A tsunami (pronounced sue-nahm-ee) is a series of travelling ocean waves caused by a sudden disturbance of the ocean floor that displaces a large mass of water. The waves spread outward in all directions from the disturbance and can travel across entire ocean basins. The word tsunami comes from theJapanese word meaning "harbor wave."

Tsunamis are sometimes incorrectly called "tidal waves" tsunamis are not caused by the tides (tides are caused by the gravitational force of the moon on the sea). Regular waves are caused by the wind.

The Development of a Tsunami:
A tsunami starts when a huge volume of water is quickly shifted. This rapid movement can happen as the result of an underwater earthquake (when the sea floor quickly moves up or down), a rock slide, a volcanic eruption, or another high-energy event.


Tsunamis are caused mainly by earthquakes, volcanic eruptions and landslides which may occur under the sea (submarine) or close to the shore.  Tsunamis can also be caused by asteroids/meteorites.

After the huge volume of water has moved, the resulting wave is very long (the distance from crest to crest can be hundreds of miles long) but not very tall (roughly 3 feet tall). The wave propagates (spreads) across the sea in all directions; it can travel great distances from the source at tremendous speeds.

The Size of a Tsunami:
Tsunamis have an extremely long wavelength (wavelength is the distance between the crest (top) of one wave and the crest of the next wave) -- up to several hundred miles long. The period (the time between two successive waves) is also very long -- about an hour in deep water.

In the deep sea, a tsunami's height can be only about 1 m (3 feet) tall. Tsunamis are often barely visible when they are in the deep sea. This makes tsunami detection in the deep sea very difficult.

The Speed of a Tsunami:
A tsunami can travel at well over 970 kph (600 mph) in the open ocean - as fast as a jet flies. It can take only a few hours for a tsunami to travel across an entireocean. A regular wave (generated by the wind) travels at up to about 90 km/hr.

Warning Signs

  • Severe shaking of the ground
  • Sea withdraws an unusual distance away from the shore
  • Roaring sound from the ocean

Persons should run to higher ground if any of these warning signs are experienced.

Before a tsunami

  • Find out if your home is in a danger area
  • If you live in a low-lying area make yourself familiar with the quickest way to retreat to high ground. Make sure all family members know the evacuation plan
  • If you are close to the sea and the water retreats by an abnormal amount, move to high ground at once. Do not stay to see what happens
  • Listen to the radio for official updates and instructions
  • Have the telephone number for your Disaster Response Agency at hand
  • Gather disaster supplies: Flashlight and extra batteries; Portable, battery-operated radio and extra batteries; First Aid kit and manual; Emergency food and water; Cash and credit cards
  • Develop an emergency plan in the event that family members are separated (for example during the workday when adults are at work and children are at school). Agree on a close friend or relative that should be contacted if children cannot reach their parents and vice-versa

NOTE: If you see the water recede quickly and unexpectedly from a beach (this is called drawback), run toward higher ground or inland there may be a tsunami coming. Also, if you are on the coast and there is an earthquake, it may have caused a tsunami, so run toward higher ground or inland. Some beaches have tsunami warning sirens do not ignore them. The first wave in a tsunami is often not the largest; if you experience one abnormally-huge wave; go inland quickly even bigger waves could be coming soon.

During a tsunami

  • Do not wait for an official warning before evacuating; authorities may not have enough time to issue a tsunami Warning
  • If you are at the beach and recognize signs of a tsunami - such as a severe ground shaking, the sea pulling back significantly from the shoreline or an unusual roar from  the ocean leave the area and move immediately to higher ground
  • Sometimes tsunami may occur without the initial pulling pack of the sea. In this case, a massive wall of water may be seen approaching land. If you can see the wave you are already too close to outrun it
  • If you are unable to move to higher ground, go to an upper floor (at least the 3rd storey) or roof of a concrete and reinforced building. As a last resort, climb a strong tree if trapped on low ground
  • If swept up by a tsunami, look for something to use as a raft
  • A tsunami is not a single wave, but a series of waves that can come ashore for hours
  • The first wave may not be the largest. Stay out of danger areas until an "all-clear" is issued by a recognized authority e.g., your local disaster management office
  • If a tsunami Warning is issued NEVER go down to the beach to watch the waves come in
  • Do not try to surf the tsunami

After a tsunami

  • Stay tuned to a battery-operated radio for the latest emergency information
  • Help injured or trapped persons and persons requiring special assistance (infants, elderly people and persons with disabilities)
  • Do not move seriously injured persons unless they are in immediate danger of fatal injury. Call for medical assistance
  • Stay out of damaged buildings. Return home only when authorities say it is safe
  • Shovel mud while it is still moist to give walls and floors an opportunity to dry
  • Check for electrical shorts and live wires. Never attempt to move live wires
  • Check for gas leaks
  • Check for damage to sewage and water lines
  • Check food supplies and have tap water tested by the local health department
  • Fresh food that has come in contact with flood water may be contaminated and should be thrown out


Earthquake magnitude and tsunami generation

Although earthquake magnitude is one factor that affects tsunami generation, there are other important factors to consider. The earthquake must be a shallow marine event that displaces the seafloor. Thrust earthquakes (as opposed to strike slip) are far more likely to generate tsunamis, but small tsunamis have occurred in a few cases from large (i.e., > M8) strike-slip earthquakes.

Note the following general guidelines based on historical observations and in accordance with procedures of the Pacific Tsunami Warning Center.

Magnitudes below 6.5

Earthquakes of this magnitude are very unlikely to trigger a tsunami.

Magnitudes between 6.5 and 7.5

Earthquakes of this size do not usually produce destructive tsunamis. However, small sea level changes might be observed in the vicinity of the epicenter. Tsunamis capable of producing damage or casualties are rare in this magnitude range but have occurred due to secondary effects such as landslides or submarine slumps.

Magnitudes between 7.6 and 7.8

Earthquakes of this size might produce destructive tsunamis, especially near the epicenter. At greater distances, small sea level changes might be observed. Tsunamis capable of producing damage at great distances are rare in the magnitude range.

Magnitude 7.9 and greater

Destructive local tsunamis are possible near the epicenter, and significant sea level changes and damage might occur in a broader region. Note that with a magnitude 9.0 earthquake, there is a possibility of an aftershock of magnitude 7.5 or greater.

Source: https://www.usgs.gov/faqs/what-it-about-earthquake-causes-a-tsunami?qt-news_science_products=0#qt-news_science_products

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