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Earthquakes and reservoir impounding: State of the art
Engineering Geology, 8(1974) 191--198 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
EARTHQUAKES AND RESERVOIR IMPOUNDING: STATE OF THE ART*
C. LOMNITZ
Institute of Geophysics, National University of Mexico, Mexico City (Mexico) (Received and accepted December 7, 1973)
ABSTRACT Lomnitz, C., 1974. Earthquakes and reservoir impounding: state of the art. Eng. Geol., 8:191--198. This review is divided into five broad questions: (1) Is there a seismic effect of reservoir impounding? (2) What are the patterns of induced seismic activity? (3) What environmental conditions predispose towards triggering of earthquakes by reservoirs? (4) What is the probable physical cause of seismic activation? (5) What avenues of research are available at this time? The following conclusions are proposed: (1) Seismic effects of reservoir impounding are probably real in those cases where the local seismicity is low and the earthquakes occur directly below the reservoir. (2) Reservoir impounding tends to produce shallow swarm patterns of low mean magnitude. (3) The most notable examples of activation by reservoirs have occurred in the marginal areas of stable blocks and have been associated with transitional relief, and with heating of the lower crust due to Cenozoic volcanism or intrusion. (4) The probable cause of earthquake swarms under large reservoirs is the reduction of bulk tectonic strength due to hydrostatic pressure, combined with the trigger effect of the water load in the reservoir. (5) It is possible to propose certain tectonic criteria, combined with certain field tests, which could be of diagnostic value for the detection of earthquake-prone sites in advance of dam construction and reservoir impounding.
INTRODUCTION
An international colloquium was called by the Royal Society on behalf of UNESCO for a specific purpose: to prepare for a larger meeting to be held at a later date, on the subject of "Seismic Effects of Reservoir Impounding". In order to comply with this difficult assignment a meeting was held in March 1973, and not less than 40 scientific papers were read and discussed. It is the intention in this article to summarize the "state of the art" in the subject, and to outline a blueprint for the discussion of future lines of research that is to follow. I am tempted to discharge my duty by simply saying "We know nothing", and leave it at that. It might be the only statement that would go unchallenged: yet it would be both untrue and unfair to the many excellent papers *Institute of Geophysics Publication No. 1065.
192 t h a t have been presented. Perhaps it is more correct t o say t hat everything we k n o w a b o u t reservoirs and earthquakes is in d o u b t . After all, d o u b t is the normal state o f science, and the cause o f scientific progress. I propose to organize this review ar ound a small n u m b e r of basic questions. These questions are the following: (1) Is there a seismic effect of reservoir impounding? (2) If so, what characteristic patterns o f induced seismic activity can be defined? (3) What environmental conditions of a t e c t o n i c or geologic nature m ay be diagnostic o f seismic effects? (4) What is the probable physical cause of seismic activation by reservoirs? (5) In case th e answers to the above questions remain d o u b t f u l , what can we do to find o u t ? IS THERE A SEISMIC EFFECT OF RESERVOIR IMPOUNDING? It has n o w b e c o m e clear t hat reservoir impounding is neither a necessary nor a sufficient cause of localized ear t hqua ke activity. Only a small prop o r tio n o f reservoirs have been r e p o r t e d to exhibit associated seismic activity, and o f course there have been m a n y localized earthquake swarms n o t associated with reservoirs. F u r t h e r m o r e , in one case report ed at the meeting, an e m p t y reservoir which had never been i m p o u n d e d was mysteriously visited with an ear t hqua ke swarm of up to 140 recorded shocks per day. I should like to look at this question f r om a di fferent angle. Where in our experience as seismologists have we come across earthquake swarms of a strictly local nature, with a very shallow focal region, with high b-values and extending r e c u r r e n t l y over t w o or three m o n t h s at a time? I submit t h a t such swarms can be f o u n d in volcanic and f o r m e r l y volcanic areas. F o r example, a sequence o f this t y p e was studied by us last year on t he edge of the Durango Plateau, a 2000 m high volcanic region in Western Mexico, which was e x t r u d e d in mid-Cenozoic times. Similar local sequences have been pr oduc e d artificially by fluid injection. E x p er imen ts r ep o r t ed at the meeting prove conclusively t hat seismic activity in the upper crust can be t ur ne d on and o f f by hydrostatic pressure changes within a focal radius of one or two kilometres. It is necessary to say, however, t h a t the correlation between n u m b e r of earthquakes and the level of impounding appears less than convincing. In a few instances the correlation looks fairly good, but in general it does n o t seem significant. The existence o f a seismic effect f r o m reservoir impounding is best docum e n t e d t h r o u g h cases such as K oyna, Kremast~, Kariba, Mont eynard, Kastraki, Bajna Basta, Grancarevo, Hendrik Verwoerd, Lake Mead, Camarillas, Talbingo, Nurek, and a n u m b e r of others. Earthquake swarms in the areas of these reservoirs were previously u n k n o w n ; and the observed seismic activity was centered in th e immediate vicinity of the reservoirs and now here else. I think th at we are all fairly convinced of the likelihood t h a t impounding has enhanced th e p r o d u c t i o n of local earthquakes in these cases.
