mvo scientific report for volcanic activity between 1 may and 30

Montserrat Volcano Observatory - P.O. Box 318 - Flemmings – Montserrat Tel : +1 (664) 491-5647 | Fax: +1 (664) 491-2423 | mvo@mvo.ms | www.mvo.ms

MVOScientificReportforVolcanicActivitybetween1Mayand30September2016

AnupstreamviewoffloodedgravelpitsintheBelhamValleyduringthelaharon24August2016.Photo:A.Stinton.

AStinton,VBass,TChristopher,MFergus,KPascal,PSmith,RStewart,RSyers,PWilliams

OpenFileReportOFR16-03

3December2016

MVOOFR16-03:MVOScientificReportforSAC21 2

ExecutiveSummaryThisreportsummarisesthevolcanicactivityattheSoufrièreHillsVolcano,Montserratfortheperiodfrom1Mayto30September2016,inclusive,includingallmonitoringandvisualobservations.

Overall,activityduringthereportingperiodhasbeenlow.Seismicactivityhasconsistedofsporadicvolcano-tectonic(VT)earthquakes,sometimesinstringsorbriefswarms.Low-frequencyearthquakes(hybrids,LPs)havebeenabsent.Rockfallactivitycontinuedataverylowlevel,consistentwithweatheringofthecollapsescarandthedome.

Grounddeformationcontinuedtoshowslowradialinflationsimilartothatduringpreviouspausesinactivity.

Duetoinstrumentproblems,dailySO2fluxwasnotmeasured.Effortsareunderwaytoobtainnewequipmenttore-establishcontinuousmonitoring.

Therehavebeennosignificantchangestothedomeorfumarolicactivity.

Thepauseinlavaextrusionthatstartedon11February2010continuesandisover79monthslongasof30September2016.

TableofContents

ExecutiveSummary..............................................................................................................................2

TableofContents.................................................................................................................................3

ListofFigures.......................................................................................................................................5

ListofTables........................................................................................................................................7

1VolcanicActivityforthePeriod1Mayto30September2016............................................................81.1Summary............................................................................................................................................81.2Seismicity.........................................................................................................................................10

1.2.1Summary...................................................................................................................................101.2.2Currentlevelsofseismicity.......................................................................................................101.2.3VTStrings..................................................................................................................................18

1.3GroundDeformation........................................................................................................................221.3.1.G.P.S.........................................................................................................................................221.3.2.‘Spider’stations.......................................................................................................................331.3.3.EDM.........................................................................................................................................331.3.4.Strain........................................................................................................................................38

1.4GasMonitoring................................................................................................................................391.4.1SO2DailyFlux............................................................................................................................391.4.2SO2DiffusionTubes..................................................................................................................40

1.5DomeVolumeandGeology.............................................................................................................431.5.1DomeVolume...........................................................................................................................431.5.2DomeMorphology....................................................................................................................431.5.3RockfallActivity........................................................................................................................431.5.4DomeIncandescenceandFumaroleTemperatures.................................................................44

2DiscussionofActivityandComparisonwithPastActivity................................................................462.1PauseinDomeGrowth....................................................................................................................462.2DomeStability..................................................................................................................................472.3CriteriaforContinuationofActivity.................................................................................................48

2.3.1Seismicity..................................................................................................................................482.3.2Gas............................................................................................................................................482.3.3Grounddeformation.................................................................................................................48

2.4PhasesandPause............................................................................................................................49

3HazardsandRisks............................................................................................................................503.1RiskAssessment...............................................................................................................................503.2HazardLevelSystem........................................................................................................................503.3AccesstoZoneV..............................................................................................................................503.4TouristVisitstoPlymouth................................................................................................................503.5GeothermalWellMON#3................................................................................................................51

4MVOMonitoringNetworks.............................................................................................................524.1SeismicMonitoring..........................................................................................................................52

4.1.1Softwareandacquisitionchanges............................................................................................524.1.2Instrumentalchangesatstations..............................................................................................52

4.1.3Ongoingproblemsandknownissues.......................................................................................534.1.4Futuredevelopments................................................................................................................54

4.2GroundDeformationMonitoring.....................................................................................................554.2.1ContinuousGPS(cGPS).............................................................................................................554.2.2CampaignGPS(eGPS)...............................................................................................................554.2.3GPSArchivingandProcessing...................................................................................................554.2.4EDM..........................................................................................................................................564.2.5Tilt.............................................................................................................................................564.2.6Strain.........................................................................................................................................564.2.7Spidersstations:single-frequencyGPS,EDMandTiltmeter....................................................57

4.3GasMonitoring................................................................................................................................574.3.1Multigas....................................................................................................................................574.3.2DOASUVspectrometernetwork..............................................................................................57

4.4DomeVolume..................................................................................................................................584.5FumaroleMonitoring.......................................................................................................................584.6RemoteCameras..............................................................................................................................584.7ThermalCameras.............................................................................................................................594.8RemoteSensingData.......................................................................................................................594.9InfrasonicMonitoring......................................................................................................................594.10WEBOBS.........................................................................................................................................60

5MVOOperations.............................................................................................................................615.1Staffing............................................................................................................................................615.2EducationandOutreach..................................................................................................................615.3Helicopter.........................................................................................................................................615.4Volunteers/PhDstudents/Visitors................................................................................................615.5Internships.......................................................................................................................................625.6CollaborationandExternalProjects................................................................................................625.7MVOArchive....................................................................................................................................635.8MVOWebsite...................................................................................................................................63

References.........................................................................................................................................64

AppendixA:25June2016RFevent....................................................................................................65A1Introduction......................................................................................................................................65A2Seismics............................................................................................................................................65

A2.1Infrasound.................................................................................................................................65A2.2Lowfrequencyonset.................................................................................................................65

A3Fieldobservations.............................................................................................................................70A4Summary..........................................................................................................................................70

AppendixB:TrendsinVTearthquakedepthssincePhase5...............................................................72B1Introduction......................................................................................................................................72B2Dataanalysis....................................................................................................................................72B3Discussion.........................................................................................................................................77

AppendixC:AdditionalGPSData.......................................................................................................79

ListofFiguresFigure1: Summaryplotofthedailyseismicitycounts(top),GPS(middle,stationGERD)andSO2flux

(bottom)fortheperiod1Mayto30September2016............................................................8Figure2: ViewsofthelargelaharintheBelhamValleyon24August2016..........................................9Figure3: DailycountsofthedifferentearthquaketypesrecordedbytheMVOnetworkfortheperiod

coveredbythisreport:i.e.between1May2016and30September2016..........................13Figure4: DailycountsofthedifferentearthquaketypesrecordedbytheMVOsincetheendofPhase

