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In 1971 researchers hoping to predict earthquakes in the sho

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bounce87 Junior | Next Rank: 30 Posts Default Avatar
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26 Mar 2016
Posted:
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In 1971 researchers hoping to predict earthquakes in the sho

Post Tue Aug 29, 2017 8:02 am
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?

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.

The passage does not state anywhere that scientists were unable to determine either the time or the place that earthquakes are likely to occur, whereas I think that there is proof for A.
Please explain!

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Post Tue Aug 29, 2017 12:27 pm
bounce87 wrote:
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?

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.

The passage does not state anywhere that scientists were unable to determine either the time or the place that earthquakes are likely to occur, whereas I think that there is proof for A.
Please explain!
Notice that we get some bad news about the efforts of the researchers in the very sentences alluded to in the question stem: Researchers initially reported success in identifying
these possible precursors, but subsequent analyses
of their data proved disheartening
.


Next we're told that while large earthquakes are often preceded by tremors, these tremors can occur without being followed by an earthquake, so the presence of a tremor can't tell researchers when an earthquake is going to happen - it's possible nothing will happen!

Later, we're given the sad story of two researchers who predict there's a 95% probability that an earthquake will occur in a certain region in a certain time... and no earthquake occurs. (Of course, if you've absorbed the fundamentals of probability, this doesn't mean that they were wrong, necessarily - there'd be no way to verify that 95% number - but it surely isn't evidence that they were able to predict when or where an earthquake might occur.)

So what is the author implying here? That researchers aren't too good at predicting when or where earthquakes are going to happen.

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