Engineering Geology30 (1991) 245-262
245
Elsevier Science Publishers B.V., Amsterdam
Reservoirs and earthquakes R o n a l d B. Meade Department of Civil Engineering, Virginia Military Institute, Lexington, VA 24450-0304, USA (Accepted after revision May 9, 1990)
ABSTRACT Meade, R.B., 1991. Reservoirs and earthquakes. Eng. Geol., 30: 245-262. In 1982 the U.S. Army Corps of Engineers published a study of reservoir-induced seismicity. The study evaluated the evidence of induced earthquakes from eight cases including Koyua, Hoover, Nurek, and Kariba. In this paper, the methods used in the 1982 study are applied to more recent data from Koyna and Aswan. The data from Aswan do not support classification of the 1981 earthquake as reservoirinduced. The data from Koyua confirm the findings of the 1982 study that the earthquakes at Koyna are not closely correlated to the reservoir operation.
INTRODUCTION
The practice of suspending judgement until the evidence can be evaluated is common to most scientific investigation. On the contrary, studies of reservoirinduced seismicity have routinely assumed a cause and effect relationship between the reservoir operation and earthquakes. In such a case the details are not evaluated but instead, form the basis for the characterization of reservoir-induced seismicity. The idea that stress changes due to reservoirs could trigger earthquakes is plausible. Reservoirs impose stresses of significant magnitudes on crustal rocks at depths rarely equalled by any other man-made work. The ability to prove a cause and effect relationship between reservoir activity and earthquakes is beyond the capabilities of scientists and engineers because of the severely limited ability to measure stress below depths of several kilometers. Despite the lack of direct evidence for evaluating a cause and effect relationship, the question of the role of reservoir activity in triggering earthquakes is sufficiently important to justify the serious review and evaluation of the available circumstantial evidence. About 1981, the US Army Corps of Engineers sponsored a study to evaluate the evidence of reservoir-induced earthquakes critically (Meade, 1982). This study took an in-depth look at all the cases of damaging earthquakes that had been linked to reservoir activity in published reports prior to 1980. A listing of these cases appears as Table I. The study used reservoir water level data and earthquake catalog data to judge 0013-7952/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
246
R,B. M E A D E
TABLE I Cases reviewed in the 1982 study Reservoir
Location
Largest Earthquake
Hoover (Lake Mead) Kariba Kremasta Koyna Kurobe Manic 3 Hsinfengkiang Nurek
USA Zambia Greece India Japan Canada China USSR
ML = 5.0 mb = 5.8 M, = 6.3 M, = 6.5 M, --4.9 rob=4.3 M, = 6.1 M,=4.5
the correlation between reservoir-induced stress changes and the occurrence of earthquakes. The correlation was measured by examining the number of earthquakes occurring during certain water level stages. In this paper the results of the study are briefly reviewed and the methods applied to data from Koyna and Aswan that appeared after publication of the study. THE METHOD
Two questions form the starting point of the investigation. First, has the occurrence of earthquakes increased since impoundment of the reservoir? Second, do the earthquakes occur at times when the reservoir is weakening the crust? In cases where the annual number of earthquakes has not been altered by filling of the reservoir, charges of induced seismicity rest on changes in location of earthquake activity or a close relationship in the time of occurrence of earthquakes with water level changes. Any active fault that is cut by a reservoir will be influenced by the changes in water level. The correlation of shallow focus microearthquake activity on active faults within reservoirs should always be detectable. Studies of active faults use microearthquake data to refine fault dimensions and location because the high rate of occurrence of microearthquakes provides a large amount of data after a few weeks of observation. The assumption that all microearthquakes represent a precursor of a large earthquake is false. Microearthquakes release so little energy that changes in the water level could account for all of the energy released (Banks and Meade, 1982). Microearthquakes found near plutons, deep mines, and volcanoes are examples of seismicity generated by local stress changes. These small earthquakes are not related to large tectonically spawned earthquakes. In contrast to small shocks, the energy released by damaging earthquakes is so large that the stress changes caused by the reservoir are minor. Although the reservoir could possibly trigger a large shock, the energy changes caused by the reservoir should not increase the size of the maximum earthquake that could take place c_',_a fault. An earthquake catalog of the area of influence of the reservoir provides the data
RESERVOIRS AND EARTHQUAKES
247
