Tsunamis Flashcards
What is a tsunami?
A tsunami is a very long ocean wave generated by sudden displacement of an equivalent volume of water on the sea floor
What causes a tsunami?
Anything which displaces water:
submarine earthquake
oceanic meteorite impact
submarine caldera collapse
pyroclastic flows entering water
landslide entering water
Tsunami Terminology
Japanese term for harbour wave, as it is only dangerous when it enters the harbour
The term tidal wave is wrong
Hazards of Tsunamis
Can hit with little to no warning
Quite common
Large tsunamis are extremely rare and unpredictable
The most prone areas are those associated with earthquakes and volcanoes, mainly in subduction zones
Structure of a wave
Crest: peak of the wave
Trough: Bottom of the wave
Amplitude: The still water line to the peak of the wave
Wave Height: Trough to the peak
Wavelength: The distance from one peak to the next
In open water, tsunamis have very large wavelength
Speed of a wave
To see how fast a wave is moving, we examine the time it takes to travel from point A to point B
Wave Period: How long it takes the wave to travel a distance
Speed of a Wave: Wavelength/Period
Tsunamis travel much faster in the open ocean than they do in shallow water
The ocean floor can also affect the speed
The Cascadia Subduction Zone
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 this coast
Tsunamis in the Open Ocean
Tsunamis in the open ocean have very large wavelengths, usually ranging from tens to hundreds of kilometers.
Their crests can undergo bending around higher seafloor topography, which can focus or defocus the wave
Distant vs. local Tsunami
Distant: A tsunami generated far away, there is plenty of time to warn people
Local: A tsunami generated in a subduction zone very close to where people live, little time to warn people (15-30 min)
Example: 2011 Tohoku Earthquake generated a local tsunami in Japan, but a distant tsunami for much of the west coast of North and South America.
Tsunamis in Shallow Water
In shallow water, tsunami waves are piled up and squished together.
As a result, velocities and wavelengths decrease.
At the same time, amplitudes increase enormously.
Tsunami Wave Arrival
Tsunamis break when the height of the wave exceeds roughly 1/7th of its wavelength.
Many tsunamis do not break.
This is because their wavelengths are so long that the height never approaches 1/7th of its wavelength.
Drawdown Effect
If the trough of the wave arrives first, the water recedes and creates the drawdown effect, which attracts people to the coastline (to their great misfortune)
If the crest arrives first, the water floods inland
How do long wavelengths impact the coast?
Because the wavelength is so long, it may take a long time for the wave to crest
The water hits the shoreline and keeps flooding inland, and only once the water crests does the water drawback
A tsunami is usually a series of waves, each wave comes in and then recedes
What factors influence wave run-up?
Water depth
Sea floor profile
Shape of coastline
What factors influence inland penetration of a wave?
Wave amplitude close to shore
The manning roughness coefficient, which varies according to the character of the land
Dense tree lines are best for stopping inland penetration
