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.
