Tsunamis
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Presentation Transcript
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)
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) • Thus, velocities are about 10 times higher for tsunamis
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 An example of wave focussing at Krakatau, 1883 • This depends on several factors: • water depth • sea floor profile • shape of coastline (focussing of energy, tsunamis travelling up rivers
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
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
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 landslide-generated
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 Each colour band represents a 1-hour tsunami travel time increment • 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)
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
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
