Permeability of Fault Gouge Under Confining Pressure and Shear Stress

Home , Fault gouge

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B5, PAGES 3193-3200, MAY 10, 1984

Permeabilityof Fault GougeUnder ConfiningPressure and ShearStress

C. A. MORROW

U.S. GeologicalSurvey

L. Q. SHI

StateSeismological Bureau, People'sRepublic of China

J. D. B YERLEE

U.S. GeologicalSurvey

The permeabilityof bothclay-rich and non-claygouges, as well as severalpure clays, was studiedas a function of confiningpressures from 5 to 200 MPa and shearstrain to 10. Permeabilityranged over four orders of magnitude,from around 10 -22 to 10-18 m 2 (1 darcy= 0.987x 10-12 m2). The lowest values were characteristicof the montmorillonite-richand finer grainednon-clay gouges. Illite, kaolinite,and chloritehad inter- mediatepermeabilities, while the highestvalues were typical of the serpentineand coarsergrained non-clay gouges.Grain sizewas an importantfactor in determiningpermeability, particularly for the clay-richsamples. The coarsegrained gouges were the mostpermeable and decreasedin permeabilityafter sheafing.Conversely, the fine grainedgouges had characteristicallylower permeabilitiesthat did not vary significantlyafter various amountsof sheafing.The permeabilitiesof the non-claysamples were not significantlydifferent than those of the clays. Therefore,comminuted rock flourscan be equallyas effectivein reducingthe flow of wateras the characteristicallylow permeabilityclay gouges.The strengthsof the sampleswere quite variable. The non-clay gougeswere consistentlythe strongest,with yield points(beginning of nonelasticbehavior) around 850 MPa, while montmorillonitehad an anomalouslylow strengthin relationto all the othergouges at 250 MPa. Strength of the saturatedsamples under drained (low pore pressure)conditions did not correlatewith high or low permeability.However, the low permeabilitiesof thesegouges could be a factorin the measuredlow shearstres- sesalong fault regionsif excesspore pressureswere createdas a resultof sheafingor compaction,and this pressurewas unable to dissipatethrough a thicksection of the material.

INTRODUCTION quakes,the fluid flow propertiesof fault gougematerials as po- tentialbarriers to the migrationof pore fluids meritsa more de- The fluid flow and mechanicalproperties of gougematerials, tailedexamination. bothclay-rich and non-clay, are of particularinterest to the dis- In a previousstudy [Morrow et al., 1981],we foundthat the cussionof fault zonebehavior. Clay-rich fault gougesare corn- permeabilityof a montmorillonite/illite-richSan AndreasFault monand have been observed todepths of 2 kmin deepmines and gougereached values of around10 -22 m 2 (about0.1 nda;1 darcy tunnels [ Brekke and Howard, 1973]. Although the extent to = 0.987x 10-12 m 2) at stressesup to 200MPa and that the per- whichclays persist at greaterdepths is difficult to verify, Wu et meabilitydid not vary significantlywith shear.We wish to deter- al. [1975] and Wang et al. [1978] argue that clay gougesmay mine whether this rather limited observation is true of different existto depthsof 10 or 12 km and thereforegreatly influence the types of gouges from other locations and whether clay-free mechanical behavior of fault zones. gougesexhibit similar propertiesto the clays. To this end, we Laboratorymeasurements [Wang et al., 1980; Summersand haveinvestigated the permeabilityand shearstrength of a variety Byedee, 1977; Morrow et al. , 1981; Byerlee, 1978] have deter- of fault gouges,including pure clays, mixed-compositionclay minedthat clay gougestypically support lower shearstresses than gouges,and clay-free comminutedrock gouges,with particular mostgranitic rocks during frictional sliding experiments, particu- interestin the effect of grain size and the comparisonbetween the larly whensaturated, and have extremely low frictionalresistance clay andnon-clay samples. when pore fluid movementis restrictedand fluid pressuresbe- comegreater than hydrostatic. GOUGE DESCRIPTIONS High porepressures at depth,regardless of rock type, can lead to seismicityin areaswhere rocks are nearfailure. This condition The gougesused in this studywere collectedfrom several1oca- has been identified as the cause of numerous reservoir induced tiog•sin Californiaalong the San Andreasand nearbyfaults (Fig- earthquakes,where diffusion of impoundedwater into underlying ure 1). The San Andreas locations included Strain Ranch (Marin rockshas triggered fault movement[Talwani, 1981;Zoback and County), CienegaValley (San Benito County), Dry Lake Valley Hickman, 1982]. In light of the obvioushazards to dams and (San Benito County), Tejon Pass(Los AngelesCounty) and Big otherengineering structures adjacent to earthquakegenerating re- Pines (Los Angeles Countymnot to be confusedwith the Big servoirs,and the potentiallydestructive nature of naturalearth- Pinesfault). Locationsnearby but not on the San Andreasfault includedthe Haywardfault at D St. in downtownHayward and This paperis not subjectto U.S. copyright.Published in 1984 by the near the Golden Gate bridge in San Francisco.Sampling was AmericanGeophysical Union. from surfaceoutcrops, with the exceptionof the CienegaValley and Dry Lake Valley gouges,which were takenat depthsof 402 Papernumber 3B 1693. and 218 m, respectively,from holesdrilled into the San Andreas

