Transcript tsunamis
Tsunamis What is a tsunami ? A tsunami is a very long ocean wave generated by sudden displacement of the sea floor or of the oceanic mass The displacement of an equivalent volume of water generates the tsunami Terminology The term “tsunami” is a Japanese word meaning “harbour wave” It was so named because the wave is harmless until it enters a harbour It is frequently called a “tidal wave”, but it has nothing to do with tides Hazards and risks of tsunamis Tsunamis can hit with little or no warning 4,000 people have been killed between 1990 and 2000 The most prone areas are those associated with earthquakes and volcanoes (mainly subduction zones) 1990-2000 26 December 2004: ¼ million fatalities Locally-generated tsunamis The subduction zone of Cascadia has potential for very large offshore quakes (M 8) There is a great danger of locally-generated tsunamis here, since they travel so fast Many large cities are found on the coast Structure of a wave Wavelength, , can exceed 200 km normal ocean waves have wavelengths of about 100 m trough; peak; wave height, h; amplitude From Murck et al. (1996) Velocities and energies Velocity = 3.132 x (water depth)½ where water depth is in meters and velocity is in meters/second (1 m/s = 3.6 km/hr) Wave energy h2 (approximately) Velocities in deep water Tsunamis travel very quickly relative to normal ocean waves This is particularly the case in open water, where velocities increase with water depth Velocities can reach 1,000 km/hr in open ocean (normal ocean wave: ~90 km/hr) Shallow water In shallow water, the tsunami waves pile up As a result, velocities and wavelengths decrease... …but at the same time, amplitudes can increase enormously... Amplitudes In deep water, wave amplitudes are generally less than 1 meter… …but in shallow water, amplitudes can reach 40 meters or more above normal sea level Arrival of a tsunami on a coast The wave will break when its height exceeds ~one seventh (1/7) of its wavelength… …so some very long waves actually may not break initially, there may be a rise or fall (drawdown) in sea level (which may attract people, to their great misfortune) Long wavelengths and the coast Due to its long wavelength, it may take a long time for a tsunami wave to crest The wave then may remain high for several minutes And it may take a while (hours) for the crests of successive waves to reach the shore…so don’t go surfing ! Wave runup - complicated This depends on several factors: water depth sea floor profile shape of coastline (focussing of energy, tsunamis travelling up rivers An example of wave focussing at Krakatau, 1883 Causes of tsunamis - all involve displacement of water Earthquakes Volcanic activity Landslides Meteorite impacts Earthquakes Mainly vertical crustal movements… …so strike-slip faults perhaps less hazardous… ...although these too can trigger mass movements such as landslides Types of faults Earthquakes In general, the larger the quake, the larger the tsunami…but not a perfect correlation Some anomalously large tsunamis generated from small quakes… ...energy released at longer periods than can be registered on normal seismometers ? Shallow quakes Quake energy seismic moment = slip x fault area x rigidity of rocks For a given quake magnitude, if displacement is large, then rigidity may be low This may indicate that the shallow parts of subduction zones are frictionally weak (unconsolidated sediments, fractures, fluids, etc.) Submarine landslides Another contributing factor to large tsunamis may be submarine landslides: -generated by shaking associated with the earthquake -cause additional displacement of water, thus a larger and more complicated tsunami event Subduction association Tsunamis typically are associated with earthquakes generated at subduction zones Rupture of sea floor surface Sediment slumps into subduction trench Volcanic activity Displacement of rock Submarine caldera collapse (e.g., along faults) (Krakatau 1883) Entrance of pyroclastic flows into water (Krakatau 1883) Subaerial lateral collapse, generating debris avalanches which enter water (Unzen 1792) Landslides Landslides often are generated by quakes or volcanoes also occur on subduction trench slopes (steep) also can occur in enclosed bodies of water (lakes, bays, reservoirs, etc.) (rockfalls, slumps of unconsolidated material, etc.) Landslides Enormous submarine landslides can occur on the flanks of ocean islands (e.g., Hawaii, Canaries) The wave washup can approach 400 meters in some cases Canary Islands Meteorite impacts Too terrible to contemplate !!! Hundreds to thousands of meters in height ? Terminal Cretaceous event Read and find out ! 