5.............................................................................................................................................14Figure5: HypocentresoflocatedVTearthquakesrecordedbytheMVOseismicnetworkbetween1

Mayand30September2016.................................................................................................15Figure6: Fault-planesolutionsoflocatedVTearthquakesrecordedontheMVOseismicnetwork

between1Mayand30September2016...............................................................................16Figure7: DailycountsofrockfalleventssincetheendofPhase5.......................................................17Figure8: VTearthquakesrecordedatSHVsincetheendofPhase5...................................................17Figure9: SeismicandstraindatafromtheVTswarmon14June2016...............................................19Figure10: StemplotshowingthenumberoftriggeredVTearthquakesineachVTstring................20Figure11: StemplotshowingthedurationsinminutesofallVTstringssincefirstidentifiedin

November2007.................................................................................................................20Figure12: StemplotsshowingindividualenergyestimatesforallVTstrings....................................21Figure13: Radialextensionrelativetothevolcanicventsince1June2015......................................23Figure14: Tangentialdisplacementsrelativetothevolcanicventsince1June2015.......................24Figure15: Verticaldisplacementsobservedsince1June2015..........................................................25Figure16: RadialextensionrelativetothevolcanicventforPause5................................................26Figure17: TangentialdisplacementsrelativetothevolcanicventforPause5..................................27Figure18: VerticaldisplacementsforPause5....................................................................................28Figure19: Radial(top)andVertical(bottom)displacementsrecordedatcampaignsitesduringPause

5.........................................................................................................................................29

Figure20: Comparisonoftheradial(top)andvertical(bottom)displacementsmeasuredatnineMVOcontinuousGPSstationsduringPause3,Pause4bandPause5(solidlines)andsubsequentextrusionphases(dashedlines).....................................................................30

Figure21: HorizontalvelocitiesobservedattheMVOGPSstations..................................................31Figure22: HorizontalandverticalvelocitiesobservedattheMVOcontinuousGPSstationsduring

Pause5...............................................................................................................................32Figure23: MSCP,MSS1,MSUH‘Spider’Stations:Radial,tangential,andverticaldisplacements

relativetothevolcanicvent..............................................................................................34Figure24: MapoftheEDMnetwork(blackfilledcircles)usedatMVO.............................................35Figure25: Slantlengthvariations(m)oftheEDMbaselines..............................................................36Figure26: Horizontallengthvariations(m)ofseveralpairsofcGPSstationsbetweenMayand

September2016................................................................................................................37Figure27: Horizontallengthvariations(m)ofseveralpairsofcGPSstationsduringPause5...........38Figure28: Dailyand30-dayaverageSO2fluxfromtheSoufriereHillsVolcanofortheperiod1

January2014to17September172015............................................................................39Figure29: MonitoredSO2concentrations(ppb)betweenFebruary2010andAugust2016.............41Figure30: TheannualmeanconcentrationsoftheSO2diffusiontubesfromsixofthemostproximal

locations.............................................................................................................................42Figure31: Dailyandcumulativerockfallcountsfortheperiod1January2015to30April2016......44Figure32: Seismic,GPSandSO2monitoringdatafortheperiod1January1995to30September

2016...................................................................................................................................46Figure33: Seismic,GPSandSO2monitoringdatafortheperiodsincetheendofPhase5...............47

ListofTablesTable1: Totalnumberofeventsandmeandailyeventratesforeachearthquaketypeduring(a)the

wholeofPause5(since11February2010)and(b)theperiodcoveredbythisreport(1Mayto30September2016)..........................................................................................................11

Table2: Totalnumberofeventsandmeandailyeventratesforeachearthquaketyperecordedat

SHVduringpausesinlavaextrusion.ThedatesofthepausesaredefinedinTable6..........11Table3: TablelistingtheVTStringsobservedatSHVbetween1Mayand30September2016........18Table4: Overviewofphasesoflavaextrusionanderuptivepauses1995-2010andthemeanSO2flux

foreach..................................................................................................................................40Table5: Maximumrecordedtemperaturesoffumarolesonthelavadomederivedfromthermal

imagesacquiredduringtheperiod1Mayto30September2016........................................45Table6: PhasesandpausesoftheeruptionoftheSoufriereHillsvolcano,withdurationscalculated

uptoandinclusiveof30September2016............................................................................49Table7: Changesmadetoinstrumentsatindividualseismicstations................................................53

1VolcanicActivityforthePeriod1Mayto30September2016

1.1 SummaryFigure1belowsummarisesthedailyseismicity,GPSandSO2fluxfortheperiod1Mayto30September2016.Duetocontinuedinstrumentissues,noSO2fluxdataisavailableforthisreportingperiod(seeSections1.4and4.3fordetails).

Figure1: Summaryplotofthedailyseismicitycounts(top),GPS(middle,stationGERD)andSO2flux(bottom)fortheperiod1Mayto30September2016.Duetoinstrumentproblems,thereisnoSO2dataavailable.

Activityremainedlowduringthereportingperiod.Seismicitywasdominatedbyvolcano-tectonic(VT)earthquakesandrockfalls(RF).AslightincreaseinRFshascoincidedwiththerainyseasoninMontserrat(seeSection1.5.3RockfallActivity).Therewasonlyonelong-periodrockfall(LPRF)eventon28August2016andnohybridorlongperiodevents.

ThetrendofoccasionalburstsofVTseismicityintheformofVTstringsorbriefswarmscontinued.ThreeVTstrings,ofvaryinglengthandintensity,occurredduringthereportingperiod,bringingthetotalnumbersinceNovember2007to82(seeSection1.2.3VTStrings).NoneofthestringswereassociatedwithanyobservablechangesinSO2flux,fumaroleactivityortheoccurrenceofashventing.

On26June2016,MVOreceivedareportofashintheupperAmershamareafromCaribbeanHelicopters.Twodayslater,MVOstaffvisitedtheareaandobservedashblowingaroundandstucktovegetation.Initially,theoriginoftheashwasnotclear,butlaterinvestigationoftheseismicrecordrevealedaparticularlyunusualRFeventon25June2016(basedonUTC)thatshowedsomesignificant

low-frequencyenergyandacoincidentsignalintheinfrasound.Thisstronglysuggestsatleastsome(small)explosivecomponenttotheRFevent.MoreinformationonthiseventispresentedinAppendixA.

