Wilks, M. T., Kendall, J. M., Nowacki, A. J., Biggs, J., Wookey, J., Birhanu,Y., Ayele, A., & Bedada, T. (2017). Seismicity associated with magmatism,faulting and hydrothermal circulation at Aluto Volcano, Main Ethiopian Rift.Journal of Volcanology and Geothermal Research, 340, 52-67.https://doi.org/10.1016/j.jvolgeores.2017.04.003

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Title:1 Seismicityassociatedwithmagmatism,faultingandhydrothermalcirculationatAlutoVolcano,2 MainEthiopianRift.3 4 Authors:5 MatthewWilksa1,J-MichaelKendalla,AndyNowackib,JulietBiggsa,JamesWookeya,Yelebe6 Birhanua,AtalayAyelec&TuluBedadac.7 8 Affiliations:9 a.SchoolofEarthSciences,UniversityofBristol,WillsMemorialBuilding,QueensRoad,Bristol,10 UK.BS81RJ.11 b.SchoolofEarthandEnvironment,UniversityofLeeds,Leeds,UK.LS29JT.12 c.InstituteofGeophysics,SpaceScience,andAstronomy,AddisAbabaUniversity,AddisAbaba,13 Ethiopia.14 15 PresentAddress16 1.NORSAR,GunnarRandersvei15,2007Kjeller,Norway.17 18 CorrespondingAuthor:19 MatthewWilks20 Email:matt@norsar.no21 Tel:+474750403922 23 AbbreviatedTitle:24 Magmatic,TectonicandGeothermalSeismicityatAluto25 26

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1.Abstract27 28 ThesilicicvolcaniccentresoftheMainEthiopianRift(MER)playacentralroleinfacilitating29 continentalrifting.Manyofthesevolcanoeshostgeothermalresourcesandarelocatedinheavily30 populatedregions.InSARstudieshaveshownseveralaredeforming,butregionalseismic31 networkshavedetectedlittleseismicity.Alocalnetworkof12seismometerswasdeployedat32 AlutoVolcanofrom2012–2014,anddetected2142earthquakeswithina24-monthperiod.We33 locatetheeventsusinga1Dvelocitymodelthatexploitsaregionalmodelandinformationfrom34 geothermalboreholesandcalculatelocalmagnitudes,b-valuesandfocalmechanisms.Event35 depthsgenerallyrangefromthenearsurfaceto15kmwithmostoftheseismicityclusteringinthe36 upper2km.AsignificantamountofseismicityfollowstheArtuJawaFaultZone,whichtrendsin37 alignmentwiththeWonjiFaultBelt,NNE–SSWandisconsistentwithpreviousstudiesofstrain38 localisationintheMER.Focalmechanismsaremostlynormalinstyle,withthemeanT-axes39 congruenttotheorientationofextensionintheriftatthislatitude.Someshowrelativelysmall40 left-lateralstrike-slipcomponentsandarelikelyassociatedwiththereactivationofNE-ENE41 structuresatthesoutherntipoftheAluto-Gedemsasegment.Eventsrangefrom-0.40to2.98in42 magnitudeandwecalculateanoverallb-valueof1.40±0.14.Thisrelativelyelevatedvaluesuggests43 fluid-inducedseismicitythatisparticularlyevidentintheshallowhydrothermalreservoirand44 aboveit.Subdividingourobservationsaccordingtodepthidentifiesdistinctregionsbeneaththe45 volcanicedifice:ashallowzone(-2–0km)ofhighseismicityandhighb-valuesthatcorrespondsto46 thehydrothermalsystemandisinfluencedbyahighfluidsaturationandcirculation;arelatively47 aseismiczone(0–2km)withlowb-valuesthatisimpermeabletoascendingvolatiles;aregionof48 increasedfluid-inducedseismicity(2–9km)thatisdrivenbymagmaticintrusionfrombelowand49 adeeperzone(below9km)thatisinterpretedasapartiallycrystalline,magmaticmush.These50 observationsindicatethatboththemagmaticandhydrothermalsystemsofAlutovolcanoare51 seismicallyactiveandhighlightstheneedfordedicatedseismicmonitoringatvolcanoesinthe52 MER.53 54 55 KeyWords:HydrothermalSystems,SeismicityandTectonics,VolcanoSeismology,Volcano56 Monitoring,Africa,ContinentalTectonics:extensional.57 58 2.Introduction59 60 TheEastAfricanRiftcapturesthetransitionfromcontinentalriftingtooceanicspreading,61 providingopportunitiestostudytheextensionalprocessesthatleadtotheformationofnew62 oceanicbasins(Chorowicz2005).SpanningembryonicriftingintheOkavango(Southwesternrift)63 andsouthwesternMozambique(Westernrift)regionstothesouth(YangandChen,2008:Ebinger64 andScholz,2012)tomaturespreadingaroundtheAfarTripleJunctioninthenorth,the65 intermediatestagesoftheprogressionareexposedatthesurfaceoftheMainEthiopianRift(MER)66 (Fig.1),wheremagmaticprocesseshaveplayedacentralroleinfacilitatinglithosphericthinning67 since~2Ma(EbingerandCasey2001).Asaconsequenceofthisfocusingofextensionalstrainto68 zonesofdikeintrusionandmagmastorageattherift’scentre,widespreadmagmatismhas69 producedanewalong-axissegmentationalongthelengthoftherift.Numeroussiliciccaldera-70 bearingvolcanoeshavesincedevelopedwithinthesesegmentsandattheiroffsets(Mohr1962;71 WoldeGabrieletal.1990).72 73 Althoughanincreasingnumberofgeophysical,includingseismic,studieshavebeenconductedin74 theMERoverthepastdecades,themajorityhavefocusedtheirattentiontowardslatitudesnorth75 of8°NandontheAfarregion(e.g.,HofstetterandBeyth2003;Caseyetal.2006;Maguireetal.76 2006;Dalyetal.2008).Incontrast,thevolcanoesoftheMERhavereceivedlittle-to-noindividual77 seismicmonitoring,whichissignificant,asvolcanicandseismichazardremainrelativelypoorly78

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constrained(Wilksetal.2017).Thisisdespitemanyofthesevolcanoesbeingvolcanicallyactivein79 theHoloceneandmanycontinuingtoshowsignificantunrestatthesurface(Biggsetal.2011).80 81 SituatedbetweenLakesZiwaytothenorthandLanganotothesouth,Alutoisoneexampleofa82 MERvolcanowheregeophysicalmonitoringhasremainedlargelyabsent(Fig.1).However,83 satelliteobservationsofsurfacedeformationhavebeenmadeviaInterferometricSynthetic84 ApertureRadar(InSAR)andrevealedmultipleepisodesofupliftandsubsidenceofupto15cmin85 displacement(Biggsetal.2011).AtAluto,thesedeformationepisodesareinterpretedtorepresent86 therepeatedinjectionofmagmaticfluidsandvolatilestoshallow(<5km)depthsdrivinginflation87 andthecoolingandflowofhydrothermalfluidscausingsubsequentsubsidence(Hutchisonetal.88 2016).89 90 Withthehypothesisthatthereplenishmentofsubsurfacemagmabodiescandrivestructurally91 controlledgeothermalsystems(Hilletal.1985;Mooreetal.2008),fluidmigrationathigh92 temperaturehasthepotentialtoprovideanabundantrenewableenergyresource.Forthisreason,93 geothermalpowerhasgainedsignificantinterestintheMER,withAlutoinparticularbeing94 identifiedasapromisingfieldforthegenerationofrenewableenergy.Explorationaldrillinginthe95 1980srevealedtemperaturesof~350°Cat2.5kmdepth,indicatingtheinfluenceofhot96 subsurfacemagmaticbodiesintheshallowcrust(Gizaw,1993;GianelliandTeklemariam1993)97 andalsoAluto’spotentialasaviableenergyresource.Ethiopia’sfirstgeothermalpowerplant(the98 Aluto-LanganoGeothermalPlant)wasconsequentlyconstructedin1999(Teklemariamand99 Beyene2002)withexpansionfromageneratingcapacityof7.3MWto70MWbeginninginlate-100 2013andcontinuingtoday.101 102 TheMERhaslongbeenrecognisedasaseismicallyactiveregionandmanyearthquakeshavehad103 structurallydamagingconsequences(Gouin1979).Duetothescarcityofstationcoveragein104 Ethiopiaduringmostofthe20thcenturyhowever,constrainingseismicsourceparametersand105 quantifyingtheassociatedhazardhasnotbeenpossibleuntilrecently(e.g.,FosterandJackson106 1998;Ayeleetal.2007;Belachewetal.2012).Earthquakesoflow-to-intermediate(M<6)107 magnitudesoccurringinrelativelydiffusepatternsacrosstheriftaxischaracterisestheseismicity108 oftheMER,althoughamaximumexpectedmagnitudeof~7hasalsobeenestimated(Hofstetter109 andBeyth2003).AtthelatitudeofAluto,theseismichazardisthoughttobelessthantothenorth110 andsouthofit(Midzietal.1999;Grünthaletal.1999)buttheMW5.3earthquakeon27thJanuary111 2017thatoccurred~70kmfromAluto(Fig.1)servesasanexamplethattheriskofground112 motionsonadamagingscalepersist.Atvolcanoes,volcanicunrestisoftencomplimentedby113 profuseseismicitythatreflectsactivitywithinthehydrothermal-magmaticsystem.Thereforeitis114 extremelyimportanttomonitorearthquakeswheredeformationhasbeenobservedtodevelop115 ourunderstandingoftheseprocesses,whichbecomesoffurtherrelevancewheneconomic116 interestforpowergenerationisalsoassociated.Inaregionwherethelocalpopulationisalso117 rapidlyincreasing,thesefactorssuggestthatmonitoringatvolcanoessuchasAlutoshouldbe118 improved(Sparksetal.2012).119 120 Inthispaperweaimtobetterunderstandthehydrothermal-magmaticinteractionsatanactively121 deformingvolcanowheregeophysicalmonitoringhasremainedrelativelysparse.Thisstudyofthe122 microseismicityaroundAlutoconstitutesonecomponentoftheAlutoResearchandGeophysical123 ObservationS(ARGOS)project,whichisamulti-disciplinaryprojectintegratingInSAR,GPS,124 magnetotellurics(MT),CO2degassingexperimentsandgeologicalmappingdatatoplace125 quantitativeconstraintsonthecausesofunrestforthefirsttime(Hutchisonetal.2015a,126 Hutchisonetal.2015b;Samrocketal.2015).127 128 WepresenttheseismicityaroundAlutowithdataacquiredfrompassiveseismicmonitoringovera129 two-yearperiod.Firstly,welocateearthquakesusinga1Dseismicvelocitymodelandexamine130 themspatiallyandtemporally.Wecomputelocalmagnitudesusinganempiricalrelationthat131