193 I do not mean to say that the case for induced seismic activity in reservoirs is definitely proven. The prior seismicity is not known well enough in any of the cases heretofore presented. In every case a small background of seismic activity could be inferred. Let us also remember that dam sites are carefully selected on the basis of special topographic and geologic features: t h e y cannot be treated as random locations. We shall see later that high topographic gradients may have a direct causal relationship with shallow earthquakes. WHAT PATTERNS OF SEISMIC ACTIVITY CAN BE DISTINGUISHED? On the basis of the evidence presented at this meeting, a statistician might be justified in claiming that there was no systematic pattern of earthquake activity associated with dams. Correlation with impounding levels may be either present or absent. Shocks may occur under the reservoir itself, upstream or downstream or on faults located near the reservoir. The earthquakes m a y increase in magnitude until a large shock occurs, or there may be swarm activity for many years without a single major earthquake. Let us look into the statistical aspects of the problem a bit further. First, the distinction between "foreshocks" and "aftershocks" in an earthquake swarm is based on hindsight. At the time of occurrence there is no w a y of telling which of these categories an event belongs to. Hence, any different patterns of foreshocks and aftershocks depend on a distinction introduced in the b o d y of data a posteriori, and is therefore statistically suspect. It seems likely that similar distinctions could be found in any b o d y of perfectly random data. The mean probable magnitude of earthquakes in a region is inversely related to the b-value. If we write the magnitude distribution as a negative exponential: f (M) = ~e-~M
(1)
in accordance with the results reported b y seismologists, then the mean magnitude is: M = 1/~, M 1> 0
(2)
The b-value as defined b y Gutenberg and Richter is b = ~ log 10 e. Thus, the higher the b-value, the lower is the mean magnitude in the region. The reported high b-values under reservoirs mean therefore that the average magnitude of shocks is rather low as compared to tectonic earthquakes in the region at large. Many earthquake swarms in active volcanic regions also tend to have a relatively low mean probable magnitude. The b-value in any active region tends to fluctuate slightly with the rate of strain release. After a large earthquake the regional stress drops, and so does the mean magnitude. This p h e n o m e n o n has also been observed in connection
194 with reservoirs, but it is certainly not characteristic of reservoir-induced earthquakes as such. On the basis of data presented so far it seems, nevertheless, that certain general stochastic models may be proposed for reservoir-induced earthquakes in preference to others. I should like to propose the following basic properties t h a t such a model ought to satisfy: (a) The long-range mean magnitude of shocks is a regionalized variable, which bears no correlation with reservoir level. (b) The mean rate of occurrence of earthquakes is correlated with the rate of reservoir impounding. (c) Any earthquake increases the probability of succeeding earthquakes, in direct proportion to its magnitude. (d) The probability of occurrence decays exponentially with the time as measured from all preceding events. Any model which satisfies these properties is non-Markovian, since the probability of an event depends on the time of occurrence of all previous events. The asymptotic properties of the process may be Poissonian, but its fine structure is similar to that of a self-exciting renewal process. It m a y be simulated by means of what I have called the Klondike analogy, which goes as follows: Suppose we wish to describe the point process of gold discoveries in the Klondike. Each find is termed an event, which is assigned a geographical location and a magnitude M, which describes the size of the find. A number of prospectors explore the area at random. After each event, a certain number of prospectors are attracted to the site in proportion to the magnitude of the find; an "after-strike sequence" is the result. Sometimes the sequence turns out to be a "fore-strike" sequence, as the major discovery may be preceded by a number of smaller