Followingthepassageofastrongtropicalwaveovernightfrom23-24August2016,severallargelaharsweregenerated,primarilyintheBelhamValley,butalsoinchannelsleadingthroughPlymouthandinGingoesGhaut.ThelaharintheBelhamValleyhadasignificantimpactonthevalley(Figure2),withboththeupper(toZoneC)andlower(toGaribaldi)crossingscutandupto4moferosionoccurring.Thelaharfloodedthemajorityofthegravelpitsinthevalley.ThisisthelargestlahartohaveoccurredintheBelhamvalleysincethosegeneratedbyTropicalStormRaphaelinNovember2013whenlaharsreachedtheseainOldRoadBay.ThesandminerstrackleadingtothejettyinPlymouthwasalsocutintwolocationsbylahars.

Figure2: ViewsofthelargelaharintheBelhamValleyon24August2016.A)Thelaharcutthroughthetrackoftheuppercrossing.B)4+moferosioninvalleydownstreamofuppercrossingcuttingthroughtrack.c)Floodedgravelpitbetweentheupperandlowercrossing.

1.2Seismicity

1.2.1SummaryThelevelofseismicactivityrecordedatSHVduringthisreportingperiodhasremainedlow.

Figure3displaysthedailycountsofthedifferentvolcanicearthquaketypesfortheperiod1May2016to30September2016inclusive,andshowsthatthepatternofsporadicVTearthquakesandoccasionalrockfallshascontinued.VTearthquakesmadeupthevastmajorityofthetotalseismicity.

AfurtherthreeVTstringsoccurredduringthereportingperiod,withthemostintenseintermsofduration,eventnumbersandearthquakemagnitudeoccurringon14June.

Low-frequencyseismicityhasbeenalmostabsent,withonlyonesinglelong-periodrockfallrecordedduringthisperiod.Thispatternremainstypicalofapauseinlavaextrusion,withthelevelofactivitylower,butstillcomparableto,thatseenduringpreviouspauses.

Alargerockfallsignaloccurredon25June2016,withalow-frequencysignalonsetandcoincidentwithaninfrasonicsignalatMBFL.ThissuggestsatleastsomeexplosivecomponenttothiseventthatwastheprobablesourceofsomeminorashfallobservedtothewestofthedomeneartoPlymouth.

1.2.2CurrentlevelsofseismicityThetotalcountsandmeandailyeventrateforeacheventtypearegiveninTable1.CountsforboththewholereportingperiodandfortheperiodsincetheendofPhase5areshown.

Rockfallactivityhasbeenlow,inkeepingwiththeslowlydecliningtrendoverthelastfewyearsasthedomehasstabilised(seeFigure3andparticularlyFigure7).Thesmallincreaseinrockfallactivitycomparedtothelastreportingperiodwaslikelyaresultofseasonaleffectsandincreasedrainfall.Low-frequencyseismicityhasbeenextremelylow,withonlyonelong-periodrockfallsignalrecordedon28August.

ThreeVTstringsoccurredduringthisreportingperiod(seeSection1.2.3VTStrings).ThemeandailyrateofVTsforthelast5monthsof0.41islessthantheaverageacrossthewholeofthecurrentpauseof0.69(Table1).Thisisalsoadecreaseonthevaluefortheprevious7monthsreportingperiodof0.60perday,aresultoffewerVTstringsoccurringduringthisperiod.

Figure5displaysthehypocentresofallVTearthquakesrecordedbytheMVOnetworkduringthereportingperiodthatwereabletobelocated,alongsideaplotoftheirfocalmechanisms(Figure6)showingtherangeoffault-planeorientations.ThefocalmechanismswerecalculatedusingFPFIT(Reasenberg&Oppenheimer,1985)andonlythosewithhigh-quality,well-constrainedsolutionsareshown.Thehypocentresaresubjecttotheusualerrorsandnetworkbiases,butareconsistentwithallrecentVTseismicityatSHV.Theyareclusteredwithinacomparativelysmallseismogenicvolumebeneaththedomeandthe2010collapsescaratdepthsbetween0.5and4km.

ComparingthecurrenteventratesinTable1withtheratescalculatedforpreviouspausesinTable2,thetotalmeandailyeventrateforthiswholepauseperiodremainsbroadlycomparablewiththeratesseenduringpreviouspauses,eveniftheratesforthisreportingperiodaresomewhatlower,indicativeofageneraldecliningtrendinoverallseismicityrates.

EventTypePause5(since11February2010) 1Mayto30September2016

Total Events/Day Total Events/DayLP/Rockfall 106 0.04 1 0.01Hybrid 138 0.06 0 0.00LP 97 0.04 0 0.00

Rockfall 1615 0.67 35 0.23VT 1673 0.69 63 0.41

Total 3629 1.50 99 0.65

Table1: Totalnumberofeventsandmeandailyeventratesforeachearthquaketypeduring(a)thewholeofPause5(since11February2010)and(b)theperiodcoveredbythisreport(1Mayto30September2016).

EventType

Pause1 Pause2 Pause3 Pause4a Pause4b

Total Events/Day Total Events

/Day Total Events/Day Total Events

/Day Total Events/Day

LPRFs 44 0.07 2 0.00 34 0.07 31 0.54 12 0.04Hybrid 627 1.00 1696 2.72 393 0.82 78 1.37 2 0.01LP 273 0.44 145 0.23 1458 3.02 74 1.30 12 0.04RF 6336 10.11 257 0.41 454 0.94 153 2.68 174 0.64VT 3689 5.88 256 0.41 432 0.89 84 1.47 172 0.63

Total 10969 17.49 2359 3.78 2797 5.80 420 7.36 372 1.36

Table2: TotalnumberofeventsandmeandailyeventratesforeachearthquaketyperecordedatSHVduringpausesinlavaextrusion.ThedatesofthepausesaredefinedinTable6.

1.2.2.1VTenergyreleaseAnalysisofearthquakenumbersalonemaybemisleadingasitdoesnotconsidertherateofenergyreleasesincethesizeoftheearthquakesarenottakenintoaccount.Figure8showsanestimateofthecumulativeVTenergycalculatedfromthelocalmagnitudesusingthefollowingequation:

log𝐸! = 1.5𝑀! + 11.8 (1)

(Gutenberg-Richtermagnitude-energyrelation).ThismethodtoestimatetotalVTearthquakeenergyshouldbemorerobustthanrelyingonamplitudemeasurementsatsinglestations,withtheimpactofsiteeffectsanddatagapsetc.minimised.