to answer the first question. Earthquakes large enough to be felt, not microearthquakes, were taken as data. Only felt earthquakes were considered in order to screen possible non-tectonic seismicity from the data. The presence of microearthquakes without a history of larger earthquakes should not influence the siting or operation of reservoirs. The rarity of damaging earthquakes occurring near reservoirs supports the decision to reject data that contains only microearthquakes. The region of the crust within 20 km of the reservoir boundary was adopted as the zone of influence for the reservoir. This zone should be considered to be threedimensional, although in the study the depth dimension was not explicitly used. This limit was assigned based on evaluation of three cases of induced seismicity due to fluid injection. Two of the cases were in the USA. One was at Rocky Mountain Arsenal, in Denver, Colorado, and the second, in the Rangely Oil Fields, also in Colorado. The third case was in Matsushiro, Japan. The question of increased earthquake activity after impoundment is difficult to answer for areas normally considered earthquake country such as Greece, Japan, and central Asia. In seismically active areas, the earthquakes occurring in the reservoir area may be unrelated to the reservoir. None of the literature summarizing reservoir-induced seismicity has attempted to separate coincidental earthquake activity from induced activity. These studies, that listed all reported cases of induced seismicity, assumed that all earthquakes near reservoirs were induced. This assumption is unwarranted and misleading. For example, the reputation of Kremasta as a case of reservoir-induced seismicity was based solely on coincidence. At Kremasta, the evaluation of the pre-impoundment and post-impoundment seismicity taken with the relatively deep focus of the one large earthquake in the reservoir zone of influence implies that triggering by the reservoir was unlikely. The Kremasta data are presented later in this paper. Water level records and an earthquake catalog for the reservoir area form the raw data to address the second question, that of correlation of the reservoir operation to earthquakes. Changes in water level are interpreted as affecting the crust in the reservoir area in one of two ways. The water level change can move the crust toward failure or not. The analysis of water level is based on the regional stress conditions as evaluated from fault type or fault plane solutions of earthquakes. The interpretation of water level effects on stress in the crust are not complex but a familiarity with the principle of effective stress is essential. When a reservoir water level rises the stresses due to the weight of the water rise. The water pressure within the crustal rocks is changed in two ways. One effect is caused by the change in shearing stresses due to water weight. This effect occurs immediately but dissipates completely after some time. The other change is due to the change in water level elevation (head). This change occurs slowly and is permanent. The water loading is applied within the reservoir boundaries. This limited loading area creates shearing stresses within the crust as well as a change in compressive stresses. Rock at depths of several kilometers or more will always experience an increase in pore water pressure with an increase in shear stress. This change in pore water pressure is temporary and diminishes as water flows out of the crustal rocks. The time for flow to take place depends on permeability of the rocks and the
248
R.B, MEADE
variability in permeability within the crust. This variability is not known with confidence at earthquake focal depths. An assumption must be made regarding the rate of dissipation of temporary changes due to shearing stresses associated with water level changes. The rising water level can directly change the water pressure within the crustal rocks just as pumping water into or out of porous rock changes the water pressure within the pores of the rock. The change in water pressure is not temporary but acts to change the steady-state water pressure. Of course, if water levels continually fluctuate no constant steady-state pressure can be achieved. A reservoir may produce effects similar to an injection well for the part of the crust adjacent to the reservoir. For the crust directly below the reservoir the changes in total stress due to water weight rather than pore water pressure will dominate. The reservoir water level will form the basis of long term water pressure if the water in the crustal rock and the reservoir water form part of the same flow regime. It is possible that some impervious layer creates a boundary between the reservoir water and the water at focal depths so that the reservoir resembles a bathtub rather than a standpipe recording the water pressure in the crust. The unity of the reservoir water and the crustal water has been described as a case with fluid communication and a bathtub situation has been described as a case with no fluid communication. Fluid communication was assumed in each case. The water level data is used to assign a stability state to the crust. When changes in the water level would weaken the crust a stability state of T is assigned, meaning triggering is possible. When the water level is changing in a manner that would not weaken the crust a state of N is assigned, meaning that triggering is not possible. The effects of water level on the stability state are shown in Fig. 1. The earthquake catalog data is examined with respect to the stability state at the
WATER
_/
STRESS
/ PONE
•
N
WATER
I 1
I
STABILITY
T
LEVEL
PRESSURE
I N
T
I 2
N i
FLUID
2
NO
COMMUNICATION
STATE FLUID
Fig. 1. E f f ~ of chan~ng wa~r level.
