThe wave killed between 14,000 and 15,000 people in coastal communities Geological map, showing 1792 debris avalanche deposit

73 The debris avalanche deposit zExtent of the 1792 debris avalanche deposit and the scar on Mt. Mayuyama zNote the islands From Siebert et al. (1987)

74 An artist’s rendition of the 1792 events Before... …and after scar deposit New islands

75 18 November 1929 Grand Banks tsunami zThis tsunami was caused by a M 7.2 quake on the Grand Banks zThe quake triggered a submarine landslide which resulted in the tsunami

76 1: 1700 quake 3: M9.5 Chilean quake in : M9.2 Alaskan quake in : 1929 Grand Banks quake 1: 1700 quake 3: M9.5 Chilean quake in : M9.2 Alaskan quake in : 1929 Grand Banks quake

77 The 1929 landslide zThe volume of the landslide was approximately 200 km 3 (big !) zIt flowed at speeds up to 70 km/hr zThe flow cut 12 trans-Atlantic cables in 28 places

78 The 1929 tsunami zThe height of the tsunami reached 5 meters in height zThe wave struck the south coast of the Burin Peninsula on Newfoundland zBetween 27 and 29 people drowned

79 Tsunami hazards zExtensive flooding zAction of wave on coastal structures, both natural and built zThe incredible force of the waves can remobilize huge objects zThe event may create drawdown

80 Effects of tsunami drawdown zRelease of dissolved gases (CH 4, CO 2, H 2 S) previously contained in shallow sediments zPotential ignition of gases by their rapid expulsion zAs a result, a wave of noxious and burning gases may engulf people BEFORE the wall of water arrives

81 Mitigation efforts

82 Warning times zEvery ~750 km of travel distance is equal to about 1 hour of warning time zSo, as discussed above, there is very little warning time for tsunami generated by local sources, compared to those from distant sources

83 Quake-generated tsunamis zIn general, the size of the quake is an approximate indication of the size of the tsunami zBut this guide doesn’t always work zTo determine the amount and orientation of crustal displacement at the surface, the moment magnitude is more useful than the Richter magnitude

84 Moment magnitudes z(fault slip) x (fault area) x (rigidity of rox) zThe point is that we cannot always rely on quake magnitude to determine the magnitude of the tsunami

85 Hawaii is particularly vulnerable, being in the middle of the Pacific

86 Warning systems zMainly based on earthquake data zPacific-wide warnings: require at least 1 hour warning time zMore local networks require warning times less than 1 hour…this is difficult

87 A proposed system of real-time detectors

88 Response to tsunami zRequires good emergency planning and preparation… z…an educated and trained public… z…which has access to information… z…so the dissemination of this info needs to be efficient and reliable

89 Personal mitigation zRun (don’t walk) to higher ground zTell your family and friends zNever go to the beach to watch tsunamis

90 Sign in the lobby of a Hawaiian hotel: zIN CASE OF TSUNAMI: yRemain calm yPay your bill yRun like hell

91 Hazard maps zAs we have seen for earthquakes and volcanoes, hazard maps are critically useful pieces of information zHere are two examples, the first from Hawaii, and the second from Eureka, California

92 Note inundation areas and arrows for evacuation centres

93 Eureka Eureka, Calif.

94 Eureka, California zLocated in northwestern California, and is part of Cascadia zHazards from tsunamis, liquefaction, ground shaking associated with liquefaction, etc.

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96 But don’t forget... zMany areas and towns do not have such maps

97 Tsunamis -reading zBillings, L.G., Some personal experiences with earthquakes. National Geographic, v. 27, no. 1, January 1915, pp zGonzález, F.J., Tsunami! Scientific American, May, zNiven, L., and J. Pournelle, Lucifer’s Hammer. New York, Fawcett Crest, 629 pp. zSimkin, T., and R.S. Fiske, eds, Krakatau 1883, the volcanic eruption and its effects. Washington, D.C., Smithsonian Institution Press, pp

98 Tsunamis - web zCanada: zhttp://atlas.nrcan.gc.ca/site/english/maps/environment/naturalhazards/natu ralhazards1999/tsunamishttp://atlas.nrcan.gc.ca/site/english/maps/environment/naturalhazards/natu ralhazards1999/tsunamis zhttp://www.pep.bc.ca/hazard_preparedness/Tsunami_Preparedness_Inform ation.htmlhttp://www.pep.bc.ca/hazard_preparedness/Tsunami_Preparedness_Inform ation.html zU.S.: zhttp://www.ess.washington.edu/tsunami/index.htmlhttp://www.ess.washington.edu/tsunami/index.html zhttp://www.tsunami.noaa.gov/http://www.tsunami.noaa.gov/ zU.K.: zhttp://www.nerc-bas.ac.uk/tsunami-risks/http://www.nerc-bas.ac.uk/tsunami-risks/


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