Figure8highlightsthedistinctseismicgap(inbothVTenergyandnumbers)thatoccurredforseveralmonthsfollowingthe23March2012episode.Asimilar,butshorter,gapoccurredfollowingthe8March2014activity,withverylittleVTenergyreleasedbetween8MarchandJune2014.Inthe2.5yearsorsosincethen,theenergyreleaseratehasremainedroughlyconstantdespitenumeroussmallerVTstringsoccurringduringthistime.

ThisfigurealsoshowsthatalthoughtheVTstringoccurringon14June2016wasthelargestinthisreportingperiod(seeTable3)itstillreleasedmorethanoneorderofmagnitudelessenergythanthoseinMarch2012andMarch2014.

Figure3: DailycountsofthedifferentearthquaketypesrecordedbytheMVOnetworkfortheperiodcoveredbythisreport:i.e.between1May2016and30September2016.Thetotalcountofalleventtypesisshownatthetop,followedbyindividualcountsforVTs,Rockfalls,Hybrids,LPeventsandLP/Rockfalls.ThethreeVT“strings”tohaveoccurredduringthisperiodaremarkedbytheredstarsontheVTplot.

Figure4: DailycountsofthedifferentearthquaketypesrecordedbytheMVOsincetheendofPhase5,i.e.from12February2010until30September2016.Tickmarksareat3monthintervals.Thetotalcountofalleventtypesisshownatthetop,followedbyindividualcountsforVTevents,Rockfalls,Hybrids,LPeventsandLP/Rockfalls.VT“strings”aremarkedbytheredstarsontheVTplot.

Figure5: HypocentresoflocatedVTearthquakesrecordedbytheMVOseismicnetworkbetween1Mayand30September2016(61eventsofthe63total).Thesizeofeachcircleisafunctionoftheearthquakemagnitudeandthelowerpaneldisplaysthedepthsasafunctionoftime.

Figure6: Fault-planesolutionsoflocatedVTearthquakesrecordedontheMVOseismicnetworkbetween1Mayand30September2016(37eventsofthe63total).ThefocalmechanismsshownwerecalculatedusingFPFITwithinSEISAN,withonlywellconstrained‘A’qualitysolutionsshownontheplot.Thelowerpaneldisplaysthedepthsasafunctionoftime.

Figure7: DailycountsofrockfalleventssincetheendofPhase5,i.e.from12February2010until30September2016.Theredlinerepresentsa60-daymovingaverage,andthegreena60-daylow-passfilterofthedailyeventcounts.Thelast5monthsofactivityduringthisreportingperiodshowasmallincreaseinrockfallactivity,duetoseasonaleffectsandaresponsetoseveralperiodsofheavyrainfall.

Figure8: VTearthquakesrecordedatSHVsincetheendofPhase5.ThebluehistogramshowsdailycountsofVTearthquakes,withVTstringsmarkedbyredstars.ApproximatecumulativeVTenergyrelease(calculatedfromthemagnitudesusingEquation(1))isshownbythegreenline.Notethetotalenergyreleaseofaround409GJduringthisperiodisapproximatelyequivalenttoasingleM4.5earthquake.

1.2.3VTStringsAsdiscussedinpreviousreports,VTstrings,definedasshortintenseswarmsofVTearthquakes(sometimesreferredtoelsewhereintheliteratureas‘Spasmodicbursts’),havebecomearelativelycommonphenomenonatSHVsincetheywerefirstidentifiedin2007(seeFigure9-Figure12).Table3liststhedetailsofthreemoresuchstringsthatoccurredduringthisreportingperiod.ThedatainthistableandFiguresFigure9-Figure12showthatthestringoccurringon14Junewasthemostsignificantintermsofthenumberofearthquakesandenergyrelease.However,nosignificantshort-termstrainsignalwasobservedcoincidentwiththisseismicity(Figure9),unlikeforsomeoftheprevioushighenergyVTstrings.

ItshouldbenotedthatTable3onlycontainsearthquakeslargeenoughtotriggertheearthwormeventdetectionalgorithm(whichcanbesubjectiveiftheeventrateishighandseveraleventsaremanually“cutout”fromasinglewaveformfile).Inallcasesmanyadditional,loweramplitudeeventswereidentifiedfromdetailedinspectionofthecontinuouswaveformdata(particularlythemoreproximalstationsMBLYandtheMSS1Spider).

# Date/Time(UTC)

TriggeringVTs

LocatedVTs

Approximateduration(minutes)

Max.LocalMagnitude

Approx.Energy(GJ)

1 14-Jun-201620:56:27 9 8 7.2 M3.2 5.98

2 28-Aug-201620:23:42 4 4 37.7 M1.7 0.04

3 17-Sep-201604:48:58 2 2 2.0 M2.5 0.36

Table3: TablelistingtheVTStringsobservedatSHVbetween1Mayand30September2016.Thetableliststhedateandonsettimeofthefirstearthquakeofthestring,thenumberofVTeventsthattriggeredtheearthwormeventdetectionalgorithm,thenumberofeventsthatwerelocated,theapproximatedurationofthestringinminutes,themaximumlocalmagnitude,andtheapproximatetotalenergyoftheearthquakes.

Figure9: SeismicandstraindatafromtheVTswarmon14June2016.ThetopplotshowstheverticalcomponentseismogramfromstationMBGH.Thelowertwoplotsshowthe50Hzand1Hzstraindatarecordedatthe3CALIPSOstations:AIRS(AirStudios),GERD(Gerald’s)andTRNT(Trant’s).Straindataamplitudeshavebeencorrectedtounitsofnanostrain.Nofilteringwasapplied,butalineartrendwasremovedfromthe24hrsofstraindatatoaccountfordrift,andthemeansubtractedfromthedatashownintheplots.

TherelationshipbetweenVTstringsandsurfaceactivityhasbeendiscussedinpreviousreports,whereithasbeenassertedthatthereisevidenceofapositivecorrelationbetweentheoccurrenceofVTs,particularlyVTstrings,andincreasedSO2output.ThestringsoccurringinthisreportingperiodcanaddnonewinformationaboutthisrelationshipasnoSO2fluxdatawasrecorded.

Noneofthestringsthatoccurredduringthisreportingperiodwerefollowedbysubsequenttremorsignalsoranyotherobservableactivityorchangesatthesurface.

ThetotalnumberofVTstringsidentifiedbetweenNovember2007andtheendofthisreportingperiodnowstandsat85,withonly17(≈20%)ofthesedirectlyprecedingobservablesurfaceactivityorchanges.

Figure10: StemplotshowingthenumberoftriggeredVTearthquakesineachVTstring.Filledwhitecirclesrepresentstringsthatprecededobservablesurfaceactivity.Thebluelineontheright-handaxisrepresentsthecumulativeVTenergyreleaseforthesameperiod(includingbothVTstringandnon-stringVTearthquakes).

