In spite of early doubts, it is now well established that large reservoirs can generate earthquakes. When the possibility of...
GMAT Reading Comprehension : (RC) Questions
In spite of early doubts, it is now well established that large reservoirs can generate earthquakes. When the possibility of reservoir-induced seismicity was first suggested during the mid-1960s, it was greeted with skepticism in some quarters, largely because detailed records of the prereservoir seismic background were incomplete at best. A scientifically rigorous examination of the evidence was therefore impossible.
But now there are no doubts, for well-documented examples of reservoir-induced seismicity are known throughout the inhabited continents.
In general terms, what happens is that the huge mass of water in a reservoir changes the stress patterns in the underlying rocks. The water load exerts a simple vertical pressure on the rocks immediately, while the pore pressure in the rocks increases gradually as the water infiltrates at a rate that depends on their permeability and prior water content.
One such example is the Nurek reservoir in the Soviet Union, which has been generating seismic activity since filling began in the early 1970's. In this case, the timing of the induced seismicity is directly related not to the absolute amount of water present but to variations in that amount. Bursts of seismic activity are triggered when the water level changes, with the frequency of the activity increasing as the rate of level change increases.
The spatial distribution of the seismic activity at Nurek is less straightforward. Most of the induced earthquakes occur beneath the central part of the reservoir and upstream from it. The region containing that half of the reservoir immediately behind the dam, on the other hand, is almost completely aseismic. Yet both the seismic and aseismic zones arc lithologically similar, consisting largely of shallow-water limestones, which are highly fractured and thus relatively permeable, interbedded with mixed components of low-permeability shale and gypsum.
What, then, governs the location of the induced seismicity?
The answer lies in the varying structures of the rock strata and the different permcabilities such structures engender. Immediately upstream from the dam the alternating layers of limestone- and gypsum-shale arc folded into a V-shape (syncline). Any water entering one of the permeable layers could migrate to the bottom of the basin formed by that layer, but would be prevented from moving any further downward by the impermeable rock layer beneath. In this region, therefore, water cannot enter the vast mass of rock below and cannot thereby change the port pressure. The water in the reservoir here acts only as a load and thus is insufficient to induce seismicity.
Further upstream from the darn, on the other-hand, the strata arc folded into an inverted V-shape (anticline). In this region water entering a permeable layer can diffuse along it and thus be carried to various depths without restriction. Here the pore pressure at given depths does change when the water level fluctuates, and this is sufficient to generate seismicity.
The passage suggests that the author would regard which of the following as most important for a "scientifically rigorous examination" of the hypothesis that large reservoirs can induct earthquakes?
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| In spite of early doubts, it is now well established that large reservoirs can generate earthquakes. | What it says: Scientists used to doubt that man-made lakes could cause earthquakes, but now we know they definitely can. What it does: Opens with the main claim - establishes the central thesis Source/Type: Author stating current scientific consensus Connection to Previous Sentences: This is the opening - no previous context Visualization: Timeline: Past (doubt) → Present (certainty) Before: "Reservoirs probably don't cause earthquakes" Now: "Reservoirs definitely DO cause earthquakes" Reading Strategy Insight: This is a classic RC opening - sets up a "then vs. now" structure. Expect the passage to explain how we moved from doubt to certainty. |
2. Passage Summary:
Author's Purpose:
To explain how scientists moved from doubting that man-made reservoirs could cause earthquakes to understanding exactly why and where these earthquakes occur, using specific scientific evidence to demonstrate the mechanism.
Summary of Passage Structure:
The author builds their explanation in a logical sequence that moves from historical doubt to current understanding:
- First, the author establishes that scientists used to doubt reservoirs could cause earthquakes in the 1960s because they lacked proper before-and-after earthquake records to prove anything.
- Next, the author explains that we now have certainty because well-documented examples exist worldwide, then describes the general two-part mechanism of how reservoir water creates earthquakes through direct pressure and gradual water infiltration.
- Then, the author uses the Nurek reservoir as a detailed case study, showing that earthquake timing depends on changes in water level rather than total water amount, but noting that earthquake location patterns are more puzzling.
- Finally, the author solves the location puzzle by explaining that identical rock types arranged in different structural patterns (V-shaped vs inverted V-shaped) allow different amounts of water penetration, which determines whether earthquakes occur in each area.
