In 1971 researchers hoping to predict earthquakes in the short term by

Line

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

(5)

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,

(10)

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

(15)

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

(20)

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

(25)

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

(30)

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

(35)

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

(40)

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

(45)

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,

(50)

from 44 to 332 years.

(Book Question: 11)
1. The passage is primarily concerned with

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



(Book Question: 12)
2. According to the passage, laboratory evidence concerning the effects of stress on rocks might help account for

A. differences in magnitude among earthquakes
B. certain phenomena that occur prior to earthquakes
C. variations in the intervals between earthquakes in a particular area
D. differences in the frequency with which earthquakes occur in various areas
E. the unreliability of short-term earthquake predictions



(Book Question: 13)
3. It can be inferred from the passage that one problem with using precursory phenomena to predict earthquakes is that minor tremors

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



(Book Question: 14)
4. According to the passage, some researchers based their research about long-term earthquake prediction on which of the following facts?

A. The historical record confirms that most earthquakes have been preceded by minor tremors.
B. The average interval between earthquakes in one region of the San Andreas Fault is 132 years.
C. Some regions tend to be the site of numerous earthquakes over the course of many years.
D. Changes in the volume of rock can occur as a result of building stress and can lead to the weakening of rock.
E. Paleoseismologists have been able to unearth and date geological features caused by past earthquakes.



(Book Question: 15)
5. The passage suggests which of the following about the paleoseismologists’ findings described in lines 42–50?

A. They suggest that the frequency with which earthquakes occurred at a particular site decreased significantly over the past two millennia.
B. They suggest that paleoseismologists may someday be able to make reasonably accurate long-term earthquake predictions.
C. They suggest that researchers may someday be able to determine which past occurrences of minor tremors were actually followed by large earthquakes.
D. They suggest that the recurrence of earthquakes in earthquake-prone sites is too irregular to serve as a basis for earthquake prediction.
E. They indicate that researchers attempting to develop long-term methods of earthquake prediction have overlooked important evidence concerning the causes of earthquakes.



(Book Question: 16)
6. The author implies which of the following about the ability of the researchers mentioned in line 18 to predict earthquakes?

A. They can identify when an earthquake is likely to occur but not how large it will be.
B. They can identify the regions where earthquakes are likely to occur but not when they will occur.
C. They are unable to determine either the time or the place that earthquakes are likely to occur.
D. They are likely to be more accurate at short-term earthquake prediction than at long-term earthquake prediction.
E. They can determine the regions where earthquakes have occurred in the past but not the regions where they are likely to occur in the future.