Figure11: StemplotshowingthedurationsinminutesofallVTstringssincefirstidentifiedinNovember2007.Filledwhitecirclesrepresentstringsthatprecededobservablesurfaceactivity.

Figure12: StemplotsshowingindividualenergyestimatesforallVTstrings(fromasummationoftheearthquakeenergiesderivedfrommagnitudes).Thelowerplotshowsthecumulativestringenergyconvertedbacktoatotalorpseudo-magnitudeforeachstring–equivalenttoplottingtheenergyonalogscale.

1.3GroundDeformation

1.3.1.G.P.STherewasnochangeinthedeformationpatternobservedbetween1Mayand30September2016.Thecloseststationsfromthedomeshownodeformationwhilethestationsfurtherawayshowaradialextensionfromthedome,ataslowrate.Allstationsshowaverticalupwardmovement.Thisisconsistentwiththecontinuationofthepauseinvolcanicactivityandthelong-terminflationoftheedifice/island.

Theslowinflationrateat‘far-field’stations(e.g.MVO1,GERD,TRNT,NWBL)impliesthatthedeformationisbestseenonlongertime-series,asplottedinFigure13-Figure15fortheperiodJune2015–September2016,andinFigure16-Figure18fortheentirePause5(since12February2010).ThetimeseriesoftheradialandverticaldisplacementsmeasuredatthecampaignstationsareplottedinFigure19forPause5.AdditionalplotscoveringtheentiredatasetarepresentedinAppendixC.

‘Close-field’stations(e.g.HERM,SPRI,FRGR),moveupwardsbutnosignificanthorizontaldisplacementsarerecorded,exceptedinrelationwithstrongeventsuchastheashventingeventinMarch2012(Figure16).NosucheventshaveoccurredbetweenMayandSeptember2016,andnodisplacementswereobservedassociatedtotheVTstringsneitherinthedailypositiontimeseries,norinhigh-rateGPSdata.OnlySPRI(SWofthedome)time-seriesshowsasignificantradialshorteningtowardsthedomesinceJuly2016.Fornow,thissignalremainsunexplainedandcouldstillberelatedtotherainyseason(June-November),whichcommonlyaffectsSPRIdatathemostamongstthenetwork.

Figure20,wherethedisplacementshavebeenfilteredtoremovepartofthenoise,alsoshowsthatthedeformationpatternisthesamesincethebeginningofPause5.Ateachstation,thedeformationrateisrelativelysimilarforthefivepauses.

ThehorizontalvelocitiesvectorsarerepresentedinFigure21forPause5,foradurationof18months(sinceJune2016)andfora12monthperiod(sinceSAC20),thelatterforindicationsincevelocitiesneedatleast18monthsdataforareliabletime-series.ForPause5,thevelocitiesofbothcontinuous(namesinred)andofcampaignstations(namesinblack)havebeencalculated.Stationsmissingasignificantamountofdataduringanyofthesethreeperiodswereomittedfortheperiodconcerned(e.g.SGH1).

ThedirectionsofthevelocityvectorsofPause5andofthelast18monthsofdataareoverallsimilar.Whilethecloseststationsvelocityvectorsdirectionsarevariableandtheiramplitudessmallifnotnegligible,themoredistalstationsshowaclearradialextensionawayfromthedome(indicatedbyaredstar).Forthesestations,theextensionrateisupto~1cm/yrisslowerforthelast18monthsperiod,becausethedeformationrateswerehigheratthebeginningofPause5(seepreviousSACreports).

ThePause5verticalvelocityvectorsarealsorepresentedinFigure22andshowanupwardmovementatallstations.Theiramplitudesvary,withthemaximumvelocitiesintheNEoftheedifice(TRNT,HARR,WTYD),aswellasSPRIontheSWflankoftheedifice.

Figure13: Radialextensionrelativetothevolcanicventsince1June2015.Thereportingperiod(May-September2016)isboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.Displacementsduetoantennachangeshavebeencorrected.HigherenergyVTstringsareindicatedwithagreyline.Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.

Figure14: Tangentialdisplacementsrelativetothevolcanicventsince1June2015.Thereportingperiod(May-September2016)isboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.Displacementsduetoantennachangeshavebeencorrected.HigherenergyVTstringsareindicatedwithagreyline.Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.

Figure15: Verticaldisplacementsobservedsince1June2015.Thereportingperiod(May-September2016)isboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.Displacementsduetoantennachangeshavebeencorrected.HigherenergyVTstringsareindicatedwithagreyline.Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.

Figure16: RadialextensionrelativetothevolcanicventforPause5(February2010-September2016).Thereportingperiodisboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.Displacementsduetoantennachangeshavebeencorrected.HighenergyVTstringsareindicatedwithagreyline,andthestrongestevents(27October2010,9July2011,22March2012,8March2014)havebeenhighlighted(reddashedline).Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.

Figure17: TangentialdisplacementsrelativetothevolcanicventforPause5(February2010-September2016).Thepresentreportingperiodisboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.Displacementsduetoantennachangeshavebeencorrected.HighenergyVTstringsareindicatedwithagreyline,andthestrongestevents(27October2010,9July2011,22March2012,8March2014)havebeenhighlighted(reddashedline).Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.

Figure18: VerticaldisplacementsforPause5(February2010-September2016).Thepresentreportingperiodisboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.Displacementsduetoantennachangeshavebeencorrected.HighenergyVTstringsareindicatedwithagreyline,andthestrongestevents(27October2010,9July2011,22March2012,8March2014)havebeenhighlighted(reddashedline).Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.

Figure19: Radial(top)andVertical(bottom)displacementsrecordedatcampaignsitesduringPause5(February2010-September2016).Thereportingperiodisboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.HighenergyVTstringsareindicatedwithagreyline,andthestrongestevents(27October2010,9July2011,22March2012,8March2014)havebeenhighlighted(reddashedline).Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.SeeAppendixC:AdditionalGPSDatafortangentialdisplacements.

Figure20: Comparisonoftheradial(top)andvertical(bottom)displacementsmeasuredatnineMVOcontinuousGPSstationsduringPause3,Pause4bandPause5(solidlines)andsubsequentextrusionphases(dashedlines).A60-daylow-passfilterwasappliedtothedata.SeeAppendixC:AdditionalGPSDatafortangentialdisplacements.