COMMUNICATION
249
RESERVOIRS AND EARTHQUAKES
time that an earthquake occurred. The numbers of earthquakes occurring during each stability state show the correlation of the reservoir and the earthquakes. In cases where the seismicity is not triggered by the reservoir, earthquakes should take place during state N as often as during state T. If most of the earthquakes take place during state T or if little earthquake activity is recorded during state N, then the reservoir is triggering the activity. A complete description of the method is given in the Corps of Engineers study (Meade, 1982). The method is simple in concept, but sometimes the earthquake catalog data is too sparse to apply the method confidently. The absence of accurate pre-impoundment data is frequent and often the locations of the epicenter and focus have a significant measurement error. Another problem is that the seasonal cycles of filling make it difficult to assign state N except in times when the water level falls below the previous year's low. The tendency to assign a state of T when conditions are uncertain due to the time lag in raising pore pressures biases the evaluation in favor of triggering. RESULTS OF THE 1982 STUDY
A judgement based on circumstantial evidence is rarely conclusive. The conclusions drawn in the 1982 study are reasoned opinions or beliefs. The author of the study depicted his beliefs as shown in Fig.2. The two-question method focussed attention on the quantity and quality of the data. The chief limitation of the data was the incompleteness of the earthquake catalogs for the reservoir area of influence. The quality of the pre-impoundment catalog was high for only one case, Nurek. The other seven cases had inconsistent or incomplete catalog data. For these cases the evaluation of the seismicity in the reservoir areas was a major effort. Data was drawn from a wide variety of published sources. Incompleteness of the catalog data was one of the principal reasons for placing cases as undecided in the results of the study.
Nurek (USSR) Nurek Dam is a 315 m high earth dam on the Vakhish River in Central Tadyikistan in the Soviet Union. The seismicity of the region has been studied since the 1930's. KARTBA HISZNFI~NQK'IrANG MANZC 3
KOYNA KREHASTA
I/--
1
\ OZSAQREE
STRONGLY [3ZSAGPIEE NCI RT8
UNDECZDED
HZLOLY D'rSAGREIE
NURWK
AGREE
HYLPLY AGREE
NO RIrSPONSTBLE DEC~[SZON POSSZBL j"
8TRONSLY A~RI[I[ R'r8
Fig.2. Decision diagram for Corps of Engineers Study (Meade, 1982). RIS = reservoir-induced seismicity.
R.B. MEADE
250
Ouerterly
number
8hocks
oe
weter /
3.50
( N )
level
(
(meters)
).,,._,,..._I
H
S
--
10®
100 0 50
I
I
60
I
I
62
I
I
64
I
I
66
YEARS
I
( 1960
Fig.3. Quarterly number of shocks (K>
=
I
68
I
I
70 --
1&J74
I
I
72
74
)
7) at Nurek (after Soboleva and Mamadaliev, 1976).
Detailed studies began in 1955 when a high sensitivity network was installed. By the mid 1960's shocks greater than magnitude 1.7 (Soviet energy class K= 7) could be located to within an accuracy of 5 km. Filling began in 1967 but the first substantial increase in water level took place in 1971. From 1960 to 1971 the average number of earthquakes per quarter (3 months) was 26. In 1971 the average number was 40 and in the last quarter of 1972 133 shocks were recorded. These data are shown in Fig.3. This catalog is the most complete of any case examined in the study. The quality of the data permitted use of a statistical test. A two-sample Wilcoxon test was performed to test the hypothesis that the median 3 month average before impoundment was the same as after impoundment. The alternate hypothesis was that the median after fiUing was larger. The null hypothesis was rejected at a 99% level of confidence (P value =0.0003). Nurek was also unique among these cases for its thrust fault setting. Simpson and Negrnatullev (1978) suggested that drops in water level cause bursts of seismic activity. During the 1976-77 filling cycle the testing of a power tunnel caused a rapid 3 m drop in the water level. A burst of shocks occurred. Simpson inferred that the rapid drop caused the shocks. At the time of the 3 m drop the water depth was about 160 m. The water level data for that period is shown in Fig.4.
