- Email: [email protected]
Reservoirs and earthquakes, by Ronald B. Meade Comments
Engineering Geology, 32 (1992): 203-206 Elsevier SciencePublishers B.V., Amsterdam
203
Discussion
Reservoirs and earthquakes, by Ronald B. Meade 1 Comments Kusala Rajendran Department of Geological Sciences, Universityof South Carolina, Columbia, SC 29208, USA (Received October 23, 1991; revised version accepted February 12, 1992)
In this paper, the author raises some fundamental questions regarding the characterization of Reservoir Induced Seismicity (RIS). According to the author, studies of RIS have routinely assumed a cause-and-effect relationship between the reservoir operation and earthquakes and they are not supported by detailed evaluation. The author develops some new criteria and applies them to some hitherto known cases of RIS, to examine their causal behavior, if any, with the reservoir. He concludes that several incidences of earthquake occurrences in the vicinity of reservoirs are casual, with no proven association of the seismicity with the reservoir. Some questions raised in this paper are important, but the methodology chosen to demonstrate the issues does not accommodate several processes that are involved in the mechanism of RIS. Two main issues addressed in this paper are: (1) the nature of pre-impoundment seismicity; and (2) temporal relation of seismicity to crustal weakening. Evaluation of background seismicity is an important issue in RIS, as pointed out by the author. It is true that lack of adequate preimpoundment data often constrains the studies of RIS, especially if the increase in seismicity is of low level. However, the author's statement that "none of the literature summarizing reservoir1 Eng. Geol., 30(3/4): 245-262. 0013-7952/92/$05.00
induced seismicity has attempted to separate coincidental earthquake from induced activity" is not true. Assignment of RIS status to incidence of earthquakes near most of the reservoirs are based on the post-impoundment seismicity and its relation to lake level fluctuations. Occurrences of RIS in some seismically active areas have been long debated before assigning an RIS status. The Oroville reservoir, California, is the best example in this context. There are excellent papers dealing with the post-impoundment seismic response and their relation to stability changes introduced by the reservoir (Bell and Nur, 1978; Roeloffs, 1988, for example). We now understand much more about reservoir-induced seismicity than the "routine assumption of cause-and-effect relationship between reservoir operations and earthquakes" as stated by the author. The procedure followed by the author involves two steps. One, screening of coincidental earthquakes, which form the background. Two, defining states of stability and instability on the basis of changes in water level. Periods during which water level is increasing are treated as stability state (T) during which triggering is possible. Stages with no increase in water level are treated as stability state (N) during which triggering is not possible. Having fixed these criteria, he examines some selected cases of RIS to decide whether they are indeed triggered by the reservoir. One basic problem in his approach is the over
© 1992-- ElsevierSciencePublishers B.V.
