The system is designed to rapidly detect the initiation of an earthquake, and it can determine how big the earthquake is and where it is located. The system can also issue a warning to people immediately through TV, short messages and emails. If quake­monitoring stations are about 30 km apart, the system can issue warning information about 10 seconds after a quake happens below ground.

　　As earthquakes cannot be predicted, getting people to safety quickly before the destructive S­waves arrive can decrease casualties and reduce damage. Experts estimate that casualties could be reduced by 14 percent with a three­second advance warning of an earthquake, and 39 percent with a 10­second warning. And the few seconds of warning can also help a train driver slow down to avoid possible derailment, and give a shutdown signal to chemical factories or nuclear plants to prevent potential leaks and fires.

　　But the system has its limitations. The S­waves generally move at a speed of 3.5 km per second, which means before the warning is issued, S­waves have already spread in a radius(半径)of 35 km from the epicenter. The area within the 35­km radius of the epicenter called the “blind area” cannot be warned. The system cannot play a role in small quakes, either.

　　 Unit 3

　　Ⅰ.1. A　考查被动语态。sounds和drop之间存在被动关系，应用被动语态形式。本句讲的是一般性的情况，应用一般现在时态。句意：在美国一些地方的英语口语中，词尾的“r”音省掉了。

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