200
W~ter
level
(m)
~ I
'
-- 5 0 bet
o
~ 1969
I
I
I 1971
OF
I
1~
pep
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I 1973
deye
I
I 1975
Yeer8 Fig.4. CoEelafion ~ m ~ t N ~ e k ( a ~ r S i m p s o n a n d N e g a m a t ~ v ,
1978).
~|.
I 1977
RESERVOIRS
^
:..
-
L
1S=8
AND
EARTHQUAKES
~g4e
251
1m45
ag~®
VEAA
Fig.5. Correlation data for Lake Mead. Stability state bar open=triggedng possible; stability state bar shaded = triggering impossible.
TABLE II Earthquakes at Lake Mead Date
Stability state
09/07/36 09/20/36 04/28/37 06/18/37 11/12/37 05/04/39
T T T T T T
06/11/39
T
06/04/42 08/11/42 09/09/42 07/30/47 09/30/47 07/30/47 09/30/47 05/09/48 11/02/48 05/07140 02/08/52 02/20/52 05/24/52 10/20/52 04/19/58 03/25/63 04/23/63 09/23/64
T N
N T N T N T N T N N T N N N N N
Stability state codes: T--triggering possible; N=triggering not possible. Source for catalog data: Rogers and Lee (1976). Note: All earthquakes had masnitudes larger than 4.0 or MMI greater than or equal to V.
252
R.B. M~,DE
Hoover (USA) Located on the Colorado River southeast of Las Vegas, Nevada, the Hoover Dam impounded Lake Mead. In 1935 the reservoir was the largest in the world until surpassed by Kariba about 1958. No earthquakes (MI >4.5) had occurred in the reservoir area for 15yr prior to impoundment. Twelve earthquakes (ML>4.5) occurred in the reservoir area in the 29 yr after impoundment. Eight of the twelve occurred at times when the reservoir was weakening the crust (stability state T). These data are shown in Table II and Fig.5 (Meade, 1985). Since 1964 no earthquakes (MI.> 4.5) have occurred in the reservoir area.
Manic 3 (Canada) Hydro-Quebec began the Manicougan-Outardes hydroelectric project in 1959. Located north of the St. Lawrence River in eastern Quebec the project used nineteen dams to create seven reservoirs on the Manicougan and Aux-Outardes Rivers. Three of these reservoirs, Manic 3, Manic 5, and Aux-Outardes 4, are large reservoirs with dams over 100 m high. The entire project area is vast, as shown in Fig.6. The seismicity of the area is low to moderate. At the southern terminus of the project near Baie Cameau there is moderate seismicity associated with the St. Lawrence River valley. Proceeding north towards the huge impact crater north of Manic 5 the seismicity becomes low. The seismic history of eastern Canada is given by Basham et al. (1979). The catalog of the region is complete for shocks of magnitude 4 and above. A list of shocks in the project area (until the end of 1981) is shown in Table III. The last of the shocks in 1975 took place near Manic 3. One sensitive instrument was installed to monitor the project area in December 1974 (Le Blanc and Anglin, 1978). The instrument could detect microearthquakes as small as M = 1 in a 200 km radius. From January 1975 to mid-September 1975, no microearthquake activity was observed in the Manic 3 area. The filling of Manic 3 began in August 1975. In mid-September a few microearthquakes were recorded from the Manic 3 area. On October 20th portable instrumentation was sent to Manic
Manic
Ma~Xc
3
-.l-me 66
/,, lie
Comes
+ 7e
/
Fig.6. Regional seismicity near Manic 3. Tfiangies=pre-filling earthquakes; pentagon -- postfilling earthquake.
253
RESERVOIRS AND EARTHQUAKES
TABLE III Regional earthquakes in the vicinity of Manic 3, 49-50°N, 67.5-69°W Date
Latitude (°N)
06/12/17 05/17/38 06/23/44 10/21/58
49.0 49.0 49.4 49.2
10/23/75
49.8
Longitude (°W) 68.0 68.0 67.8 68.5 Filling of Manic 3 68.6
Magnitude 4.0 4.0 5.0 4.0 4.0
Note: Prior to 1963 only earthquakes of magnitude 4 or higher could have been recorded for this area.
