Denver Area Earthquakes and Rocky Mtn Arsenal Disposal Well-evans 1970-2nd Folder

8/12/2019 Denver Area Earthquakes and Rocky Mtn Arsenal Disposal Well-evans 1970-2nd Folder

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f l i E D E N V E R A R E A E A R T H Q U A K E S AN Dr i l E ROC KY MOUNT AIN ARSE NA L D ISPOSAL WE L L

i V l D M . EV AN S : Co n s u l t i n g Ge o l o g i s t , D e n v er . Co l o ra d o

ual'RACT: During 1961, a deep well was drilled at the Rocky Mountain Arsenal northeastofV n v e r , Colorado, to dispose of contaminated waste water. The well is bottomed in 75 feetof

, , ly fractured Precambrian gneiss. Pressure injection of waste water into the fra ctur ed Pre-. i b r i a n rock was begun in March 1962. Since the start offluid injection, 710 Denver-area earth-

ikes have been recorded. The majority of these earthquakes had epicenters within a five-mile. ' u i i u s of the Arsenal well. The volume of fluid and pressure offluid injection appear to be directly- . - a t e d to the frequency of eart hquakes. Evidence also suggests that rock movement is due to the-crease offluid pressurewithin thefractured reservoir and that openfractures may exist at depthsthan previously considered possible.- ,r a o D U C T i O N

Products for chemical warfare have been...inufactured on a large scale under the direc-

j n of the ChemicalCorps of the U. S. Army.; -.he Rocky Mountain Arsenal since 1942. A;.-product of this operation is contaminatedi i = te water and, until 1961, this'waste water. 1 5 disposed of by evaporation from dirtreservoirs (Scopel, 1964).

W h e n it was determined that Arsenalj . . i d t e water was contaminating the local ground- a t e r supplyand endangeringcrops (Gahr,,:ol; Walker, 1961), the Chemical Corps tried-.aporation of the contaminated waste fromliter-tight reservoirs. This proved unsuccess-:.::. The Chemical Corps and the Corps ofEngineers then decided todrill an injection dis-j o s a l wel for the purpose of disposing of thejontaminated waste water (Scopel, 1964).

The U. S. Army Corps of Engineers,Jmaha. District, commissioned the firm of.. A. Polumbus, Jr., and Associates, Inc.,~ design the well, supervise the drilling and^Jmpletion, provide thenecessary engineering.eological services, and manage the project.-ouis J. Scopel, as anassociate, was theP r o j e c t Geologist and was responsible for all.?ological aspects of the operation. Another.cological associate was George R. Downs,

Figure 1. Structural map of a port ion of theDenver- Jul esbur g Basin (after Anderman andAckman, 1963), showingthe location of theRocky Mountain Arsenal well.

- Mountain Geologist , v. 3, no. 1, p. 23-36 23

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Figure 3. Diagram and log of the Rocky Mountain Arsenal Injection Disposal Well (Scopel, 1964).

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FOUNTAIN

REGO LI THM A R O O N - R E D B R O W N S H A L EMAROON Q UARTZITEORDOVICIAN ? CAMBRIAN?

fra ctur es observed were partially tocompletelycemented with quartz.

ORANGE QUARTZ CONGLOMERATE TESTIN GOF THE WELL

11 970FRACTURES

PURPLE S H A L E

PINK DOL OM I T EPURPLE SH A L E P R E - C A M B R I A N SCHISTGREEN, MICACEOUS

h- P R E - C A M B R I A N GNEISS

12 45 TOTAL DEPTHFigu re 4. Log of pre-Pennsylvanian portionof disposal well (Scopel, 1964).

tains brown tocopper-colored mica andkaolinite. The pre-Pennsylvanian sedi-ments and the Precambrian werenotcored.The Precambrian schist is immediatelyabove highly fractured hornblende granitegneiss which contains pegmat iteintrusions.The top eight-foot section of the gneiss wascored. Hedge and Walthall ( 1 9 6 3 ) havedated the gneiss to be 1, 350 x 10 yearsold.A portion of the core mentioned above was

examined by the present author. The fracturesobserved were almost vertical and from one-half-inch to twoinches apart. Whentakenfrom the core barrel, the core was found to besplit apart along one fracture plane, and thelack of cementingmaterial su ggeste d that thismight have been an open fracture. The other

A drill stem test was taken of the basalFountain Formation, the pre-Pennsylvanianrocks and Precambrian rocks from the bottomof the 8-5/8-inch casing at 11, 171feet to thetotal depth of 11,985feet. Recovery was5,400 feet of salt wat er, in addition to 2 000feet of water cushion, in 156minutes . Ninety-thr ee-mi nut e final shut-in pr ess ur e was 4, 128pounds, measured at 11,002 feet . Densityofthe water was 1.05 gm./cc.