ones. These sequences decay exponentially with time, as a lode or field is depleted. The Klondike analogy satisfies the basic properties of earthquake processes as found in m a n y regions. Triggering of earthquakes by reservoirs would be roughly equivalent to small areas of the Klondike being set apart for intensified prospecting. Many of these areas will prove barren; some will show minor activity which might otherwise have been overlooked, and one or two might yield a major find. The basic insight to be retained from this analogy is the independence of the magnitude distribution from the time distribution of events. Events m a y be induced or accelerated or triggered by external forces, but their magnitude is a slowly-varying function of local geological factors only. WHAT LOCAL GEOLOGIC FACTORS MAY BE DIAGNOSTIC OF POTENTIAL SEISMIC HAZARDS FROM RESERVOIR IMPOUNDING? There is an obvious risk in attempting to generalize from a small number of case histories. Suppose that we have 20 k n o w n cases of seismicity associated
195 with reservoir impounding. Since the listof possible geologic factors exceeds the number of cases by at least two orders of magnitude it is extremely likely to find one or more factors in c o m m o n at all 20 sites.Of course, the statisticalsignificance of such coincidences is quite small. What I a m trying to say is that geology is not an exact science, and that we could do worse than to rely on the intuition and the field experience of trained geologists. The main problem is'to explain the origin of quite high tectonic stresses in areas such as the Deccan, which were formerly believed to be among the most stable in the world. Now, the Deccan traps were extruded into Eocene time and perhaps even later, as the trap basalts are not generally overlain by any more recent formations. Abnormally high temperatures persisting under the edge of the Deccan are expressed through hot springs which well up, among other places, in the area of Koyna Dam. Furthermore, the gradient of the reliefis steep, as the Western Ghats drop nearly 2000 m to the coast of the Arabian Sea, and there is an abrupt continental slope offshore. Artyushkov has computed devlatoric stresses of order 150 bars under the margin of the Western Ghats, assuming that the competent crust has been reduced to an effective thickness of 20 k m by heating of its basal part. This computation is based on the assumption that the Deccan Plateau tends to spread under its o w n load, and that lateral movement is impeded by the crustal plate which surrounds the Indian Peninsula on all sides. This configuration leads to compressive stress concentrations along the edges of the uplift. Indeed, Guha et al. have reported induced seismic activity under some other reservoirs such as Kinnersani, Sholayar, Mangalam, Parabikulam, Sharavathy, Ghirny and Ukai, located mostly in the marginal areas of the Indian Shield. Reservoirs situated well within the shield were not affected by seismic activity. Tectonic stresses computed by Artyushkov's method range from 300 bars at Kariba to 1000 bars at Lake Mead. However, the load effect of these reservoirs at the focal depth of the earthquakes is of order 1-10 bars, which is comparable to the probable range of stress drops due to the observed earthquakes. In the reports on fluid injection, hydrostatic pressures of the order of 100 bars were necessary to trigger earthquakes; when the pressure dropped below this level the activity ceased altogether. Hence a very special geologic environment is necessary if such small triggering forces as were present at Koyna, Kremast~i or Kariba are to cause earthquakes. W e m a y surmise that this environment resembles one which would cause seismic swarms to arise spontaneously. High tectonic stresses and the reduction of competent thickness of the crust through heating m a y be of diagnostic value: notice that hot springs have been reported to occur near many active d a m sites,including Koyna, Kariba and Kremast~. In conclusion, tectonic environments likely to produce seismic effects due to reservoir impounding may share some of the following features: (a) they may be associated with steep gradients of the earth's relief; (b) they may be regions of residual heating in the lower crust of the Earth, as expressed by hot springs and other post-volcanic manifestations.
