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. |
The author implies which of the following about the ability of the researchers mentioned in line 18 to predict earthquakes?
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