简介:Weidentifytwointerrelatedbutindependentspeciesofmicrocrackswithdifferentoriginsanddifferentdistributions.Onespeciesistheclassichigh-stressmicrocracksidentifiedinlaboratorystress-cellsassociatedwithacousticemissionsasmicrocracksopenwithincreasingstress.Theotherspeciesisthelow-stressdistributionsofclosely-spacedstress-alignedfluid-saturatedmicrocracksthatobservationsofshear-wavesplitting(SWS)demonstratepervadealmostallinsiturocksintheuppercrust,thelowercrust,andtheuppermost400kmofthemantle.Onsomeoccasionsthesetwosetsofmicrocracksmaybeinterrelatedandsimilar(hence'species')buttheytypicallyhavefundamentally-differentproperties,differentdistributions,anddifferentimplications.TheimportanceforhydrocarbonexplorationandrecoveryisthatSWSinhydrocarbonreservoirsmonitorscrackalignmentsandpreferreddirectionsoffluid-flow.TheimportanceforearthquakeseismologyisthatSWSabovesmallearthquakesmonitorstheeffectsofincreasingstressonthepervasivelow-stressmicrocrackdistributionssothatstress-accumulationbefore,possiblydistant,earthquakescanberecognisedandimpendin~earthquakesstress-forecast.