3 to locate the activity. Three days later a magnitude 4.1 shock took place near the dam. The portable array had not been installed before the shock but the instruments were operating within 5 h after the shock. Filling was temporarily halted in response to the shock. Aftershock studies indicated that the events were located in a small area about 8 km upstream of the dam. Filling was resumed and the water level rose slowly to maximum pool and remained at that level regulated by the discharge from Manic 5. No other shocks have been felt.
Hsinfengkiang (China) This reservoir is located in southern China in an area described as unstable by the Chinese. The reservoir area is well populated. In the 25 yr prior to filling, four earthquakes, intensity V-VI were reported. Small shocks (M, = 2 or 3) were felt in the reservoir area during October 1959, one month after filling began. An array of instruments was installed in July 1961 and recorded about five shocks per month in July. This rate increased to eleven per month by February 1962. A large shock (M,=6.1) occurred on March 19, 1962. A large earthquake may have foreshocks and aftershocks that are components of the one large quake. The foreshocks and aftershocks can distort the catalog data. The evaluation of seismicity should use independent events as the raw data for any discussion of correlation of water level and activity. The occurrence of one large event makes the data ambiguous. Foreshocks and aftershocks should be culled out of the catalog data but no clear rules are available. Almost all events were located within 5 km of the reservoir. Data were published by Shen et al. (1974) and are shown in Fig.7. If microearthquakes are discounted by drawing a line at I0 Joules then a few significant periods of seismicity can be seen. Stability decreased from October 1959 to mid 1963. In late 1963 there was an increase in activity that was not due to the reservoir. During 1964 the water level rose and seismicity increased and included a magnitude 5.3 (M,) shock. During 1967 the lake level was relatively low for the year. In 1968 the lake level rose abruptly and remained high for about a year with no increase in seismicity. After 1965 nearly all of the events were located in the narrow canyon near the dam and seem unrelated to reservoir level.
254
R.B. MEADe We~cer"
level
( m )
"3 i ~ 0
>"
0 L @ E
w /3 o J
[I 12
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( J/month
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)
~ ___
10
8
I 1981
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I 1863
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I 1'967
'I
I
I
1969
I
I
1971
Yeer
Fig.7.Correlation data for Hsinfengkiang (after Shen et al.,1974). Kariba (Zambia) Little pre-impoundment data was available. There were active faults in the reservoir area but they had produced no large quakes. Gough and Gough (1970) report that a tremor was felt near Binga in 1956 (see Fig.8). A visitor to the area observed that the locals were not alarmed by the shock. In 1958 Lake Kariba was the largest sustained loading that man had created. Filling occurred slowly over several years
+
(+
~29°
16°s N
8rlba
f .18° S --/--~27 o E
Osm
81no a "JF
Fig.&Epicenters at Kariba. A triangle marks the vicinityoflarge earthquakes, M~5.
RESERVOIRS AND EARTHQUAKES
255
(1958-1963). Seismic instruments were installed in 1959 after filling had begun. The array began recording small shocks within weeks of installation. In 1963 several large quakes occurred near the deepest portion of the reservoir. Most of the activity took place in a burst of seven large quakes in a 3-day period of September 23 through 25th and two more large quakes in October and November. No large earthquakes occurred again until 1966 when they struck in April 1966, April 1967, and June 1968. During these years the water level varied little. The activity in 1963 occurred during filling but the later events seem unrelated to the reservoir.
