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 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

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

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

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

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

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

The End