3193 3194 MORROWET AL.' PERMEABILITYOF FAULT GOUGE

HAYWARD FAULT STRAIN RANC D STREET HAYWARD GOLDEN GATE BRIDGE

CIENEGAVALLEY • ? '•,DRYLAKE VALLEY "•,,

•- .•._•.• BI g PINES ",,,

'kilo:!:ers ' •____;.I.,)

Fig. 1. Samplelocations nearby and along the SanAndreas fault in California. fault [Zobacket al., 1980]. Pure clayswere also includedin the Mineralogicalcompositions of the clay-richgouges determined study for comparison.These were montmorillonite(Chambers, by X ray diffractionare given in Table 1. The CienegaValley, Arizona), kaolinite (Lincoln, California), and illite (Fithian, Il- Dry Lake Valley, and Tejon Passsamples are all similarand are linois). composedlargely of montmorilloniteand mixed layer (inter-

TABLE 1. Mineralogical Composition of Gouges and Clays Mineral Percentages (wt% + 2%)

Mixed Layer Name of Gouge or Clay Chlorite Kaolinite Illite Montmorilionite Mont/Illite Serpentine

Strain Ranch San 72 15 8 -- 5 Andreas Gouge Marin County Cienega Valley San 6 14 8 26 46 Andreas Gouge San Benito County Dry Lake Valley San 11 28 12 16 33 Andreas Gouge San Benito County Tejon Pass San 10 trace 20 38 32 Andreas Gouge Los Angeles County D St. Hayward 100 Fault Gouge Golden Gate Bridge 100 Fault Gouge Kaolinite 100 Lincoln, CA Montmorillonite 100 Chambers, AZ Illite 100 Fithian, IL MORROWET AL.' PERMEABILITYOF FAULTGOUGE 3195

TABLE 2. Big Pines Gouges

Sample Description*

BP-1 Massive gouge zone of comminuted grainite gneiss BP-3 Black comminuted rock from a I cm thick shear zone in gneiss BP-4 Pelona Schist fault slice gouge BP-5 Granite gneiss fault slice gouge

*Whole rock analyses are given in the work of Anderson et al. [1980]

