GEOPHYSICAL RESEARCH LETTERS, VOL. 17, NO. 12, PAGES 2109-2112, NOVEMBER 1990 FRICTION, OVERPRESSURE AND FAULT NORMAL COMPRESSION J. Byerlee U.S. GeologicalSurvey, Menlo Park, CA 94025 Abstract. More than twenty-five years ago Miller clay. In a seriesof experimentsto measurethe strengthof and Low reportedthe existenceof a thresholdpore somewater saturated clays [Radney and Byerlee, 1988], pressuregradient below which water would not flow we found that the effectivestress law for frictional sliding •hroughclay. Recentexperimental observations of the was obeyedand that the tz for montmorillonitewas _>0.2. shearstrength of structuredwater on biotite surfaces Thus even if the San Andreas fault was filled with pure haveprovided a physicalbasis for understaaxdingthis montmorillonite, which seems unlikely, the heat flow t•o!d gradient. The existenceof this phenomenon constraints would be exceeded. hasprofound implications for the rheologicalproperties The only way for the fault to have the necessarylow of mature fault zones, such as the San Andreas, that strength is for pore pressureto exceed75 percentof the containlarge thicknessesof fault gouge. For example,a lithostatic stressin the San Andreasfault zonewhile pore day-filledfault zoneabout I km wide at the baseof the pressurefollows the hydrostaticgradient everywhereelse s•mogeniczone decreasing to zero width at the surface (A.H. Lachenbruch,personal commun., 1990). couldsupport core fluid pressureequal to the maximum We will denotean effectivestress component •i = •ri-p principalstress over the entire seismogeniczone. As where p is fluid pressure. Then, for fluid pressures a result, the fault would have near-zero strength and followingthe hydrostat it is commonlyfound [Zoback and t•hemaximum principal stressmeasured on the flanks of Healy,1989] that the maximumand minimumeffective t.•e fault, would be oriented normal to the fault surface. principalstreses are related by •s = •/3. If a• is An.otherconsequence of the threshold gradient is that approximately equal to the lithostatic stress and the normalhydrostatic fluid pressuresoutside the fault zone densityof the overlyingrock is 2,500kg/m s, thenas = couldcoexist with near-lithostatic fluid pressuresin the 0.6 a•. Thus, from conventionalrock mechanicswe would •nterior of the fault zone without the need for continual expect hydrofracture to occur if ,•, the ratio of pore replenishmentof the overpressuredfluid. In addition,the pressure to lithostatic stress, in the fault zone exceeds porepressure at any point shouldnever exceedthe local 0.6. This wasalso pointed out by Zobacket al. [1987]. minhnumprincipal stressso that hydrofracture will not If the country rock is so strong that hydrofracturedoes not occur, then it is necessaryfor the countryrock to have very low permeabilty to prevent the high pressuresin the introduction fault zone from bleeding off. Calculations by Hanshaw and Bredehoeft,[1963] show that evenif a high pore Recent determinatons of the stress directions indicate pressureregion is surrounded by materiM 1 km thick, thatthe maximumprincipal stress•rx is almostnormal to witha permeabilityof 10-21m2 (whichis anexceedingly ß e SanAndreas fault [Mountand Suppe,1987; Zoback, low permeabilityfor geologicalmaterials) A can not be eta/., 19.87]. This is consistentwith the low shear maintained above 0.75 for more than 10,000 years. This strengthof the fault inferred from the heat flow data. is becausethe specificstorage, which is a functionof the Theseobservations have proven to be very difficult to pore volume and the compressibilityof water and rock, .•plain in terms of our conventional understanding of is so low that only a very small amount of water needs f•u!t theology. in most regionsof California,faults escapeto drastically lower the pore pressure. • with almost all orientations. If all the faults A clue to explaining the apparent paradox of the 'ma regionhave approximatelythe samecoefficient of mechanics of large fault systems comes from crustal friction/•, then elementarycoulomb analysis demands drilling projects and the oil exploration industry. It has :tZatthe maximumangle that ax can make with the been found that overpressuresare commonin the earth most active fault is 45ø , even if /• is zero. It has even in rocks as old as Cambrian. In fact, in some beensuggested, [Zoback ½t al., 1987]that • for sliding casesthe pore pressurecan even exceedthe lithostatic •onthe San Andreas may be very low, but high for stress[Fertl, 1976]. In someoil fieldsthese very high thesubsidiary faults. Calculations(A.H. Lachenbruch,pressures in oil or gascan be explainedby the very high ,personalcommun., 1990) show that/• onthe SanAndreas capillary pressuresthat are required in the non-wetting needs.... to be _• 0.1 to satisfythe heat flow constraints. phase to displacewater from the narrcrwchanne!.ways T• weakestmineral that can be expectedto occurin in the overlyingrocks [Watts, 1987]. In russetplaces, '•y significantquantity in fault zonesis montmorillonite however, water is the only liquid phase present, so we cannot appeal to capillary forcesMone to explainthe his paper is not subject to U.S. copyright. Pub- overpressure. I!s'hedin 1990 by the AmericanGeophysical Union. Threshold Gradient number 90GL02295 An •planation of how overpres.sure can be maint•ed !