Figure21: HorizontalvelocitiesobservedattheMVOGPSstations.Velocitiesarecalculatedrelativetoatectonicplatemodeldefinedusingvelocitiesfromlocalandreferencestationsthatprovideastatisticallyconsistentplaterotationpole/velocity.ThevelocitiesvectorscomparedarethosecomputedfortheentirePause5(black),forJune2015-April2016(blue),andforthereportingperiod(red).Continuousandcampaignsitesareindicatedwithredandblacknames,respectively.Stationsmissingasignificantamountofdatahavebeenremovedforthecorrespondingperiod.Theredstarmarksthedomelocation.

Figure22: HorizontalandverticalvelocitiesobservedattheMVOcontinuousGPSstationsduringPause5.HorizontalVelocitiesarecalculatedrelativetoatectonicplatemodeldefinedusingvelocitiesfromlocalandreferencestationsthatprovideastatisticallyconsistentplaterotationpole/velocity.Whenlackingasignificantamountofdataoveroneoftheperiodstudied,thestationisomitted.Theredstarmarksthepositionofthedome.

1.3.2.‘Spider’stationsThedeployed‘Spider’continuousGPSstationsareprocessedincombinationwiththerestofthenetwork.ThedeformationrecordedatMSUH(UpperHermitage),MSS1(Scar),andMSCP(Chance’sPeak)betweentheirrespectivedeploymentin2014until15April2016ispresentedinFigure23,inparallelwiththedisplacementsrecordedatoneclose-field(FRGR)andonefar-field(GERD)station.Unfortunately,sinceMay-June2015,thedataqualityofMSUHandMSCPhasbeendegrading,explaininginparticularthestepinthetime-seriesinMay2015.ThesetwostationshavenotbeenfunctioningsinceNovember2015.

Noneofthe‘Spiders’haveshownanysignificantdeformationsincetheirdeployment,eveninrelationwiththestrongerVTstringswhichoccurredduringthisperiod.TheradialandtangentialdisplacementsrecordedatMSUHarelikelyrelatedtothespiderssettlingdown,sincenosimilardisplacementswererecordedatHERMwhichisonlytensofmetersaway.

1.3.3.EDMAmapoftheEDMbaselinesmeasuredsinceFebruary2010isgiveninFigure24(filledblackcircled).EDMmeasurementsprovidedataonthenear-fielddeformationofthevolcano.InFigure25,theslantdistanceschangesrecordedovertheEDMnetworkarerepresentedforPause5andforMaytoSeptember2016.

SignificantdisplacementshavebeenrecordedsincethebeginningofPause5,inrelationwithstrongVTstrings.NosignificantdeformationhasbeenrecordedduringtheperiodMaytoSeptember2016atthebaselineslocatedintheSWofthedome.Duetoweatherandtechnicalissues,itwasnotpossibletotakeanymeasurementsofEDMbaselinesintheNorthofthedome

TheEDMplotsarealsocomplementedwithhorizontaldistancevariationsmeasuredatseveralpairsofcontinuousGPSstations,acrosstheedificeandacrosstheisland(Figure25),fortheperiodMay–September2016(Figure26)andforPause5(Figure27).

DuringPause5,thestepinthehorizontaltime-seriesoccurringinrelationwiththeMarch2012ashventingeventisvisibleforbaselinesinvolvingclose-fieldstations.Theextensionisemphasizedbetweenthesouthernandnorthernstations(SSOU-NWBL,FRGR-NWBL).Somedistancesdonotvary,forexample,thebaselineSPRI-WTYD,twoclose-fieldstationsSWandNEofthedome,respectively.Thecontractionseenattheendofthetime-seriesisduetotheunexplainedradialdisplacementofSPRImentionedearlier.

BetweenMayandSeptember2016,nodeformationisclearlyvisible.

Figure23: MSCP,MSS1,MSUH‘Spider’Stations:Radial,tangential,andverticaldisplacementsrelativetothevolcanicventsinceMSUH,MSS1andMSCPdeployment(June2014-September2016).FRGRandGERDdisplacementsarealsoshown.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Theplatevelocityhasbeenremovedfromthedata.Displacementsduetoantennachangeshavebeencorrected.HighenergyVTstringsareindicatedwithagreyline.Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.

Figure24: MapoftheEDMnetwork(blackfilledcircles)usedatMVOduringthereportingperiodandPause5andofcomplementingbaselinescomputedbetweenthecontinuousGPSstations(blackcircles).SeealsoFigure25Figure27.

Figure25: Slantlengthvariations(m)oftheEDMbaselinesduringthereportingperiod(2upperpanels)andduring Pause 5 (5 bottom panels). See Figure 24 for network map. The strongest events (27October2010,9 July2011,22March2012,8March2014)havebeenhighlighted (reddashedline).

Figure26: Horizontallengthvariations(m)ofseveralpairsofcGPSstationsbetweenMayandSeptember2016.SeeFigure24fornetworkmap.

Figure27: Horizontallengthvariations(m)ofseveralpairsofcGPSstationsduringPause5(February2010–present).SeeFigure24fornetworkmap.Thestrongestevents(27October2010,9July2011,22March2012,8March2014)havebeenhighlighted(reddashedline).

1.3.4.StrainAlthoughstrainmeasurementsduringtheentirereportingperiodneedtobestudiedforconfirmation,itisunlikelythananysignificantdeformationwasrecordedinrelationtothesmallseismiceventswhichhappenedduringthereportingperiod.TherewasnostrainsignalrelatedtothehigherenergyVTstringswhichoccurredduringMay-September2016,anddescribedinSection1.2.

1.4GasMonitoring

1.4.1SO2DailyFluxUnfortunately,duetofailureofthelastworkingDOASinstrumenton17September2015,ithasnotbeenpossibletomeasurethedailySO2fluxfromtheSoufriereHillsVolcanoduringthisreportingperiod.Priortothis,theDOASnetworkhadbeenrunningononeinstrumentlocatedatBroderick’s.ThishadbeenthecasesinceFebruary2013.

Figure28belowshowsthedailySO2fluxforthe20monthspriortothefailureofthelastDOASinstrumentatBroderick’s(1January2014to17September2015).Duringthisperiod,theaveragedailySO2fluxwas340tonnesperday.Thisisslightlybelowthelongtermaverageof375tonnesperdayforPause5andconsiderablylowerthantheaveragesforpreviouspauses(Table4;Christopheretal.,2014).