The well was drilled ahead to 12, 045 feetwhere it was completed in Precambrian gneissConsiderable lost circulation was experiencedwhile coring, testing, anddrilling the Precam-brian gneiss from 11,970 to 12,045 feet.

A 5-1/2-inch liner was cemented five feetinto the Precambrian gneiss from the bottom64feet of the 8-5/8-inch casing. Five-and-one-half -inch t ubing was run to 9,011 feet tocomplete the well.

During November and December 1961aconventional oil field pump was run in the well,and pumpingtests were condu cted. Afterpumping 1, 100barrels of wat er, a quantity inexcess of the amount of fluid that had been lostinto the formation duringdrilling operations,the well pumpeddown and fluid recovery be-came negligible. It was concluded, at the timeof testing, that fluid recovery was from frac-tures. It was further believed that as fluidwas withdrawnfrom these fra ctu res, theywere squeezed shut bycompressive forceswhich restricted fluid entry into the well bore.Pressure injection tests were conductedonthe well dur ing January 1962 to determine therates and injection pressures at whichthe Pre-cambria n would take the fluid. As a resultofthese tests, it was noticed that calculationsofthe drainage radius andformation capacity in-creased as fluid was injected (se e Calhoun,1953, for more on reservoir calculations).

As a result of the testing program, it wasconcluded that the formation would take freshwater at 400 gallons per minute under 650pounds pressure, and that the reservoir con-sisted of fra ctu res which expanded as additionavolumes of fluid wer e injected.THE PRESSUREINJECTIONPROGRAM

Contaminated waste from the Arsenal

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plants is first collected and allowed to settle ina two-hundred-million-gallon waste- sett lingbasin that is sealed with anasphaltic mem-brane to prevent seepage. It is then flocculatedand clar ified. Next it is filtered toless than20 parts per million ofsuspended solids lessthan 5microns in diameter. It is sterilizedandmonitered for bacteri a, then pumped intothe well. Four 130-horsepower positive-displacement electric pumps are available.Normally, two or three pumps are u sed.

The first contaminated wastewas injectedinto the well dur ing March 1962, when 4.2million gallons of waste were injected into thewell. The monthly volume ofwaste injectedinto the well is shown in the lower halfoffigure 5. During the first year ofoperation,considerable trouble was experienced with thefilter plant with the result that the injectionwell was often shut down for a fewdays orweeks at a time. From March 1962 untilSeptember 1963the maximum injection pressureis reported to have been about 550 pounds, witha fluid injectionrate of 200 gallons per minute.

At the end of September 1963the injectionwell was shut down, and no fluid was injecteduntil operations were resumed 17 September1964. During the shut-down period, surfaceevaporation, from the sett ling basin, wassufficient to handle the plant output .

From 17 September 1964 until the end ofMarch 1965, injection operations were resumedby gravity discharge into the well. Nowell-head pressure was necessary to inject themaximum of 2. 4million gallons ofwaste permonth into thewell.

Beginning inApril 1965 larger quantitieso f fluid were injected. The filter plant operatedefficiently, and fluid was usually injected 16 to24 hours daily. During Apriland May a maxi-mum pump pressure of 800poundswas required.From June to the end of September 1965 amaximum pressure of 1,050 pounds was re-quired to inject 300gallons per minute into thewell.THE DENVEREARTHQUAKES

The U. S. Coast and Geodetic Survey re-ports that on 7 November 1882an earthquakewas felt in the Denver, Louisville, George-town, and S. E. Wyoming area (Wang, 1965).From that date until April 1962no earthquake

epicenters were recorded in the Denverareaby either the U. S. Coast and Geodetic Surveyor by the Regis CollegeSeismological Observa-tory, located ten miles southeast of theRockyMountain Arsenal well (Joseph V. Downey,personal communication, 1965).

During Phe period from April 1962to theend of September 1965, 710 earthquakes wererecorded with epicenters in the vicinity of theArsenal at theCecil H. Green Observatory,Bergen Park, Colorado, operated by theColorado School of Mines (Pan, 1964; Wang,1965; Jones, 1965, Mines Magazine, 1965).

