A tsunami is a series of ocean waves that are generated by large movements or other disturbances on the ocean's floor. Such disturbances include volcanic eruptions, landslides and underwater explosions, but earthquakes are the most common cause. Tsunamis can occur close to the shore or travel thousands of miles if the disturbance occurs in the deep ocean.
Tsunamis are important to study because they are natural hazard that can occur at any time in coastal areas around the world. In an effort to gain a more complete understanding of tsunamis and generate stronger warning systems, there are monitors throughout the world's oceans to measure wave height and potential underwater disturbances. The Tsunami Warning System in the Pacific Ocean is one of the largest monitoring systems in the world and it is made up of 26 different countries and a series of monitors placed throughout the Pacific. The Pacific Tsunami Warning Center (PTWC) in Honolulu, Hawaii collects and processes data gathered from these monitors and provides warnings throughout the Pacific Basin.
Causes of Tsunamis
Tsunamis are also called a seismic sea waves because they are most commonly caused by earthquakes. Because tsunamis are caused mainly by earthquakes, they are most common in the Pacific Ocean's Ring of Fire - the margins of the Pacific with many plate tectonic boundaries and faults that are capable of producing large earthquakes and volcanic eruptions.
In order for an earthquake to cause a tsunami, it must occur below the ocean's surface or near the ocean and be a magnitude large enough to cause disturbances on the sea floor. Once the earthquake or other underwater disturbance occurs, the water surrounding the disturbance is displaced and radiates away from the initial source of the disturbance (i.e. the epicenter in an earthquake) in a series of fast moving waves.
Not all earthquakes or underwater disturbances cause tsunamis - they must be large enough to move a significant amount of material. In addition, in the case of an earthquake, its magnitude, depth, water depth and the speed at which the material moves all factor into whether or not a tsunami is generated.
Once a tsunami is generated, it can travel thousands of miles at speeds of up to 500 miles per hour (805 km per hour). If a tsunami is generated in the deep ocean, the waves radiate out from the source of the disturbance and move toward land on all sides. These waves usually have a large wavelength and a short wave height so they are not easily recognized by the human eye in these regions.
As the tsunami moves toward shore and the ocean's depth decreases, its speed slows quickly and the waves begin to grow in height as the wavelength decreases (diagram) This is called amplification and it is when the tsunami is the most visible. As the tsunami reaches the shore, the trough of the wave hits first which appears as a very low tide. This is a warning that a tsunami is imminent. Following the trough, the peak of the tsunami comes ashore. The waves hit the land like a strong, fast tide, instead of a giant wave. Giant waves only occur if the tsunami is very large. This is called runup and it is when the most flooding and damage from the tsunami occurs as the waters often travel farther inland than normal waves would.
Tsunami Watch versus Warning
Because tsunamis are not easily seen until they are close to shore, researchers and emergency managers rely on monitors that are located throughout the oceans that track slight changes in the height of waves. Whenever there is an earthquake with a magnitude greater than 7.5 in the Pacific Ocean, a Tsunami Watch is automatically declared by the PTWC if it was in a region capable of producing a tsunami.
Once a tsunami watch is issued, PTWC watches tide monitors in the ocean to determine whether or not a tsunami was generated. If a tsunami is generated, a Tsunami Warning is issued and coastal areas are evacuated. In the case of deep ocean tsunamis, the public is normally given time to evacuate, but if it is a locally generated tsunami, a Tsunami Warning is automatically issued and people should immediately evacuate coastal areas.
Large Tsunamis and Earthquakes
Tsunamis occur all over the world and they cannot be predicted since earthquakes and other underwater disturbances occur without warning. The only tsunami prediction possible is the monitoring of waves after the earthquake has already happened. In addition, scientists today know where tsunamis are most likely to occur to due to large events in the past.
Most recently in March 2011 a magnitude 9.0 earthquake struck near the coast of Sendai, Japan and generated a tsunami that devastated that region and caused damage thousands of miles away in Hawaii and the west coast of the United States.
In December 2004, a major earthquake struck near the coast of Sumatra, Indonesia and generated a tsunami that damaged countries all over the Indian Ocean. In April 1946 a magnitude 8.1 earthquake struck near Alaska's Aleutian Islands and generated a tsunami that destroyed much of Hilo, Hawaii thousands of miles away. The PTWC was created in 1949 as a result.
To learn more about tsunamis, visit the National Oceanic and Atmospheric Administration's Tsunami Website and "Prepare for Tsunami" on this website.
National Weather Service. (n.d.). Tsunami: The Great Waves. Retrieved from: http://www.weather.gov/om/brochures/tsunami.htm
Natural Hazards Hawaii. (n.d.). "Understanding the Difference Between a Tsunami 'Watch' and 'Warning'." University of Hawaii at Hilo. Retrieved from: http://www.uhh.hawaii.edu/~nat_haz/tsunamis/watchvwarning.php
United States Geological Survey. (22 October 2008). Life of a Tsunami. Retrieved from: http://walrus.wr.usgs.gov/tsunami/basics.html
Wikipedia.org. (28 March 2011). Tsunami - Wikipedia, the Free Encyclopedia. Retrieved from: http://en.wikipedia.org/wiki/tsunami