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.
1. The passage suggests that the author would regard which of the following as most important for a "scientifically rigorous examination" (Highlighted) of the hypothesis that large reservoirs can induct earthquakes?(A) Systematic surveys of seismic activity under a reservoir after filling began
(B) Detailed geological maps of the composition of rock strata underlying a reservoir
(C) Comparable records of seismic activity in the area of a reservoir prior to and after construction and filling of the reservoir
(D) Precise calculation of the amount of vertical pressure exerted by the reservoir's water load
(E) Accurate records of the rate of increase in pore pressure in the rock strata underlying the reservoir
2. 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
3. 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, 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 exerted 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.
4. Which of the following best describes the organization of the passage?(A) An assertion is made, briefly explained, and then an illustrative example is examined in detail.
(B) A hypothesis is advanced, carefully qualified, and then supporting data are presented.
(C) A scientific dispute is introduced and the c:asc for one side is analyzed and illustrated_
('D) A generally accepted principle is stated in order to introduce a detailed examination of a case that violates the principle.
(E) A theory is explained and a recent example that makes the theory obsolete is presented.
5. 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 changes.
(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.
6. 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 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 and hold water, thus increasing the load on the rocks below.
7. 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 of 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
8. 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 under-lying the reservoir would be less affected by vertical load.
9. The author of the passage is primarily concerned with(A) advancing a theory
(B) resolving a dispute
(C) explaining a relationship
(D) correcting an oversimplification
(E) reinterpreting data