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Tsunami Strikes Sri Lanka
A tsunami (tidal wave) is actually a series of water waves(tsunami wave train) caused by the displacement of a large volume of water. It usually happens in an ocean(80% in the Pacific Ocean), but can occur in large lakes. The immense volumes of water and energy involved can devastate coastal regions. There are many possible causes for a tsunami: earthquakes, volcanic eruptions, landslides, impact events, or the underwater detonation of a nuclear device.
Common waves have a crest to crest wavelength of about 100 meters and a height of roughly 2 meters. A tsunami may have a wavelength of about 200 kilometers and travels at speeds in excess of 800 kilometers per hour, but has an amplitude of only about 1 meter in the open ocean. Shipboard passengers rarely notice their passage. When the tsunami approaches the coast and the waters become shallow its velocity slows below 80 km. Its wavelength drops under 20 km, but its height grows substantially. A tsunami wave does not ‘break’ on the shore, but will continue across the land. When the tsunami’s wave peak reaches the shore, the temporary rise in sea level is called run up. Run up is measured in meters above a known sea level. A large tsunami may feature multiple waves arriving over a period of hours. The first wave to reach the shore may not have the highest run up.
A tsunami cannot be accurately predicted, even if the magnitude and location of an earthquake is known. Scientists analyze each earthquake and, based on many factors, may issue a tsunami warning. There are some warning signs of an impending tsunami, and automated systems can provide warnings in time to save lives. One of the most successful systems uses bottom pressure sensors that are attached to buoys. The sensors constantly monitor the pressure of the overlying water column. The Pacific Tsunami Warning System monitors Pacific Ocean seismic activity. A large earthquake magnitude and other information triggers a tsunami warning. While the subduction zones around the Pacific are seismically active, not all earthquakes generate tsunami. Computers assist in analyzing the tsunami risk of every earthquake that occurs in the Pacific Ocean and the adjoining land masses. While no warning system is sure-fire, the ones that are in place help save hundreds of life.
We have written many articles about tsunami for Universe Today. Here’s an article about the causes of tsunamis, and here’s an article about the tsunamis predicted after the 8.8 magnitude earthquake in Chile.
If you’d like more info on tsunami, check out Visible Earth Homepage. And here’s a link to NASA’s Earth Observatory.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.
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