204
simplification of the key issue in RIS v& ~ v&, the poroelastic deformation of the saturated or partially saturated media under the influence of the reservoir. These, including the deformation under the influence of cyclic load, have been studied by several workers (Bell and Nut, 1978; Roeloffs, 1988, for example). These papers have dealt not only with time-dependent destabilization but also outlined specific regions of instability, in different tectonic settings. None of these papers have been mentioned by the author. The author does not seem to recognize the process of diffusion as a potential mechanism for delayed instability. In any reservoir, there can be time-dependent weakening, controlled by the hydraulic properties of the medium. In a study by Talwani and Acree (1984) the time lag between water-level change and earthquakes at several reservoirs was explained by the process of diffusion. These correspond to stages of stability (N) according to the author, because he totally disregards diffusion of pore pressure. If pore-pressure diffusion is considered as a probable mechanism, it is easy to see how these stages represent instability. It is true that the lack of pre-impoundment seismicity limits the understanding of RIS, but at least two cases presented by the author Koyna, and Aswan, are too compelling, even in the absence of adequate background data. The Koyna region has been largely aseismic prior to filling of the Sivaji Sagar Lake (Gupta and Rastogi, 1976). In the absence of a local network, the estimate of background seismicity may be questionable, but earthquakes of the order of magnitude 4.0-5.0 could have hardly been unnoticed in this densely populated area. Since the occurrence of the first large earthquake in 1967, seismicity has been monitored there. Two important observations at Koyna are: (1) The occurrence of earthquakes to the south of the dam (fig. 9, Meade, 1991). (2) The continued occurrence of earthquakes, particularly after the seasonal fluctuations in lake level (Gupta, 1983) suggested that the rate of filling is the controlling factor in triggering RIS at Koyna. Gupta observed that most earthquakes of magni-
KUSALA RAJENDRAN
rude >4.0 occurred after a lag of 2-3 months after the largest yearly fluctuations. These periods, in fact, coincide with the declining water level and thus show a significant delay from the time of lake level rise. Seismicity at Koyna, and several other reservoirs where such delays are noticed, seem to be showing a "delayed response" as suggested by Simpson et al. (1988). Porepressure increase by diffusion and subsequent destabilization is considered to be the controlling mechanism in such cases. The Aswan reservoir is unlike any known cases of RIS - - its uniqueness being the peculiar hydrogeologic setting of the area. Unlike most other reservoirs which are built across valleys, Aswan occupies a flat terrain, the subsurface of which was mostly unsaturated when the reservoir was built. Simpson et al. (1990) reported that the water level in deep wells to the west of the reservoir was 90 m below the ground elevation. Filling of the reservoir effectively raised the water level by about 75 m, completely saturating the sandstone formations. Thus, the effective increase in subsurface pressure was mostly due to the rise in water table. The long delay in this case represents the time for the water to flow into these formations. Response analyses of several seismogenic reservoirs suggest that the filling of a reservoir changes the stress environment in three ways: (a) by changing normal and shear stresses beneath the reservoir under the influence of the weight of the water; (b) by increase in pore pressure without establishing a hydraulic connection with the substratum; and (c) by increasing pore pressure through diffusion. In a recent study, Rajendran et al. (1991) outlined the influence of each of these in triggering seismicity at Monticello reservoir, one of the best documented cases of RIS. Their study suggested that the Monticello region was more active after the lake was full, and the major mechanism of destabilization was diffusion of pore pressure to seismogenic depths. The above arguments suggest the need to recognize the various mechanisms operating in each geologic and hydrogeologic setting. Once a reser-
205
RESERVOIRS AND EARTHQUAKES
voir is built, there is continuous hydraulic interaction between the reservoir operations and the environment. Presence of heterogeneities (lithologic, in-situ stress, etc.) further complicate the various processes that buildup/diffuse pore pressure. Thus, the processes that lead to instability due to filling of a reservoir are far more complicated than what is presented in this paper. Any model that does not accommodate these processes can hardly explain the mechanism of induced seismicity.
References Bell, M.L. and Nur, A., 1978. Strength changes due to reservoir induced pore pressure and application to Lake Oroville, J. Geophys. Res., 83: 4469-4483.
Gupta,, H.K. and Rastogi, B.K., 1976. Dams and Earthquakes. Elsevier, Amsterdam, 230 pp. Gupta, H.K., 1983. Induced seismicity hazard mitigation through water level manipulation at Koyna, India: A suggestion. Bull. Seism. Soc. Am., 73: 679-682. Rajendran, K., Talwani, P. and Acree, S., 1991. The relative roles of undrained and diffusion in triggering earthquakes at Monticello reservoir, S. Carolina. Bull. Seism. Soc. Am., (in review) Roeloffs, E.A., 1988. Fault stability changes induced beneath a reservoir with cyclic variations in water level. J. Geophys. Res., 93: 2107-2124. Simpson, D.W., Leith, W.S. and Scholz, C.H., 1988. Two types of induced seismicity. Bull. Seism. Soc. Am., 78: 2025-2040. Simpson, D.W., Gharib, A.A. and Kebeasy, R.M., 1990. Induced seismicityin water level at Aswan Reservoir, Egypt. Gerlands Beitr. Geophys., 99: 191-204. Talwani, P. and Acree, S., 1984. Pore pressure diffusion and the mechanism of Reservoir Induced Seismicity. Pageoph, 122: 947-965.