Koyna (India) The seismicity at Koyna was examined in detail in the original study. Koyna is located in India near the central west coast in a region of lava flows known as the Deccan plateau. The pre-impoundment seismicity (ML > 4) was negligible based upon the records of the Benioff seismograph installed about 100 km away at Poona. The seismograph at Poona had been operating for about 12 yr before filling began in 1962. Gupta and Rastogi (1976) state that earthquakes as large as magnitude 4 would have been recorded at Poona. Mild tremors began with filling and the size of the earthquakes increased, capped by a magnitude 5.5 earthquake in September 1967 and a magnitude 6 earthquake in December of 1967. No nearby instrumentation was available until 1964. The earthquake epicenters were downstream of the dam within 20 km of the reservoir as shown in Fig.9. The post-impoundment seismicity is substantial, so the answer to the first question regarding the increase in earthquake activity is yes. The answer to the question of correlation is more difficult. The reservoir completed four complete seasonal cycles from 1963 to 1967 before the largest earthquake occurred as shown in Fig.10. The lake level was highest in 1965. During the first loading in 1963, the lake level reached elevation 2145 ft and did not fall below 2040 ft. T h i s 30 m (100 ft) of seasonal fluctuation could raise pore pressures a maximum of about 3 bars. The typical cycle was a rapid rise from of about 30 m within 8 to 16 weeks followed by a slow decline during the remaining 36 to 44 weeks each year. The 1967 filling cycle was similar to previous years but the maximum level was maintained slightly longer than usual, potentially allowing for an increase in pore pressure above that of previous years. Assignment of a stability state of T for all periods of rising water level was considered. The assignment of a stability state of N during all cases of declining water levels was not justified. In those years where the water level declined below the previous year's minimum a state of N was imposed until the water rose past the previous year's minimum. In June, July and August of 1966 and from April to July of 1967 the circumstances permitted an assignment of state N. A catalog of earthquakes was assembled by Guha et al. (1974). The magnitude of located earthquakes ranged from 2.1 to 3.9 plus the magnitude 7 earthquake of December 10, 1967. No earthquakes larger than magnitude 3.9 were located until the largest earthquake occurred. Of these small earthquakes recorded from 1965, 1966, and 1967, most took place in the brief 2-day episodes during the months of November and December. These periods of earthquake activity take place when the
256
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Fig.9. Epicenters at Koyna (adapted from Guha et al., 1974). w ~ r 2 ~1 O e~ /
Level
( f`~
)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I Number" of" l o ~ J ................................................................................................... . [ 6 ...........................................................
1963
1965
Shocks
......... M
>=
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YEAR E i g . 1 0 . C0rl~|aLlon
(~UL a t K.0yna ] 9 6 3 - ] 9 7 0
(adapted f r o m G u h a et 8]., 1974).
water level had been declining for weeks. Any triggering theory had to postulate a delay effect to make this sequence plausible. These shocks are unrelated to the reservoir. Kremasta
Located in central Greece, Kremasta is within earthquake country. The dam was built in 1964 and upon filling small shocks were felt near the dam. On February 5, 1966, a large shock (M, ffi 6.3) took place within the reservoir zone of influence. This large quake was located outside of the reservoir boundary and had a focal depth
257
RESERVOIRSAND EARTHQUAKES
probably in excess of 20 km. The large quake fits easily into the normal seismicity pattern of central Greece as shown in Fig. 11. Small quakes felt at the dam (Therianos, 1.974) may have been induced but they were unrelated to the quake of February 5, 1966.
Kurobe (Japan) Kurobe Dam was begun in 1956 in a mountainous region of the central island of Honshu. On August 1961 a magnitude 4.9 earthquake occurred within 10 km of the dam. Several days later on August 21 a magnitude 4 shock took place at nearly the same location. The earthquakes at Kurobe were consistent with the regional seismicity. Three quakes (M>3.8) were associated with the reservoir. The two largest quakes occurred in August 1961 within 20 km of epicenters of large (Mffi 6) historic quakes as shown in Fig.12. The first quake on August 19, 1961 took place just 8 h after the Kita Mino earthquake (M= 7.2). The epicenter of the Kita Mino quake was about 100 km from the dam but the earthquake caused "rather strong" shaking in the Kurobe area as shown by the isoseismals (Fig. 13). ADDITIONALRESULTS
Koyna (India) Intermittent activity has continued as reported by Gupta (1985). In the years 1975 through 1980, 27 earthquakes of magnitude 4 or larger have occurred. Of these 27, SCALE 25
0
25
KM -N-
O
5.5 0 s.s
•
o .,aN,ruo~
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.=
Fig. 11. Regional seismicity near Kremasta.
o,.o
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2._._
258
R.B. MEADE
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"~~-t'..~,,. "
(
"-"
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,~ MATSU.OTO.
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•
SCALE
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10 •
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Fig.12. Regional seismicity near Kurobe (from Hagiwara and Ohtake, 1972).
