Until a few decades1 ago, there were many theories2 about the cause of earthquakes, but none could explain why such regions  as California3 and Japan have earthquakes so often, while Sweden4 and Australia seldom do.
　　The upper part of the earth is divided into a number of plates5. They are about 48 km thick under land and about 1.6 km to 8 km thick beneath6 the ocean. The plates float7 on molten lava8 which move in a circular9 motion causing the plates to move.
　　The point at which two plates meet is called a fault10. The most famous fault in the world is in America's California. It is where the Pacific and North American plates meet.
　　Earthquakes happen when two plates grind11 and scrape12 against each other at a fault line.
　　Scientists estimate13 that there are 500,000 earthquakes in the world each year. Most of them are too small to be felt.
　　A measure14 called Richter Scale15 is usually used to tell the size of an earthquake. Each one-point increase on the scale is ten times stronger.
　　Although scientists can measure earthquakes, they can't predict16 them. Maybe, some animals can. Dogs, cats, snakes and   horses have been known to behave17 strangely just before an earthquakes.
　　If you are caught in an earthquake, drop to the floor and get under something for cover. If you smell gas and hear a hissing18 sound, get out immediately. A gas line might have broken. If you're outdoors, move away from buildings, streetlights and  wires—anything like that could fall on you.

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几十年前， 人们对于地震产生的原因众说纷纭，但没有一种理论能够解释清楚——为什么加利福尼亚和日本这些地区地震如此频繁，而瑞典和澳大利亚则很少发生地震？
　　地球的上层（即地壳）被分割成若干板块。在陆地下的板块厚度大约是48公里，在海洋下的板块厚度从1.6公里到8公里不等。这些板块漂浮在熔岩上面,而熔岩呈圆周状运动，并引起板块运动。
        板块相遇的地方叫断层。世界上最著名的断层在美国的加利福尼亚州。太平洋板块同北美洲板块就在这里相遇。
　　当两个板块在断层沿线互相磨擦时就会产生地震。
       科学家估计，地球上每年有50万次地震——但大多数地震都很微弱，人们感觉不到。
       人们常用“里氏震级”来衡量地震的强度。里氏震级每增加一级，地震强度就增加10倍。
　　虽然科学家能测出地震的强度，但却不能预报地震。也许，某些动物倒是可以报警的。狗、猫、蛇、马在地震即将来临时行为反常，这是人所共知的。
　　如果你遇上了地震，卧倒在地上，并躲在某个物体下面以作掩体。如果嗅到有煤气味或听到有咝咝声，应立即离开，因为可能是煤气管道破裂了。如果你在户外，应远离建筑物、路灯和电线等任何坍塌后可能压在你身上的东西。