layeredmontmorillonite and illite) clays. The StrainRanch gouge EXPERIMENTAL PROCEDURE is chlorite-rich,and the D St. Hayward and Golden Gate Bridge samplesare composedalmost entirely of serpentine(chrysotile). Cylindricalsamples of Berea sandstone25.4 mm in diameter Althoughthere may be other types of clay-rich gougesfound and 63.5 mm long were cut in half alonga planeoriented at an alongthe San Andreasfault, thesegouges are probablyrepresen- angleof 30ø to the axis (Figure 2). The sandstonehalves were tativeof the wide rangeof mineralogiesthat we would expectto separatedby a 1.0-mm thick layer of gougealong the saw cut. find. The gougewas presaturatedwith distilledwater, usingthe same The non-claygouges described in Table 2 werecollected from volumeof water and the sameprocedure each time. A 0.13-mm theBig Pineslocation, where the San Andreas fault separatesgra- thick coppersleeve held the two piecesof sandstoneand the nite gneissfrom the PelonaSchist. These gouges are essentially gouge together. The entire assembly was then jacketed in a rockflour equivalent of oneor theother of the parentrock com- polyurethaneto isolatethe samplefrom the confiningpressure positions.Their petrogenesisis describedin moredetail by An- fluid. dersonet al. [1980], who conductedthe whole rock analysesof The confiningpressure and inlet pore pressurewere held con- the gouges.We havemaintained the samesample labeling as in stantby a computercontrolled servo-mechanism. Pore pressure their study. was maintainedat 2 MPa at the inlet side, and atmosphericpres- Grain size analysesdetermined by both dry and wet sieving sureat the outlet, creatinga gradientacross which distilled water techniquesare givenin Table 3 for all the materialsstudied. The flowed. The flow rate of water throughthe gougewas determined CienegaValley, Dry Lake Valley, Tejon Pass, and pure clay by measuringthe changein volume of the pore fluid reservoir sampleswere similar, being composed chiefly of clay sizedparti- with time. Temperaturewas maintainedat 27ø -+ 0.5øCthrough- clesless than 0.045 ixm in diameter.The othergouge specimens out the systemto ensureaccurate pore volume measurements. hada widerdistribution of grainsizes. In addition,a seivedfrac- Permeabilitywas calculatedby usingDarcy's law: tion of the D St. Haywardgouge was madesuch that all grains were lessthan 45 mm in diameter. In this way one can compare theeffect of variablegrain size between samples of like composition. (1)

PORE PRESSURE INLET whereq is the volumetricflow rate,ixis the dynamicviscosity of water, A is the cross sectional area normal to the direction of flow, and dP/dx is the fluid pressuregradient across the gouge. O- RING SEAL Sincethe permeabilityof Bereasandstone is manyorders of mag-

STEEL PLUG nitudehigher than that of the gouge[Zoback and Byerlee, 1975], thepressure gradient in the sandstonewas essentially zero. Permeabilitywas measuredas a functionof confiningpressure for eachof the specimensby increasingthe hydrostaticload on the samesample in a stepwisefashion up to a pressureof 200 BEREA FAULT GOUGE MPa, and then stepwisedown to low pressure.After each in- SANDSTONE LAYER creaseor decreasein pressure,the flow rate wasrecorded for sev- eralhours to ensureflow equilibrium.

COPPER Using the same sampleassembly as in the hydrostatictests, SLEEVE - POLYURETHANE permeabilitywas determinedas a functionof sheardisplacement JACKET (O. lSmm) alongthe sawcutwhile at a confiningpressure of 200 MPa. An axial loadwas appliedto the sampleat a constantaxial strainrate of 10-6/sover the 63.5-mm long sample, resulting in a sliprate STE EL END PORE PRESSURE alongthe gouge layer of 7.33 x 10-5 mm/s.Permeability was PLUG OUTLET measuredafter 0.0, 0.5, 1.0, 3.0, 5.0, 7.0, and 9.0 mm of axial Fig. 2. Sampleassembly. shortening,corresponding to a maximum shear strain in the 3196 MORROWET AL.: PERMEABILITYOF FAULTGOUGE

TABLE 3. Grain-Size Distribution of Gouges and Clays

Grain-Size Distribution (wt%) Name of Gouge 0.25- 0.125- 0.09- 0.063- 0.045- or Clay 2 mm 1-2 mm 0.5-1 mm 0.5 mm 0.25 mm 0.125 mm 0.09 mm 00.063 mm 0.045 mm

Strain Ranch, 15.2 4.3 3.7 3.8 5.9 2.5 3.4 2.2 59.0 San Andreas Cienega Valley, 0.2 .... 0.1 0.4 0.3 0.7 0.5 97.8 San Andreas Dry Lake Valley, -- 0.1 .... 0.4 0.8 3.3 3.3 92.1 San Andrea s Tejon Pass, 0.1 0.3 0.3 0.8 2.2 1.3 1.5 2.2 91.3 San Andreas D St. Hayward 5.1 2.1 2.6 6.3 25.2 17.5 16.8 • 24.•------• Hayward Fault Golden Gate 0.5 1.3 3.4 13.1 37.8 18.5 25.4 Bridge Kaolinite ...... 0.2 0.2 99.6 Montmorillonite ...... 1.0 4.4 1.9 2.8 89.9 Illite ...... 1.3 3.0 1.4 6.i 88.2 Big Pines 1' 1 5 5 8 12 8 5 i5 4i Big Pines 3* 1 4 5 10 16 8 7 14 35 Big Pines 4* 1 1 2 3 5 7 8 • 64 Big Pines 5* 2 3 4 7 10 5 5 8 56

