4) Tsunamis

Tsunami

= the seismic energy transfer from the release of strain energy into a liquid medium
... creating more energy waves travelling through the ocean

how does a tsunami form?

1. tectonic impact
2. build
3. travel
4. approach
5. impact it has

1. tectonic

= the vertical displacement of the ocean floor along a fault line

- tectonic upthrust in the form of an earthquake and ocean floor volcanoes cause vast quantities of water to be displaced in short space of time

... generates massive amounts of energy

2. build

= the energy from the quake/impulse causes a series of simple, oscillating waves to be propelled over the ocean surface in increasingly widening circles at speeds as fast as 500mph

- in deep water, results in a wave that has a wavelength of 100km and a wave speed of 700km/h
... only a wave height or around 1m
+
is known as the energy wave

3. travel

= the wavelength of the tsunami continues to grow with the waves periods

... this varies from 5 minutes to more than an hour

(waves period = the length pf time for a successive crest and trough to pass through a single point)

4. approach

= as the tsunami waves approach the coastline of a landmass, they are slowed dramatically by friction of their collision with the rising seabed (shallower water)

- as velocity decreases. wavelength becomes shortened/compressed and amplitude (height) increases

... the base of the wave starts to slow down as water gets shallower which causes the wave to rush over it causing and the amplitude to increase

= 10-30m high wave

5. impact

= finally, with wavelength compressed and heightened to large levels, the giant waves collide with the shore and cause massive damage

- succeeding outflow of water then continues the destruction
... uproots trees, washes people and property away

always followed by at least 3 waves

main wave of tsunami is always followed by...

a series of waves
= never only 1 wave AT LEAST 3

origins of tsunamis

earthquakes = 72%
landslides = 10% (submarine debris slides)
volcanoes = 5% (submarine volcanoes)
atmosphere = 2%
other/unknown = 11% (meteors)

features of tsunamis

= generated by seismic activity (e.g., displacement of earths crust/ocean flood

- wave height is very low to begin with (1m) but can rise to around 30m when approaches shore

- wavelength is very long, 100-1000km

- travel quickly, speeds of 640-960km/h

- usually consist of series of waves with long wait times between them (10-60mins)

- on approach of coast, will slow down and pile up into a massive wall of water before breaking

- 90% of tsnusmis associated with seismicity along pacific ring of fire (along fault lines)

other causes of tsunamis

landslides
earthquakes
volcanic eruptions
meteors/asteroids

other causes of tsunamis - LANDSLIDES

= can occur on surface or complete underwater (submarine landslides)
... sea bottom collapses which displaces a large body of water

- waves then travel rapidly across the ocean until they reach shallower water and compress to increase amplitude of wave

... forms a wall of water

other causes of tsunamis - EARTHQUAKES

= fault movement shifts oceanic plate in seconds, causing large bodies of water to be displaced

other causes of tsunamis - VOLCANIC ERUPTIONS

= sides of volcano collapse during eruption and the landslide falls into ocean, causing a tsunami

other causes of tsunamis - METEORS/ASTEROIDS

= displace large bodies of water on impact

first warning sign of a tsunami

the apparent draining away of the sea in the from of the tsunami
= the "drawdown"

effects of tsunamis

sweep away victims
uproot vegetation
destroy buildings, boats and bridges

pacific tsunami warning system

hawaii
= an effective warning system which gives hours of warning of waves approaching following important or major seismic activity