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accountsforattenuationintheMainEthiopianRift(Keiretal.2006b)andinvestigatetheir132 distributionswithrespecttotheGutenberg-Richterrelationship.Wethencalculatesource133 mechanismsforasubsetoftheearthquakecataloguetoinvestigatefaultingandthesubsurface134 stress-stateandfinallycombinetheseobservationstoproduceamodeltodescribeour135 interpretationsofthehydrothermal-magmaticsystem.136 137 3.TheMainEthiopianRiftandVolcanismatAluto138 139 TheCentralMER(CMER)isboundedbylarge,~50km-long,widely-spacedborderfaultsthat140 formedduringtheprimitivestagesofrifting8–6Ma(WoldeGabrieletal.1990;Boninietal.2005)141 andwhosemeantrendisatN032°Eatthislatitude(Agostinietal.2011)(Fig.1).Earlyrifting142 duringthistimewascomplimentedbywidespreadbimodalvolcanism,withlargeignimbritic143 eruptionsoccurringatMunesaat3.5Ma(WoldeGabrieletal.1990)andatGademotta,whichlast144 eruptedat1.3Ma(LauryandAlbritton1975)forexample(Fig.2).Extensionwasfacilitatedbythe145 borderfaultsfortheremainderoftheMiocene,thePlioceneandintothePleistocenebutat2Ma,146 therewasadrasticshiftinriftingstylewherestrainlocalisedatthecentreoftherift(Boccalettiet147 al.1998;EbingerandCasey2001;Keiretal.2015).Concentratedepisodesofmaficmagmatic148 intrusion,dikingandfaultingledtotheformationofshorter(~20km-long),NNE-SSW(~N012°E)149 trendingfaultsthatformedadenseseriesofright-steppingen-echelonstructures(Keiretal.150 2006a;Pizzietal.2006).Thedevelopmentoftheseinternalriftfaults,calledtheWonjiFaultBelt151 (WFB)coincidedwithbasalticfissureeruptionsatthesurfaceandproducedtheBofabasalt152 sequencetooverlietheignimbritesbelow(Kebedeetal.1985;ELCElectroconsult1986).153 154 WidespreadfloodingoftheevolvingZiway-Shalabasinat570kaledtothedepositionofclay-rich,155 lacustrinesediments,whichcontinueduntil330ka(LeTurduetal.1999).Thissedimentationwas156 interspersedwiththefirstsilicicdepositsintheareaoftrachytictuffsandlavas,eruptiveproducts157 thatmarktheonsetoftheformationoftheAlutocomplex(Hutchisonetal.2015b).Assilicic158 depositsaccumulated,theAlutovolcaniccentrecontinuedtogrowuntil~310ka,whenan159 explosiveeruption(s)occurred(Hutchisonetal.2015a)andformedweldedignimbritesthatare160 thickestatAluto’scentre(DakinandGibson1974).Atpresent,onlytheeasternrimofthecaldera161 ispreserved,withthewesternportioninferredfromthealignmentofvolcaniccentresandcraters162 (Kebedeetal.1985).163 164 Aperiodofvolcanicquiescencefollowedfor~240kaatAluto,inwhichtimeothercalderaforming165 eventsoccurredintheMERtothesouthatShala(O’acaldera)andthenCorbetti(Mohretal.1980;166 Hutchison2015).Post-calderavolcanismrecommencedwithinthemainedificeofAlutoat~60ka167 throughrhyoliticlavaflows,pumicefalloutsandpyroclasticdensitycurrents.Theseeruptions168 produceddistinctsequencesthatcontinueduntilatleast10ka,perhapsuntil2ka(ELC169 Electroconsult1986;GianelliandTeklemariam,1993),andareseparatedbypalaeosolsthat170 indicatewhenvolcanismtemporarilyceased(Hutchisonetal.2015b).171 172 Oxygenisotopemeasurementshaveshownthatthemajority(90%)offluidswithinAluto’s173 geothermalsystemarederivedfromrainfallfallingontheriftshoulderstotheeast(Darlingetal.174 1996).Thisobservation,alongwithgeochemicalevidencethatgeothermalfluidsinteractwith175 rhyoliticvolcanicproducts(GianelliandTeklemariam1993)andtheknowntemperaturesand176 stratigraphywithindeepexplorationwells(Gizaw1993;Teklemariametal.1996)suggestthatthe177 geothermalreservoirresidesintheNeogeneignimbriteunit,whichisthelowermostandoldest178 unitinthegeothermalwells(Teklemariametal.1996).Thelowporosityandlowpermeability179 lake-derivedsedimentsthenactasa‘claycap’tosealthereservoir,whichhaveexperienced180 intensehydrothermalalteration(Teklemariametal.1996)andhavebeenimagedbyMT181 surveying(Samrocketal.2015).182 183

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AlthoughthemajorityofstrainintheCMERhasbeenaccommodatedbytheWFBsince~2Ma,the184 majorityofthemappedfaultstructuresaroundAlutoandparticularlytotheeast,aretheolderand185 relativelyinactiveborderfaults(Agostinietal.2011)(Fig.3).WithinandaroundtheAlutoedifice186 however,shortlineamentstrendingNNE-SSWandfollowingthetrendoftheWFBhavebeen187 identified(Kebedeetal.1985).Theseincludeafaultscarpthatcross-cutsthemainedificeofAluto188 inthesameorientationcalledtheArtuJawaFaultZone(AJFZ)(Hutchisonetal.2015a).This189 structurehasbeenidentifiedastheprimarypathwayalongwhichhydrothermalfluidsandgases190 (steamandCO2)ascendfromthegeothermalreservoirtothesurfaceandiswheretheproductive191 geothermalwellsarelocated.192 193 4.Methods194 195 4.1.SeismicNetworkandData196 197 TomonitortheseismicityaroundAluto,aseismicnetworkof12GüralpCMG-6TDthree-198 componentseismometerswithalowcornerof30sweredeployedinJanuary2012.Thestations199 wereinstalledbothwithinthevolcanicedificeandencirclingit,withstationsalsodeployedonthe200 surroundingriftvalleyfloorandtowardstheeasternriftescarpment(Fig.3).Stationspacings201 werebetween2and10kmandthearraycoveredanareaof20×20km.Asamplingrateof100Hz202 wasmaintainedthroughouttheexperimentandtheinstrumentsremainedinthefielduntil30th203 January2014.TwostationsatA06EandA02Ewererelocatedduringthedeployment(tobecome204 A13EandA14Erespectively)duetonoiseandlogisticalconsiderations.Amaximumofeleven205 stationswererecordingdataatanyonetimebutthenetworkgeometryprovidedgoodcoverageof206 theseismicitybeneathAlutoandprovidedarichseismicdatasetforanalysis.FiveGPSstations207 werealsodeployedaroundthevolcanoaspartofthewiderARGOSproject.208 209 4.2.EventDetectionandLocations210 211 P-andS-wavearrivalsarepickedmanuallyonseismogramsusingtheSeismicAnalysisCode212 (GoldsteinandSnoke2005;Helffrichetal.2013).AButterworthbandpassfilterofhighandlow213 cornersof15and2HzisappliedtoenhancetheclarityofthearrivalsandP-wavearrivalpicksare214 madewherecoherentfirst-breaksareobservedataminimumof3stations.P-waveonsetsare215 initiallyidentifiedontheverticalcomponentoftheseismograms,withallthreecomponents216 subsequentlyusedtorefinethepicksandidentifyS-wavearrivals.Thepicksaremanually217 attributedweightingstoreflecttheirqualityandclaritywithassignedvaluesof0,denotingthe218 highestquality,to3,thelowest.ThevaluesarethenmappedtoP-wavearrivaltimeuncertainties219 of0.05,0.1,0.2and0.5sandtoS-waveuncertaintiesof0.1,0.2,0.3and0.5s.Theseerrorsare220 basedonasemi-quantitativescale,whereifinspectionoftheseismogramledtoconfidenceinthe221 picktowithin±0.05s(duetonoise,anemergentonset,etc.),itwasassignedaqualityof‘0’,andso222 forth.Experienceandthesamplingratealsoguidedthesevalues.223 224 Thedeterminationofearthquakeparameterssuchaslocationsandfocalmechanismsishighly225 dependentonanaccurateseismicvelocitymodel.Thiscanbecomeparticularlydifficultinregions226 withsignificanttopographicreliefsuchasaroundvolcanicedifices.Well-logrecordsacquired227 fromindustrialdrillingareusedtoconstrainseismicvelocitiesfromthesurfacetoadepthof300228 mbelowsealevel(b.s.l.)(ELCElectroconsult1986;Gizaw1993;GianelliandTeklemariam1993;229 Teklemariametal.1996).Thelithologiesofthelogsaremappedtotheappropriateseismic230 velocity(Press1966;Christensen1984)toconstructthetop~3kmofa1Dlayer-cakemodel.At231 greaterdepthsthevelocitystructureistakenfromapassivesourcetomographicinversionthat232 wasdeterminedusinglocalearthquakesinthenorthernMER(Dalyetal.2008).Thetwovelocity233 structuresarethenamalgamatedtoproducethe1DvelocitymodelpresentedinTable1.234 235