The total number of earthquakes reportedin the Denverarea is plotted in the uppe r halfo f figure 5. The magnitudeof the earthquakesreported range from 0. 7 to 4. 3 on the Richterscale. Table 1lists the earthquakes in Colo-rado of magnitude 3 and lar ger, a ccording tothe U. S. Coast and Geodetic Survey reports(Wang, 1965). Wang ( 1 9 6 5 ) calculated the epi-centers and hypocenters of the 1963-65 Denverearthquakes, and the results of his calculationsare shownin figure 6.

The majority of the earthquake epicentersare within a five-mile radius of the well. Al lepicenters calculated from four or more re-cording stations are within sevenmiles of thewell.

Wang (1965) calculated the best-fittingplane passing through the zone of hypocenterscalculated from four or more recording stations.He concluded that this plane might be a faultalong which movement was taking place. Theplane dips to the east, and passes beneath thearsenal well at a depthofabout six and one-half miles (fig. 6).ROCK MOVEMENTAND EARTHQUAKES

An at tempt has been made to develop amethod of estimating, directl y from seismo-grams, wave energy radiated duringa n earth-quake. Using the formula favored byTocher(1964) and Richter (1958), the elasticwaveener gy of a magnitude 3 earthquake could beprovided by dropping a 100foot cube of rock adistance of a few feet.

Admit tedly, the formula applies to distantearthquakes and is not routinely applicable tolarge number of earthquakes, but it does suggestthat the Denver earthquakes may be caused byrelatively minor rock movements.

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E X P L A N A T I O N

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Figure 6. Earthquakehypocenters during 1963-64 from local seismological stations in theDenverarea (afte r Wang, 1965). All epicenters calculated from four or more reco rdi ng stations are withinseven miles of the Arsenal well. Al l hypocenters calculated from four or more recording stationsare *ithin area indicated on section A - A.

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A V E R A G E N U M B E R O F D E N V E R E A R T H Q U A K E SP E R M O N T H

AVERAGEGALLONSOFWASTEINJECTEDPERMONTHINRSENALDI SPOSALWELL

_MAXIMUM INJECTION-PRESSURE 550LBS- NO WASTEINJECTED

^INJECTEDY GRAVITYMAXIMUMINJECTION}PRESSURE1050 LBSA

1962 1963 1964 1965Figure 7. Earthquake fre quenc y - waste injection relationships during five characteristic periods.

indicates that as fluid was pumped out of thereservoir thefractur es closed, and as fluidwas injected into the reservoir the fracturesopened. In other words , the pumping and in-jection tests indicated that rock movementoccurred as fluid was withdrawnor injected atrelatively lowpressures.

The pressure-depthrelations of the Pre-cambrian reservoir, showing hydrostaticandlithostatic pressure variations with depth, areshown in figure 8. These data weredeter-mined from thedrill stem test. As shownonthe chart, the observed pressure of the Pre-cambrian reservoir is almost 900poundslessthan hydrostatic pressure.

Hubbert and Rubey(1959) devised a simpleand adequate means of reducing by the requiredamount thefrictional resistance to the slidingof large overthrust blocks down very gentle

slopes. This arises from the circumstancethat the weightof such a block is jointly sup-ported by solid stress and the pressure ofinter-stitial fluids. As the fluid pressure approachesthe lithostatic pressure, correspondingto aflotation of the overburd en, the sheer stressrequired tomove the block approaches zero.

If highfluid pressures reduce frictionalresistance and permit rocks to slide down verygentle slopes, it follows that, as fluid pressureis decreased, frictional resistance betweenblocks ofrock is increased, thus permittingthem tocome torest onincreasi ngly ste epslopes. The steeper the slope upon whichablock of rock is at rest, the lower the requiredraise influid pressure necessary toproducemovement.

Inthe case of the Precambrian reservoirbeneath theArsenal well, these rocks were at

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F O R M A T I O N P R E S S U R E(Ib./in2)2000 4000 6000_8000 10.000 12,000 14,000

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OBSERVED ^uvnoncTATirPRESSURE \\HYDROSTAT1C^oFpc W A T E R \ PRESSUREpC W TERFigure 8. Pressure-depth relations, Pre-cambri an reser voir, Rocky Mountain Arsena lDisposal well.

equilibrium onhigh-angle fracture planes witha fluid pressure of 900 pounds less than hydro-static pressure before injection began.

