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.
It can be inferred from the passage that one problem with using precursory phenomena to predict earthquakes is that minor tremors
选项 A、typically occur some distance from the sites of the large earthquakes that follow them
B、are directly linked to the mechanisms that cause earthquakes
C、are difficult to distinguish from major tremors
D、have proven difficult to measure accurately
E、are not always followed by large earthquakes
答案E
解析 Inference
This question asks what can be inferred from certain information in the passage. The second paragraph explains two problems with using minor tremors to predict earthquakes. First, minor tremors provide no information about how large an impending earthquake will be. Second, the minor tremors that occur prior to a large earthquake are indistinguishable from other minor tremors. Thus, it can be inferred that minor tremors sometimes occur when no large earthquake follows.
A The passage does not mention the distance between minor tremors and ensuing earthquakes.
B The passage implies that minor tremors sometimes occur without an ensuing earthquake, so the phenomena are most likely not directly linked.
C The passage suggests no difficulty in distinguishing between minor tremors and major tremors.
D The passage does not mention any difficulties in the measurement of minor tremors.
E Correct. The passage indicates that minor tremors occurring prior to a large earthquake are indistinguishable from minor tremors that are not followed by large earthquakes. So the fact that minor tremors are not always followed by large earthquakes, together with the inability to distinguish between those that are and those that are not, poses a problem for a ny attempt to predict large earthquakes on the basis of this type of precursory phenomena.
The correct answer is E.
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