AsdiscussedinpreviousMVOreports,thedatainFigure28stillshowstheabsenceoftheSO2pulse/cyclepatternontheorderofyears(seeSAC13report).Instead,theSO2fluxisdominatedbyvariationsontimesscalesontheorderofdaystomonths,whichattimesstillcorrelatewithVTearthquakes,e.g.,thelargespikeinSO2outputfollowingtheVTstringsinMarch2014.PriortothefailureoftheinstrumentinSeptember2015,itappearedthatthegasfluxwasdescendingintoatroughofa6-8monthcycle,thesecondsinceJanuary2014.ThisreflectstheoveralltrendinSO2fluxsincethestartofPause5(seeFigure32andFigure33).

Figure28: Dailyand30-dayaverageSO2fluxfromtheSoufriereHillsVolcanofortheperiod1January2014to17September172015whentheDOASinstrumentatBroderick'sfailed.The6-8monthpulses

arepresentinthedata,asistheinfluenceoftheelevatedSO2fluxfollowingtheMarch2014VTstringonthe30-dayrunningaverage.

Eruptivephase n Meandailyflux(t/d) Pausefollowing n Meandailyflux

(t/d)I–15thNov1995 68 569 I–10thMarch1998 112 699II–27thNov1999 563 471 II–1stAugust2003 698 561III–8thAug2005 604 429 III–4thApril2007 482 620

IVa–29thJul2008 77 923 IVa–14thOctober2008 50 800

IVb3rdDec2008 32 451 IVb–4thJanuary2009 258 627V–4thOct2009 7 379 V–11thFebruary2010 2270 376

Table4: Overviewofphasesoflavaextrusionanderuptivepauses1995-2010andthemeanSO2fluxforeach.“Pausefollowing”istheeruptivepausethatimmediatelyfollowedtheeruptivephaseinthesamerow;nisthenumberofdaysintheeruptivephaseorpause(updatedto30April2016,fromChristopheretal.,2014).

1.4.2SO2DiffusionTubesMVOmeasurestheaverageground-levelconcentrationofSO2usingdiffusiontubespositionedattwelvedifferentlocationsonMontserrat.Figure29showstheaverageconcentrationofSO2fromalltwelvesitesaroundtheislandandcoverstheperiodMarch2010toAugust2016.TheconcentrationvaluesforChancesPeakareconsiderablyhigherthanalloverlocationsduetoitscloseproximitytothelavadome(1.5km).

Duringthecurrentreportingperiod,measuredSO2concentrationscontinuetoshowanoveralldecliningtrend.Measuredconcentrationsatthemostdistallocationsarenowfallingbelowtheminimumdetectionlevelof0.28ppb.

Figure29: MonitoredSO2concentrations(ppb)betweenFebruary2010andAugust2016.

Figure30showsacomparisonofSO2concentrationfrom6ofthemostproximallocationstothatofTheUnitedStatesNationalAmbientAirQualityStandards(USNAAQS)annuallimitof30ppb.Allofthelocations,whichareineitherZoneCorZoneVarenowshowingconcentrationsbelowtheUSNAAQSlimit.ThisincludesPlymouth(measurednearAngelo’sSupermarket)whichpriorto2015hadbeenconsistentlyoverthe30ppblimit.Since2015,Plymouthhasnowdroppedbelowthe30ppblimitandreflectingacontinueddecreasingtrendsince2008.Chance’sPeak,notshownontheplotduetothehighconcentrationslevels,hasconsistentlyremainedoverthelimit.

centra^o

PoliceHeadQuarters,Plymouth RichmondHillSugarMill St.GeorgesHillWeekes VuePointeHotel St.Augus^nesPrimarySchoolSt.Johns(DMCA) JackBoyHill Brodericks(DOASSite)SalemClinic BradesPrimarySchool ChancesPeak

Figure30: TheannualmeanconcentrationsoftheSO2diffusiontubesfromsixofthemostproximallocations.Chance’sPeak,themostproximallocation,isnotshownduetothehighconcentrationsofSO2recordedatthatsite.

2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

PlymouthRichmondHillStGeorgesHillWeekesVuePointe30ppbBrodericks

1.5DomeVolumeandGeology

1.5.1DomeVolumeTherehasbeennoextrusionoflavasincetheendofPhase5on11February2010.Therefore,therehasbeennoincreaseinthedomevolume.Therehas,however,beensomeminorrockfall(RF)activity.Duringthecurrentreportingperiod,therehavebeennopyroclasticflows.Thelastrecordedpyroclasticflowwason29September2012.

1.5.2DomeMorphologyWithnoextrusionoccurringsince11February2010,thelavadomehasundergoneverylittlechange.Throughoutthereportingperiod,fewobservationsofthewholedomeweremadeduetopersistentcloudcover.Whenobservationswerepossible,littletonoobvioussignsofsignificantchangewereidentified.LargefracturescontinuetobeobservedinthestepcliffintheTarRiver(east)flankofthedomewithsomeaccumulationoflargeblocksatthebaseofthecliff.Thenear-continuouspresenceofcloudpreventsmoreregularobservationofthedomesummit.

1.5.3RockfallActivityFigure31showsthedailyandcumulativerockfall(RF)countsfortheperiod1January2015to30September2016.AlthoughitshowsacontinuedlowlevelofRFactivity,therehasbeenaslightincreaseinRFactivityduringthecurrentreportingperiodwithatotalof35RFeventstriggeringtheseismicmonitoringsystem.Ofthese,therewere4RFeventson25August,associatedwithheavyrainfallduringthepassageofastrongtropicalwave,andafurther5on1September2016.Overall,theincreaseisrelatedtotheonsetoftherainyseasoninMontserrat,atrendwhichhasbeenobservedinpreviousyears.

AlargeRFeventoccurredon25June2016(UTC).Noobservationswereimmediatelypossibletoduetocloudcover,butareportoffreshashfallintheupperAmershamareawasreceivedfromCaribbeanHelicopterson27June.Itisassumedthattheeventoccurredinsidethe2010collapsescar.SeeAppendixA:25June2016RFeventformoredetail.

ObservationsduringhelicopterflightshaveshownthattheRFactivityhasbeenconcentratedintwomainlocations:insidethe2010collapsescarwhereRFsoriginatefromtheclifffaceintheSWcorner;andfromtheTarRiverclifffacewithlargeblocksaccumulatingonthetalusslopeatitsbase.

Figure31: Dailyandcumulativerockfallcountsfortheperiod1January2015to30April2016

1.5.4DomeIncandescenceandFumaroleTemperaturesNolong-exposurephotographshavebeenacquiredfromMVOduetounfavourablenight-timeconditions.Incandescencehas,however,beenrecordedonoccasionbytheremotecameraatHarrisLookout.Thesephotographsrevealnochangestothelocationoftherecordedincandescence.