As fluid was injected into the Precambrianreservoir, the fluid pressure adjacent to thewell bore rose, and the frictional resistancealong the fracture planes was thereby redu ced.When, final ly, enough fluid pressure wasexerted over a large enough areamovementtook place. The elastic wave energy releasedwas recorded as an earthquake.

Since the formation fluid pressure is 900pounds sub-hydrosta tic, merely filling the holewith contaminated waste (mostly salt water)raises the formation pre ssu re 900 pounds, orto the equivalent of hydrostatic pressure. Anyapplied injection pressureabove that of gravityf l o w brings about an increase in press ure re-sulting in a total in excess of hydrosta ticpressure. For example, an injection pressureo f 1, 000 poundswould raise the reservoirpressure adjacent to the well bore 1, 900pounds, or by the amount neces sa ry to bri ngthe pressure to hydrostatic (byfilling the hole)plus 1,000 pounds.

Apparently a rise influid pressure withinthe Precambrian reservoir of from 900 to1,900pounds is sufficient toallow movementto take place.

OPENFRACTURESThe hypocenters in the Arsenalarea

plotted from data derived from four or morerecording stations indicate that movement istaking place beneath the Arsenal well at depthso f from 1-1/2 to 1Zmiles. If the Precambrianfracture system extends to a depth of 12miles,thenfluid pressure could be transmitted to thisdepth bymoderate sur face injection pr essur eas long as the fracture system was open for thetransmission of this pressure.

Secor (1965) concluded that open fracturescan occur to great depths even with only mod-erately high fluid pr essur e-overbur den weightratios. Itappears possible that high-angle,open fractures may be present beneath theArsenal well at great depths with much lowerfluid pressure-overburden weight ratios thanha s formerly been considered possible.

Almost 150 million gallons of contaminatedwaste had been injected into theArsenal wellby the end of September 1965. Since thisamount ofwater would be enoughto fill fourcontinuous I/16-i nch fra ctu res each sevenmiles long (the maximum distance of epicentersfrom the welllocated byfour or more record-ing stations) and five miles deep, it can beseem that a relatively small area is beingaffected by the injection program.TIME LAG BETWEEN FLUID INJECTION ANDEARTHQUAKES

The correlation offluid injected withearth-quake frequency (fig. 5) suggests that a timelag exists between the two. Bardwell (1966)notes that the frequency of Denver earthquakesappear s to lag injected waste by approximatelyone to four months. This phenomenon is prob-ably the same as that described by Serafim anddel Carnpo (1965). They describe theobservedtime la g between water levels in reservoirsand the pressures mea sur ed in the foundationsof dams, and ascribe this to an unsteady stateof percolation through open joints in the rockmass due to the opening and closing ofthesepassages resulting from internal and externallyapplied pressures.

The time la g between waste injected in t h e^_ _Arsenal welland earthquake frequency isprobably due to an unsteadystate ofpercolationthrough fractures in the Precambrian reservoir

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due to the openingand closing ofthese fract uresresulting from the applied fluid pressure of theinjected waste. The delayed application of thispressure at a distance from the wellbore isbelieved totrigger themovement recorded asan earthquake.EARTHQUAKES DURING SHUT-DOWNPERIOD

In consideringthe earthquake frequencyduring the year the injection well was shut down,it is unf ortu nat e that nei the r periodic bottom-hole pres su re te sts nor checks of the fluidlevel in the hole wer e run. Had these measure-ment s been made , then specul ation as to howlong it took for the bottom-hole pressure todecline would have been un nece ssa ry.

By the end of September 1963, 10 2.3million gallons offluid had been injected intothe well. It is believed that this injection hadraised the fluid pressure in the reservoir forsome distance s urro undin g the wellbore. Dur-ing the shut-down period this elevated pressurewas equalizing throughout the rese rvoir and atincreasing distances from the well bore. Asthis fluid pre ssu re redu ced the frictional re-sistance in fra ctu res fart her away from thewell, movement occurre d, andsmall earth-quakes were th eresult.SUMMARY AND CONCLUSION

Th e Rocky Mountain Arse nal Pressure In-jecti on Disposal well was dril led for the pur-pose ofdisposingof contaminated waste water,which is a by-produ ct of chemical wa rfa reproducts manufacturedat the Arsenal.

During the monthfollowing the initiation ofinjection of waste water into the almost verti-cally fract ure d Precambrian rocks there weretwo earthquakes with epicenters in the Arsenalarea.

