BEIJING, March 11 (Xinhua) -- An 8.9-magnitude earthquake struck northeastern Japan on Friday afternoon, the largest temblor ever recorded by the Japanese Meteorological Agency. The earthquake triggered a tsunami that swamped hundreds of kilometers around the epicenter.
The following is a brief introduction of the relationship between earthquakes and tsunamis.
A tsunami is a series of destructive waves, sometimes tens of meters high, caused by the displacement of a large volume of water, usually an ocean. With gigantic energy and fast movement, the waves are catastrophic to the affected coastal areas.
Tsunamis are usually triggered by earthquakes, volcanic eruptions and underwater explosions, landslides and other mass movements. Underseas earthquakes have generated nearly all the major tsunamis in history.
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of temblor associated with the earth's crustal deformation.
When these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position.
However, undersea earthquakes do not necessarily lead to tsunamis.
Statistics from the China Earthquake Administration show that of the past 15,000 undersea tectonic earthquakes, only about 100 generated tsunamis. Some experts hold that only earthquakes of above 6.5 magnitude and with a focal depth of less than 25 km underground can cause tsunamis.
Sometimes even strong earthquakes, such as the 8.5-magnitude qukae that occurred near Sumatra in 2005, do not trigger tsunamis because the quake intensity can be largely compromised by the great focal depth, experts say.
In addition to the earthquake magnitude, global climate change may also have a bearing on the occurrence of tsunamis.
According to experts from the China Meteorological Administration, the 2004 tsunami that struck Southeast Asia was partially linked to the rising sea level caused by global climate change.