196
WHAT IS THE PROBABLE PHYSICAL CAUSE OF ASSOCIATED SEISMIC ACTIVITY FROM RESERVOIR IMPOUNDING? In geophysics it is generally f o u n d that the cause of any effect is seldom d o u b t f u l provided that there is a single d o m i n a n t cause. Randomness and uncertainty are indicative of multiple causation. Convincing evidence has been presented for either pore pressure or gravitational loading as mechanisms of induced seismic activity: hence it is plausible that both pore pressure and Boussinesq loading play a part in this phenomenon. This seems all the more likely as the two effects are of the same order of magnitude. However, one cause m a y be excluded in certain cases. The Camarillas Reservoir in Spain was impounded to a height of less than 30 m when it triggered seismic activity in 1961. The dam is built on a limestone diapir, probably caused by an intrusion at depth. Here the added hydrostatic head may well have been sufficient to reduce the strength of the basement rock for yield to take place. In the case of the Hendrik Verwoerd Reservoir on the Orange River, earthquakes began six months after impounding and were concentrated around the region of m a x i m u m computed strain. However, there was no correlation between earthquake activity and fluctuations in loading. Talbingo Reservoir in southeastern Australia has exhibited a good correlation between seismic activity and the rate of impounding. The epicenters appear to lie on geologic faults in the immediate vicinity of the reservoir. The longest range of data was provided by Lake Mead on the Colorado River, where over 10,000 small earthquakes have been recorded. A weak correlation between seismic activity and level of impounding has now been reported. The epicenters also are located along probable tectonic faults. Experience in highly active regions, including Japan, has shown that some high dams do increase the incidence of small local shocks in proportion to the level of impounding. However, this effect remains small in terms of the background seismicity of the region, and has not been a cause for concern. I am personally acquainted with at least two large dams, Infiernillo and Malpaso in Western and Southern Mexico, which are instrumented with short-period vertical seismometers and have shown no associated local activity. The stress fluctuations in such areas are presumably so high that the trigger effect of reservoirs is negligible in comparison. Hence we do not normally observe the enchancement of local seismicity in reservoirs located in such areas. AVENUES OF FUTURE RESEARCH Current research related to reservoir impounding includes the following areas: dam and reservoir engineering, rock mechanics, mechanics of porous media, experimental geology, elasticity, statistics and thermodynamics. At present most researchers seem to accept the t h e o r y of rock failure first
197 proposed by Hubbert and Rubey (1959)*, according to which the bulk shearing strength of a r o c k depends on the effective normal stress (o--p), where o is the mechanical stress across a potential fracture and p is the pore pressure. Thus the strength of the rock would decrease due to pore pressure from reservoir impounding. Goguel has given us a lucid discussion of this argument, in which he pointed out that the pore pressure at the focus of an earthquake should be governed by the water-table elevation over an area which includes the high ground adjacent to the reservoir; the normal water table in this area is often much higher than the level of impounding. He also argued that Boussinesq loading, if significant, should tend to decrease any differential stresses at depth caused by the lithostatic pressure o f the adjacent mountain masses. Hence the effect of a reservoir should be one of increasing the stability of the region. Since any a t t e m p t of forecasting future research trends is likely to be soon overtaken by reality, I merely point out the feasibility of certain experiments which could be performed in order to get a better grip on the problem of reservoir earthquakes. Computer simulation o f the earthquake process Since reservoir earthquakes originate in a very small volume of the Earth's crust, usually surrounded by a quiescent area, the influence of neighboring seismic processes can be neglected. The shocks are also accessible to precise location. Hence we m a y have the best kind of data for simulation studies (both mechanical and stochastic), since the loading function at the Earth's surface is well known. Source parameters and stress drop A well-planned instrumentation of active dam sites, both in the near field and the far field, should yield uniquely valuable data on source dimensions, slip vectors, and the magnitude of the stress drop in these earthquakes. Comparisons between different active dam sites could lead to conclusions about the values of absolute stresses in the Earth's crust, and about the trigger forces required to activate an earthquake fault. Pre-construction tests A fairly simple way of early detection of seismic hazard at a dam site would be the following. If the general reasoning presented in this paper is correct, there are only two possibilities with respect to a given site: either it is susceptible to trigger a significant seismic activity, or it is not.
*Hubbert, M. K. and Rubey, W. W., 1959. Mechanics of fluid-filled porous solids and its application to overthrust faulting. Bull. Geol. Soc. Am., 70:115--166.
198
Th e way to find o u t is t o drill a hole in t he center of t he prospective dam site, p u m p water t o depths of t he order of 1000 ft. or more, and gradually increase th e pressure until earthquakes start t o occur. The critical threshold o f induced ear th qua ke activity will give us a reliable measure of relative e ar th q u ak e risk for t h e reservoir.
