• .. • ~ p " ••

• ~ " ,.. I • .,__ -, . . . . . .. 1 . -··-- . .. ,_, . -... ; ....... . . ' . . -· . . t . . '--, . , • • .. . ' . " • I

iJ

T :mi..rCED SE 1 S~1 I C I T1 STL'D f ES

'·

,. ..

... ...-:uz

-. ';

........... _ .......

;; 'f : ·:t ~ -"

" '•I

~l

' . . ..

.J

f..

• • c;_: >.

CJc >'\)

"·

_,_,- .-.............. ,_._. w .....

~ .

f;~o' ~·· .

J:

.. 1\

,. ~-:,.;,_ m_:~.~·~~ .. {tf;~ t?.f.£..-ff~~ _..._ .l:.. 1"':;'f' .-ot-~~-,t~..V:S ....... ·S:.,

~-. ~} . ~:~~~< ~;· ..... ·. ~

. -.. .~ . . .. ~)-" .. :..t:'-

,.

..

·~.

Induced

Seismicity Studies • zn Soviet Central Asia

by David W. Simpson, Lanzom-Doherty Geological Observatory.

Columbia University, Palisades, N.Y., and .:.

S. Kh. Negmarullaev, Tadjik Institute of Seismoresistant

Construction and Seismology, Dushanbe. Tadjikistan, U.S.S.R.

This report is Contribudon No. 2712 of the Lamont-DohPrty Geological Observatory. Co­lumbia T1 niversity. The research is supported by lhe National Sdence Foundation.

Articles in previous issues of the Earthquake Informadon Bu.lledn have described the seismic activity that has sometimr~ occurred when large reservoirs have been impounded. !'iot all reser­voirs trigger earthquake activity. :-..bny of the world's largest res~rvoil!: have shown no ch:mge in seismicity: some relatively small reser;oirs have produced earthqua.l<es far in ex.ce-;s of th<'-;e estimated from the seismic hi,tory of the region. If dams are to be designed for ':lfl!ty. •.ve mu::1· understand what factors ..:rmtrnl \\ ht:~h\!r 1'r iOt

induced seismicity will occur. C.m '-"c! 11entify "safe" sites where there i;< Hrr:e pc•.::"iJi fa induced. st:is-..:i~h} '! It .1 .J:..r:: ·:::::-t ~.,t: .... ;•. =~ .!:' area thar may he c;ubjected to induced ,;c:,;)iJ;,.;-

Nurek Dam in Soviet Cer.t·1! -~.;.:= earth and rock fill, which s mcst earthquake 3haking.

~ ....... .:

T~.:: ~,..-:,..,... s •es;st3~"'t ·o

..

,,~

~ I

..

t. I

.. ..

;

~·I

I I

0

"'--

il

; ;

:} • I

,. I

II

I l I I ' j l ) ; ~

i l : I l I

I I I I . I . I

I I I

I I I I

I

I , I .I I ,

'•'

---

largest indu.::e..! !!ar!hquuke'? Are there pre\ en­tive measure!> that can be taken. such as proper m:tnipulation of the water level in the reservoir, that will reduce the possibility of a large induced earthquake'! To an~·.,er the!-e question!>. we must understand the mechunism bj which 1he rc.!.er­voir can modify seismicity.

Srudies of induced seismicity are also jmpor­tant in terms of earthquake prediction. Current models of the earthquake process stress the im­portance of fluids migrating within the Earth's crust. At sizeable reservoirs, we can set up large-scale experiments where the hydrological regime within the crust can be artificially mod­

ified. As part of a joint Soviet-American program

in earthquake prediction (see "Odyssey to Tad­zik," by C. V. K. Wesson and R. L. Wesson, Earthquake l.riformation Bulletin, vol. 7, no. l, p. 8-15; and "The Talas-Fergana Fault," by R. E. Wallace, Earthquake Information Bulle­tin, vol. 8. no. 4, p. 4-13), we have had a uruque opportunity to study induced seismicity associated witll the filling of the world's deep­est reservoir, the Nurek Reservoir in Soviet Central Asia {fig. 1). In a cooperative project between the Lamont-Doherty Geological Ob­servatory and the Tadjik Institute of Seismores-

'/

iswnt Construction and Sei~.molvg} rTI:-,.3:s we have installed a network of radiotelemct-;:rc~ 5eismograph starions around the Nurek Rc~er. voir, 60 k.Horneters southeast of the Tadjik car.. ital of Dushanbe (fig. 2). The 315-meter-h~<:L.