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Depth(km) VP(km/s) VS(km/s) ρ(kg/m3) Geology-2.5 3.27 1.88 2380 SilicicVolc.-1.5 4.12 2.41 2380 Seds.&Tuffs-1.0 4.22 2.49 2500 BofaBasalt-0.2 4.50 2.50 2500 Ignimbrite0.3 4.62 2.58 2790

CrystallineBasement

2.0 4.70 2.58 27904.0 4.95 2.77 27906.0 5.12 2.78 27908.0 5.85 3.36 279010.0 5.99 3.53 279012.0 6.09 3.61 297014.0 6.26 3.61 297016.0 6.31 3.61 279020.0 6.35 3.68 279025.0 6.70 3.80 2940

LowerCrust30.0 6.89 3.92 294040.0 7.50 4.28 3190 UpperMantle

236 Table1:TheinputP-andS-wavevelocitymodelinferredfromwell-logdataandatomographic237 inversioninthenorthernMER.Thereferencedepthat0kmissealevel.Densitiesarederivedfrom238 Cornwelletal.(2006).Theinferredgeologiesaredenotedintheright-handcolumnwiththeunits239 derivedfromthewell-logscolouredasinFig.2.240 241 Arrivaltimesareusedasinputtotheearthquakehypocentrelocationsoftwarepackage242 NONLINLOC(Lomaxetal.2000)toperformanonlinearglobal-searchtoattainabsolute243 earthquakelocations.TheinversionfollowstheprobabilisticapproachofTarantolaandValette244 (1982)andTarantola(2005)tosolveforthehypocentrallocationofmaximumlikelihoodintime245 andspace,whichisrepresentedbyanaposterioriprobabilitydensityfunctionofmodel246 parameters.First,synthetictraveltimesfromeachstationtoeverygridnodeofa3Dmodelgrid247 arecomputedusingaHuygen’sprinciplefinite-differenceeikonalsolver(PodvinandLecomte248 1991).Inthisapproach,thevariabletopographyofthestudyregionisaccountedforbyfirst249 shiftingthemodelspaceverticallyby2km(thehigheststationisat1955melevation).Anoct-tree250 3DimportancesamplingmethodisthenimplementedwithinNONLINLOC,whichrecursively251 subdividesgridcellstodeterminethelocationPDFsandhypocentresofmaximumlikelihoodinan252 efficientandrobustmanner(LomaxandCurtis2001;Lomax2005).253 254 4.3.Magnitudes255 256 4.3.1.LocalMagnitudeScale257 258 Inseismicallyactivezonesitisessentialtoimplementanobjectiveandquantitativemethodof259 estimatingthesizeofanearthquake.Weusealocalmagnitude(ML)scaleconsistingofan260 attenuationtermspecifictotheMERthatisformulatedas:#$ = log )*+ +261 1.196997 log 2 17 + 0.001066 2 − 17 + 2.0,where)*+isthezero-to-peakdisplacement262 amplitudeofthehorizontalcomponentsonaWood-Andersonseismographinmmandristhe263 hypocentraldistanceinkm(Keiretal.2006b).264 265 Theinstrumentresponsesfromthehorizontalcomponentsofseismogramsareremovedand266 convolvedtothatofaWood-Andersoninstrument.Sincemaximumamplitudedeflectionsaremost267

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associatedwithS-wavesforlocalevents,amplitudesaremeasuredinatimewindow268 encapsulatingtheonsetoftheS-wavearrival.Forasingleeventthecomputedmagnitudesacross269 anetworkshouldtheoreticallybeequalbutvariationsinradiationpatterns,stationnoiseandlocal270 geologycanamplifyordiminishthesignalataparticularstation(SavageandAnderson1995).We271 consequentlyapplystationcorrectionsbyfindingthemagnituderesiduals,whicharedefinedas272 thedifferencebetweenthecalculatedandaverageeventmagnitudesforeachcomponentandat273 eachstation.Thearithmeticmeanisthentakenacrosseachcomponentateachstationforall274 eventsandproducescorrectionsrangingfrom−0.284and0.218MLunits.Throughtheadditionof275 thecorrectionsthevarianceofthemagnituderesidualsisreducedby22.4%from0.092to0.072.276 Oncethestationcorrectionshavebeenapplied,weaveragethetwomagnitudevaluesateach277 stationtoattainasinglevaluethatdefinestheML.Theprecisionofthedeterminedmagnitudesis278 constrainedbytheuncertainty,whichisthestandarddeviationofthevariationinmagnitudesthat279 iscomputedateachstation.280 281 4.3.2.b-Values282 283 TheGutenberg-Richterrelationship,whichisdefinedas:7(#) = 10:;<= ,describesthe284 distributionofearthquakesinagivenregionandtimeframewithrespecttothemagnitudeM285 (GutenbergandRichter1942).Nisthenumberofearthquakesofmagnitudegreaterthanorequal286 toMandaandbarepositiveconstants.adescribestheseismicactivityandisdeterminedbythe287 eventrate,whereasb(termedtheb-value)isatectonicparameterthatdescribestherelative288 abundanceoflargetosmallmagnitudeevents.Weusethemaximum-likelihoodmethodofAki289 (1965)andUtsu(1965)tofindbandalsocalculatetheminimummagnitudeofcompleteness(MC),290 whichisdefinedastheminimumeventmagnitudeatwhichtheearthquakecatalogueisassumed291 tocontainallevents.ThisisperformedautomaticallyusingKolmogorov-Smirnovteststochoose292 thesmallestmagnitudeatwhichthesimilaritybetweenthedatacurve(magnitudedistribution)293 andthestraightlinefit(ofgradient−b)reachesapredeterminedsignificancelevel(Kagan1995).294 ThereisthereforenorequirementtorestricttheearthquakecataloguetoeventsaboveMCinthe295 subsequentcalculationofb.296 297 4.4.FocalMechanisms298 299 Calculatingfocalmechanismsisusefulforcharacterisingtheseismicityofaregionastheyaidin300 determiningtheorientationofstressthatleadstoruptureandcanhelptoinferthestyleof301 faulting.Here,double-couplesourcemechanismsareassumed(Belachewetal.2012)andare302 derivedfromP-andSH-wavefirst-motionpolaritiesandamplituderatiosusingFOCMEC(Snokeet303 al.1984),whichsystematicallysearchesthefocalsphereforacceptableorientationsofthenodal304 planes.Weselecteventsthatarerecordedataminimumof6stationswitharrivaltimepick305 weightsof0or1andthathaveamaximumazimuthalgapof180°.WeallowzeroP-polarityerrors306 andamaximumofoneerrorintheSH-polarity.307 308 WechoosetouseinformationfromSH-wavepolaritiesandamplituderatiosinadditiontoP-wave309 polarities.P-wavefirstmotionsarepickedontheverticalcomponentofthetracesandSH-310 polaritiesarepickedonthetransversecomponents.Thisisperformedonseismogramsconvolved311 tovelocitybutwithnofurtherprocessinginordertonegateprocessingartefacts.Particularcareis312 takenwhenpickingS-wavepolaritiesastheyaremoresensitivetolocalstructure(Snoke2003).313 ForthisreasonSV-polaritiesarenotconsideredastheyaremorepronetophaseconversions.For314 S-waves,thereferenceframeoftheobserverfacingthestationwiththeirbacktothesourceis315 used,whereafirst-motiontotherightisdenotedbyapositivepolarityandafirst-motiontothe316 leftisdenotedbyanegativepolarity.317 318 Fortheamplituderatios,wefollowthemethodofHavskovandOttemoller(2010)andexclude319 amplitudesmeasuredontheradialcomponentsastheirassociatedfree-surfacecorrectionsvary320