Reply R o n a l d B. M e a d e
Department of Civil Engineering, Virginia Military Institute, Lexington, I/A 24450, USA
I appreciate the comments made by Ms. Rajandran. Let me respond to her comments in the order she presented them. Regarding my statement that: " N o n e of the literature summarizing reservoir-induced seismicity has attempted to separate coincidental earthquake activity from induced activity." I went on to say: "These studies, that listed all reported cases of induced seismicity, assumed that all earthquakes near reservoirs were induced. This assumption is unwarranted and misleading." I am referring to the summary studies that contain compiled lists of reservoir sites having published reports that claimed or suggested that earthquake activity was triggered by impoundment. A few examples of such studies are the National Academy of Sciences report entitled "Earthquakes Related to Reservoir Filling" (1972), the report by Woodward-Clyde Consultants for the U.S. Geological Survey (1979) and G u p t a (1985). I did not intend to suggest that all studies focussing on specific sites made no
attempt to discriminate induced earthquakes from background seismicity. I disagree with the commenter in the statement that "we now understand much more about reservoir-induced seismicity". I contend that the principles governing effective stress have been known for m a n y years. Our understanding of the processes is unchanged. Our ability to model the processes has improved drastically. I recognize that diffusion is a mechanism for delaying change in pore pressure. I also recognize that our knowledge of crustal properties at focal depths of significant earthquakes ( > 3 km) is minimal. In my fig. 1, I show the continued rise of pore pressure after m a x i m u m water level was reached, when there is fluid communication. I see diffusion in the crust as similar to the process of consolidation in clay soil. I have no way to estimate the parameters involved (layer thickness and the coefficients of consolidation in the case of clay). Most reservoirs have seasonal maximums. I
203
Discussion
Reservoirs and earthquakes, by Ronald B. Meade 1 Comments Kusala Rajendran Department of Geological Sciences, Universityof South Carolina, Columbia, SC 29208, USA (Received October 23, 1991; revised version accepted February 12, 1992)
In this paper, the author raises some fundamental questions regarding the characterization of Reservoir Induced Seismicity (RIS). According to the author, studies of RIS have routinely assumed a cause-and-effect relationship between the reservoir operation and earthquakes and they are not supported by detailed evaluation. The author develops some new criteria and applies them to some hitherto known cases of RIS, to examine their causal behavior, if any, with the reservoir. He concludes that several incidences of earthquake occurrences in the vicinity of reservoirs are casual, with no proven association of the seismicity with the reservoir. Some questions raised in this paper are important, but the methodology chosen to demonstrate the issues does not accommodate several processes that are involved in the mechanism of RIS. Two main issues addressed in this paper are: (1) the nature of pre-impoundment seismicity; and (2) temporal relation of seismicity to crustal weakening. Evaluation of background seismicity is an important issue in RIS, as pointed out by the author. It is true that lack of adequate preimpoundment data often constrains the studies of RIS, especially if the increase in seismicity is of low level. However, the author's statement that "none of the literature summarizing reservoir1 Eng. Geol., 30(3/4): 245-262. 0013-7952/92/$05.00
induced seismicity has attempted to separate coincidental earthquake from induced activity" is not true. Assignment of RIS status to incidence of earthquakes near most of the reservoirs are based on the post-impoundment seismicity and its relation to lake level fluctuations. Occurrences of RIS in some seismically active areas have been long debated before assigning an RIS status. The Oroville reservoir, California, is the best example in this context. There are excellent papers dealing with the post-impoundment seismic response and their relation to stability changes introduced by the reservoir (Bell and Nur, 1978; Roeloffs, 1988, for example). We now understand much more about reservoir-induced seismicity than the "routine assumption of cause-and-effect relationship between reservoir operations and earthquakes" as stated by the author. The procedure followed by the author involves two steps. One, screening of coincidental earthquakes, which form the background. Two, defining states of stability and instability on the basis of changes in water level. Periods during which water level is increasing are treated as stability state (T) during which triggering is possible. Stages with no increase in water level are treated as stability state (N) during which triggering is not possible. Having fixed these criteria, he examines some selected cases of RIS to decide whether they are indeed triggered by the reservoir. One basic problem in his approach is the over
© 1992-- ElsevierSciencePublishers B.V.
