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.
The extent of the disaster in an earthquake depends on many factors. If you carefully build a toy house with an erect set (坚立架), 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 was not strong, 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.
There is one type of earthquake disaster that little can be done about. This is the disaster caused by seismic (地震的) sea waves, or tsunamis (海啸). These are often called tidal waves, but the name is incorrect. They had 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 (相当大的) height only in harbors.
Tsunamis travel fairly slowly, at speeds up to 500 miles an hour. An earthquake 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.
66. Which of the following can NOT be concluded from the passage?
A. The number of earthquakes is closely related to the depth.
B. Roughly the same number of earthquakes occur each year.
C. Earthquakes are impossible at depths over 460 miles.
D. Earthquakes are most likely to occur near the surface.
67. The destruction of Agadir is an example of .
A. faulty building construction
B. an earthquake’s strength
C. widespread panic in earthquakes
D. ineffective instruments
68. According to the passage, the waves caused by submarine earthquakes are not noticeable out at sea because of .
A. their high speed
B. the wide shores
C. their silent movements
D. their long wave length
69. The significance of the slow speed of tsunamis is that people may .
A. help reduce fear
B. find ways to stop them
C. be warned early enough
D. develop warning systems
70. According to the passage, the number of factors that may determine the extent of the disaster in an earthquake is .
A. two B. five C. four D. three