Main Point:
Reservoirs definitely do cause earthquakes, and scientists now understand both the general mechanism (water pressure changes in rocks) and the specific factors that determine when and where these earthquakes happen (changes in water level trigger them, and rock structure determines location).
Question Analysis:
The question asks what the author would consider most important for a "scientifically rigorous examination" of whether large reservoirs can induce earthquakes. This is asking us to identify what kind of evidence or methodology the author believes is necessary for proper scientific investigation of this phenomenon.
Connecting to Our Passage Analysis:
Our passage analysis reveals several key insights relevant to this question:
- The author establishes that early skepticism in the 1960s was justified because "detailed records of the prereservoir seismic background were incomplete at best"
- This incomplete data made "a scientifically rigorous examination of the evidence therefore impossible"
- The author emphasizes that current certainty exists because we now have "well-documented examples of reservoir-induced seismicity"
- The passage structure moves from "then vs. now" - from doubt due to insufficient data to certainty based on proper documentation
Prethinking:
The passage clearly establishes that the fundamental problem with early investigations was the lack of proper before-and-after data. The author explicitly states that without "detailed records of the prereservoir seismic background," scientists couldn't conduct rigorous examination. This suggests that for scientifically rigorous examination, you need to be able to compare seismic activity before reservoir construction with seismic activity after construction. Only by having both sets of data can you establish whether the reservoir actually caused changes in earthquake patterns.
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Why It's Wrong:
- This only provides data from after the reservoir was filled, missing the crucial "before" component
- The passage emphasizes that incomplete prereservoir records were the core problem preventing rigorous examination
- Surveys only after filling began cannot establish causation - you need comparison data
Common Student Mistakes:
- Thinking that detailed post-construction data alone proves causation?
→ Remember that correlation requires comparison - you need to know what was happening before the reservoir to prove it caused changes - Focusing on the detailed Nurek examples in the passage?
→ The Nurek data works as evidence precisely because scientists already established the general causation through before/after comparisons
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Why It's Wrong:
- While geological composition is mentioned in the Nurek example, it's not presented as the key requirement for rigorous examination
- The passage shows that similar rock types (limestone, shale, gypsum) can have different earthquake patterns based on structure, not composition
- The author's emphasis on inadequate prereservoir records suggests data collection, not geological mapping, was the missing piece
Common Student Mistakes:
- Getting distracted by the detailed geological descriptions in the Nurek section?
→ Those details explain WHY earthquakes happen in specific locations, but the question asks about proving the general phenomenon exists - Thinking geological mapping would solve the 1960s skepticism problem?
→ The skepticism wasn't about rock types but about whether reservoirs actually cause earthquakes at all
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Why It's Right:
- Directly addresses the core problem identified in the passage - lack of prereservoir seismic background records
- Enables the comparison necessary to establish causation between reservoir construction and earthquake activity
- Matches the author's explanation of why early examination was impossible and why current certainty exists
Key Evidence: "When the possibility of reservoir-induced seismicity was first suggested during the mid-1960s, it was greeted with skepticism in some quarters, largely because detailed records of the prereservoir seismic background were incomplete at best. A scientifically rigorous examination of the evidence was therefore impossible."
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Why It's Wrong:
- This is a specific measurement detail, not the fundamental requirement for rigorous examination
- The passage mentions vertical pressure as part of the mechanism, but doesn't suggest measuring it precisely was the key to proving causation
- The focus is on proving the phenomenon exists, not on quantifying specific aspects of the mechanism
Common Student Mistakes:
- Confusing detailed mechanism study with basic causation proof?
→ Precise calculations help understand HOW reservoirs cause earthquakes, but the question asks what's needed to prove THAT they cause earthquakes - Thinking scientific rigor always means precise quantification?
→ The most rigorous approach depends on what question you're trying to answer - here it's about establishing causation
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Why It's Wrong:
- Like choice D, this focuses on specific mechanism details rather than the fundamental requirement for proving causation
- Pore pressure changes are part of understanding HOW the process works, not evidence THAT the process exists
- The passage suggests these detailed measurements became possible only after the basic causation was established
Common Student Mistakes:
- Thinking the most technical-sounding answer must be correct?
→ Technical precision is valuable, but the question asks about the most important requirement for rigorous examination of the basic hypothesis - Focusing on the detailed explanations of pore pressure in the passage?
→ Those explanations demonstrate current understanding, but the question asks what was needed to move from skepticism to acceptance