2. 外语
  3. In 1971 researchers hoping to predict earthquakes in the short term by identifying precursory phenomena (those that occur a few

In 1971 researchers hoping to predict earthquakes in the short term by identifying precursory phenomena (those that occur a few

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问题 In 1971 researchers hoping to predict earthquakes in the short term by identifying precursory phenomena (those that occur a few days before large quakes but not otherwise) turned their attention to changes in seismic waves that had been detected prior to earthquakes. An explanation for such changes was offered by "dilatancy theory," based on a well-known phenomenon observed in rocks in the laboratory: as stress builds, microfractures in rock close, decreasing the rock’s volume. But as stress continues to increase, the rock begins to crack and expand in volume, allowing groundwater to seep in, weakening the rock. According to this theory, such effects could lead to several precursory phenomena in the field, including a change in the velocity of seismic waves, and an increase in small, nearby tremors.
Researchers initially reported success in identifying these possible precursors, but subsequent analyses of their data proved disheartening. Seismic waves with unusual velocities were recorded before some earthquakes, but while the historical record confirms that most large earthquakes are preceded by minor tremors, these foreshocks indicate nothing about the magnitude of an impending quake and are indistinguishable from other minor tremors that occur without large earthquakes.
In the 1980s, some researchers turned their efforts from short-term to long-term prediction. Noting that earthquakes tend to occur repeatedly in certain regions, Lindh and Baker attempted to identify patterns of recurrence, or earthquake cycles, on which to base predictions. In a study of earthquake-prone sites along the San Andreas Fault, they determined that quakes occurred at intervals of approximately 22 years near one site and concluded that there was a 95 percent probability of an earthquake in that area by 1992. The earthquake did not occur within the time frame predicted, however.
Evidence against the kind of regular earthquake cycles that Lindh and Baker tried to establish has come from a relatively new field, paleoseismology. Paleoseismologists have unearthed and dated geological features such as fault scarps that were caused by earthquakes thousands of years ago. They have determined that the average interval between ten earthquakes that took place at one site along the San Andreas Fault in the past two millennia was 132 years, but individual intervals ranged greatly, from 44 to 332 years.
The passage is primarily concerned with
选项 A、explaining why one method of earthquake prediction has proven more practicable than an alternative method
B、suggesting that accurate earthquake forecasting must combine elements of long-term and short-term prediction
C、challenging the usefulness of dilatancy theory for explaining the occurrence of precursory phenomena
D、discussing the deficiency of two methods by which researchers have attempted to predict the occurrence of earthquakes
E、describing the development of methods for establishing patterns in the occurrence of past earthquakes
答案D
解析 Main idea
To answer this question, focus on what the passage as a whole is trying to do. The first paragraph describes a method for predicting the occurrence of earthquakes, and the second paragraph explains problems with that method. The third paragraph describes a second method for predicting the occurrence of earthquakes, and the fourth paragraph explains problems with that method. Thus, the passage as a whole is primarily concerned with explaining the deficiencies of two methods for predicting the occurrence of earthquakes.
A The passage does not compare the practicability of the two methods.
B The passage does not discuss combining long-term and short-term methods.
C Only the first half of the passage discusses dilatancy theory; the second half discusses a different method for predicting the occurrence of earthquakes.
D Correct. The passage describes two methods for predicting the occurrence of earthquakes and explains the shortcomings of each method.
E Only the second half of the passage discusses patterns in the occurrence of past earthquakes; the first half discusses a different method for predicting the occurrence of earthquakes.
The correct answer is D.
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