Tsunamis: Violent Ocean
What is a Tsunami
Tsu = harbour, nami= waves
A wave of water that is generated by the sudden displacement of the seafloor or the disruption any body of standing water
Physical Characteristics of Tsunamis
Wavelength: distance between two identical points on a wave
Wave height: distance between the trough and the peak of the wave
Wave amplitude: refers to the height of the wave above the still waterline, usually half the wave height
Wave period: amount of time it takes for one full wavelength to pass through a stationary point
Wave velocity: the speed of the wave, wavelength divided by wave period
Tsunami vs. Wind-caused waves
Wind waves:
Entire mass of water being moved forms a single wave
Wave velocity depends on the period of the wave , usually 90km/hr
Wavelength of normal ocean waves is 100-200m
Tsunami waves:
A much bigger mass moving with tremendous motion
Wave velocity depends on gravitational acceleration and depth of water. Can reach up to 950km/hr
The wave height is only the leading edge of the sheet of water
Series of waves separated by 10-60 minutes, first wave is not most destructive
May comprise up to 12 large wave crests, with much longer wavelengths, from a few to 500 kilometres
The rate at which a wave loses its energy is inversely related to wave length
The Sequence of a Tsunami Landfall
When crest hits first, the sea level rises (run-up), usually expressed in metres above the high tide
When trough hits first, the sea level drops (drawdown) which is followed immediately by the crest, catching people off guard
Common triggers
When a mass of water is hit with a massive jolt of energy
Earthquakes, volcanoes, mass movements, impacts (secondary effect of other natural hazards)
Earthquake-Caused
Necessary condition: Submarine fault rupture that causes a vertical movement of the seafloor (uplifting/downdropping), offsetting huge mass of water
Usually with earthquakes 7-7.5 magnitude or higher
Must also have a shallow hypocentre so that seafloor gets disrupted more
Volcano-Caused
Common at convergent plate boundaries, especially subduction zones in Pacific Ocean
Do not occur at transform fault or divergent plate boundaries because horizontal motion and too small vertical motion
cause tsunamis by adding volcanic debris to ocean, or creating large blasts that lead to a flank collapse
The newly added material cause a sudden change in the seafloor topography that can produce a massive displacement of water
Landslide-caused
sudden addition of massive amounts of of material to the ocean can produce a tsunami
Closely related to: rockfalls, volcanic flank collapses, earthquakes triggering mass movements of aquatic sediment
Impact-caused
75% of earth is water, so an asteroid would probably hit the ocean, generating a massive tsunami
Primary and secondary effects
Primary: destructive nature of waves
Secondary: debris as projectiles, erosion, fires from disruption of gas lines, loss of crops and water and electrical systems
Disaster Mitigation
Seismographs, pressure sensors on seafloor
Construction of seawalls and breakwater structures to decrease momentum
Planting of pine tree belts to act as a filter
DART buoys read inconsistencies in pressure readings, and send it to a satellite
Coastal tide gauges: monitor unusual changes in normal tides