From Anderson et al. [1980] wt% values + 1%

gougelayer of 10.4 at the endof the experiment.The axial load low permeabilityrange. Progressivelyhigher permeabilities were wasremoved for eachpermeability measurement. In thisway, the characteristicof the illite, kaolinite,and StrainRanch samples, (5 permeabilitywas determined as a functionof shearstrain without x 10-2• to5 x 10-•9 m2).The highest permeability gouges were the added effect of differential stress. There was a small amount the serpentine-richGolden Gate Bridge and D St. Haywardsam- of strainrecovery during unloading due to theelastic nature of the plesand BP-1 and BP-3, with permeabilities ranging from 10 -•9 gouges,therefore measurements reflected a "maximum past to 10-n8m 2, atpressures upto 200 MPa. strain".During theseshearing experiments, the strengthof the Permeabilityas a functionof sheardisplacement is shownin saturatedgouge was also recorded.The testswere preformed Figure4. There are three distincttypes of behaviorexhibited under drained conditions as described in the work of Morrow et here.First, a markedreduction in permeabilitywith displacement al. [ 1981]. Measurementswere madeafter transientpore pressure thatis typicalof the higherpermeability samples (the serpentine, changeshad died away at eachnew confiningpressure. Therefore chloriteand kaolinitetypes, plus BP-1 and BP-3). Thesegouges excesspore pressure was not createdin the thin gougelayer dur- all experiencedan orderof magnitudedrop in flow rate as a result ing loadingthat may cause a loweringof theshear strength. of shearing.Thin sectionanalysis of the shearzones showed evi- dence of grain crushingand rotation in these coarser-grained gouges(particularly the serpentine).A secondtype of behavior RESULTS was observedwith the low permeabilityfiner-grained montmoril- 1onite-richgouges (Tejon Pass,Dry Lake Valley, CienegaVal- Permeability ley) and to a lesserextent the pure illite sample.Here therewas little changein permeabilityduring shear. The only significantre- The permeabilityof the fault gougesand pure clays ranged ductionin flow rate occurredduring the first millimeterof slid- over four ordersof magnitude,as seenin Figure3. Loadingand ing, as the samplescompacted during the onsetof shearing.Al- unloadingpaths are indicatedby the arrowson selectedcurves. though the BP-4 and BP-5 gouges (Pelona Schist and granite Permeabilitycharacteristically decreased with higher confining gneiss)had initial permeabilitieswithin the same range as the pressureas the samplesbecame more compacted.Upon unload- montmorillonitegouges, they did not exhibit a comparablebe- ingpermeability values were consistently less than they were dur- haviorwith strain.These two gouges,comprising the third group, ing loading. showeda markedincrease in flow rate after 5 mm of sliding. The montmorillonite/mixedlayer gouges(Tejon Pass, Dry Lake Valley, CienegaValley, andpure montmorillonite) had the Strength lowestpermeabilities; around 10 -2• m2 at 200 MPa confining pressure.This setof gougesall hadremarkably similar permea- From the differentialstress data recordedduring the displace- bility valuesat all confiningpressures regardless of the percen- mentexperiments we can plot strengthcurves under drained con- tagesof thefour basic mineral constituents in eachsample (Table ditionsat 200 MPa confiningpressure (Figure 5). The strength 1). The clay-freesamples BP-4 and BP-5 were alsowithin this and strainhardening characteristics of the clay samplesunder a MORROWET AL. ßPERMEABILITY OF FAULTGOUGE 3197

- • - "'"'""'""'""•ß BPI

_ - -• GOLDEN GATE

_ . D ST. HAYWARD _ BP:3

E - + v

• STRAIN RANCH I.- I0 -ao-- , KAOLINITE

_03 -_

O._ - ß ILLITE - iO_?_l___ • BP4

_ BP5 _ TEJON PASS _ DRY LAKE VALLEY - MONTMORILLONITE

CIENEGA VALLEY

0 50 I00 150 200 CONFINING PRESSURE (MP(]) Fig. 3. Permeabilityof pureclays, clay-rich fault gouges,and non-clay Big Pinesgouges as a functionof confiningpressure. The loadingand unloading paths exemplary of all the samplesare illustratedwith arrowson the illite curve.The permeabilities of theunloading paths are consistently lower than the loading permeabilities because compaction 6f thegrains is notcompletely reversible.