204
simplification of the key issue in RIS v& ~ v&, the poroelastic deformation of the saturated or partially saturated media under the influence of the reservoir. These, including the deformation under the influence of cyclic load, have been studied by several workers (Bell and Nut, 1978; Roeloffs, 1988, for example). These papers have dealt not only with time-dependent destabilization but also outlined specific regions of instability, in different tectonic settings. None of these papers have been mentioned by the author. The author does not seem to recognize the process of diffusion as a potential mechanism for delayed instability. In any reservoir, there can be time-dependent weakening, controlled by the hydraulic properties of the medium. In a study by Talwani and Acree (1984) the time lag between water-level change and earthquakes at several reservoirs was explained by the process of diffusion. These correspond to stages of stability (N) according to the author, because he totally disregards diffusion of pore pressure. If pore-pressure diffusion is considered as a probable mechanism, it is easy to see how these stages represent instability. It is true that the lack of pre-impoundment seismicity limits the understanding of RIS, but at least two cases presented by the author Koyna, and Aswan, are too compelling, even in the absence of adequate background data. The Koyna region has been largely aseismic prior to filling of the Sivaji Sagar Lake (Gupta and Rastogi, 1976). In the absence of a local network, the estimate of background seismicity may be questionable, but earthquakes of the order of magnitude 4.0-5.0 could have hardly been unnoticed in this densely populated area. Since the occurrence of the first large earthquake in 1967, seismicity has been monitored there. Two important observations at Koyna are: (1) The occurrence of earthquakes to the south of the dam (fig. 9, Meade, 1991). (2) The continued occurrence of earthquakes, particularly after the seasonal fluctuations in lake level (Gupta, 1983) suggested that the rate of filling is the controlling factor in triggering RIS at Koyna. Gupta observed that most earthquakes of magni-
KUSALA RAJENDRAN
rude >4.0 occurred after a lag of 2-3 months after the largest yearly fluctuations. These periods, in fact, coincide with the declining water level and thus show a significant delay from the time of lake level rise. Seismicity at Koyna, and several other reservoirs where such delays are noticed, seem to be showing a "delayed response" as suggested by Simpson et al. (1988). Porepressure increase by diffusion and subsequent destabilization is considered to be the controlling mechanism in such cases. The Aswan reservoir is unlike any known cases of RIS - - its uniqueness being the peculiar hydrogeologic setting of the area. Unlike most other reservoirs which are built across valleys, Aswan occupies a flat terrain, the subsurface of which was mostly unsaturated when the reservoir was built. Simpson et al. (1990) reported that the water level in deep wells to the west of the reservoir was 90 m below the ground elevation. Filling of the reservoir effectively raised the water level by about 75 m, completely saturating the sandstone formations. Thus, the effective increase in subsurface pressure was mostly due to the rise in water table. The long delay in this case represents the time for the water to flow into these formations. Response analyses of several seismogenic reservoirs suggest that the filling of a reservoir changes the stress environment in three ways: (a) by changing normal and shear stresses beneath the reservoir under the influence of the weight of the water; (b) by increase in pore pressure without establishing a hydraulic connection with the substratum; and (c) by increasing pore pressure through diffusion. In a recent study, Rajendran et al. (1991) outlined the influence of each of these in triggering seismicity at Monticello reservoir, one of the best documented cases of RIS. Their study suggested that the Monticello region was more active after the lake was full, and the major mechanism of destabilization was diffusion of pore pressure to seismogenic depths. The above arguments suggest the need to recognize the various mechanisms operating in each geologic and hydrogeologic setting. Once a reser-
205
RESERVOIRS AND EARTHQUAKES
voir is built, there is continuous hydraulic interaction between the reservoir operations and the environment. Presence of heterogeneities (lithologic, in-situ stress, etc.) further complicate the various processes that buildup/diffuse pore pressure. Thus, the processes that lead to instability due to filling of a reservoir are far more complicated than what is presented in this paper. Any model that does not accommodate these processes can hardly explain the mechanism of induced seismicity.