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rapidlywithincidenceangle.Wethereforeusethethreeamplituderatiosof@ABCB,@DECBand@AB

@DEonly,321

whereFG ,HIGandHJK aretheP-waveamplitudesontheverticalcomponents,SV-onthevertical322 andSH-onthetransverserespectively.DuetotherelativelyshorttraveltimesoftheARGOSevents323 underconsideration,theattenuationqualityfactorsaresmallincomparisontotheuncertaintiesin324 measuringtheamplitudesandareignored(HavskovandOttemoller2010).Thefree-surface325 correctionsareassumedtobe1.7,0.8and2.0forthe@AB

CB,@DECBand@AB

@DEratiosrespectively(Akiand326

Richards2002).Fromtheoutputlistofpossiblesolutionsweselectthemechanismwiththeleast327 polarityerrorsandthelowestRMSerroroftheacceptedamplituderatios(seeTableC.1).328 Uncertaintiesindipandstrikearealsopresented.329 330 5.Results331 332 5.1.SpatialVariationinSeismicity333 334 UsingtheAlutoseismicnetworkwelocate2142earthquakes,ofwhich1361aresituatedwithin335 15kmofthecentreofthecaldera,whichisdefinedasstationA01E(Fig.4).Fortheseevents336 within15km,weobtainmedianlocationuncertainties,definedastheonestandarddeviation337 standardconfidencelevelofthePDFscattercloud,of±3.31kminlongitude,±3.17kminlatitude338 and±2.57kmindepth,with50.9%ofthecataloguecontainingamaximumuncertainty<4km(Fig.339 4c).Wenotethatthelowermedianuncertaintiesindepthcomparedtoinlongitudeandlatitude340 maybeinfluencedbystationtopography,whereforshallowevents(themajorityofthecatalogue),341 raypathspropagatelaterallyandtheuncertaintyindepthisapproximatelyequaltothehorizontal342 uncertainty(e.g.foreventsbelow5kmb.s.l.themedianuncertaintiesare2.96,2.71and3.24km343 inlongitude,latitudeanddepthrespectively).344 345 Asubsetof693eventswithmaximumuncertaintiesof4kmispresentedinFig.4.Inthissubset,346 abundantseismicityisevidentatshallowdepthswith369eventsoccurringbetweenthesurface347 andsealevelofwhich,202eventsoccurwithintheuppermost500m.Thereisarelatively348 aseismicregionbeneaththis,with39earthquakesoccurringbetween0and2kmb.s.l.butamore349 seismicallyactiveregionbetween2and9kmb.s.l.containing239events.Thisseismogeniclayer350 deepensdownto~15kmwithlatitude,toboththenorthandsouthofthevolcanicedifice.Little351 seismicityisdetectedbelow9km,whereonly48eventsoccur.352 353 ThereisanalignmentofeventsalongtheAJFZthattrendsparalleltotheWFBat~N012°E354 (Agostinietal.2011).Thisprovidesanarrowpeakcentredat38.79°Einthehistogramofevents355 withlongitude,where44.9%occurbetween38.77°Eand38.81°E.Thepatternofseismicitywith356 latitudeproducesaweaklybimodaldistribution,wherefewereventsoccurbeneaththecaldera357 relativetotheregionsdirectlytothenorthandsouthofthecalderarim.Relativelyfeweventsare358 locatedontheborderfaultstotheeastofAluto,althoughwenotethatthisregionisoutsidethe359 apertureoftherecordingnetwork.Theoverallspatialdistributionofeventsisfurthersupported360 throughthedouble-differencedearthquakerelocations(WaldhauserandEllsworth2000)361 presentedinAppendixA.362 363 5.2.SeismicityandAnthropogenicActivity364 365 FromFig.4b,wedonotobserveanyconsistentspatio-temporalpatternsinseismicitybeneath366 Alutowiththeeventdistributionappearingscattered.Overthetwoyearstheaverageseismicity367 rateofeventsoccurringwithin15kmoftheedificewas1.81earthquakesperday.Duringthe368 experiment,thepowerplantwasinoperationfrom14thJanuary2012until4thJuly2012,a369 relativelysmallproportionoftherecordingspanofthenetwork.Unfortunatelynodataregarding370 fluidinjectionorextractionratesareavailableduringthisoperationalperiodanditistherefore371 difficulttocommentontheplant’simpactonlocalseismicityasaresult.Nonetheless,wedonot372

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observeanysignificantchangesbetweenperiodsofpowergenerationandperiodsofelevated373 seismicity.374 375 5.3.Magnitudesandb-Values376 377 LocalmagnitudesfortheAlutoeventcatalogueof1361earthquakes(Fig.4)rangefrom-0.39to378 2.98(Fig.5a).Duetotheirrelativelysmallsizes,noneoftheAlutoeventswererecordedbythe379 InstituteofGeophysics,SpaceScienceandAstronomy(IGSSA)basedatAddisAbabaUniversity.380 Wecalculatetheseismicenergyreleaseforthe1361locatedearthquakesaroundthecalderausing381 empiricalrelationshipsforML(Kanamori1977;Keiretal.2011),whichtotals3.80×1014Nmover382 thespanofthedeployment(1.85×1014Nm/yr).Thisvalueandthatofthetotalmomentrelease383 fromall2142ARGOSevents(4.49×1015Nm/yr)areapproximately3—5ordersofmagnitudeless384 thanthepredictedmomentreleasedfromgeodeticmodellingintheMER(Déprezetal.2013)and385 agreewiththedominantaseismicreleaseofmomentwithintherift(HofstetterandBeyth2003).386 WedonotobserveanyclearspatialrelationshipsbetweenMLandeventlocation(Fig.4).387 388 Intectonicsettings,theb-valueisgenerallycloseto1(FrohlichandDavis1993)butanomalous389 valuesashighas3havebeenobservedinvolcanicareaswheretheaccumulationofstrainmaybe390 preferentiallyreleasedbynumerousrelativelysmallevents(e.g.,WiemerandMcNutt1997;Murru391 andMontuori1999;Wyssetal.2001).FortheseAlutoeventswedetermineab-valueof1.40±0.14392 tothe95%confidencelevelandamagnitudeofcompleteness(MC)equalto1.35(Fig.5).Wefind393 thatonlyconsideringasubsetofthebestlocatedevents(<4kmuncertainty)intothecalculation394 doesnotimprovetheuncertaintiesofthederivedb-value.395 396 Wenextcalculatetheb-valueincrementallywithdepth(Fig.6).Eventsarebinnedinto397 overlappinghorizontalslicesof2kmthicknessat0.5kmspacingsdownto10km,belowwhich398 seismicityistoosparsetosuccessfullyfittheGutenberg-Richterrelationshipwithineachbin.The399 a-valueisalsopresentedandreflectstherelativeseismicityrate.Thisdepthmappingshowsthree400 regionsofcontrastingbintheuppermost10km(Fig.B.1):401 402 -2–0km:Ab-valueof2.55±0.55andhighseismicityrate(a=5.64)where796eventsoccur.403 0–2km:Areducedb-valueof0.82±0.21andalowera-valueof2.49where90eventsoccur.404 2–9km:Elevatedb-valuesrangingfrom0.95to1.33andincreasedseismicity,wherea-valuesare405

2.69–3.53and111–145eventsoccurwithineachdepthbin.406 407 5.4.FocalMechanisms408 409 Wecomputesourcemechanismsfor21earthquakesaroundAluto(Figs.7andAppendixB),410 findingthatincorporatingSH-wavepolaritiesandamplituderatioshelpstoreduceambiguityin411 thegrid-searches.ThefocalmechanismsbeneathAlutoarepredominantlynormaleventswith412 rakeslessthan−22.2°andanaveragerakeof−56.9°.Generally,theeventsthatwedetermine413 mechanismsforoccurclosetotheAJFZandtoQuaternarytoRecentfaultsassociatedwiththe414 WFB.Someevents(e.g.,11,14,17,18,20and21)haveleft-lateralstrike-slipcomponentsthat415 occuron~NEstrikingfaultplaneshowever.416 417 TheoverallmeanazimuthoftheT-axesisN098°E,whichisapproximatelyperpendiculartothe418 strikeoftheWFBandtheaverageT-axesplungeis10.8°(Fig.8).TheP-axesofthefaultsolutions419 clustertowardsthecentreofthehemisphericalprojections,withameanazimuthofN276.0°Eand420 anaverageplungeof62.0°.421 422 6.Discussion423 424 6.1MagmaticSystem425