variety of different conditionsare describedin more detail in the with the time for fluid drainageor the gougeis boundedby less work of Morrow et al. [1982]. From Figure 5, we see that the permeablerocks). strengthsof the rock-flour and clay sampleswere quite distinct from eachother. The rock-floursamples, derived from a number DISCUSSION of different parentmaterials, all have very similar stress-strain curves,and supportedsubstantially higher frictional stressesthan The grain size distributions(Table 3) appearto be a factor in any of the clay samples.These values of shearstrength are con- the variablepermeability of all the gougesillustrated in Figure3. sistent with those previously reported for granitic type rocks The clay-rich low permeabilitygouges were all predominantly [Byedee, 1978]. composed(>90%) of fine grainsless than 0.045 mm in diameter. There appearsto be no systematicrelation between strength The higherpermeability clay gouges,on the otherhand (Golden and permeabilityfor these samples.Low permeabilitygouges Gate Bridge, D St. Hayward), had a more even distributionof coverthe entirerange of frictional stress,from the lower-strength grain sizes, from granulesto clay-sizedparticles. Only a small montmorillonite,to the predictably strong rock-flour gouges. percentof thesegrains were lessthan 0.045 mm in diameter.The Low strengthcan be equatedwith low permeabilityonly if the differencein the rangeof grain size is not as strikingwith the Big samplesare undrained(e.g., the gouge is thick in comparison Pines gougesas comparedwith the clays. But here again, the 3198 MORROW ET AL.: PERMEABILITY OF FAULT GOUGE

SHEAR STRAIN , 7' o 4 6 8 I0 12 I I I i I i I I I

* v•v BPI • I0-2ø

GOLDEN GATE

BP5

BP3

D ST HAYWARD

10-21_• STRAIN RANCH

• ILLITE(.)

KAOLINITE (+) %••-k••l-••. TEJONPASS •••••• • x MONTMORILLONITE(o)

i0-22 I I I I i i i i o 4 6 8 I0 DISPLACEMENT , • (mm) Fig.4. Permeabilitiesofpure clays, clay gouges, and non- clay gouges asa function ofshear strain (upper axis) and displacement(lower axis). The confining pressure during all experiments was 200 MPa. There are three types of behavior exhibited here: permeabilitydecreases after sliding for the coarse-grained higher-permeability gouges,permeability doesnot change significantly forthe fine-grained lower-permeability gouges, and two of the nonclay gouges show dilatant behavior after 4 mmof sliding. more coarsesamples BP- 1 and BP-3 (41% and 35% grains suitof dilatancyin thesamples, although we did notmeasure vol- <0.045mm, respectively) had higher permeabilities than BP-4 umetricstrain in orderto substantiatethis hypothesis.We can andBP-5 (64% and 56% grains <0.045 mm,respectively). imaginethat the rounder-shaped rock flour grains are more likely This grainsize-permeability relationship has been demonstrat- to causedilation than the flat clayplatelets as theyslide past one ed in the past[Beard and Weyll, 1973;Lambe, 1954], for both anotherto accommodateshear. It mustbe notedthat these experi- sandsand clays. We also observedthis effect with the sieved ,mentshave been run on samples that have been disaggregated and fraction(all grains<45 Ixm)of theD St. Haywardgouge, where then"reassembled" for studies in thelaboratory. Therefore, the permeabilityas a functionof confiningpressure paralleled the un- lowerpermeabilities observed after shearing due to grainalign- sievedgouge curves but was63% lower.These experimental re- mentmay be morerepresentative of in situpermeabilities than sultsare all consistentwith the Kozeny-Carmenequation [Bear, thosevalues obtained after hydrostatic tests. 1972]which states that permeability is proportionalto porosity The presenceof montmorillonitemay also be a factorin de- and inverselyproportional to specificsurface area. Bear [1972] terminingpermeability. All of the clay gougeswith per- derivesa numberof modificationsto the Kozeny-Carmenequa- meabilitiesof 10 -2• m:z or lesshave some major percentage of tionshowing that permeability is alsoproportional to grainsize, thishighly expandable clay. The degree to whichmontmorillonite poresize, and crack width. Unfortunately, we havenot measured swellswhen in contactwith fluids (thereby reducing flow through all theparameters necessary to testthese relationships. the sample),is in part dependenton fluid chemistry.Previous We notedin the last sectionthat the flow behaviorduring studies[Moore et al., 1982] on pure montmorilloniteshave shearing(Figure 4) seemedto fall intogroups depending on per- shownthat strong electrolyte solutions suppress the waterbond- meabilityrange and grainsize. The coarse-grainedgouges de- ingproperties of clayplatelets, and as a result,permeabilities can creasedin permeabilitywith shear, while the fine grained gouges be variedby ordersof magnitudein differentelectrolyte solu- didnot. One interpretation of this finding may be thatthe coarse tions.Since our studies on claygouges were conducted with dis- gougesare approaching ihepermeability of the finer ones as they tilledwater as the fluid medium, we feel thatthese results repre- are crushedinto smallerparticles and becomemore densly senta lowerbound on the permeabilitiesexpected from gouges packed.Grain crushing was noted in thin sectionanalyses of the containingexpandable clays. coarsersamples, particularly the serpentines.This doesnot ex- With informationon the strengthand fluid flow propertiesof plainthe rise in permeabilitywith shearin samplesBP-4 andBP- faultgouges, we shouldbe ableto addressthe question of what 5. Theseruns were repeatedto verify their differingbehavior. rolegouges play in areasof inducedseismicity. Talwani [1981] The increasesin permeabilityafter 4 mm of slidingcould be a re- found that at severalsites of inducedseismicity in North MORROWET AL. ' PERMEABILITYOF FAULT GOUGE 3199

