What Is a Tsunami ?
A Japanese word that means harbor wave; a sea wave of local or distant origin that results from large-scale
seafloor displacements associated with large earthquakes, major submarine slides, or exploding volcanic
islands. Typically generated by seismic or volcanic activity or by underwater landslides.
- Not all earthquakes produce tsunamis. To generate a tsunami, an earthquake
must occur underneath
or near the ocean, be very large (approximately Richter magnitude 7 or greater), and create vertical
movement of the sea floor.
- However, recent studies regarding the potential for a great Cascadia Subduction zone
the Washington, Oregon, and Northern California coastlines indicate the local tsunami waves may
reach nearby coastal communities within minutes of the earthquake thereby giving little or no time
to issue warnings.
- A tsunami is not a single wave but a series of high-energy waves, that radiate outward like pond ripples,
also known as a wave train. The first wave in a tsunami is not necessarily the most destructive.
Tsunamis are not tidal waves
- Tsunami waves can be very long (as much as 60 miles, or 100 kilometers) and be as far as one hour
apart. They are able to cross entire oceans without great loss of energy. The Indian Ocean tsunami
traveled as much as 3,000 miles (nearly 5,000 kilometers) to Africa, arriving with sufficient force
to kill people and destroy property.
- Tsunamis are often barely visible when they are in the deep sea. This makes tsunami detection in
the deep sea very difficult.
- As a tsunami wave approaches the coast (where the sea becomes shallow), the trough (bottom) of a wave
hits the beach floor, causing the wave to slow down, to increase in height (the amplitude is magnified
many times) and to decrease in wavelength (the distance from crest to crest).
- At landfall, a tsunami wave can be hundreds of meters tall. Steeper shorelines produce higher tsunami waves.
- In addition to large tsunami waves that crash onto shore, the waves push a large amount of water onto
the shore above the regular sea level (this is called runup). The runup can cause tremendous damage
inland and is much more common than huge, thundering tsunami waves.
- A Tsunami can easily wrap around islands and be just as dangerous on coasts not facing the source of
- Some types of animals have an innate sixth sense which enables them to detect when a tsunami is
going to occur, they will then head inland in the opposite direction.
- About 80 per cent of all tsunamis take place in the Pacific Ocean.
- The Indian Ocean tsunami generated by the most powerful earthquake in decades on December 26 2004
is believed to have killed approx 300,000 people and made millions homeless.
How will I know if a Tsunami is coming ?
If you're near a coastal beach, here are ways to know a tsunami may be
imminent and you need to
seek higher ground:
- Seawater may recede quickly.
- The ground may shake, indicating an earthquake has occurred.
- A warning siren may sound, no early warning sirens UK
NOAA National Weather Service
NOAA National Ocean Service
NOAA National Tsunami Hazard Mitigration Program
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