海啸

巨大的海浪与风暴潮截然不同，人们通常称之为海啸。海啸一词起源于日语“港口满朝”。科学家们通常觉得把海啸称作地震海浪比较恰当，因为它是由海底地震运动引起的。

海啸是由于海床的突然移动造成的，比如水下发生地震或火山爆发时，而且移动陆地以上的海水会突然发生位移。所以水的突然变化产生了连续的海浪。这些海浪可以以每小时接近700公里的速度快速移动。在远洋海域中，这些海啸的规模还很小，通常移动距离不到一两米，只有当其移动到海岸附近撞击到浅海时，才会突然升高，海浪可以达到大约40米高。

海啸通常发生于太平洋等地震频繁活动地区。我们知道，太平洋有两个地区，日本和夏威夷，经常遭到海啸侵袭。日本之所以发生海啸，是因为该岛国附近的海床经常发生地震，而且这些海啸几乎没有预警，因此通常造成灾难性的后果。但是，夏威夷岛发生的海啸却可以提前发出警告，因为这些海啸发生于千里之外的阿拉斯加海岸，因此还有很长的距离才能到达夏威夷岛，所以那里的居民通常有时间知道海啸要来的警告。

Tsunamis

Quite different from storm surges are the giant sea waves called tsunamis, which derive their name from the Japanese expression for “high water in a harbor”. Scientists often refer to them as seismic sea waves, far more appropriate in that they do result from undersea seismic activity.

Tsunamis are caused when the sea bottom suddenly moves, during an underwater earthquake or volcano for example, and the water above the moving earth is suddenly displaced. This sudden shift of water sets off a series of waves. These waves can travel great distances at speeds close to 700 kilometers per hour. In the open ocean, tsunamis have little noticeable amplitude, often no more than one or two meters. It is when they hit the shallow waters near the coast that they increase in height, possibly up to 40 meters.

Tsunamis often occur in the Pacific because the pacific is an area of heavy seismic activity. Two areas of the Pacific well accustomed to the threat of tsunamis are Japan and Hawaii. Because the seismic activity that causes tsunamis in Japan often occurs on the ocean bottom quite close to the islands, the tsunamis that hit Japan often come with little warning and can therefore prove disastrous. Most of the tsunamis that hit the Hawaiian Islands, however, originate thousands of miles away near the coast of Alaska, so these tsunamis have a much greater distance to travel and the inhabitants of Hawaii generally have time for warning of their imminent arrival.