SHEARSTRAIN, 0 I 2 3 4 5 6 I I I I I I

220 240• Big P•nes 3 SATURATED CLAYS AND FAULT GOUGES Big Pines 1 Big Pines 5 200 MPo CONFINING PRESSURE 200 Big P•nes 4

180

160

Te]on Pass

•-' 14o Dry Lake Valley Strain Ranch Illire 120

D St Hayward ]00 C•enegaValley

Kaolln•te

80 GoldenGate Bridge

• Montmor •11omte

• i i i ,i øo ] 2 3 4 5 6 7 8 9 AXIAL DISPLACEMENT, • (ram) Fig. 5. Frictionalstrength of all samplesup to 9 mm of axialshortening, or a shearstrain of approximate!y10.4 in the gouge layer.

Carolina,the growthof the epicentralarea was linearwith time, can be estimatedby using (2) above. Talwani [1981] found that suggestingthat a diffusionalflow processwas operating.This for over 40 casesof earthquakescaused by reservoirsand fluid statecan be describedby the equation injection,the hydaulicdiffusivity was within an order of magnitudeof 5x 104cm2/s. He concludedthat this may be a charac- t2 (2) teristic value associatedwith induced seismicity. For typical C valuesof rock porosity,fluid compressibility,and viscosity,this diffusivitycorresponds toa permeabilityof 10-16 to 10-14m 2. wheret is the time betweenreservoir impoundment and seismic- Using a different approachto determine C, Zoback and ity, •eis the distancefrom the reservoirto the hypocenterof the Hickman[ 1982], in their studyof the MonticelloReservoir earth- earthquake,and C is hydraulicdiffusivity. C is related to other quakesin North Carolina,,measured the in situ permeabilitiesof physicalproperites by therocks. These values were t•ypically around 10-•5 m2 (1 mil- lidarcy),corresponding to a diffusivityof 3 x 104cm2/s, using k (3) C- (3) withporosity, fluid viscosity, and compressibility appropriate to theirsite. This valueof diffusivityis well withinthe rangethat Tal'Wanifound as characteristic of induced earthquakes. In addi- where k is bulk permeability,• is the averageporosity of the tion, the time historyof the Monticelloearthquakes was consis- rock, and • and [5 are the viscosityand compressibilityof the tent with the diffusion model of (2). porefluids, respectively.The growthof the epicentralregion with The permeabilityof 10-15 m2 measuredby Zobackand time in these caseswould suggestthat the earthquakeswere Hickman [1982] and inferred by Talwani using the diffusional causedby a pore pressurefront that migratedaway from the re- fluid flow model is m•y ordersof magnitudegreater than the servoirand not by the loadof the reservoiron the rocks[Talwani, permeabilitiesmeasured for our fault gougesand is more typical 1981]. In areas where rocks are stressedto near failure levels, of jointed rocks [Brace, 1980]. The high diffusivities would thisincrease in porepressure can triggerearthquakes. suggestthat joints and fracturesare the pathwaysthrough which Where only time and locationdata for a singlereservoir in- fluids migrate away from reservoirs.In contrast,gouge-filled ducedearthquake event are available,the hydraulicdiffusivity fault zonesare more likely to restrictfluid flow, becomingbar- 3200 MORROWET AL.: PERMEABILITYOF FAULTGOUGE tiersthat may restrictthe migrationof porefluids and thus inhibit fine-grainednon-clay gouges (BP-4 and BP-5) exhibitedan in- the growthof epicentralzones in areasof inducedseismicity. creasein permeabilityafter 4 mm of sliding,possibly because The state and extent of gouge materialswithin active fault thesegouges dilated in orderto accommodatethe shearingof the zones is also importantto the discussionof stressesat