EARTHQUAKES AND RESERVOIR IMPOUNDING: STATE OF THE ART*
C. LOMNITZ
Institute of Geophysics, National University of Mexico, Mexico City (Mexico) (Received and accepted December 7, 1973)
ABSTRACT Lomnitz, C., 1974. Earthquakes and reservoir impounding: state of the art. Eng. Geol., 8:191--198. This review is divided into five broad questions: (1) Is there a seismic effect of reservoir impounding? (2) What are the patterns of induced seismic activity? (3) What environmental conditions predispose towards triggering of earthquakes by reservoirs? (4) What is the probable physical cause of seismic activation? (5) What avenues of research are available at this time? The following conclusions are proposed: (1) Seismic effects of reservoir impounding are probably real in those cases where the local seismicity is low and the earthquakes occur directly below the reservoir. (2) Reservoir impounding tends to produce shallow swarm patterns of low mean magnitude. (3) The most notable examples of activation by reservoirs have occurred in the marginal areas of stable blocks and have been associated with transitional relief, and with heating of the lower crust due to Cenozoic volcanism or intrusion. (4) The probable cause of earthquake swarms under large reservoirs is the reduction of bulk tectonic strength due to hydrostatic pressure, combined with the trigger effect of the water load in the reservoir. (5) It is possible to propose certain tectonic criteria, combined with certain field tests, which could be of diagnostic value for the detection of earthquake-prone sites in advance of dam construction and reservoir impounding.
INTRODUCTION
An international colloquium was called by the Royal Society on behalf of UNESCO for a specific purpose: to prepare for a larger meeting to be held at a later date, on the subject of "Seismic Effects of Reservoir Impounding". In order to comply with this difficult assignment a meeting was held in March 1973, and not less than 40 scientific papers were read and discussed. It is the intention in this article to summarize the "state of the art" in the subject, and to outline a blueprint for the discussion of future lines of research that is to follow. I am tempted to discharge my duty by simply saying "We know nothing", and leave it at that. It might be the only statement that would go unchallenged: yet it would be both untrue and unfair to the many excellent papers *Institute of Geophysics Publication No. 1065.
192 t h a t have been presented. Perhaps it is more correct t o say t hat everything we k n o w a b o u t reservoirs and earthquakes is in d o u b t . After all, d o u b t is the normal state o f science, and the cause o f scientific progress. I propose to organize this review ar ound a small n u m b e r of basic questions. These questions are the following: (1) Is there a seismic effect of reservoir impounding? (2) If so, what characteristic patterns o f induced seismic activity can be defined? (3) What environmental conditions of a t e c t o n i c or geologic nature m ay be diagnostic o f seismic effects? (4) What is the probable physical cause of seismic activation by reservoirs? (5) In case th e answers to the above questions remain d o u b t f u l , what can we do to find o u t ? IS THERE A SEISMIC EFFECT OF RESERVOIR IMPOUNDING? It has n o w b e c o m e clear t hat reservoir impounding is neither a necessary nor a sufficient cause of localized ear t hqua ke activity. Only a small prop o r tio n o f reservoirs have been r e p o r t e d to exhibit associated seismic activity, and o f course there have been m a n y localized earthquake swarms n o t associated with reservoirs. F u r t h e r m o r e , in one case report ed at the meeting, an e m p t y reservoir which had never been i m p o u n d e d was mysteriously visited with an ear t hqua ke swarm of up to 140 recorded shocks per day. I should like to look at this question f r om a di fferent angle. Where in our experience as seismologists have we come across earthquake swarms of a strictly local nature, with a very shallow focal region, with high b-values and extending r e c u r r e n t l y over t w o or three m o n t h s at a time? I submit t h a t such swarms can be f o u n d in volcanic and f o r m e r l y volcanic areas. F o r example, a sequence o f this t y p e was studied by us last year on t he edge of the Durango Plateau, a 2000 m high volcanic region in Western Mexico, which was e x t r u d e d in mid-Cenozoic times. Similar local sequences have been pr oduc e d artificially by fluid injection. E x p er imen ts r ep o r t ed at the meeting prove conclusively t hat seismic activity in the upper crust can be t ur ne d on and o f f by hydrostatic pressure changes within a focal radius of one or two kilometres. It is necessary to say, however, t h a t the correlation between n u m b e r of earthquakes and the level of impounding appears less than convincing. In a few instances the correlation looks fairly good, but in general it does n o t seem significant. The existence o f a seismic effect f r o m reservoir impounding is best docum e n t e d t h r o u g h cases such as K oyna, Kremast~, Kariba, Mont eynard, Kastraki, Bajna Basta, Grancarevo, Hendrik Verwoerd, Lake Mead, Camarillas, Talbingo, Nurek, and a n u m b e r of others. Earthquake swarms in the areas of these reservoirs were previously u n k n o w n ; and the observed seismic activity was centered in th e immediate vicinity of the reservoirs and now here else. I think th at we are all fairly convinced of the likelihood t h a t impounding has enhanced th e p r o d u c t i o n of local earthquakes in these cases.