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426 Atdepthsbelow9kmthereislittleseismicity,whichagreeswiththehypothesisthatbeneaththis427 depth,riftextensionintheMERisaccommodatedaseismicallyandassociatedwithductile428 stretching(Danielsetal.2014)and/ormagmaticinjectionintothelowertomid-crust(Ebinger429 andCasey2001;Keiretal.2006a).IntheBoset-Konevolcanicsegmenttothenorth,earthquakes430 deeperthan10kmarealsorelativelysparseandareindicativeofanelevatedheatflowthat431 inhibitsbrittledeformation(Maggietal.2000).Thisissuggestiveofahot,ductilemagmaticmush432 zoneasAluto’sprimarymagmaticreservoirwherelittleseismicityoccursasaconsequence(Fig.433 9).WeacknowledgehoweverthatwiththeuncertaintyofthepreciselocationofAluto’smagma434 reservoir(Samrocketal.2015),thatmeltmaybepresentatgreaterdepthsthanthis.Thismay435 consequentlyraisethebrittle-ductiletransitiondepthaboveitandproducerelativelyshallower436 earthquakesbeneaththecalderathantothenorthandsouth.437 438 Modellingofedifice-wideinflationoccurringatAlutoin2008suggestsadeformationsourceat3.1439 kmdepthandisinterpretedtoreflecttheepisodicintrusionofmagmaintoavolatile-richcap440 betweenthemagmaticandhydrothermalreservoirs(Hutchisonetal.2016).Sincedeformation441 modellingismostsensitivetothetopsurfacesofinflatingordeflatingsources(Yunetal.2006),442 thisimpliesthatmagmaascentdoesnotpropagateshallowerthanthisdepth,andisbroadly443 consistentwithMTsurveying,whichhasfailedtoimageconductivemagmaticphasesabove5km444 (Samrocketal.2015).445 446 Between2and9kmb.s.l.weobservearegionofincreasedseismicityandhighb-valuesofupto447 1.33.Elevatedb-valuesassociatedwithmagmaticintrusionhavebeenpreviouslyattributedto448 increasesinporepressures(reducingtheeffectivestress)andthepropagationofcracks449 (increasingheterogeneity)asmagmasmigrate(Mogi1962;Wyss1973).Withintrusion450 interpretedtodriveinflationepisodesatthesurface,wesuggestthattheseismicitydistribution451 withinthiszonealsoreflectsthesevolcanicprocesses.Inthiscase,highstrainratesandan452 elevatedb-valueareinducedviamagmaticinjectionfromalargermagmareservoirbelow.453 454 Furthermore,themappingofb-valuesatMt.St.HelensandMt.Spurr(WiemerandMcNutt1997)455 andattheLongValleyCaldera(Wiemeretal.1998)hasalsoshownthatregionsofhighb-values456 correlatewellwithregionsofgasvesiculationfromascendingmagmas.Wespeculatethattowards457 thetopofthemagmaticsystem(2–3kmb.s.l.),thattheexsolutionofvolatilesalsocontributesto458 theobservedb-valueanomaly,whereviolentvesiculationfracturestherockvolumeatthe459 deformationsourceandmicroseismicityisinducedaroundit.460 461 6.2HydrothermalSystem462 463 Abovethisregionofvolatilereleaseandbetween0and2km,wedetectaregionofreduced464 seismicitywhereelasticdeformationisdiminished.Theb-valuealsodecreasestobelowunity,465 whichimpliesanincreaseinfrictionandthatseismicityisnotfluidinduced.AlthoughCO2gas466 samplingoffumaroleshasshownthatthemagmaticandhydrothermalsystemsareconnected467 (Teklemariametal.1996;Hutchisonetal.2016),thesparseseismicityandrelativelylowb-value468 suggeststhatduring2012and2013thatthispathwaywasrelativelyimpermeable.469 470 ThemodellingofunrestatCampiFlegreiinItalyhassuggestedthatdeformationcyclesbeginwith471 theinjectionofmagmaticphasesintoanimpermeablecapthatexsolvevolatilesasthemagma472 crystallises(Battagliaetal.2006).Vesiculatedfluidshavebeenshowntopreferentiallyformin473 horizontallenses,inwhichcasepermeabilityanisotropymayinhibitthevolatiles’ascentintothe474 hydrothermalsystem(Fournier1999).Aself-sealedzoneofprecipitatesprovidesthe475 impermeablebarrierthatthensegregatestheshallowerhydrothermalfieldfromthemagmatic476 reservoirandcausessurfaceupliftasvolatilesaccumulatebeneaththebarrier.477 478

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Atsomecriticalpointtheimpermeablebarrierisbreachedundertheincreasingpressureand479 volatilesascendtothebrittle,lowerpressuredaquiferthroughfaultingandbrecciationofthe480 mediumandinducesseismicity.Thisincreasestheporepressureandtemperaturewithinthe481 shallowaquifer,withtheefficientdischargeoffluidsvialateralflowfacilitatingsubsequent482 subsidence(DeNataleetal.2001).AsimilarmodelofunresthasalsobeensuggestedatAlutoby483 Biggsetal.(2011)whereinflationepisodesareattributedtomagmaticactivityperturbingthe484 hydrothermalsystemandsubsidenceepisodesattributedtothehydrothermalsystemonly.485 486 ThedeformationsourceofsubsidenceatAlutobetween2009and2010hasbeenmodelledasa487 Mogipointsourceat1.4kmdepth(Fig.9),whichlikeatCampiFlegrei,representsthelossoffluid488 fromthehydrothermalsystem(Hutchisonetal.2016).Assumingthesamesourcelocationforthe489 2012subsidenceplacesthesourcewithintheobservedaseismicregionbutthefaultingassociated490 withthereleaseofvolatilesattheonsetofsubsidenceisnotobservedintheseismicity491 distributionwithinthisdepthregion.AtAluto,thecontinuedsubsidenceobservedfromDecember492 2009(Biggsetal.2011)suggeststhatitislikelythatthefaultingandvolatileascentassociated493 withtheonsetofsubsidenceoccurredbeforethedeploymentoftheseismicnetwork.The494 associatedfaultinghadsubduedby2012,withtheself-sealingofthevolatilecaprecommencing495 quicklyasfurthervolatilesaccumulated.Therelativelyprolongeddeflationsuggeststhatlateral496 flowandhydrothermalprocessescontinuedtodominatethesurfaceexpressionofthesystem497 however,andthatdespitethefastresealingoftheimpermeablelayer,inflationwasnotobserved498 againuntilmid-2013(Hutchisonetal.2016).499 500 Areasofenhancedthermalgradients,suchaswithingeothermalregionsareoftenrelatedtorocks501 ofweakrheologieswherefailuremayensueatrelativelylowshearstresses(WarrenandLatham502 1970;Wyss1973).Thiscanleadtob-valuesgreaterthan1(Bachmannetal.2012;Trugmanetal.503 2016).Theoverallb-valueatAlutoof1.40±0.14indicatesanelevatednumberofsmallmagnitude504 eventsrelativetolargeronesandsitsinagreement.Thisisparticularlyevidentintheuppermost2505 km,wheretheb-valueis2.55±0.55andissignificantlygreaterthanavaluecalculatedforthe506 widerMERregionof1.13±0.05(Keiretal.2006b).AtAlutoitislikelythatthehighlyfractured507 natureoftheAJFZ,assumedringfaultingandhighthermal-gradientdrivingthegeothermal508 system(Hutchisonetal.2015a),allaidinpromotingfailureatlowshearstressesthatgenerates509 thehighestseismicityratesandsignificantlyelevatestheb-value.510 511 Well-logdatahasrevealedthatthegeothermalreservoirresidesinaNeogeneignimbriteunitat512 approximatelysealevel(Teklemariametal.1996)(Figs.2&9).InthehottestwellsatLA-4and513 LA-6,temperaturesinexcessof340°Chavebeenrecordedat2100mbelowthesurface(~-150m514 b.s.l.,Gizaw1993),hencesuggestingatemperaturegradientof~160°C/km.Highb-values515 associatedwiththemigrationandcirculationofhydrothermalfluidshavealsobeenpresentedat516 theYellowstonecaldera(Farrelletal.2009),wherearegionofb=1.3±0.1correlateswellwith517 hydrothermalfeaturesmappedatthesurface.Althoughourderivedvaluewithinthe518 hydrothermalsystemismuchgreaterthanthisvalue,theestimatedtemperaturegradientatAluto519 isuptotwicethatatYellowstone(50–80°C/km),suggestingamoreenergeticgeothermalsystem520 thatinducesrelativelymoremicroseismicevents.521 522 Elevatedb-valueshavealsobeenpresentedatgeothermalfieldsinCalifornia,whereatthe523 Geysers,SaltonSeaandCoso,highvaluesofover1.3usingamovingwindowof400eventshave524 beencorrelatedwithgeothermalpowerproductionateachfield(Trugmanetal.2016).AtCoso,a525 b-valueashighas2.4hasalsobeendeterminedusingaone-yearmovingwindowapproachthat526 incorporated1000sofeventsintoeachcalculation(Kavenetal.2013).Inaddition,b-valuesofup527 to3.5weredeterminedattheBaselEGSexperimentduringco-andpost-injectionstages528 (Bachmannetal.2012),furtherhighlightingtheprevalenceofshortearthquakerecurrencetimes529 inregionsoflowfrictionatgeothermalsites.530 531