depth. morerounded grains. Lachenbruchand Sass[1980] arguethat shearstresses must be The gougesand pure claysexhibited a wide variety of shear low (around10 MPa) along the San AndreasFault, basedon the strengths.Montmotillonite was substantiallyweaker than the absenceof a heat flow anomaly acrossthe fault. O'Neil and otherclay gougesand pure clay samples.The clay-freerock-flour Hanks [ 1980], on the otherhand, suggestthat thesestresses may samplesexhibited consistently higher frictional strengths that are be high but that watercirculation convects the heat away from the typicalof otherfractured granitic rocks. Strength did notcorrelate fault, maskingthe high stressesand giving the appearanceof a with permeability,as theseexperiments were run underdrained low-heat flow, low-stress environment. Zoback et al. [1980] conditionsand therefore excess pore pressures were probably not measuredin situ stressesto a depth of 1 km along the San An- generatedin the gougezones. dreas in Southern California and found that stresses increased with depthto near 10 MPa in their deepest•hole. Whetherthis REFERENCES trend can be extrapolatedto greaterdepth, or whetherstresses level off at this point as Lachenbruchand Sasswould have, is a Anderson,J. L., R. H. Osborne,and D. F. Palmer,Petrogenesis of clas- tic rockswithin the San Andreasfault zone of southernCalifornia, mostimportant question. U.S.A., Tectonophysics,67, 221, 1980. The low permeabilityof our fault gougesamples would indi- Bear, J., Dynamics of Fluidsin PorousMedia, p.166, Elsevier,New catethat convectionis probablyan inefficientmeans of transport York, 1972. throughrocks that contain numerousdeep faults. These results Beard,D.C., andP. K. Weyl, Influenceof textureon porosityand permeabilityof unconsolidatedsand, Am. Assoc.Pet. Geol. Bull., 57, show that even non-clay gougesare effective in reducingflow 349-369, 1973. acrossfault zones. If fault gougesare extensiveat depth (a point Brace,W. F., Permeabilityof crystallineand argillaceous rocks, Int. J. that is not yet resolved),then the low permeabilitieswould place RockMech. Min. Sci. Geomech.Abstr., 17, 241, 1980. restraintson the amountof heat transportwe couldexpect away Brekke,T. L., andT. R. Howard,Functional classification of gouge ma- from faultsby convection. terialsfrom seams and faults in relationto stabilityproblems in under- groundopenings, Final reportsubmitted to U.S.Bureauof Reclama- Laboratorystudies of frictionalsliding [Summersand Byeflee, tion,Dept. of Civil Eng., Univ. of Calif., Berkeley,1973. 1977; Wang et al., 1980] indicatethat fault gougescan sustain Byedee,J. D., Frictionof rocks,Pure AppI. Geophys., 116, 615, 1978. muchhigher stresses than the heatflow constraintwould suggest. Lachenbruch,A. H., andJ. H. Sass,Heat flow andenergetics of theSan However, undrainedtests of shear strengthpoint out that high AndreasFault zone, J. Geophy.Res., 85, 6185, 1980. Lambe,T. W., The permeabilityof fine grainedsoils, in Proceedingsof pore pressuresare quite effective in reducingthe resistanceto theASTM Symposium on Permeabilityof Soils,vol.56, 1954. shear,particularly in samplesthat containexpandable clays such Moore,D. E., C. A. Morrow,and J. D. Byerlee,Use of swellingclays as montmotillonite [Morrow et al., 1982]. to reducepermeability and its potentialapplication to nuclearwaste In order to reconcilethe opposingpoints of view presented repositorysealing, Geophy. Res. Lett., 9, 1009, 