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  3. Man has long wished to predict earthquakes. Recent findings indicate that reliable earthquake prediction is almost a reality. B

Man has long wished to predict earthquakes. Recent findings indicate that reliable earthquake prediction is almost a reality. B

admin2013-11-29  90
问题     Man has long wished to predict earthquakes.  Recent findings indicate that reliable earthquake prediction is almost a reality. Before earthquakes occur, there frequently are changes in rock behavior that affect the velocities of other earthquake waves passing through the rocks. The former Soviet seismologists have used earthquake waves from other unrelated earthquakes to meas- ure the alterations in wave speed through rocks around a fault zone. For months to years before a particular earthquake, the former Soviet scientists observed that the strained rock in the fault zone was deformed in a way that slowed other earthquake wave that passed through the zone. Similar behavior has preceded earthquakes in California and New York.
    Changes in electrical resistance, water pressure l rock motion, and leakage of gas also can accompany the lowering of wave velocity. Fractures in the fault zone apparently open, which leads to a lowering of water pressure. When the fractures are filled by underground water, the continuing stress on the rocks is also exerted on the water in pores, which contributes to pressure within the rocks and ultimately causes further fault movement and earthquakes.
    These preliminary events have been observed and studied for many earthquakes. The larger the earthquake, apparently the longer the time during which the preliminary events take place. Careful observation and measurement of the early events will precede reliable forecasts. The preliminary events are most evident along normal and reverse fault systems, and strike-slip faults may not produce the same effect. Although earthquake forecasting is in its infancy, both American and the former Soviet scientists have been able to predict the occurrence of a few earthquakes.
    Understanding of the causes of earthquakes has opened several possibilities for their control. Underground nuclear explosions in Nevada have released strain energy stored in certain rocks. In some instances, the shock wave from the explosion has raised the strain on nearby fractures and faults enough to initiate fault movement. All of the resulting earthquakes have been small, but a large earthquake could conceivably be initiated.
    In the future, a situation may arise where it is desirable to deliberately initiate an earthquake near a heavily populated area because too large an amount of strain has accumulated on an active fault zone in the vicinity. If hazardous areas were left empty and if emergency services were standing at the ready, such action might be considered necessary to prevent a later much more damaging earthquake. However, the legal, environmental, and human problems would be large in- deed, perhaps too great for such action to be taken.
    Another possibility for earthquake control is much more exciting. Increasing water pressures can initiate faulting, as was unintentionally demonstrated by a deep well at the U. S. Army’ s Rock Mountain Arsenal near Denver, Colorado, in the early 1960s. Disposal of nerve gas wastes in the well triggered movement along deeply buried inactive faults in the region. The liquid waste reduced frictional resistance along fault flat surface in the rocks surrounding the well, leading to movement along the faults. Earthquake activity in the area connected closely with the times of pumping of wastes into the disposal well, as was demonstrated convincingly by a Denver geologist, David Evans. Strain energy stored along the fault flat surface was apparently released by the fluid injection.
    Experiments by the U. S. Geological Survey in the Rangely oil field of northwestern Colorado have added to the experience gained from the study of the Denver earthquakes. The Survey geologists injected water in some of the Rangely wells, causing very small earthquakes. By withdrawing the water, the earthquakes were stopped.
    Although it is premature, many geologists believe we could eventually restrain earthquakes by injecting fluid into fault zones to permit slippage to take place gradually or in a series of small earthquakes. However, means must be found to control the areas affected and to be certain a major destructive quake is not initiated.
Which of the following titles best suits the passage?
选项 A、The Causes of Earthquakes
B、The Importance of Preliminary Events
C、Earthquake Experiments in the U. S.
D、Earthquake Forecasting and Control
答案D
解析
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