•eu

!\urek Dam h scheduled for completion in i9"iY Although the reservoir is only two-thirds fu::: activity during the early stages of filirlg h:..t!>

given us one of the best documented C;:tse~ Jf induced seismicity.

The exploitation of the water resources of the Vakhsh River is an integr-c1l part of the develop­ment of the resources of Tadjikistan. The use of the Nurek Reservoir to control water for irriga. tion pl)rposes will open extensive areas of land for cotton. the major export of the republic, and electricity from the dam's 2.7-milJion-hllowan generators will provide power for new alumi. num and chemical processing plants.

Tadjikistan is one of the most seismically ac. tive areas of the Soviet Union. The collision be. tween Asia and the Indian subcontinent that has taken place in the last 35 mi1lion years, and formed the Himalaya mountajn belt, continues to deform the Earth's crust, giving rise to exten­

sive mountain ranges and earthquake activity in Centra] Asia. The Jesign and construction of a high dam in such an area present impressive en-

KAZAKHSTAN

----·-····, ' ., . ,., .

) . UZBEKISTAN '··:

~-

TURKMENIA

KYZYL KUM O£SERT

AFGANISTAN

0 100 200km

KIRGHIZIA TIEN SHP..~ '•\. ,:o.\OUNTAINS

•'

CHINA

60"' 66• 72" 78•

Figure 1.-Map of Soviet Central Asia showing location of Nurek Reservoir on Vakhsh River in Tadjik­istan.

210

. . ...

-

......

·~

.. .. ..

,-:;:

KRA 0

Figure 2.-c.i liDOS. Th~ A ! to a centml i locations of ; the deepes< l the reservo1 1

gineering d' ble for mos· de£ign. Ext !>tl.ldies wer seismic risk! took part ir., ing out mot; LoEarth ~h:.l model tech·!

i and rock-fi 1 an earthquat

1

I

-si,_;." ~-' ,_, . .,..ri<,-~:,;,_ , . , . l -' }Eht;'ll;rlfijy·l;,:;:~·l' '';i)ittiJ:' -~-. -~I:.: .......:....., ___ ~_:_j_ _____ _:__..._ __ --'--_-~-...... -~--------:----------~-~"'----~.----} i . . . ' ' . . -~~~"~-:..:..- .. _ --~~t"'~...:.- ·_

I ' ---· ·'-~ I

I l j Seismol(lg~ tTISSS). i ori ;iioLel:::metered j )Unci Surd; Re~er-1 1e~st of the T<.:Jjik .:~ii·

""-.,...,.:J. The 315-::~el!:':-!"j~!--. i for completion in 1979. J·.S only t\\ o-rhirds ful!. ·:y stage~ of filing ha;. 'I

'~r documented ca~es of ... I~

;.: water resources of the i'ral pan of the deve]op­/.Tadjik:istan. The use of i ] fi . . ,~ontro water or Imga-r i:!xtensive areas of land I• .i ort of the republic, and I•

;'··s 2. 7-million-kilowan ;i:power for new alumi­f~;sing plants. l•" • • pe most seJsmJcally ac~ )· nion. The collision be­[11 subcontinent that has i-"~5 "}}" 1·:. mi Ion years. and :luntain belt, continues :; t: giving rise to ex ten­~ earthquake activity in -, and construction of a

CO j!present impressive en­ll I'

H-.. '!'1"'!~~--· )1 !l r i. h1 ;11'

:':~ AJ.,M~-ATA, i.a ..... ___ ._ ..... _1" P!llf ·····-· ;~. :r #"

I< (; I~ ~· l"

·~ lh I' .~ ... 1 .. ~

L ~~CHINA '. i' i• { ~

1 . .,

.. .... ~ . .

..::,_,.;. ·;;::;···it~~·..::.::;;.i.:.:..::;;.,~~~~~':::.::7:.:::::.:~.~~;~.;~~r; .:: ~"·.:..:.;:.::.:. ~·-::., ..... -..... ~:....i. , . ., ..

KRA l':il

c

TNG ;.