•<94-8276 /90 / 90GL-02295503ßO0 for long times comesfrom the r•u!ta of !&borato• 2!• 2110 Byer!ee, J.' Friction, Overpressureand Fault Normal Compression experiments carried out in the field of soil science. It whentwo surfaces, separated by a liquidare brought has been found that at very low pressure gradients closertogether than about10 timesthe diameterof the water will not flow through dense clay. This is shown molecules,the liquid becomes ordered into discrete lay- schematically in Figure 1. For Darcian flow the flow rate ers. Whenthis occurs the liquidhas a finitestrength. increaseslinearly with pressuregradient. The slope of Furthermorethe normalstress required to expela layer the straight line through the origin is a measure of the of the liquidincreases as the numberof layersdecreases. permeability of the material. With denseclay the flow If a shearstress is applied,shear displacement will not is non-Darcian at low pressure gradients. In this case occurbetween the surfacesuntil the shearstress reaches a thresholdlevel. The magnitudeof the thresholdshear stressincreases as the numberof layersof the liquidde. creases.In addition,the shearstress required to shearthe liquidis •ndependentof the shearvelocity, [Isra•lach•'•i et al., 1988]. With thin filmsof a liquidbetween atomi. callysmooth surfaces the conceptof Newtoninnviscosity breaks down, and attempts to calculate an effectivevis- cositylead to valuesthat are many ordersof magnitude larger than the bulk values. This appliesto water as as all the otherliquids so far studied,[McGuiggan • •., 1989].In the caseof waterits shearstrength is directly proportionalto the normal stressthat canbe supported [Homolaet al., 1989]. It should make no differencewhether shearingof 'the water is caused by a shear displacement of one of the TG surfaces relative to the other or whether the two surfaces are held fixed and the water is forced to move between PRESSURE GRADIENT them by a pressure gradient. The first case requires Fig. 1. A schematic diagram of flow rate as a function a threshold shear stress and the secondcase, requires of pressuregradient for flow through a Darcian material, a threshold gradient. Because the threshold stressis and a non darcian material with a threshold gradient TG. zero for greater than 10 layers of moleculesit wouldbe expected that the threshold gradient would not occ-ur when the water-clay ratio is large, but wouldbe man•ffest the zero intercept on the pressuregradient axis is called whenthis ratio is low,as found by Miller andLow, [!• the thresholdgradient {TG). At pressuregradients below With dense clay it has been found that the TG starts the TG, flow does not occur so that the permeability of to be measurablewhen the void ratio, the volumeof the the material is zero. The experiments to measurethe TG voids to the volume of the solids,is about 2 and increases are very difficult to carry out, and somehydrologists have to 0.5MPa/m at a voidratio of about0.75 [Ping, 1963]. assumedthat the observedTG is an experimental artifact From physical considerationsall that shouldbe re- mainly becausein some experiments no TG is observed, quiredfor a TG to existin fault gougeis that thewidth of whereas in others it is, even with the same material the passagewaysoccupied by water not exceedten although of different porosity. In addition, until now the diameter of a water molecule. If this is true, then there has been no satisfactory physical explanation for it may not be necessaryfor the walls of the passageways the phenomena.It has been suggestedœu•z and Kemper throughwhich the watermoves to be perfectlyparallel [1959],that the streamingpotential generated by the flow they are in clays,such as montmorillonite,illire, s•- can producesmall countercurrents along the pore walls tine, etc. Thusa TG mayalso exist in a gougewith pa•- in a direction opposite to the main flow, and it may be clesof claysize regardless of theircomposition, possiblefor this mechanismto reduce the flow rate at experimentsare required to confirmthis. In anycase it low pressuregradients, but it can not explain why flow doesappear reasonable at this stageto assumethat ceasesaltogether below the TG. Others[Miller andœow, manygouges the TG