'\,.~." ~ ~ 34
°
~/) '~
N
K_., .1.3S o
E
142
o
E
Fig. 13. Isoseismals of the Kita Mino earthquake (adapted from Hagiwara and Kayano, 1961), isoseismals on Japan Meterological intensity scale.
259
RESERVOIRS AND EARTHQUAKES
eight occurred on September 20, 1980. These eight plus three others took place in August and September during or immediately after a period of summer tilting. The one other earthquake of 1980 occurred during declining water level. Of the 15 earthquakes that occurred from 1975 through 1979, five occurred during or immediately after summer filling and 10 occurred during the slow water level decline that occurs during most of the year. This data is summarized in Table IV and Fig.14. In a typical year (1975-1980), most of the earthquakes took place during a nontriggering stability state. Any claim that the reservoir is triggering earthquakes looks selectively at the 1980 experience and ignores the bulk of the data. The recent data supports the conclusion of the original evaluation (Meade, 1982) that the Koyna seismicity was unrelated to the reservoir.
Aswan (Egypt) The rapids at Aswan have been dammed since 1932 when a 52 m high dam was raised. In 1970, a high d a m located 7 km upstream of the old dam was built to a height of 111 m. The reservoir level began to rise in 1970 and by 1975 the reservoir depth was 93 m. The typical variation in water level since 1975 was 6 m/yr with a maximum of 10 m (Gibowicz et al., 1983). The historical seismicity of southern Egypt is low, but damaging earthquakes are not unknown in the region. Early records of a large earthquake in the area in 1210 B.C. caused cracks in a temple at Abu-Simbel (Gibowicz et al., 1983). Seventeen TABLE IV Koyna earthquake data 1975-1980 Total number of earthquakes ML > --4 Number occurring in 1980 Number occurring on September 20, 1980 Number during 1975 through 1979 Number occurring 1975 through 1979 during stability state N Number occurring 1975 through 1979 during stability state T
27 12 8 15 10
Data adapted from Gupta (1985).
t~
t
LEVEL
I
Fig. 14. Correlation data at Koyna, 1975-1980 (adapted from Gupta, 1985). N and T=earthquakes with magnitude > 4; N -- triggering impossible; T = triggering possible.
260
R.B. MEADE
years after filling, a magnitude 5.5 earthquake took place on the Kalabsha fault on November 14, 1981. This fault runs through an arm of the reservoir near AbuSimbel (Fig.15). The Kalabsha Wadi area was within the reservoir of the old 1932 dam but the water depth was rather shallow and varied seasonally. Aftershock activity continued during the next year, the largest being a magnitude 4.5 shock on August 20, 1982. The single episode of 1981 was not accompanied by any unique reservoir activity. The reservoir had been filled initially l? yr before the earthquake and the mild seasonal fluctuations of l0 m or less had boon repeated for years as well. This earthquake was not triggered by filling. The initial rise in water occurred more than l0 yr prior to the earthquake. The seasonal fluctuations were modest, compared with Koyna for example. The reservoir operations have continued without any large earthquakes since 1982. The continued presence of microearthquakes on an active fault do not provide proof of a relationship between the reservoir and the Kalabsha fault. The microseismic activity of the Kalabsha fault was not investigated prior to impoundment, so it is not possible to conclude that the seismicity has changed in location due to impoundment. The lack of earthquakes triggered by filling in 1975 and the continued lack of large earthquakes since the one episode in 1981 show that the reservoir at Aswan is not an example of induced seismicity.
320
330
ASWAN 0 24 °
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in
el- ~ /
bOX
8howl'h
1 I~.~..~
"C
/ Fig.15. Epicenter near Aswan of the November 14, 1981 earthquake.