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Thehighfluidsaturationandpermeabilityoftheshallowhydrothermalsystemregionislikely532 amplifiedbyitshighlyfracturednaturewithsomecontributionfromthecondensationof533 geothermalgasesthatascendalongtheAJFZ(Gizaw1993).Thetemporaldistributionofshallow534 (<2km)Alutoeventsinrelationtothehydrostaticloading(precipitation)andelasticresponseof535 thesurface(deformationinferredfromGPS)hassuggestedthatperiodsofhighrainfallreplenish536 thehydrothermalsystem,whereacross-correlationofshallowseismicityandrainfallsuggests537 thatpeakseismicityfollowspeakrainfallafter~4months(Birhanuetal.2017,).Thisestimated538 lagtimeisbroadlyconsistentwithresultsseenelsewhereintheMER(BirhanuandBendick2015)539 andprovidesfurtherevidencethatfluidsaturationofthesubsurfaceandcirculationinthe540 geothermalreservoirplaysanimportantroleintheseismicresponseofthevolcano.541 542 6.3.RiftStructures543 544 Since2Ma,strainintheMERhaslocalisedtoright-steppingen-echelonriftsegmentsthatare545 largelycontrolledbytheintrusionofmaficmagmasasriftinghasevolvedtowardscontinental546 break-up(Boccalettietal.1998;EbingerandCasey2001;Keiretal.2015).Thismigrationofstrain547 localisationtothecentreoftheriftiscommonplaceduringmagma-assistedrifting(Buck2004),548 wherethedecreasingstrengthofthelithospherepreventsthestresslevelsofboundaryfaults549 fromreachingfailure.ThesparsenessofseismicityontheborderfaultstotheeastofAluto550 suggeststhatthisisthecase,whilethealignmentofseismicityalongtheAJFZandWFBat551 ~N012°E,ratherthanalongtheborderfaultsat~N032°E,highlightsthatthesearecurrentlythe552 activestructures(Agostinietal.2011).553 554 ThesourcemechanismsatAlutoarepredominantlynormalornormalwithsomesmallcomponent555 ofstrike-slip.Theseeventsareincloseagreementwithmechanismsproducedpreviouslyalonga556 largeproportionoftheMER(e.g.,FosterandJackson1998;HofstetterandBeyth2003;Keiretal.557 2006a),whereeventshaveoccurredonsteeply-dippingnormalfaultsstrikingapproximatelyNNE.558 TheaverageT-axisazimuthoftheseeventsofN098°Eisperpendiculartotheaveragestrikeofthe559 WFB(Agostinietal.2011)andsuggeststhattheoverallorientationofminimumcompressionat560 Alutoiscongruenttothatofpresent-dayriftingat~N100°E(Stampsetal.2008).561 562 Exceptionstothistypicaltrendofnormaldip-slipintheMERincludestrike-slipeventsonNE–ENE563 structures,whereleft-lateraloffsetshavebeenobservedbyFosterandJackson(1998),Ayele564 (2000)andKeiretal.(2006a).Thishasbeenparticularlyevidentatthetipsofmagmaticsegments565 andisthoughttorepresenttranscurrentmotionwheredeformationisadditionallycomplex566 (Caseyetal.2006;Beuteletal.2010).BeneathFentaleandBosetforexample,left-lateralsliphas567 beenassociatedwithdisplacementonpre-3.5Mastructuresthatformedpriortotheonsetof568 magma-assistedriftingintheCMERat2Ma(Keiretal.2006a).AtAluto,whichislocatedatthe569 southerntipoftheAluto-Gedemsamagmaticsegment,wesuggestthatthecomponentsofstrike-570 slipincertaineventslikelyarisesinthismannerandmaybeindicativeofthereactivationofsuch571 structuresobliquetothedirectionofpresent-dayextension.572 573 7.Conclusions574 575 Usingalocalnetworkof12seismicstations,theseismicityatAlutohasbeenmonitoredforthe576 firsttime.Priortothis,thevolcanowasknowntobedeformingbutnoseismicityhadbeen577 detectedwithregionalornationalnetworks.Wedetect2142earthquakesaroundAlutooveratwo578 yearperiod,ofwhich1361occurwithin15kmofthevolcanicedifice.Shalloweventsare579 interpretedintermsofhydrothermalactivityintheuppermost2km,whilsttherelativelysparser,580 deepereventsaremostlikelyassociatedwithmagmaticprocesses.Ofthebestlocatedevents,focal581 depthsrangefromthenearsurfacetoamaximumof15kmwithasignificantamountofseismicity582 followingNNE–SSWfaultingthroughthecalderaandinalignmentwiththeQuaternarytoRecent583

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riftingoftheWFB.Focalmechanismsarenormalornormalwithsmallcomponentsofstrike-slip584 andshowshallowdippingT-axesorientatedparalleltocurrentextension.585 586 Weutilisealocalmagnitudescalethatprovideseventsizesbeneaththevolcaniccentreranging587 between−0.40and2.98.Resemblingmanyvolcanicandgeothermalsettings,weobserveahigh588 overallb-valueof1.40±0.14 thatsuggeststhatseismicmomentreleaseoccursthroughnumerous589 relativelysmallevents.Thisisparticularlyevidentinthehydrothermallayer(abovesealevel),590 wherebreaches2.55andtheadditionalinfluenceofhighprecipitationandsubsequent591 gravitationalloadinglikelycontributestothereleaseofaccumulatedstraininnumerous592 microseismicevents.593 594 Theb-valuevarieswithdepthhowever,withlowvaluesandarelativelylowseismicityrate595 between0and2kmsuggestingacomparativelystrongerrheologyandincreasedfriction.Thelow596 seismicitywithinthislayerlikelyindicateslowpermeabilitythatsuppressestheascentof597 magmaticfluidstothehydrothermalsystemandimpliesthatprocesseswithinthehydrothermal598 systemaretheprimarycontrolonsurfacedeformation.599 600 Theb-valueandseismicityrateincreasesbetween2–9km,whichsuggeststhathighstrainrates601 andtheinteractionofmagmaticfluidspromoteelasticdeformation,magmaticintrusionandgas602 vesiculationthatinturn,driveshortperiodsofupliftatthesurface.Littleseismicityoccurs603 beneaththesedepthsnortotheeasternborderfaults,supportingthetheorythatstrainlocalised604 towardsthecentreoftheriftat2Maandwasheavilyassociatedwithmagmaticprocesses.605 606 Alutoisanactivevolcanothathashighlighteditsseismogenicnaturedespitenovolcaniceruptions607 foratleast2000years.Localseismicityobservationshavebeenpresentedandalongwithprevious608 andon-goinggeodeticobservationsandtheareas’growingeconomicimportance,werecommend609 furthergeophysicalmonitoringofAlutoandofothervolcanoeswithintheMER.610 611 8.Acknowledgements612 613 WewouldliketothankSEIS-UKfortheuseoftheirequipment,fortheirhelpwhilstinthefieldand614 uponmanagingthedatawhenbackintheUK.Likewise,wethankvariouscollaboratorsfromthe615 EthiopianElectricPowerCorporation(EEPCo)andtheGeologicalSurveyofEthiopia(GSE)for616 theircontributionstotheproject.TheBristolUniversityMicroseismicProjectS(BUMPS)provided617 fundingfortheseismicexperimentandfieldworkandtheseismicequipmentwasloanedfrom618 SEIS-UKwithGEFloan962.TheseismicnetworkisXMandthedatasetisopenaccessand619 availableonIRIS.M.W.wasfundedbyanEPSRCstudentship.Theresearchleadingtotheseresults620 hasreceivedfundingfromtheEuropeanResearchCouncilundertheEuropeanUnion’sSeventh621 FrameworkProgramme(FP7/2007-2013)/ERCgrantagreement240473“CoMITAC”.A.N.was622 fundedbyBUMPSandCoMITACandJ.B.wasfundedbyNERCCOMET.623 624 9.Appendices625

9.A.Double-DifferenceEarthquakeLocations626

Tonegatetheeffectsofapoorlyconstrainedvelocitymodelcausinginaccuraciesintheevent627 locations,itiscommonpractisetorefinehypocentrelocationsusingthedouble-differencing628 techniquedescribedbyWaldhauser&Ellsworth(2000).Thistechniquecanreviseinitialevent629 locationstohighlighttheclusteringofearthquakesandbetterresolvesubsurfacefeaturesincases630 wherethehypocentralseparationbetweenanearthquakepairissmall,incomparisontothe631 source-stationdistance.Inthisscenario,itisassumedthattheraypathsbetweentheeventsand632 thecommonstationaresimilarandthatthedifferenceintraveltimesisduetospatialoffset633 betweentheevents.634

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First,cataloguephasedata(outputfromNONLINLOC)ispre-processedtoderivetraveltime635 differencesbetweenearthquakepairs.A‘networkoflinks’iscreatedbyexaminingeveryevent636 andcomparingitspatiallytootherproximaleventswithinsomemaximumradius.Themaximum637 radiusshouldhoweverbesmallincomparisontothesource-stationdistanceandthelengthofthe638 subsurfacevelocityheterogeneitybutsimilartotheuncertaintiesoftheinitiallocations.639 ‘Neighbouring’eventsarethendeterminedusinganearestneighbourapproach.Throughsolving640 thedouble-differencingequationswiththecomputeddifferentialtraveltimesasoutlinedby641 Waldhauser&Ellsworth(2000),clusterswithintheinitialcataloguearethenrelocated.642