0 ® 12

0 DO

0 0

0

0

0

0 0

OSGC

0

0

c;SGZ

0 station locations

0 Soviet American

A 0 10 ICJLOMETER5

o three -component 0

seismograph station o OCT 25-DEC20,1976

~--~-------------------------------------------------------------~38" 69" 69"45'

Figure 2.-0ut/ine of the Nurek Reservoir and the location of Soviet and American .'\eismograph sta­tions. The American stations operated unattended for 1 year, cgnunuously transmiwng data by radio to a central recording station in Dushanbe, 60 km northwest of the dam site. The squares are the locations of earthquakes recorded during 1976. Most of the activity is concentrated directly beneath the deepest part of the reservoir. Earthquakes in 1971-72 began approximately 15 km southwest of the reservoir, and activity gradually migrate:i towards the presently active area.

gineering challenges. TISSS has been responsi­ble for most of the seismic aspects of the dam's design. Extensive seismological and geological studies were started in 1955 to determine the sei!;mic risk for the dan1 site. In addition, TISSS took pan in the d~sigr of the darn itself, carry~ ing out model studies ot the response of the dam to Eartit shaking, using beth analytical and scale­model :echniques. The Nurek Dam is an earth and roc.k~fiiJed structure designed to withstand an earthquake of at least magnitude 6.5.

.. . . .. .·

-

Uniikc most other reservoirs where seismic instruments have been installed only after in­duced earthquake activity began, the seismolog­ical studies carried out by TISSS before con­struction of the dam have provided a unique se< of data defining the seismic regime before im­pounding. Since the water level first exceeded 60 min 1971, there has been a tenfold increase in the number of earthquakes occurring close to the reservoir. The strongest srquence, two earthquakes of magnitude 4.5, oc~urred in No-

2ll

. •· . ..

. . ... ··

~I i

; I

,l

..

•:

ik t t *'"' (/~b...'

. ,. !

vember 1972 just after the first stage of filling to 125m.

In addition to the increase in the number of earthquakes~ other changes were seen. These in­cluded the ratio between the numbers of large and small earthquakes, changes in the types of faulting that caused the earthquakes, and a mi­gration of epicenters toward the reservoir. None of the earthquakes at Nurek has yet been large enough to pose any threat to the safety of the dam.

The impounding of a large reservrsir modifies the stress conditions in the Earth· s ctust beneath the reservoir in two ways. First, there is an in­crease in elastic stress due to the dead weight of the reservoir. Second, there is an increase in pore pressure resulting from the infiltration of water into the surrounding rock. In a region of thrust faulting, such as is the case at Nurek, the natural tendency is for blocks of the Earth's crust to be thrust up and over adjacent blocks. Under such conditions, the weight of the reser­voir r•·c;!-:'lg downward opposes the natural stress system, strengthening fault zones and inhibiting earthquake activity.

Increased pore pressure, however, tends to force fault zones apart, weakening faults and in­creasing earthquake activity. Following a change in water level, there is a delay between the ef­fect of the load and pore pressure: the load stresses increase immediately, whereas the in­creased pore pressure is delayed by the time re-

!

~ • < • ••• •• : :. ; • :t· . . .

quired cor diffusion of water into fault zones. At Nurek we can actually see this time delay. When the water level increases, there is an ini­tial decrease in seismic activity, followed by an increased level of activity as the pore pressure rises.

One of the most interesting results from Nu­rek has been the effect which rapid changes in water level or rate of filling have on the seismic activity. The largest earthquakes, in November 1972, appear to be related to the abrupt stop in filling when the reservoir reached a depth of 100 m. When the water level is abruptly decreased (for example, in early March 1975 when the water level dropped by more than 3 meters per day), there is a pronounced increase in seismic activity as the stabilizing load is removed and the high pore pressure remains. When the water level is decreased less rapidly (for example, the two downdraws in I 974 when the rate did not exceed. 2 mid), the pore pressure appears to have time to dissipate with the load and no increase in seismicity is observed.

Based on our interpretation of the relationship between increased seismicity and rapid changes in water level, we recommended that the second stage of filling in late I 976 be carried out as smoothly as possible without any rapid fluctua· tions in either the water level or rate of filling. As figure 3 shows, the filling and emptying of the reservoir in 1976-77 was smoother than ear­lier fillings, except for a rapid 3-m change in

People of Central Asta. 212

-

•

I · .

200

180

160

-1<40]: :z:

:n 120:;: ~ .... 0 "? ICO ~ 9 .... "'

'10 80 ... c( .... ~ 0.. 30- 60 ..

