6. tsunamis

what initiates tsunamis

undersea earthquakes, landslides and volcanic eruptions

what charectarises tsunamis

• long wavelengths (typically 150 to 1000km)

• low amplitude (0.5 to 5m)

• fast velocities (up to 600km/h in deep water)

stages in tsunami formation

1. generation in deep water

2. tsunami run up

3. landfall

generation in deep water

• difficult to detect by ships in deep ocean due to low wave heights and wave lengths

• sea floor uplifts leading to displacement of water column above

tsunami run up

nature of the wave depends on

• cause of wave ie volcano or earthquake

• distance travelled from source (energy is lost as they travel)

• water depth over route travelled (energy is lost as friction)

• offshore topography and coastline orientation affects their impact

• seafloor irregularities can reflect wave energy (so less energy reaches coast)

landfall

• as coast shallows, friction with wave base slows it down which increases wave height (ie tsunamis 1m in deep ocean can reach 30m at coast)

• sequence of waves called a wave train

• first wave is not necessarily most destructive and there is usually a space of a few mins between the waves (wave period)

• in rare cases this can be an hour

• 40% wave energy is scattered back to sea, 60% expanded at or near coast

water column

area of sea water from the surface to sea floor

water displacement

when ocean floor rises and displaces water above it creating a tsunami wave

wave train

a series of waves created by a tsunami event

why does the water retreat

vacuum effect - water retreats from beaches and harbors

bathymetry

study of the underwater depth of an ocean