193 I do not mean to say that the case for induced seismic activity in reservoirs is definitely proven. The prior seismicity is not known well enough in any of the cases heretofore presented. In every case a small background of seismic activity could be inferred. Let us also remember that dam sites are carefully selected on the basis of special topographic and geologic features: t h e y cannot be treated as random locations. We shall see later that high topographic gradients may have a direct causal relationship with shallow earthquakes. WHAT PATTERNS OF SEISMIC ACTIVITY CAN BE DISTINGUISHED? On the basis of the evidence presented at this meeting, a statistician might be justified in claiming that there was no systematic pattern of earthquake activity associated with dams. Correlation with impounding levels may be either present or absent. Shocks may occur under the reservoir itself, upstream or downstream or on faults located near the reservoir. The earthquakes m a y increase in magnitude until a large shock occurs, or there may be swarm activity for many years without a single major earthquake. Let us look into the statistical aspects of the problem a bit further. First, the distinction between "foreshocks" and "aftershocks" in an earthquake swarm is based on hindsight. At the time of occurrence there is no w a y of telling which of these categories an event belongs to. Hence, any different patterns of foreshocks and aftershocks depend on a distinction introduced in the b o d y of data a posteriori, and is therefore statistically suspect. It seems likely that similar distinctions could be found in any b o d y of perfectly random data. The mean probable magnitude of earthquakes in a region is inversely related to the b-value. If we write the magnitude distribution as a negative exponential: f (M) = ~e-~M
(1)
in accordance with the results reported b y seismologists, then the mean magnitude is: M = 1/~, M 1> 0
(2)
The b-value as defined b y Gutenberg and Richter is b = ~ log 10 e. Thus, the higher the b-value, the lower is the mean magnitude in the region. The reported high b-values under reservoirs mean therefore that the average magnitude of shocks is rather low as compared to tectonic earthquakes in the region at large. Many earthquake swarms in active volcanic regions also tend to have a relatively low mean probable magnitude. The b-value in any active region tends to fluctuate slightly with the rate of strain release. After a large earthquake the regional stress drops, and so does the mean magnitude. This p h e n o m e n o n has also been observed in connection
194 with reservoirs, but it is certainly not characteristic of reservoir-induced earthquakes as such. On the basis of data presented so far it seems, nevertheless, that certain general stochastic models may be proposed for reservoir-induced earthquakes in preference to others. I should like to propose the following basic properties t h a t such a model ought to satisfy: (a) The long-range mean magnitude of shocks is a regionalized variable, which bears no correlation with reservoir level. (b) The mean rate of occurrence of earthquakes is correlated with the rate of reservoir impounding. (c) Any earthquake increases the probability of succeeding earthquakes, in direct proportion to its magnitude. (d) The probability of occurrence decays exponentially with the time as measured from all preceding events. Any model which satisfies these properties is non-Markovian, since the probability of an event depends on the time of occurrence of all previous events. The asymptotic properties of the process may be Poissonian, but its fine structure is similar to that of a self-exciting renewal process. It m a y be simulated by means of what I have called the Klondike analogy, which goes as follows: Suppose we wish to describe the point process of gold discoveries in the Klondike. Each find is termed an event, which is assigned a geographical location and a magnitude M, which describes the size of the find. A number of prospectors explore the area at random. After each event, a certain number of prospectors are attracted to the site in proportion to the magnitude of the find; an "after-strike sequence" is the result. Sometimes the sequence turns out to be a "fore-strike" sequence, as the major discovery may be preceded by a number of smaller