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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

admin2016-11-21  68
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. Another 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 harbors, they pile up into walls of water 6 to 60 feet high. The Japanese call them "tsunamis" , meaning "harbor waves" , because they reach a sizable height only in harbors.
    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 United Nations’ experts are supposed to______.
选项 A、construct strong buildings
B、put forward proposals
C、detect disastrous earthquakes
D、monitor earthquakes
答案B
解析 事实细节题。文章第三段提到联合国专家与当地地质学家和工程师一起研究地质构造，制定最实用的房屋建筑规范。一旦这些建议得以实施，毁灭性地震灾害几乎将成为历史。由此可见，联合国专家的任务是提建议，故答案为B。
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