WefollowthisapproachatAlutousingthe1DvelocitymodeldescribedinTable1.Clusteringthe643 693best-locatedevents(uncertainties<4km)fromtheAlutocataloguebygroupingeventsinto644 binsof1kmradiusrelocates218eventsin79clusters(Fig.A.1).Withahighproportionofthe645 Alutocataloguebynature,occurringclosetothevolcaniccentreandatrelativelyshallowdepths,646 notallearthquakepairshaveahypocentralseparationsignificantlylessthanthesourcetostation647 distance.Thisisreflectedbytherelativelylownumber(31%)ofearthquakesthatarerelocated648 whenusingthistechnique.649

TherelocationsclusteralongtheAJFZandhighlightsthatseismicitytrendsNNE-SSWin650 agreementwiththelocationsfromNONLINLOC.Asharppeakat38.79–38.80°isobservedthat651 emphasisesthisfurther.Feweventsarerelocateddirectlybeneaththecalderaincomparisonto652 regionstothenorthandsouthoftherim,creatingabimodaldistributionwithlatitude.653

9.B.b-ValueswithDepth654

Estimatingb-valuesforfourdepthrangesforthe1361earthquakesaroundAlutoshowsthree655 regionsofcontrastingb(Fig.B.1).Betweenthesurfaceandsealevelandwhereseismicityis656 highest,wecalculateab-valueof2.55,whichevenwithrelativelylargeuncertaintiesof±0.55657 suggestsab-valuewellabove1thatisindicativeofswarmactivity.Intherelativelyaseismic658 regionbetween0and2km,bisreducedto0.82±0.21andismorealignedtotheglobaltectonic659 average.Below2km,thetworegionsaboveandbelow9kmproduceb-valuesof1.25±0.28and660 1.20±0.34thatareagainsuggestivethatstrainispreferentiallyreleasedvianumerousrelatively661 smallmagnitudeevents,relativetotectonicsettings.662

9.C.FocalMechanisms663 TheparametersoutputfromFOCMECforthe21focalmechanismsarepresentedinFig.C.1and664 TableC.1.ThebestfittingfaultplanesolutionsforeacheventarepresentedwiththefractionsofP-665 andSH-wavepolaritiesthatcorrectlyfittheoutputquadrants.Fortheamplituderatios,the666 fractiondenotesthenumberofacceptableratiosdividedbythetotalnumberofratiosinputinto667 thegrid-search,whiletheRMSerroristheRMSoftheacceptedratioobservation668 669

670 10.References671 672 Agostini,A.,Bonini,M.,Corti,G.,Sani,F.,Mazzarini,F.,2011.FaultarchitectureintheMain673 EthiopianRiftandcomparisonwithexperimentalmodels:Implicationsforriftevolutionand674 Nubia-Somaliakinematics.EarthandPlanetaryScienceLetters301(3-4),479–492.675 676 Aki,K.,1965.MaximumLikelihoodEstimateofbintheFormulalogN=a-bManditsConfidence677 Limits.In:UniversityofTokyoEarthquakeResearchInstituteBulletin.Vol.43.Earthquake678 ResearchInstituteoftheUniversityofTokyo,Tokyo,Japan.,pp.237–239.679 680 Aki,K.,Richards,P.G.,2002.QuantitativeSeismology,2ndEdition.UniversityScienceBooks,681 Sausalito,California.682

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1018 Figure1:SRTM(http://www2.jpl.nasa.gov/srtm)elevationmapoftheMainEthiopianRift.1019 Borderfaultsoftherift’smarginsandinternalfaultsoftheWonjiFaultBelt(Agostinietal.2011)1020 areredandblacklinesrespectively.Magmaticsegments(EbingerandCasey2001)aregreyand1021 volcanoesactiveintheHoloceneareorangetriangleswithAlutoenlarged(SiebertandSimkin1022 2003).Populationcentresofgreaterthan20,000people(2007)arebluestarsandlabelled:mk:1023 Meki;bt:Butajira;zw:Ziway;as:Assela;hs:Hosaena;sh:Shashemene;hw:Hawassaandya:Yirga1024 Alem.MERlakesare:KO:LakeKoka;ZW:LakeZiway;LN:LakeLangano;AB:LakeAbijta;SH:Lake1025 ShalaandHW:LakeHawassa.ISCreviewedearthquakessince1960(InternationalSeismological1026 Centre2016)andseismicityrecordedbytheEAGLEprojectfromOctober2001toJanuary20031027 (Keiretal.2006a)arewhiteandbluecircleswiththeMW5.3.UpperLeftInset:Thestudyarea’s1028 contextwithintheEastAfricanRift.Volcanoesareblacktrianglesandvectorsarethecurrentday1029 platemotions,scaledtoextensionalvelocityandrelativetotheNubianPlate(Stampsetal.2008).1030 1031

37.5˚ 38˚ 38.5˚ 39˚ 39.5˚6.5˚

7˚

7.5˚

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AY

zw

btmk

as

hwsh

ya

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Border FaultsWonji FaultsMagmatic Segs.

Pop. CentresISC EQsEAGLE EQs

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1032 Figure2:Across-sectionalsummaryofthestratigraphybeneathAlutoofthetransectA-A’inFig.1033 3(afterHutchisonetal.2015a).Geothermalwellsareprojectedontothecross-sectionand1034 labelledwithtemperaturesrecordedbyGizaw(1993).TheAluto-LanganoGeothermalPlantisthe1035 yellowsquare.Faultsandinferredfaultsaresolidanddashedlinesrespectivelywitharrows1036 denotingtheup-flowofgeothermalfluidsalongtheAJFZ.1037 1038

1039 Figure3:TopographicmapofAlutovolcano.ThestationsoftheARGOSseismicnetworkandGPS1040 stations,theAluto-LanganoGeothermalPowerPlantandgeothermalwells.Borderfaultsarered1041 (Agostinietal.2011)whiletheArtuJawafaultzone(AJFZ)andotherfaultsoftheAlutovolcanic1042 complexareblue(Kebedeetal.1985;Hutchisonetal.2015a).Thecalderaringstructureisthe1043 magentadashedline(Hutchisonetal.,2015a).ThetransectA-A’inFig.2isthedashedbackline.1044

A06EA A’

A09E

A12E/G

LakeLangano

LakeZiway

7.7°

7.75°

7.8°

7.85°

38.7° 38.75° 38.8° 38.85°

Seismic StationsGPS StationsGeothermal WellsPower PlantBorder FaultsAJFZ and Aluto FaultsCaldera Ring StructureRoads

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Adami Tulu

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1045

1046 Figure4:(a)TheseismicityaroundAlutowithmaximumlocationuncertainties<4kmplottedin1047 mapviewandinaN-Sdepthprofilewiththeassociatederrorsbars.Eventsarecolouredbyfour1048 depthregionsandsizedbylocalmagnitude.Zerodepthissealevel.The15kmradiusfromstation1049 A01Eisthewhitedashedline.Seismicstationsareinvertedwhitetriangles,borderfaultsarered1050 andthefaultsoftheAJFZandtheAlutovolcaniccomplexareblue.Stackedhistogramsinlongitude1051 andlatitudeusebinsof0.01°widthandarecolouredbydepth.(b)693earthquakehypocentresof1052 eventswithin15kmofA01EalongtransectA–A’.Eventsarecolouredbyorigindate.The1053 histogramplotseventdepthsin0.5kmbins.(c)Boxplotsoftheestimatedlocationuncertainties1054 inlongitude,latitudeanddepthforalllocatedevents.1055 1056

0 10 15Depth (km)

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Latit

ude

(°N

)a)

b)

Longitude (°E)

Longitude (°E) Number of Events

0

5

10

15

20

204A A’

38.6˚ 38.7˚ 38.8˚ 38.9˚ 39˚

0 10

Kilometres

7.6

7.7

7.8

7.9

8.00 5 10 15 20

Depth (km)

7.6

7.7

7.8

7.9

8.00103050

20406080

100120

38.6 38.7 38.8 38.9 39.0

0 1 2 3

ML

205

A A’

Numberof

Events

Longitude Latitude Depth0

2

4

6

8

10

12

Unc

erta

inty

(km

) c)

25

1057 Figure5:Distributionsofearthquakemagnitudesfor1361earthquakesaroundAluto.(top)1058 Magnitudedistributionsplottedin0.1sizedbinswithanormaldistributionalsofitted.(bottom)1059 TheGutenberg-Richterdiagramfortheevents.b-valuesaresolidredlinesandthe95%confidence1060 levelsarereddashedlines.MCisthesolidblueline.1061 1062