1982. Morrow,C. A., L. Q. Shi, andJ. D. Byedee,Permeability and strength above, we can envision two different scenarios.First, if stresses of SanAndreas fault gougeunder high pressure, Geoph. Res. Lett., alongthe fault are low, then the gougematerials must be in an 8, 325, 1981. undrainedstate. This requireslow permeabilitygouges (which we Morrow,C. A., L. Q. Shi, and J. D. Byedee,Strain hardening and havefound at leastat shallowdepths), and a wide anddeep fault strengthof clay-richfault gouges, J. Geophy.Res., 87, 6771, 1982. O'Neil, J. R., and T. C. Hanks, Geochemicalevidence for water-rockin- zone that will sustainhigh pore pressuresover time. Second,if teractionalong the San Andreasand GarlockFaults of California,J. stressesalong the fault are high, thenthe gougematerials must be Geophys.Res., 85, 6286, 1980. in a drainedstate. In this case,pore fluids must be free to flow Summers,R., andJ. D. Byerlee,A noteon the effect of faultgouge com- out of the fault zone, which placesrestraints on the geometryof positionon the stabilityof frictionalsliding, Int. J. RockMech. Min. Sci. Geomech.Abstr., 14, 155, 1977. the zone in questionif permeabilitiesare low. Clearly, these Talwani,P. D., Hydraulicdiffusivity and reservoir induced seismicity, questionscannot be resolvedwithout further details on the geol- FinalTechnical Report, 48 pp., U.S. Geol. Surv.,Reston, Va., 1981. ogy andstress distribution of fault zonesat depth. Wang, C, W. Lin, and F. Wu, Constitutionof the San Andreasfault zone at depth,Geophy. Res. Lett., 5, 741, 1978. Wang,C., N. Mao, andF. T. Wu, Mechanicalproperties of claysat high SUMMARY OF RESULTS pressure,J. Geophys.Res., 85, 1462-1468, 1980. Wu, F., L. Blatter,and H. Roberson,Clay gougesin the San Andreas Permeabilitiesof all the gougesunder confining pressures up to fault systemand theirpossible implications, Pure AppI. Geophys., 200 MPa ranged over four orders of magnitude.The lowest 113, 87, 1975. valuesof around 10-21 m2 were characteristicof the montmoril- Zoback,M.D., andJ. D. Byedee,Permeability and effective stress, Am. Assoc. Pet. Geol. Bull., 59, 154-158, 1975. 10nite-richand finer-grainedBig Pinesgouges, and the highest Zoback, M.D., and S. Hickman, In situ study of the physical valuesof between10 -19 to 10-18 m 2 weretypical of theserpen- mechanismscontrolling induced seismicity at MonticelloReservoir, tine and coarser-grainedBig Pinesgouges. The permeabilityof SouthCarolina, J. Geophys.Res., 87,6959, 1982 Zoback, M. D., H. Tsukahara, and S. Hickman Stress measurementsat the comminutedrock flours was not significantlydifferent than depthin the vicinityof the San Andreasfault: Implicationsfor the thatof the clay samples. magnitudeof shearstress at depth,J. Geophys.Res., 85,6157, 1980. Grain size appearsto be an importantfactor in controllingper- meability, particularlyfor the clay-rich samples.The coarse- J. D. Byeflee and C. A. Morrow, U.S. GeologicalSurvey, 345 grainedgouges were the most permeable and showed significant Middlefield Road, Menlo Park, CA 94025. L. Q. Shi, StateSeismological Bureau, Beijing, People'sRepublic of decreasesin permeabilitywith shearing.Rearrangement of the China. grainfabric through grain rotation, cracking and crushing was ob- servedinthin sections ofthese coarse gouges layer after shearing. (ReceivedApril8, 1983; Thefine-grained clay gouges, on the other hand, had characteris- revisedOctober 6, 1983; tically lower permeabilitiesthat did not vary with sheafing.The acceptedOctober 14, 1983.)