A summary of the evidence relating theArsenal injection program with the earthquakesis:

1. The fi rs t earthqua kes obser ved dur ingthe presen t century with epicent ers inth e Denver area were recor ded duringthe monthfollowing the initiation of theArsenal injection program.

2. Since the initia tion of the inject ion pr o-gram in Mar ch 1962, 150 million gallonsof waste have been injected into theArsenal well, and ther e have been 710earthquakes (to 1October 1965).

3. The majority of the earthquake epi-centers are located within five milesofthe Arsenal well. All epicenters deter -mined from four-or-more station dataare within sevenmiles of the well.

4. There is evidence that the earthquakeactivity is taking place along a planethat dips eastward and passes beneaththe Arsenal well at a depth of 6. 5miles(Wang, 1965).

5. When theArsenal injection program isconsidered on thebasis of high, medium,low, or no injection, the re is acorrela-tion between the fluid injected and earth-quake frequency.

6. Thebest correlation ofearthquake fre-quency with fluid injected occurredduring July, August and September 1965,when relatively large amounts of fluidwere injected at higher pressuresa ndfor longer per iods of time than pre-viously.

7. Astatistical analysis (Bardwell, 1966)is cited that suggests a mathematicalrelat ionship bet ween Arsenal earth-quakes and volumes ofwaste injectedinto theArsenal well.

The volume of fluid injected appears to beaffecting the Precambrian reservoir only for alimited distance from the well bore, and rou ghestimates of the energy released by a singleearthquake suggest that relatively minor rockmovement is involved.

The Precambrian reservoir receivingt heArsenal waste is highly fractured granitegneiss of very low permeability. The fracturesare nearly vertical. The fra ctu re porosityofthe reservoir is filled withsalt water. Reser-voir pressure is 900 pounds sub-hydrostatic.

Itappears that movement is taking placein this fractured reservoir as a resultof theinjection ofwater at pressures from 900 to1,950 poundsgreater than reservoir pressure.

Hubbert and Rubey (1959) point out thatrock masses in fluid-filled reservoirs aresupported by solid stress and the pressureofinterstitial flu ids. A s fluid pressure a pproacheslithostatic pressure, th eshear stress requiredto move rock masses down very gently slopesapproaches zero.

It appears that these principles offer anexplanation of the rock movement in theArsenal res ervoir . The highly fractured rockso f the reservoir are at rest on steep slopesunder a condition of sub-hydrostatic fluid

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press ure. As the fluid pressure is raisedwithin the res ervoi r, frictional resistancealong fracture planes is redu ced and, even-tually, movement takes place. Theelasticwave energy released is recorded as an earth-quake.If earthquake hypocenters indicate thepoint atwhich movement is taking placeandinjected fluid is triggering this movement, thenthere is evidence that open fractures exist atdepths of 12miles under conditions oflowerf l u i d pressure-overburden weightratios thanhas formerly been considered possible (Secor,1 9 6 5 ) . It is believed that the highangle of thisfracture system is an important factor. Be-cause the fra ctu res a re almost vertical , only asmall par t of the lithostatic pre ssu re is actingtoforc e the fra ctu res closed, and they can re-main open under conditions oflower fluidpressure, and at greater depth than if theywere horizontal or inclined at a lower angle.

The time la g betweenfluid injection andearthquake frequencyis believed to be due tothe unsteady state of percolation of fluid throughthe fractures in the reservoir due to the open-ing and closing of these passages resultin'gfrom the applied pressure of the injected waste.

It is believed that as fluid continues to beinjected into this reservoir fluid pressure willbe increased at greater distances from the wellbore, and rock movement will be occurring atever increasing distances.

In the present case it is believed that astable situation in this Precambrian reservoiris being made unstable by the application off l u i d pressure. However, it is interesting tospeculate that the principle ofincreasing fluidREFERENCESAnderman, G. G. , and Ackman, E. J. , 1963,

Structure of the Denver-Julesburg Basinand surroundingareas: inRocky Mtn.Assoc. Geologists Guidebook to the Geologyof the northern Denver Basin and adjacentuplifts.

Anonymous, 1965, Geophysical observatory.Mines Mag., v. 55, no. 5, p. 24-26.

Bardwell, G. E. , 1966, Some statisticalfeature of the rela tionship between RockyMountain Arsenal waste disposal andfrequency of earthquakes: TheMountainGeologist , v. 3, no. 1, p. 37-42.

