AZUZ: Just after midnight on Tuesday, the earth jolted beneath the Gulf of Alaska. This is a Pacific body of water surrounded by southern Alaska.
The earthquake that hit there was powerful, magnitude 7.9. The U.S. Geological Survey says only about three quakes that strong hit the earth in an average year. And it was felt about 175 miles away in the southern Alaska city of Kodiak.
Residents described the tremor as a long slow roll. And though siren sounded and classes were cancelled so schools could serve as emergency shelters, no major damage was reported. Still, people in Kodiak evacuated their homes and headed for high ground.
The reason: a tsunami warning. Earthquakes at sea don't just bring the threat of shaking. This one brought fears of dangerous tsunami waves from the Gulf of Alaska to Hawaii, and the west coast of the U.S. and Canada.
Some people in California were warned to avoid the coastline for 12 hours. Thankfully, the only tsunami waves actually seen were in Alaska and they were small, less than a foot high. Meteorologists say the type of quake that occurred yesterday was the result of a strike-slip fault, when two plates move horizontally against each other. It's not believed to be as dangerous as a thrust fault, when the plate jolts vertically. That can produce major tsunami waves.
SUBTITLE: What is a tsunami?
JENNIFER GRAY, AMS METEOROLOGIST: Tsunami waves or seismic sea waves are a series of waves, not just one that are created when there's an underwater disturbance, like an earthquake or a volcanic eruption.
When an earthquake strikes, the erratic movement of the ocean floor can generate a tsunami. The first wave in this series can occur within minutes of the earthquake or hours. Areas that are closest to the epicenter will experience the waves much sooner than areas far away.
One warning sign of a tsunami is water right along the coast rapidly receding. If you see this or find yourself in a tsunami warned area, seek higher ground immediately and get as far away from the coast as you can. Make sure you stay in that safe place until the tsunami threat has ended.
阿拉斯加湾发生的强震震级为7.9级 。美国地质调查局表示，地球平均每年只发生三起如此强烈的地震 。距离震中约175英里的南阿拉斯加科迪亚克市有震感 。
居民描述称地震时晃动很慢，但是持续时间长 。当地拉响了警报，取消了课程，使学校可以当作应急避难所，目前没有重大损害事件发生 。不过，科迪亚克居民还是从家园撤离，前往高处 。
原因：海啸预警 。海上地震不只是带来震动威胁 。这起地震还引发了从阿拉斯加湾到夏威夷的危险海啸警报，另外美国西海岸和加拿大也可能受影响 。
加州一些民众收到警告，在12小时内不要前往海岸沿海地区 。幸运的是，阿拉斯加唯一一起海啸规模很小，高度不到1英尺 。气象学家表示，昨天发生的那种地震是由走滑断层引起的，走滑断层是两个板块平行移动时发生碰撞所产生的断层 。据信这种断层没有冲断层危险，冲断层指板块垂直震动所形成的断层 。这可能引发大海啸 。
当地震发生时，海底的不规则运动可能引发海啸 。这种系列波浪的第一次浪潮可能在地震发生几分钟或是几小时内出现 。离震中最近的地区经历浪潮的时间要远远早于远离震中的地区 。
海啸的其中一个警告信号是，沿海水位迅速下降 。如果你看到这种情况，或是发现你自己处于海啸警报区，你要马上寻找更高的地方，尽可能远离沿海地区 。确保你待在安全的地方，直到海啸威胁结束 。
1. describe as 把…说成；把…称为；认为；
例句：He described it as an extraordinarily tangled and complicated tale.
2. serve as 用作；可当…使用；充作；
例句：The museum will serve as a memorial to the millions who passed through Ellis Island.
3. a series of 一连串；一系列；连续；接连；
例句：The Jamaica Festival is planning a series of workshops and business seminars.
4. make sure 一定要；设法保证；
例句：Make sure you are strapped in very well, or else you will fall out.