ones. These sequences decay exponentially with time, as a lode or field is depleted. The Klondike analogy satisfies the basic properties of earthquake processes as found in m a n y regions. Triggering of earthquakes by reservoirs would be roughly equivalent to small areas of the Klondike being set apart for intensified prospecting. Many of these areas will prove barren; some will show minor activity which might otherwise have been overlooked, and one or two might yield a major find. The basic insight to be retained from this analogy is the independence of the magnitude distribution from the time distribution of events. Events m a y be induced or accelerated or triggered by external forces, but their magnitude is a slowly-varying function of local geological factors only. WHAT LOCAL GEOLOGIC FACTORS MAY BE DIAGNOSTIC OF POTENTIAL SEISMIC HAZARDS FROM RESERVOIR IMPOUNDING? There is an obvious risk in attempting to generalize from a small number of case histories. Suppose that we have 20 k n o w n cases of seismicity associated
195 with reservoir impounding. Since the listof possible geologic factors exceeds the number of cases by at least two orders of magnitude it is extremely likely to find one or more factors in c o m m o n at all 20 sites.Of course, the statisticalsignificance of such coincidences is quite small. What I a m trying to say is that geology is not an exact science, and that we could do worse than to rely on the intuition and the field experience of trained geologists. The main problem is'to explain the origin of quite high tectonic stresses in areas such as the Deccan, which were formerly believed to be among the most stable in the world. Now, the Deccan traps were extruded into Eocene time and perhaps even later, as the trap basalts are not generally overlain by any more recent formations. Abnormally high temperatures persisting under the edge of the Deccan are expressed through hot springs which well up, among other places, in the area of Koyna Dam. Furthermore, the gradient of the reliefis steep, as the Western Ghats drop nearly 2000 m to the coast of the Arabian Sea, and there is an abrupt continental slope offshore. Artyushkov has computed devlatoric stresses of order 150 bars under the margin of the Western Ghats, assuming that the competent crust has been reduced to an effective thickness of 20 k m by heating of its basal part. This computation is based on the assumption that the Deccan Plateau tends to spread under its o w n load, and that lateral movement is impeded by the crustal plate which surrounds the Indian Peninsula on all sides. This configuration leads to compressive stress concentrations along the edges of the uplift. Indeed, Guha et al. have reported induced seismic activity under some other reservoirs such as Kinnersani, Sholayar, Mangalam, Parabikulam, Sharavathy, Ghirny and Ukai, located mostly in the marginal areas of the Indian Shield. Reservoirs situated well within the shield were not affected by seismic activity. Tectonic stresses computed by Artyushkov's method range from 300 bars at Kariba to 1000 bars at Lake Mead. However, the load effect of these reservoirs at the focal depth of the earthquakes is of order 1-10 bars, which is comparable to the probable range of stress drops due to the observed earthquakes. In the reports on fluid injection, hydrostatic pressures of the order of 100 bars were necessary to trigger earthquakes; when the pressure dropped below this level the activity ceased altogether. Hence a very special geologic environment is necessary if such small triggering forces as were present at Koyna, Kremast~i or Kariba are to cause earthquakes. W e m a y surmise that this environment resembles one which would cause seismic swarms to arise spontaneously. High tectonic stresses and the reduction of competent thickness of the crust through heating m a y be of diagnostic value: notice that hot springs have been reported to occur near many active d a m sites,including Koyna, Kariba and Kremast~. In conclusion, tectonic environments likely to produce seismic effects due to reservoir impounding may share some of the following features: (a) they may be associated with steep gradients of the earth's relief; (b) they may be regions of residual heating in the lower crust of the Earth, as expressed by hot springs and other post-volcanic manifestations.