Boos, C. M. , and Boos, M. F., 1957,Tectonics of eatern flank and foothills ofthe Front Range, Colorado: Am. Assoc.Petroleum Geologists Bull., v. 41, p.7.603-2676.

pressure to release elastic wave energy mighthave an application in the subject of earthquakemodification. That is, it might someday bepossible to relieve the stresses along somefault zones in urbanareas by increasing thefluid pressures along the zone using a seriesof injection wells. The accumulated stressmight thus be released at will in a seriesofnon-damaging earthqua kes instead of eventuallyresulting in one large event that might cause amajor disaster.ACKNOWLEDGEMENTS

The author is especially obligated to BenH. Parker, whocritically read the manuscriptand made many valuable sugges tions. Thanksarealso due to Lt. Col. Martin J. Burke, Jr.,Commanding Officer of the Rocky MountainArsenal, Lt. Col. William J. Tisdale, Direc-tor of IndustrialOperations, and to the ArsenalEngineeringDepartment, for aid incompilingthe well and inject ion data; John C. Hollisterand Maurice W. Major of the Colorado Schoolo f Mines Geophysical Engineerin g Department,for help in compiling the earthquake data;Joseph V. Downey, S. J. , Director of theRegis College Seismological Observatory, forearthquake data prior to 1962; George E.Bardwell, for undertaking the statisticalanalysi s, present ed els ewhere in this iss ue;L. Tro wbridge Grose and David T. Snow forinformation concerning the role of fluid pres-sure in jointing; Harry C. Kentand David A.Moore for critically reading the manuscript;and to John A. Rathbone and Charles C. Worksfor suggestions concerning the statisticalanalysis.Calhoun, J. C . , 1953, Re ser voir rocks and

rock-fluid systems, Part II, Fundamentalso f Reser voir Engineering, Norman,University ofOkla. Press.

Gahr, W. M. , 1961, Contamination ofgroundwater, vicinity of Denver (abstract):Symposium on Water Improvement, Am.Assoc. Adv. Sci. , p. 9-20.

Hedge, C. E., and Walthal l, F. G. , 1963,Radiogenic Strontium-87 as an indexofgeologic proces ses: Science, v. 140,p. 1Z14-1217.

Hubbert, M. K., and Rubey, W. W. , 1959,Role of fluid pressure in mechanics ofoverthrust faulting: Pt. I, Mechanicsoffluid-filled porou s solids and its appl ica-tion to overthrust faulting: Geol. Soc.America'Bull., v. 70, p. 115-166.

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, 1961, Role of flu id pressurein mechanics of overthrust faulting, areply: Geol. Soc.America Bull. , v. 72,p. 1445-1452.

Jones, P. B., 1965, Derb yarea earthquakes,1964 : Colorado School of Mines, Dept.of Geophysics, unpublished report.

Mechem, O. E. , and Garr et t, J. H. , 1963,Deepinjection disposal well for liquidtoxic \vaste: Proc. Am. Soc.Civil Eng. ,Jour. ConstructionDivision, p. 111-121.

Pan, Poh-Hsi , 1963, The 1962 ea rthqu akesand micro-ea rthqua kes near Derby,Colorado: unpu blished M. Sc. thesis,Colo. School of Mines.

Richter, C. F., 1958, Elementa ry seismology,SanFrancisco, W. H. Freeman & Co.

Scopel, L. J. , 1964,Pressure injectiondis-posal well, Rocky Mountain Arsenal,Denver, Colorado: The Mountain Geologist,v. 1, no. 1, p. 35-42.

Secor, D. T., Jr., 1965,Role of fluidpressure in jointing: Am. Jour. Sci. ,v. 263,p. 633-646.

Serafim, J. L. , and del Campo, A ., 1965,Interstitial pressures on rock foundationsof dams: Jour, of the Soil Mechanics andFoundations Div., Proc. Am. Soc. ofCivil Engrs., v. 91, no.SM5.

Tocher, D. , 1964, Earthquakes and ratingscales: GeoT imes , v. 8, no. 8, p. 15-20.Walker, T. R. , 1961, Ground wat er conta mi-nation in the Rocky Mountain Arsenal area,Denver, Colorado: Geol. Soc. AmericaBull., v. 72, p. 489-494.

Wang, Yung-Hang, 1965, Localhypocenterdetermination in linearly vary ing la yersapplied to earthquakes in the Denver area:unpublished D. Sc. thesis, Colo. SchoolofMines.

Manuscript received December 20, 1965.

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Denver Area Earthquakes and Rocky Mtn Arsenal Disposal Well-evans 1970-2nd Folder

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