Lecture 15 - Tsunamis Flashcards
What does the word tsunami mean?
The word tsunami means ‘harbour wave’ in Japanese, so called because fishermen would return home to find their harbours destroyed, having seen no large waves while at sea. The word also emphasises the large heights these waves can reach in inlets and harbours as the bathymetry focuses the wave into a smaller space.
How are tsunamis created?
These giant waves are generated geologically, whenever a large volume of water is displaced by a sudden movement of rock (or ice).
What are examples of events that can cause tsunamis?
volcanic eruptions
landslides
meteorite impacts
earthquakes
What does the geographical reach of a tsunami depend on?
It depends upon the volume of water displaced.
For example a great earthquake that involves 1 m of sea-floor uplift over an area of 100 km × 100 km (displacing a water volume of 10 km³) could generate an ocean-wide tsunami, whereas a 1 km × 1 km × 100 m landslide (displacing just 0.1 km³) could produce a large local or regional tsunami but the waves would die off quickly with distance.
What is shoaling?
As a water wave propagates through a water body, individual molecules move in a prograde ellipse, the size of which decreases with increasing depth. As the wave enters shallow water, friction along the sea-floor causes it to slow down. The wavelength thus shortens, and to compensate, the wave height increases.
What happens when normal wind waves experience shoaling?
Normal wind waves, caused by shearing of the ocean surface by wind, have wavelengths of 100s of meters in the open ocean; shoaling in shallow waters reduces the wavelength to 10s of meters and increase wave heights to up to ∼15 m. However, the run up and retreat is confined to the beach, and there is no additional water mass behind the wave front.
What happens when tsunami waves experience shoaling?
Tsunami waves have wavelengths of up to 100s of kilometers in the open ocean, across which they travel at tremendous speed. As they move into shallow water they may be no taller than wind waves, but behind the leading edge is an elevated mass of water. The danger lies in their momentum: they run up and over the beach, flooding further inland and across populated areas.
How are tsunamis able to refract around islands?
Tsunamis have long wavelengths and periods.
How can bathymetry in the deep ocean affect waves?
Wave paths across the deep ocean are distorted by uneven bathymetry, causing focusing where waves are bent inwards and defocusing where they are bent outwards.
What is an example of bathymetry affecting tsunami waves?
Tsunami waves from South American megathrust earthquakes are focused towards Japan and away from Australia.
How many wave peaks can tsunamis consist of? By how long are they separated?
They can consist of up to ten wave peaks separated by 10–60 minutes.
Why are later waves are sometimes the deadliest?
The largest wave is often not the first to arrive and the later waves are sometimes the deadliest, as people assume the danger has passed.
When does ocean drawback occur?
When the first arriving wave is a trough.
What do run up heights depend on?
the nearshore bathymetry
the shape of the coastline
the onshore topography
Which locations are especially prone to devastation, regardless of the tsunami source, because of high run up heights?
Hilo, Hawaii
Port Alberni, Vancouver Island, Canada