4 case histories Alaska 1964 (earthquake-generated) Krakatau 1883 (caldera-generated) Unzen 1792 (landslide-generated) Grand Banks 1929 (submarine landslidegenerated 1964 Alaska quake and tsunami Prince William Sound epicenter Old Valdez 1964 events 27 March 1964, 5:36 PM local time (early evening, people in their homes) Magnitude 9.2 quake…largest ever recorded in North America…second largest ever Shaking lasted 4-5 minutes (to compare, the 1906 San Francisco event lasted 45-60 seconds Tectonic setting Subduction in the Aleutian region results in very large quakes Between 1899-1965: 7 quakes with M 8 60 quakes with M 7 Tsunami generation In this region, tsunamis are generated by two mechanisms: 1) large vertical movements of the sea floor along faults (local and distant tsunamis) 2) slumping of material, both underwater and from land to water, by ground shaking Nature of the 1964 tsunami 106 people were killed by the wave, 114 people total (consider the small coastal population of the area) The extensive ground deformation caused by the quake triggered tsunamis Destructive force of the wave Avalanches and landslides were generated Some of these generated locally damaging tsunamis The force of such a wave can be seen in this picture Boat runups Carried inland by tsunami waves, boats acted as battering rams, efficiently destroying buildings Here is a beached boat at Seward after the events Submarine sliding at Valdez, Seward, and Whittier These towns were built on unconsolidated sediments Seismic shaking ruptured petroleum storage tanks in these towns, causing fires The shaking also initiated submarine landslides, causing tsunami waves Effects at Valdez The landslides carried burning oil out into the bays… …while the tsunamis returned the burning oil to the harbours and townsites, exacerbating the fires Unconsolidated sediments Old and new Valdez Wave runup This is Valdez Inlet after the main tsunami hit Here the wave runup was the highest, reaching 67 meters At Kodiak, tsunami effects were made worse by tectonic subsidence (faulting) Wave runup Valdez It took 2-3 minutes to generate the tsunami from the landslide 30 people died $ 15 million US in damage Distant effects As you can see, the wave affected the entire Pacific basin The 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 The eruption of Krakatau 1883 Krakatau is a volcano located between Java and Sumatra It is mainly a submarine volcano, with its top sticking out of the water Krakatau Caldera collapse The cataclysmic eruption occurred on 26-27 August 1883 A submarine caldera was formed Displacement of material during collapse generated a series of devastating tsunamis Two views of the caldera margin on Rakata, one soon after the eruption and the other in 1979 This is Anak Krakatau, which emerged through the sea in 1928. It is within the caldera Tsunami 36,000 people were killed by the tsunami along the coasts of Java and Sumatra At least 3 great waves occurred 165 coastal villages were destroyed by the waves The largest waves were recorded by tide gauges up to 7,000 km away on the Arabian Peninsula Tsunami Coral blocks up to 600 tons were carried inland… …these served efficiently as natural battering rams Runup heights reached 40 meters Maximum runup heights in meters (from Simkin and Fiske, 1983) Telok Betong Telok Betong From Simkin and Fiske (1983) Before... …and after Shaded grey is submerged area red=boat yellow=buoy blue=hill buoy hill The District Hall in Telok Betong. The tsunami stopped just before this building, sparing the people cowering inside The hill near Telok Betong. The lower part of the hill has been cleansed of its vegetation by the tsunami Boat runup…the Berouw... This boat, named the Berouw, was carried 2.5 km inland at Telok Betong by the wave, which reached 24 m in height …and inland emplacement of its mooring buoy This is the Berouw’s mooring buoy, also carried inland It is now a visually pleasing monument overlooking Telok Betong From Simkin and Fiske (1983) Refraction diagram of the tsunami, showing transport times in minutes Krakatau 26 December 2004 earthquake and tsunami Magnitude 9.0-9.3 From Brumbaugh (1999) A warning to Indonesians: Kerry Sieh’s poster and efforts to educate people beforehand Plate tectonics of the eastern Indian Ocean region Courtesy USGS Tectonics and previous great earthquakes From Lay et al 2005, Science Cumulative energy from global seismicity From Lay et al 2005, Science Tsunami runups (blue) and maximum tsunami heights (black) in Sri Lanka From Liu et al 2005, Science Global propagation of the 26 December 2004 tsunami based on a model by Titov et al 2005 in Science Tsunami wave heights around the world (from Titov et al 2005 Science) Unzen volcano, Japan: 1792 collapse of