Pore fluid in experimental calcite-bearing faults: Abrupt weakening and geochemical signature of co-seismic processes

Abstract

While itiswidelyrecognizedthatfluidsinfluencefaultstrengthandearthquakenucleation,propaga- tion andarrest,theireffectsonco-seismicslidingfrictionareonlyconjectured.Toshedlightonthese effects,55highvelocity(41 m s 1) frictionexperimentswereconductedatroomtemperatureon Carraramarblesamplesinthepresenceofporefluid(upto15MPaporepressure),room-humidityand ‘‘vacuum’’(10 4 mbar)conditions.Inalltheexperiments,thefrictioncoefficientevolvedfromapeak value of0.6–0.8toasteady-statevalueof0.1inabout1–1.5mofslip.However,experiments performedinthepresenceofporefluidhadalargeandmoreabruptdecreaseinfrictionatthe initiationofsliding(65%after20mmofslip),whereasexperimentsperformedundervacuumandroom humidityconditionsshowedinitialvelocity-strengtheningbehaviorfollowedbyamoregradual reductioninfriction.Thisindicatesthatcalcite-bearingrocksaremorepronetoslipinthepresence of water.Underroom-humidityconditions,CO2 was detectedduringtheentiredurationofthe experiment.Inthepresenceofporefluid,HCO3 and Ca2þ were detectedforslips 40.1 m.Thelack of decarbonationproducts(HCO3 and Ca2þ) inporefluidexperimentsforslip o0.1 mimpliesthat the abruptweakeningisnotrelatedtodecarbonation(orthattheabundanceofthereactionproductsis below theresolutionoftheanalyticalmethods).Giventhemodestthermalexpansionofwater, the estimatedthermalpressurizationduringtheabruptweakeningappearstobenegligible.Instead, we suggestthatabruptweakeningisduetosubcriticalcrack-growth,hydrolyticweakeningandbrittle failureoftheasperitiesontheslidingsurfaces.Modelingshowsthattheoccurrenceinnatureof co-seismic(water-present)decarbonationreactionssimilartothosetriggeredinthelaboratorycould yield sufficientreactionproducttobedetectedinaquiferslocatedintheproximityofactivefaults.