地震是一种自然现象。
地震是一种自然现象，经常集中发生于地球上某些地区。
Earthquake is a phenomenon in nature which tends to cluster in some parts of the world.
地震是一种经常发生的自然现象，地表之下每天要发生几千次的震动，但大地震还是比较少见的。
Earthquakes are a common occurrence, rumbling below earth's surface thousands of times every day. But major earthquakes are less common.
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