References Bell, M.L. and Nur, A., 1978. Strength changes due to reservoir induced pore pressure and application to Lake Oroville, J. Geophys. Res., 83: 4469-4483.
Gupta,, H.K. and Rastogi, B.K., 1976. Dams and Earthquakes. Elsevier, Amsterdam, 230 pp. Gupta, H.K., 1983. Induced seismicity hazard mitigation through water level manipulation at Koyna, India: A suggestion. Bull. Seism. Soc. Am., 73: 679-682. Rajendran, K., Talwani, P. and Acree, S., 1991. The relative roles of undrained and diffusion in triggering earthquakes at Monticello reservoir, S. Carolina. Bull. Seism. Soc. Am., (in review) Roeloffs, E.A., 1988. Fault stability changes induced beneath a reservoir with cyclic variations in water level. J. Geophys. Res., 93: 2107-2124. Simpson, D.W., Leith, W.S. and Scholz, C.H., 1988. Two types of induced seismicity. Bull. Seism. Soc. Am., 78: 2025-2040. Simpson, D.W., Gharib, A.A. and Kebeasy, R.M., 1990. Induced seismicityin water level at Aswan Reservoir, Egypt. Gerlands Beitr. Geophys., 99: 191-204. Talwani, P. and Acree, S., 1984. Pore pressure diffusion and the mechanism of Reservoir Induced Seismicity. Pageoph, 122: 947-965.
Reply R o n a l d B. M e a d e
Department of Civil Engineering, Virginia Military Institute, Lexington, I/A 24450, USA
I appreciate the comments made by Ms. Rajandran. Let me respond to her comments in the order she presented them. Regarding my statement that: " N o n e of the literature summarizing reservoir-induced seismicity has attempted to separate coincidental earthquake activity from induced activity." I went on to say: "These studies, that listed all reported cases of induced seismicity, assumed that all earthquakes near reservoirs were induced. This assumption is unwarranted and misleading." I am referring to the summary studies that contain compiled lists of reservoir sites having published reports that claimed or suggested that earthquake activity was triggered by impoundment. A few examples of such studies are the National Academy of Sciences report entitled "Earthquakes Related to Reservoir Filling" (1972), the report by Woodward-Clyde Consultants for the U.S. Geological Survey (1979) and G u p t a (1985). I did not intend to suggest that all studies focussing on specific sites made no
attempt to discriminate induced earthquakes from background seismicity. I disagree with the commenter in the statement that "we now understand much more about reservoir-induced seismicity". I contend that the principles governing effective stress have been known for m a n y years. Our understanding of the processes is unchanged. Our ability to model the processes has improved drastically. I recognize that diffusion is a mechanism for delaying change in pore pressure. I also recognize that our knowledge of crustal properties at focal depths of significant earthquakes ( > 3 km) is minimal. In my fig. 1, I show the continued rise of pore pressure after m a x i m u m water level was reached, when there is fluid communication. I see diffusion in the crust as similar to the process of consolidation in clay soil. I have no way to estimate the parameters involved (layer thickness and the coefficients of consolidation in the case of clay). Most reservoirs have seasonal maximums. I