1. Tsunamis

2. 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

3. 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

4. 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)

6. 26 December 2004: ¼ million fatalities

8. 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

9. 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)

10. 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

11. 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...

12. 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

14. 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)

15. 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 !

16. 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

17. Causes of tsunamis - all involve displacement of water
Earthquakes
Volcanic activity
Landslides
Meteorite impacts

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

19. 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

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

21. 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

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

23. 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

24. Personal mitigation Run (don’t walk) to higher ground
Tell your family and friends
Never go to the beach to watch tsunamis

25. The End