196
WHAT IS THE PROBABLE PHYSICAL CAUSE OF ASSOCIATED SEISMIC ACTIVITY FROM RESERVOIR IMPOUNDING? In geophysics it is generally f o u n d that the cause of any effect is seldom d o u b t f u l provided that there is a single d o m i n a n t cause. Randomness and uncertainty are indicative of multiple causation. Convincing evidence has been presented for either pore pressure or gravitational loading as mechanisms of induced seismic activity: hence it is plausible that both pore pressure and Boussinesq loading play a part in this phenomenon. This seems all the more likely as the two effects are of the same order of magnitude. However, one cause m a y be excluded in certain cases. The Camarillas Reservoir in Spain was impounded to a height of less than 30 m when it triggered seismic activity in 1961. The dam is built on a limestone diapir, probably caused by an intrusion at depth. Here the added hydrostatic head may well have been sufficient to reduce the strength of the basement rock for yield to take place. In the case of the Hendrik Verwoerd Reservoir on the Orange River, earthquakes began six months after impounding and were concentrated around the region of m a x i m u m computed strain. However, there was no correlation between earthquake activity and fluctuations in loading. Talbingo Reservoir in southeastern Australia has exhibited a good correlation between seismic activity and the rate of impounding. The epicenters appear to lie on geologic faults in the immediate vicinity of the reservoir. The longest range of data was provided by Lake Mead on the Colorado River, where over 10,000 small earthquakes have been recorded. A weak correlation between seismic activity and level of impounding has now been reported. The epicenters also are located along probable tectonic faults. Experience in highly active regions, including Japan, has shown that some high dams do increase the incidence of small local shocks in proportion to the level of impounding. However, this effect remains small in terms of the background seismicity of the region, and has not been a cause for concern. I am personally acquainted with at least two large dams, Infiernillo and Malpaso in Western and Southern Mexico, which are instrumented with short-period vertical seismometers and have shown no associated local activity. The stress fluctuations in such areas are presumably so high that the trigger effect of reservoirs is negligible in comparison. Hence we do not normally observe the enchancement of local seismicity in reservoirs located in such areas. AVENUES OF FUTURE RESEARCH Current research related to reservoir impounding includes the following areas: dam and reservoir engineering, rock mechanics, mechanics of porous media, experimental geology, elasticity, statistics and thermodynamics. At present most researchers seem to accept the t h e o r y of rock failure first
197 proposed by Hubbert and Rubey (1959)*, according to which the bulk shearing strength of a r o c k depends on the effective normal stress (o--p), where o is the mechanical stress across a potential fracture and p is the pore pressure. Thus the strength of the rock would decrease due to pore pressure from reservoir impounding. Goguel has given us a lucid discussion of this argument, in which he pointed out that the pore pressure at the focus of an earthquake should be governed by the water-table elevation over an area which includes the high ground adjacent to the reservoir; the normal water table in this area is often much higher than the level of impounding. He also argued that Boussinesq loading, if significant, should tend to decrease any differential stresses at depth caused by the lithostatic pressure o f the adjacent mountain masses. Hence the effect of a reservoir should be one of increasing the stability of the region. Since any a t t e m p t of forecasting future research trends is likely to be soon overtaken by reality, I merely point out the feasibility of certain experiments which could be performed in order to get a better grip on the problem of reservoir earthquakes. Computer simulation o f the earthquake process Since reservoir earthquakes originate in a very small volume of the Earth's crust, usually surrounded by a quiescent area, the influence of neighboring seismic processes can be neglected. The shocks are also accessible to precise location. Hence we m a y have the best kind of data for simulation studies (both mechanical and stochastic), since the loading function at the Earth's surface is well known. Source parameters and stress drop A well-planned instrumentation of active dam sites, both in the near field and the far field, should yield uniquely valuable data on source dimensions, slip vectors, and the magnitude of the stress drop in these earthquakes. Comparisons between different active dam sites could lead to conclusions about the values of absolute stresses in the Earth's crust, and about the trigger forces required to activate an earthquake fault. Pre-construction tests A fairly simple way of early detection of seismic hazard at a dam site would be the following. If the general reasoning presented in this paper is correct, there are only two possibilities with respect to a given site: either it is susceptible to trigger a significant seismic activity, or it is not.
*Hubbert, M. K. and Rubey, W. W., 1959. Mechanics of fluid-filled porous solids and its application to overthrust faulting. Bull. Geol. Soc. Am., 70:115--166.
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Th e way to find o u t is t o drill a hole in t he center of t he prospective dam site, p u m p water t o depths of t he order of 1000 ft. or more, and gradually increase th e pressure until earthquakes start t o occur. The critical threshold o f induced ear th qua ke activity will give us a reliable measure of relative e ar th q u ak e risk for t h e reservoir.