1.8 Making waves

Question 1

keychain of how a tsunami develops?

Answer 1

1. plate boundary
2. subduction zone
3. EQ
4. thrust fault - displacement
5. wave is fine until the sea-bed shelves
6. friction drags at bottom as bed shelves
7. wave frequency/ amplitude changes and accelerates towards lands
8. backwash out , swash in (wave crash)

Question 2

Tohoku Tsunami geographical facts

Answer 2

• 2011
• formed at the junction of the oceanic Pacific plate and the continental Okhotsk micro-plate (subduction zone)
• EQ 9.0 Mw at a shallow depth of 25km off the coast of Honshu Island
• fault thrusted to cause 50m horizontal and 35m vertical displacement
• wave height reaches 30-40m in places along Japanese coastline

Question 3

Tohoku damage

Answer 3

• 18,500 deaths
• 6150 injured
• $220 billion damage
• 127,500 houses destroyed
• 273,800 houses damaged

Question 4

What does the intensity of damage depend on?

Answer 4

• land use
• population density
• warning systems

Question 5

What does run-up depend on?

Answer 5

• distance travelled (energy is lost over long distance)
• offshore bathymetry (deeper water allows for waves to retain energy as more space to roll)
• coastline orientation (East Coast faced waves)
• extent of lowland
• shape of coastline (some esturies funnel waves)

Question 6

How do subduction zones occur?

Answer 6

less dense oceanic crust (Pacific Plate) descends under less dense continental crust (Okhotsk Micro Plate)

Question 7

What happens at subduction zones?

Answer 7

• melting temperatures lowered in nearby asthenosphere due to addition of sea water into ocean trench in which it crosses geothem and partial melt begins

Question 8

How are tsunamis initated?

Answer 8

• EQ causes the seabed to thrust upwards and tsunami’s waves are caused by the displacement of the water column
• formed by landslides (large scale movements can occur on continental shift) displaced water becomes tsunami waves

Question 9

How do tsunamis develop?

Answer 9

• waves are radiated outwards from the epicentre travelling at 800km per hr
• low amplitudes (heights) in deep water
• as waves move towards more shallow coastal water they begin to slow down due to the increased friction with seabed - 400km per hour which causes wavelengths to shorten as the following waves catch up and the amplitude to increase until a threshold when the wave breaks