The Spatial Distribution of the Seismic Activity at Nurek is Less Straightforward.

Reading Passage Question

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 are 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 permeabilities such structures engender. Immediately upstream from the dam the alternating layers of limestone and gypsum-shale are 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 pore pressure. The water in the reservoir here acts only as a load and thus is insufficient to induce seismicity.

Further upstream from the dam, on the other hand, the strata are folded into an inverted V-shape (anticline). In this region water entering a permeable layer can diffuse along with 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 spatial distribution of the seismic activity at Nurek is less straightforward.”- is a GMAT reading comprehension passage with answers. Candidates need a strong knowledge of English GMAT reading comprehension.

This GMAT Reading Comprehension consists of 6 comprehension questions. The GMAT Reading Comprehension questions are designed for the purpose of testing candidates’ abilities in understanding, analyzing, and applying information or concepts. Candidates can actively prepare with the help of GMAT Reading Comprehension Practice Questions.

Solution and Explanation

1. According to the passage, earthquake activity in the area of the Nurek reservoir is most frequent when the

(A) reservoir is filled to capacity
(B) water level in the reservoir is exceptionally low
(C) reservoir is subjected to sudden drops in temperature
(D) water level in the reservoir fluctuates
(E) water level of the reservoir area behind the dam remains constant

2. According to the passage, which of the following proves that earthquake activity in the area of the Nurek reservoir cannot be accounted for exclusively by the composition of the rock strata underlying the reservoir?

(A) There are two kinds of rock underlying the reservoir, a, but only the shallow-water limestones are permeable.
(B) The same rock strata underlie the whole reservoir ‘area, but only one part of the area is subject to earthquake activity.
(C) The earthquake activity in the area is evenly distributed between the regions behind the dam and the central portion of the reservoir.
(D) The vertical load exhorted by the water in the reservoir has different effects on shallow-water limestones than on mixed shale and gypsum.
(E) When the frequency of earthquake activity increases, the locations of such activity become more diffuse.

3. Which of the following sets of causes and effects most closely parallels reservoir-induced seismicity as it is described in the passage?

(A) Environmental stresses in an area combine to produce an effect; the introduction of an artifact to that area restructures the stresses and reduces the incidence of the effect.
(B) An artifact is introduced into a stable environment; the environmental effects of that artifact cannot be separated from general environmental change.
(C) An artifact is introduced into an environment; local conditions peculiar to that environment have little impact on the effects generally produced by such artifacts.
(D) An artifact creates certain initial effects on its environment; these environmental effects are gradually mitigated by environmental adaptation to the artifact.
(E) An artifact is introduced into an environment; the artifact generates spatially uniform stress but the environment responds differentially to that stress.

4. Which of the following best describes the interaction of water and rock strata in the region of the anticline under the Nurek reservoir?

(A) Shale and gypsum form the top layer of rock in an anticline and thus the water cannot penetrate to the permeable limestone layer.
(B) Fractured and highly permeable limestone forms a basin in an anticline through which water penetrates to the rock below.
(C) The shale and gypsum layer beneath the permeable limestone basin in an anticline prevents water from penetrating to deeper rock strata.
(D) The shape of an anticline allows water to travel along with the permeable layer and penetrate into deep rock strata
(E) The structure of limestone strata alternating with shale and gypsum strata in an anticline allows the region to collect handhold water, thus increasing the load on the rocks below.

5. It can be inferred from the passage that an accurate prediction of reservoir-induced earthquake activity at the site of the Nurek reservoir prior to its construction would have required all the following data EXCEPT

(A) a general geological survey of the area, establishing the lithological composition of the rock strata
(B) a detailed examination of the structure of the rock strata underlying the site of the reservoir
(C) a projection of the number and magnitude of the fluctuations in the level of the water in the reservoir
(D) an estimate of the load that would be created by the total mass of water in the reservoir
(E) a determination of the chemical properties of the water that would be used to fill the reservoir

6. The passage implies that which of the following would be true in the area of the Nurek reservoir if the rate of change in the water level in the reservoir were very gradual?

(A) Stress patterns in the rock strata beneath the reservoir would be the same as before its construction.
(B) The seismic activity beneath the reservoir would not be very frequent.
(C) The spatial distribution of seismic activity beneath the reservoir would be altered.
(D) The rate of infiltration into rock strata below the reservoir would increase.
(E) The stress patterns in the rocks underlying the reservoir would be less affected by vertical load.

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