-0.5 0 0.5 1 1.5 2 2.5 3Magnitude

100

101

102

103

Cum

ula

tive N

um

ber

of E

vents

b-value = 1.40±0.14

MC = 1.35

a = 4.41

-0.5 0 0.5 1 1.5 2 2.5 30

50

100

150N

um

ber

of E

vents

M̄L =0.99

26

1063 Figure6:b-valuesplottedatincreasingdepthincrementsbeneathAluto.Thecalculatedb-values1064 (redline)andtheassociatedupperandlowererrorbars(dashedbluelines)areplottedatthe1065 midpointofthe2kmthickdepthbinsat0.5kmspacings.Thea-valuesarethegreenline,the1066 numberofeventsineachbinaretheblackcirclesandthetectonicaverageof~1isthereddashed1067 line.1068 1069

27

1070 Figure7:The21focalmechanismscomputedaroundAlutowithdilatationalquadrantscoloured1071 bydepth.BorderfaultsareredandtheAJFZandotherfaultsoftheAlutocomplexareblue.1072 1073

38.65˚ 38.7˚ 38.75˚ 38.8˚ 38.85˚ 38.9˚ 38.95˚7.65˚

7.7˚

7.75˚

7.8˚

7.85˚

7.9˚

7.95˚

−2 0 2 4 6 8 10Depth (km)

0 10km

2 4 6 8

�

�

�

�

�

�

�

�

�

�

�

�

�

�

�

�

�

�

�

�

�

9

2

13

7

10

4

53 19 8 15

1814

20

17

16

6

21

12

11

1

28

1074 Figure8:LowerhemisphereprojectionsoftheT-(blue)andP-(red)axesfor21Alutoevents.1075 Dashedlinesarethecircularmeansoftheaxesazimuthsandcontoursmarkthedensity1076 distributionoftheT-axes.1077 1078

1079 Figure9:Aschematiccross-sectionofthehydrothermalandmagmaticsystemsbeneathAluto.An1080 underlyingductilemagmaticmushfromwhich,magmaintrudesintotheregionaboveit,1081 generatingseismicityandelevatedb-values.Intrusionintoavolatile-richcapcausesthe1082 exsolutionofgasesthatdriveperiodsofinflationatthesurface.Duringperiodsofprolonged1083 deflation,heatandvolatiletransferissuppressedandlittleseismicityoccursbetweenthe1084

N

N098°E

WFBP axesT axes

N276°E

29

magmaticandgeothermalreservoirsasthevolatilecapsealsquickly.Thelargetemperature1085 gradientsacrossthegeothermalreservoircirculatesubsurfacehydrothermalfluidswithinand1086 aboveit,withtheAJFZprovidingtheprimarypathwayofascent.Thereservoirisreplenishedby1087 rainfallthatuponheatingcirculatesenergeticallyandinducesseismicityatshallowdepths.1088 Conversely,theborderfaultstotheeastremainrelativelyinactive,withthemajorityofstrain1089 localisingalongtheWFB.Note:theverticalaxisisnottoscale.1090 1091

1092 FigureA.1:The218double-differencedeventlocationsaroundAlutousingHypoDD.Eventsare1093 colouredbyhypocentraldepthintofourdepthsectionsandsizedbylocalmagnitude.Histograms1094 withlongitudeandlatitudearealsopresented.1095

Numberof

Events

0 10

Kilometres

0 5 10 15 20Depth (km)

7.6

7.7

7.8

7.9

8.00 5 10 15 20

Depth (km)

7.6

7.7

7.8

7.9

8.0

Latit

ude

(o N)

05101520251020304038.6 38.7 38.8 38.9 39.0

Longitude (oE)

0

5

10

15

20

Dep

th (k

m)

38.6 38.7 38.8 38.9 39.0

ML Scale

43210

30

1096

FigureB.1:Gutenberg-Richterdistributionsandb-valuecalculationsforfourdepthranges1097 beneathAluto:(a)abovesealevel,(b)betweensealeveland2kmdepth,(c)between2and9km1098 and(d)below9km.1099

1100

Cum

ulat

ive

Num

ber o

f Eve

nts N = 796

b = 2.55±0.55MC = 1.41

a) b)

c) d)

ML

1

10

100

1000

-0.5 0 0.5 1 1.5 -2.5 32

Cum

ulat

ive

Num

ber o

f Eve

nts

1

10

100

1000

-0.5 0 0.5 1 1.5 -2.5 32

Cum

ulat

ive

Num

ber o

f Eve

nts

ML

1

10

100

1000

-0.5 0 0.5 1 1.5 -2.5 32

Cum

ulat

ive

Num

ber o

f Eve

nts

ML

1

10

100

1000

-0.5 0 0.5 1 1.5 -2.5 32

ML

N = 359b = 1.25±0.28

MC = 1.33

N = 90b = 0.82±0.21

MC = 0.74

N = 135b = 1.20±0.34

MC = 1.41

31

1101 FigureC.1:Lowerhemisphericalprojectionsofthe21focalmechanismsatAluto.Redoctagons1102 arecompressionalP-wavefirst-motionswhilebluetrianglesaredilatational.Arrowsindicatethe1103 SH-first-motiondirections.Amplituderatiosarecrossesandsizedbymagnitude.1104 1105

1 2 3 4 5 6

7 8 9 10 11 12

14 15 16 17 18

���

�

19 20 21

13

ID Date Time Lat(°N)

Lat(°N)

Dep(km) ML

AzGap(°) N Dip

(°)Strike(°)

Rake(°)

P-WavePols

SH-WavePols

AmpRatios

RMSError

1 120117 02:28:12.137 7.726 38.785 3.86 1.8 126 36 50.18±5.90 359.18±21.70 -83.48 7/7 3/4 7/9 0.302 120430 01:24:05.369 7.731 38.767 3.77 1.6 145 6 51.62±2.08 243.68±2.86 -70.72 8/8 3/3 11/12 0.323 120625 07:04:26.045 7.857 38.782 9.53 3.0 113 1 40.26±0.00 205.93±0.00 -82.25 8/8 6/7 20/21 0.364 120625 07:05:22.611 7.848 38.779 7.84 2.9 118 3 35.53±1.95 263.58±2.35 -53.95 9/9 7/8 17/21 0.255 120625 12:30:53.481 7.859 38.780 8.08 3.0 116 5 45.86±2.65 17.46±51.50 -76.00 9/9 5/6 14/17 0.346 120625 12:39:29.379 7.821 38.792 8.50 1.9 170 32 46.03±4.20 51.66±36.43 -54.04 6/6 4/5 15/15 0.247 120625 13:16:20.426 7.823 38.778 7.00 2.0 159 4 36.22±1.76 188.86±3.12 -72.91 7/7 5/6 16/16 0.278 120820 20:51:07.549 7.858 38.798 8.57 1.3 135 39 60.50±4.79 23.47±9.06 -78.49 6/6 2/2 6/6 0.199 120924 22:07:37.458 7.718 38.786 8.90 2.2 176 1 35.53±0.00 226.52±0.00 -53.95 9/9 6/6 16/16 0.3310 121001 21:12:30.669 7.845 38.767 6.15 1.5 136 33 60.13±16.28 7.88±55.53 -84.23 6/6 5/5 15/15 0.2611 121211 23:33:52.714 7.729 38.794 7.96 1.3 138 2 61.98±0.88 0.53±0.68 -49.48 6/6 5/5 12/12 0.3112 120217 05:31:34.960 7.743 38.811 8.32 2.3 140 11 61.98±12.44 349.99±49.18 -67.20 8/8 6/7 8/9 0.3513 120723 02:01:04.324 7.744 38.787 6.63 0.9 138 2 78.56±2.95 346.86±3.24 -49.02 6/6 3/4 3/4 0.3014 120803 18:39:58.390 7.830 38.798 3.28 1.6 126 6 43.96±17.11 31.03±54.14 -22.18 8/8 6/7 17/18 0.3615 120825 15:04:02.665 7.847 38.809 8.93 2.6 171 3 22.27±7.09 251.14±64.52 -62.73 9/9 8/8 25/27 0.3216 131016 00:10:21.490 7.826 38.832 3.92 0.8 135 4 43.96±1.58 212.76±11.29 -60.48 8/8 3/4 16/17 0.3317 131016 00:23:50.055 7.828 38.821 2.95 0.3 137 6 63.94±16.38 236.96±52.12 -44.31 6/6 1/1 3/4 0.3518 131025 11:37:18.850 7.838 38.803 4.70 2.0 146 2 56.17±1.25 219.95±2.15 -22.76 9/9 5/6 17/22 0.3219 131211 00:07:38.704 7.851 38.795 6.78 1.1 167 1 44.81±0.00 230.91±0.00 -35.53 8/8 4/5 11/14 0.3920 131212 01:00:48.491 7.830 38.810 4.22 0.8 136 2 54.60±4.69 246.46±42.49 -29.84 7/7 2/2 8/9 0.3921 131214 00:16:08.242 7.754 38.791 6.03 1.4 189 5 69.30±5.18 46.91±8.19 -40.89 10/10 6/6 15/16 0.33

1106 TableC.1:FocalmechanismparametersoutputfromFOCMECforthe21earthquakesaroundAluto.Nisthenumberofpossiblesolutions.Dipandstrike1107 uncertaintiesarethecircularstandarddeviationsofthepossiblesolutions.TheRMSerrorisfortheamplituderatiosoftheacceptedsolution.1108 1109