Mt. Mayuyama In addition to its recent lava dome and pyroclastic flow activity (19901995), the volcano also has collapsed catastrophically in the past Mt. Mayuyama scar Pyroclastic debris, 1991-1995 islands The 21 May 1792 collapse A debris avalanche occurred from Mt. Mayuyama in 1792 about 1 month after lava stopped flowing from Fugen-dake (site of recent activity) The avalanche was triggered by two quakes Fugen-dake Mt. Mayuyama Tsunamis The debris avalanche entered the Ariake Sea, generating a tsunami The wave killed between 14,000 and 15,000 people in coastal communities Geological map, showing 1792 debris avalanche deposit The debris avalanche deposit From Siebert et al. (1987) Extent of the 1792 debris avalanche deposit and the scar on Mt. Mayuyama Note the islands An artist’s rendition of the 1792 events scar deposit New islands Before... …and after 18 November 1929 Grand Banks tsunami This tsunami was caused by a M 7.2 quake on the Grand Banks The quake triggered a submarine landslide which resulted in the tsunami 1: 1700 quake 1: 1700 quake 3: M9.5Chilean Chilean 3: M9.5 quake 1960 quake inin1960 4: 4: M9.2 M9.2Alaskan Alaskan quake in 1964 quake in 1964 2: 2:1929 1929Grand Grand Banks Banksquake quake The 1929 landslide The volume of the landslide was approximately 200 km3 (big !) It flowed at speeds up to 70 km/hr The flow cut 12 trans-Atlantic cables in 28 places The 1929 tsunami The height of the tsunami reached 5 meters in height The wave struck the south coast of the Burin Peninsula on Newfoundland Between 27 and 29 people drowned Tsunami hazards Extensive flooding Action of wave on coastal structures, both natural and built The incredible force of the waves can remobilize huge objects The event may create drawdown Effects of tsunami drawdown Release of dissolved gases (CH4, CO2, H2S) previously contained in shallow sediments Potential ignition of gases by their rapid expulsion As a result, a wave of noxious and burning gases may engulf people BEFORE the wall of water arrives Mitigation efforts Warning times Every ~750 km of travel distance is equal to about 1 hour of warning time So, as discussed above, there is very little warning time for tsunami generated by local sources, compared to those from distant sources Quake-generated tsunamis In general, the size of the quake is an approximate indication of the size of the tsunami But this guide doesn’t always work To determine the amount and orientation of crustal displacement at the surface, the moment magnitude is more useful than the Richter magnitude Moment magnitudes (fault slip) x (fault area) x (rigidity of rox) The point is that we cannot always rely on quake magnitude to determine the magnitude of the tsunami Hawaii is particularly vulnerable, being in the middle of the Pacific Warning systems Mainly based on earthquake data Pacific-wide warnings: require at least 1 hour warning time More local networks require warning times less than 1 hour…this is difficult A proposed system of real-time detectors Response to tsunami Requires good emergency planning and preparation… …an educated and trained public… …which has access to information… …so the dissemination of this info needs to be efficient and reliable Personal mitigation Run (don’t walk) to higher ground Tell your family and friends Never go to the beach to watch tsunamis Sign in the lobby of a Hawaiian hotel: IN CASE OF TSUNAMI: Remain calm Pay your bill Run like hell Hazard maps As we have seen for earthquakes and volcanoes, hazard maps are critically useful pieces of information Here are two examples, the first from Hawaii, and the second from Eureka, California Note inundation areas and arrows for evacuation centres Eureka, Calif. Eureka Eureka, California Located in northwestern California, and is part of Cascadia Hazards from tsunamis, liquefaction, ground shaking associated with liquefaction, etc. But don’t forget... Many areas and towns do not have such maps Tsunamis -reading Billings, L.G., 1915. Some personal experiences with earthquakes. National Geographic, v. 27, no. 1, January 1915, pp. 57-71. González, F.J., 1999. Tsunami! Scientific American, May, 1999. Niven, L., and J. Pournelle, 1983. Lucifer’s Hammer. New York, Fawcett Crest, 629 pp. Simkin, T., and R.S. Fiske, eds, 1983. Krakatau 1883, the volcanic eruption and its effects. Washington, D.C., Smithsonian Institution Press, pp. 69-81. Tsunamis - web Canada: http://atlas.nrcan.gc.ca/site/english/maps/environment/naturalhazards/natur alhazards1999/tsunamis http://www.pep.bc.ca/hazard_preparedness/Tsunami_Preparedness_Inform ation.html U.S.: http://www.ess.washington.edu/tsunami/index.html http://www.tsunami.noaa.gov/ U.K.: http://www.nerc-bas.ac.uk/tsunami-risks/