261
RESERVOIRS AND EARTHQUAKES
KAR'r BA
-i \ / / KFIEHASTA
HSZNFI[NGKZANG HANZC 3
KOYNA 1
AE~/AN
STRONGLY OZGAGREE
AGREE DTSAGREE UNDECIDED HIrLDLY AGREE HILDLY OISAGFtEE
NO RZG
NO RESPONEZBLE DECXE~ON POSSIBLE
HOOVER NUREK
\ STRONGLY AGREE RZG
Fig. 16. Reused decision diagram (1990). RIS= reservoir-induced seismicity. CONCLUSIONS
Koyna and Aswan have been put forth as cases of induced seismicity (Gupta, 1985; Simpson et al., 1985). The evidence of induced seismicity is lacking in each of these examples. The merits of these two cases relative to the findings in the first study is shown in Fig.16. Use of these cases as examples of induced seismicity has been built on the false assumption that every earthquake occurring in or near a reservoir is induced by the reservoir. REFERENCES Banks, D.C. and Meade, R.B., 1982. Slope Stability; Control and Remedial Measures; and Reservoir Induced seismicity: Considerations at Natural and Man-Made Lakes. Proc., 4th Int. Congr. Eng. Geol., New Delhi. Basham, P.W., Weichert, D H. and Barry, M.J., 1979. Regional assessment of seismic risk in Eastern Canada. Bull. Seismol. Soc. Am., 69 (5): 1567-1602. Gibowicz, S.J., Droste, Z., Kebeasy, R.M., Ibrahim, E.M. and Albert, R.N.H., 1983. A microearthquake survey in the Abu-Simbel area in Egypt. Eng. Geol., 19 (2): 95-109. Gough, D.I, and Gough, W.I., 1970. Load induced earthquakes at Kariba--II. Geophys. J. R. Astron. Soc., 21: 211-217. Guha, S.K., Gosavi, P.D., Nand, K., Padale, J.G. and Marwadi, S.C., 1974. Koyna Earthquakes (October 1962 to December 1973), Report of the Central Water and Power Research Station, Khadakwasha (South), Poona-24, India, 340 pp. Gupta, H.K., 1985. The present status of reservoir induced seismicity investigations with special emphasis on Koyna earthquakes. In: S.J. Duda and J. Van~k (Editors), Quantification of Earthquakes. Tectonophysics, 118 (3/4): 257-279. Gupta, H.K. and Rastogi, B.K., 1976. Dams and Earthquakes. Elsevier, New York. Hagiwara, T. and Kayano, I., 1961. Seismological observations of the Kita Mino earthquake, August 19, 1961 and its aftershocks. Bull. Earthquake ReG. Inst., 39:873-880 (in Japanese). Hagiwara, T. and Ohtake, M., 1972. Seismic activity associated with the filling of the reservoir behind Kurobe Dam, Japan, 1963-1970. Tectonophysics, 15: 241-254. Le Blanc, G. and Anglin, F., 1978. Induced seismicity at the Manic 3 reservoir, Quebec. Bull. Seismol. Soc. Am., 68 (5): 1469-1485. Meade, R.B., 1982. The evidence for reservoir-induced macroearthquakes. Rep. 19, State-of-the-art for assessing earthquake hazards in the United States. Misc. Pap. S-73-1, US Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Miss., 192 p p . Meade, R.B., 1985. The Hoover Dam earthquakes reconsidered. Proc., Water Power '85, Las Vegas, NV, pp.1308-1315.
262
R.B. MEADE
Rogers, A.M. and Lee, W.H.K., 1976. Seismic studies of earthquakes in the Lake Mead, Nevada-Arizona region. Bull. Seismol. Soc. Am., 66 (5): 1657-1681. Shen, C., Cben, H., Chang, C., Huang, L., Li, T., Yang, C., Wang, T. and Lo, H., 1974. Earthquakes induced by reservoir impounding and their effect on the Hsinfengkiang Dam. Sci. Sinica, 17: 239-272. Simpson, D.W. and Negmatullaev, S. Kh., 1978. Induced seismicity studies in Soviet Central Asia. Earthquake Inf. Bull., 10 (6): 208-213. Simpson, D.W., Kebeasy, R.M., Maamoun, M., Ibrahim, E.M. and Albert, R.N., 1985. Induced seismicity around Aswan Lake (abstract). In: S.J. Duda and J. Van6k (Editors), Quantification of Earthquakes. Tectonophysics, 118 (3/4): 281-282. Soboleva, O.V. and Mamadaliev, U.A., 1976. The influence of Nurek Reservoir on local earthquake activity. Eng. Geol., 10 (2-4): 293-305. Therianos, A., 1974. The seismic activity of The Kremasta area--Greece--between 1967 and 1972. Eng. Geol., 8: 49-52.