تقرير عن تسونامي
A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over 100 feet (30.5 meters), onto land. These walls of water can cause widespread destruction when they crash ashore.
These awe-inspiring waves are typically caused by large, undersea earthquakes at tectonic plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly it displaces the water above it and launches the rolling waves that will become a tsunami.
Most tsunamis, about 80 percent, happen within the Pacific Ocean’s “Ring of Fire,” a geologically active area where tectonic shifts make volcanoes and earthquakes common.
Tsunamis may also be caused by underwater landslides or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging into an ocean.
Tsunamis race across the sea at up to 500 miles (805 kilometers) an hour—about as fast as a jet airplane. At that pace they can cross the entire expanse of the Pacific Ocean in less than a day. And their long wavelengths mean they lose very little energy along the way.
In deep ocean, tsunami waves may appear only a foot or so high. But as they approach shoreline and enter shallower water they slow down and begin to grow in energy and height. The tops of the waves move faster than their bottoms do, which causes them to rise precipitously.
A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. When it does, it produces a vacuum effect that sucks coastal water seaward and exposes harbor and sea floors. This retreating of sea water is an important warning sign of a tsunami, because the wave’s crest and its enormous volume of water typically hit shore five minutes or so later. Recognizing this phenomenon can save lives.
A tsunami is usually composed of a series of waves, called a wave train, so its destructive force may be compounded as successive waves reach shore. People experiencing a tsunami should remember that the danger may not have passed with the first wave and should await official word that it is safe to return to vulnerable locations.
tsunamis (killer waves)
PASADENA, Calif. – A wave of new NASA research on tsunamis has yielded an innovative method to improve existing tsunami warning systems, and a potentially groundbreaking new theory on the source of the December 2004 Indian Ocean tsunami.
In one study, published last fall in Geophysical Research Letters, researcher Y. Tony Song of NASA's Jet Propulsion Laboratory, Pasadena, Calif., demonstrated that real-time data from NASA's network of global positioning system (GPS) stations can detect ground motions preceding tsunamis and reliably estimate a tsunami's destructive potential within minutes, well before it reaches coastal areas. The method could lead to development of more reliable global tsunami warning systems, saving lives and reducing false alarms.
Conventional tsunami warning systems rely on estimates of an earthquake's magnitude to determine whether a large tsunami will be generated. Earthquake magnitude is not always a reliable indicator of tsunami potential, however. The 2004 Indian Ocean quake generated a huge tsunami, while the 2005 Nias (Indonesia) quake did not, even though both had almost the same magnitude from initial estimates. Between 2005 and 2007, five false tsunami alarms were issued worldwide. Such alarms have negative societal and economic effects.
Song's method estimates the energy an undersea earthquake transfers to the ocean to generate a tsunami by using data from coastal GPS stations near the epicenter. With these data, ocean floor displacements caused by the earthquake can be inferred. Tsunamis typically originate at undersea boundaries of tectonic plates near the edges of continents.
JPL is managed for NASA by the California Institute of Technology in Pasadena.
Media contact: Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
NASA Tsunami Research Makes Waves in Science Community
his is the California home of the Tsunami Research Center :
The TRC is actively involved with all aspects of tsunami research; inundation field surveys, numerical and analytical modeling, and hazard assessment, mitigation and planning.
The TRC has developed the tsunami inundation maps for California and the tsunami code MOST, now used by NOAA. MOST is the only validated code used in the US for tsunami hazard mapping with detailed inundation predictions. TRC faculty and students have surveyed all except one of the "modern" tsunamis since 1992, and have been working on mega-tsunami surveys for the 1946 Aleutian and 1956 Amorgos, Greece events. The TRC is a unit of the Department of Civil Engineering.
We acknowledge the support of the National Science Foundation (NSF), the Federal Emergency Management Agency (FEMA), the Governor's office of Emergency Services (OES) and the Earthquake Engineering Research Institute (EERI) that have made our field and modeling work possible. This site started in 1995 with the generous encouragement and support of NSF's Dr. Cliff Astill and is dedicated to showcasing the impact of recent tsunamis.
Please use our interactive map above to learn about past tsunamis by dragging the blue square on the left map and viewing details on the right. Click on the pulsating circles on the right detail map to learn about a specific location.
tsunami research center