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  3. Most earthquakes occur within the upper 15 miles of the earth’s surface. But earthquakes can and do occur at all depths to about

Most earthquakes occur within the upper 15 miles of the earth’s surface. But earthquakes can and do occur at all depths to about

admin2014-01-07  37
问题     Most earthquakes occur within the upper 15 miles of the earth’s surface. But earthquakes can and do occur at all depths to about 460 miles. Their number decreases as the depth increases. At about 460 miles one earthquake occurs only every few years. Near the surface earthquakes may run as high as 100 in a month, but the yearly average does not vary much. In comparison with the total number of earthquakes each year, the number of disastrous earthquakes is very small.
    The extent of the disaster in an earthquake depends on many factors. If you carefully build a toy house with an Erector set, it will still stand no matter how much you shake the table. But if you build a toy house with a pack of cards, a slight shake of the table will make it fall. An earthquake in Agadir, Morocco, was not strong enough to be recorded on distant instruments, but it completely destroyed the city. Many stronger earthquakes have done comparatively little damage. If a building is well constructed and built on solid ground, it will resist an earthquake. Most deaths in earthquakes have been due to faulty building construction or poor building sites. A third and very serious factor is panic. When people rush out into narrow streets, more deaths will result.
    The United Nations has played an important part in reducing the damage done by earthquakes. It has sent a team of experts to all countries known to be affected by earthquakes. Working with local geologists and engineers, the experts have studied the nature of the ground and the type of most practical building code for the local area. If followed, these suggestions will make disastrous earthquakes almost a thing of the past.
    There is one type of earthquake disaster that little can be done about. This is the disaster caused by seismic sea waves, or tsunamis.(These are often called tidal waves, but the name is incorrect. They have nothing to do with tides.)In certain areas, earthquakes take place beneath the sea. These submarine earthquakes sometimes give rise to seismic sea waves. The waves are not noticeable out at sea because of their long wave length. But when they roll into harbours, they pile up into walls of water 6 to 60 feet high. The Japanese call them "tsunamis", meaning "harbour waves", because they reach a sizable height only in harbours.
    Tsunamis travel fairly slowly, at speeds up to 500 miles an hour. An adequate warning system is in use to warn all shores likely to be reached by the waves. But this only enables people to leave the threatened shores for higher ground. There is no way to stop the oncoming wave.
The significance of the slow speed of tsunamis is that people may
选项 A、notice them out at sea.
B、find ways to stop them.
C、be warned early enough.
D、develop warning systems.
答案C
解析 文章最后一段提到，由于海啸移动比较缓慢，(警报系统)让人们得以离开受海啸威胁的地区并转移到高地上去，所以C为正确选项。此题容易误选D，海啸移动缓慢，使人们得以利用警报系统提前预报，但警报系统的开发本身并不是海啸移动缓慢的意义所在，所以D不正确。
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