Tsunamis
Geography: Edexcel A Level, topic 1.1.7
Cause of Tsunamis
Tsunamis are a big hazard for many places in the world. They are a secondary hazard following some earthquakes.
Cause
- Most tsunamis are generated by submarine earthquakes at subduction zones.
- The sea bed is displaced vertically (up or down) - this motion displaces a large volume of water in the ocean column which then moves outwards from the point of displacement.
Examples
- In 2004, the Boxing Day tsunami happened in Indonesia. The epicentre took place in Banda Aceh, western Sumatra. The tsunami was so large its devastating effects reached far beyond.
- A magnitude 9 earthquake off the coast of Tōhoku, Japan resulted in a tsunami in 2011.
Characteristics of Tsunamis
Tsunamis can be identified by their specific characteristics:
Long wavelength at sea
- The wavelength is the distance measured between two identical points of a wave.
- When tsunamis are at sea, the wavelength is often over 100 km long.
Short amplitude at sea
- Tsunamis have very short amplitudes at sea - they are usually about 1 m in height and are barely noticeable.
- But, when tsunamis reach land, their height can increase to over 25 m in some cases.
Velocity
- Tsunamis move very quickly and can reach speeds of up to 700 kph in deep water.
- Tsunamis slow down as they reach shallow water.
Drawback
- One of the identifying factors before a tsunami happens is a localised drop in sea level (drawback), as water is drawn back and up for use by the approaching tsunami.
Wave-train
- The way that a tsunami hits a coastline is like a series of waves that we call a wave-train.
Test yourself on Tsunamis
Which direction is the seabed displaced after a submarine earthquake?
When a tsunami happens, the seabed is displaced vertically.
What is this followed by?Which of the following is true of the Boxing Day tsunami?
What happens to the amplitude of a tsunami at the shore?
Which of the following are characteristics of tsunamis?