~20 40_,' ......... IW

' -.. > IW 10 20

... 1..- •

"' )o

i! 1!5 2 ""

_.. ..... ·; a.. .. ... ro :;

"' .!a c( ..g "' -J

"' !5 . IZ:

'Z • IZ: ... "'

1969

Figure 3.-A con water level and 1 the bottom grapl: of the earthquak largest events (f• creass in activity large earthquake

water level at a outlet tunnel was t

in water level apf level of microear in August 1976. peaked at 215m . no large earthqunl the frrst filling in to prove conclush quake may have b ulation of the war,

Induced seismit phenomenon. In o large earthquakes l ervoir impounding curred soon after f then graJually dis. ~on that. after a sr haps tens of years. and the reservoir h mic regime than "' 'iUits from :'-iurek: :;1

of the way in whi may be possible tc in a safe manuer. '

I '

t :).1

_ _,:'.:;;J.~ .:;,r;, e;- \f.~.: '

.• tl: .. --· .. .,.-_ ...... *¢±

". . ~- ~ ::_.;:~~

on of water into fault zones. At ar.tually see this time delay level increases, there is an ini­::ismic activity, followed by an ·f activity as the pore pressure

1st interesting results from Nu-effect which rapid changes in

e of filling have on the seismic ~est earthquakes, in November be related to the abrupt stop in

' ;eservoir reached a depth of i 00 : ter level is abruptly decrea!'t:t.l ;: early March 1975 when the Sed by more than 3 meters per 1Jronounced increase in seismic abilizing load is removed ami ssure remains. When the water l less rapidly (for example, the

. in 1974 when the rate did not Lt1e pore pressure appears to have

with,-the load and no increase >bse( r

nterPremtion of the relationship )d seismicity and rapid changl!s 'e recommended that the second n late 1976 be carried out as ;ible without any rapid flucrua· e water level or rate of filling. vs. the filling and emptying of .976-77 was smcoth~r than ear­'!pt for a rapid 3-m change in

· . ...;::-, .

-«::!... .. .. :; .2.

~ .

1969 1970 1971 1972

\ (

1973 1974 l'+.d.4 .....

197!5 1976 1977

YEARS

Figure 3.-A comparison of water-/eve! and earthquake activity. The top part of the graph shows the water level and the number of earthquake's, greater than magnitude 1.5, at each 10-day interval. In the bottom graph, showing the square root of the energy released by the earthquake activity, the size ,;f the earthquake is included, so that the peaks in the graph show the tim~ of occurrence of the fa,gest events (for example, the two magnitude 4.5 earthquakes in Novemb~r 1972). Note the in­crease in activity in March 1975 at the time of the rapid emptying of the reservoir and the absence of large earthquakes as the reservoir reached 215m in December 1976.

water level at a depth of 160 m when a new outlet tunnel v·as tested. This small, rapid change in water level appears to have initiated the high level of mil.roearthquake activity which began in August 1976. As the water level smoothly peaked at 215 mat the end of 1976, there were no large earthquakes such as those at the end of the ftrst filling in 1972. Althoubil. it is difficult to prove conclusively, we feel that a large earth­quake may have been avoided by proper manip­ulation of the water level.

Induced seismicity appears to be a transient phenomenon. In other parts of the world where large earthquakes have been associated with res­ervoir impounding, the greatest activity has oc­curred soon after filling of the reservoir and has then gradually disappeared with time. We rea· son that, after a sufficient length of time, per­haps tens of years, a new equilibrium is reached and the reservoir has no more effect on the seis­mic regime than would a natural lake. Our re­sults from Nurek sug2est that bv careful comrol -- .. of the way in which the rl!servoir is filled. It may be possible to reach this new equilibrium in a safe manner. We could control the rate of

stress buildup and the pore water pressure so that seismic energy is released as small earth­quakes and larger, more destructive ones can be avoided.

Fu. ·~er reading Gough, D. 1., and Gough, W. I., 1970, Load-In·

duced Ea!"thquakes at Lake Kariba: Geophysical Jour­nal of the Royal i!rtronomical Society, val. 21, p. 79-101.

Gupta, H. K .. and Rastogi, B. K., 1976, Dams and Earthquakes: Elsevier, Amsterdam. :!29 p.

Judd, W. R., editor, 1974, Seismic Effects of Res­ervoir Impounding: EngiMering Geology, special i~­sue, vol. 8, p. 1-22.

Milne. W. G., editor. 1976. Induced Seismicit;.: Engineering Geology. special issue. val. 10, p. ~3-385.

Rothe, I. P .• 1973, A Geophy<;ics Retxm: In Man ~fad~ Lakes-Their Problems .md Environmental Ef­fect'i: American Geophy~·ical Union. Geophysic:U ~tonograph 17, p. 41-454.

Simp~on. D. W .. 1976, S.:tsmicity Changt:s ,·\"'o· ciart:d with Reservoir Lottding: En~ineering Geolo~t.'. vol. to. p. 123-150.

. ~"·· ---· -~·.- _,.,. ·- .•.. ,._. ~·~-~--~· »~·~-··"'

' . •

r

·.1 'I ,'l 'I

;I

..

..

1 ' I

' l r i

I (

.J .;.,., . ,-:-. ~

_j!

'