Thermalimagesacquiredduringbriefclearperiodshavecontinuedtoshownochangesinthenumber,distributionorintensityoffumarolesandhotspotsonthedome.Temperaturesofindividualfumarolesderivedfromthethermalimagesshowsomevariation(primarilyduetoatmosphericconditionsandscattering/absorptionofIRradiation),butarenotoutsideofnormalranges.AlltemperaturesreportedbelowinTable5aretheresultofbasiccorrectionsforatmospherictemperatureandhumidity,anddistancetofeature.

CumulapveRFCount

DailyRFCo

DailyCumulapve

Location MaxT(°C) LastimagedGasVent,2010collapsescar 422 21Sept2016Headwall,2010collapsescar 356 21Sept2016

23March2012crater,(inside2010collapsescar) 64 26Aug2016

TarRiver 372 16June2016SummitCrater - 30Sept2015

Summit2006-7dome - 31March2015EandNflankofGalway’sMtn - 25February2015

Southflankofdome - 30Sept2015

Table5: Maximumrecordedtemperaturesoffumarolesonthelavadomederivedfromthermalimagesacquiredduringtheperiod1Mayto30September2016.

2DiscussionofActivityandComparisonwithPastActivity

2.1PauseinDomeGrowthThecurrentpause,thefifthsincetheonsetofactivityinJuly1995,isnowmorethan79monthslong.

Aswithpreviousperiods,thisreportingperiodhasbeenoneofverylowactivitywithnorestartoflavaextrusionordomegrowth.DegradationrockfallshavebecomeextremelyrareandtherehavebeennopyroclasticflowssinceSeptember2012.

Figure32showsthekeymonitoringdata(seismiccounts,GPSandSO2flux)fortheSoufrièreHillsVolcanofrom1995to30April2016.Figure33showsthesamedatafortheperiodsincetheendofPhase5inFebruary2010.Althoughthecurrentpauseisthelongestbyfar,thedataissimilartothatduringthepreviouspauses;seismicityislow,island-wideradialinflationcontinuesandthegas(uptoSeptember2015whenmonitoringstopped)isconsistentlyabovebackgroundlevels,maintainingdailyfluxesinexcessof300t/d.

Figure32: Seismic,GPSandSO2monitoringdatafortheperiod1January1995–30September2016.Extrusivephasesandpausesareinshownredandgreenrespectively.Top:Numberofseismiceventsdetectedandidentifiedbytheseismicsystem.Middle:RadialdisplacementofcGPSstationsMVO1(red)andGERD(blue)smoothedwith7-dayrunningmeanfilter,Black:GPSHeightofHARR.Bottom:MeasureddailySO2flux,filteredwith7-dayrunningmedianfilter.Green:COSPEC,Blue:DOAS.Nodataavailablesincemid-September2015.

Figure33: Seismic,GPSandSO2monitoringdatafortheperiodsincetheendofPhase5:12February2010–30April2016.Top:Numberofseismiceventsdetectedandidentifiedbytheseismicsystem.Middle:RadialdisplacementofcGPSstationsMVO1(red)andGERD(blue)smoothedwith7-dayrunningmeanfilter.Bottom:DailySO2fluxmeasuredbytheDOASnetwork,filteredwith7-dayrunningmedianfilter.

Thisprolongedperiodofpersistentlow-levelunrest,(i.e.,continuedinflation,sporadicseismicityandpersistentde-gassing)clearlydemonstratesthatthemagmaticsystemhasnotshutdown.Thus,thepotentialforarestartremains.

2.2DomeStabilityThelavadomeatSoufrièreHillsvolcanoremainsstabledespitecontainingheavilyfracturedregions,particularlyontheeastflankwhererepeatedfracturinginducedbycoolingandweatheringgeneratesoccasionalrockfalls.Therehavebeennopyroclasticflowsinthecurrentreportingperiodandthenumberofrockfallevents,whileslightlyelevatedcomparedtothepreviousreportingperiod,isstillextremelylow(seeabove).Clearly,thedomehasbecomeverystable.However,itisclearthevariationsinRFactivity,showninFigure31,indicatethatthedomemaybecapableofrespondingtoexternalfactors,suchasheavyorprolongedrainfall.Researchintopossiblelinksbetweentherockfallcounts,rainfallandseismicitycontinues.

Thepersistentincandescentfeaturesobservedinsidethe2010collapsescararefurtherevidenceforastabledome.ThepersistenceofthesefumarolesandothersonthesummitofthedomeandontheTarRiverflank,indicatesthatverystablepathwaysexistinthedomeandmaterialsurroundingtheconduitallowinggastoescape,suggestingthatthesecouldbesuitablepathwaysformagmatoreachthesurface,shouldtherebearestart.Anyrestartofextrusioncouldpossiblycausesignificantdisruptiontothepresentdome,assuggestedbytheincreaseinRFactivitypriortotheonsetofashventinginMarch2012(seeMVOOFR12-01).

2.3CriteriaforContinuationofActivitySinceSAC16(November2011),thepotentialforcontinuingactivityhasbeenconsideredagainstthefollowingthreecriteriathatindicatethepotentialforfutureactivity:

1. Seismicity–thepresenceoflowfrequencyseismicswarmsandofseismictremor2. Gas–dailySO2ratesabove50tonnesperday3. Grounddeformation–significantgrounddeformation

Asdiscussedbelow,Criteria3iscurrentlybeingmet.Althoughthereisnodatafromthepast11months,itisassumedthat,basedonvisualobservations,Criteria2isalsobeingmet.Therefore,thereisevidencethatthedeepplumbingsystembeneaththevolcanoisstillactive.

2.3.1SeismicityThepresenceofswarmsoflow-frequencyseismicityorthepresenceofseismictremorindicatethepotentialforfuturevolcanicactivity.

Low-frequencyseismicityhascontinuedtobeextremelylowandhasbeencompletelyabsentduringthecurrentreportingperiodwithnolow-frequencyswarmsortremorobserved.

2.3.2GasDailySO2ratesabove50tonnesperdayindicatethepotentialforfuturevolcanicactivity.

Duetoinstrumentfailure,therehasbeennoSO2fluxmeasuredduringthecurrentreportingperiod.However,intheperiod1May2015to31October2015,SO2levelsaveraged436tonnesperdaywithspikesinthefluxabove1,000tonnesperday.Visualobservationsinthecurrentreportingperiodindicateastronggasplumeisstillpresent.

2.3.3GrounddeformationSignificantgrounddeformationindicatesthepotentialforfuturevolcanicactivity.

ThecGPSnetworkcontinuestoshowsignificantslowinflation,similartothatduringpreviouspauses.

2.4PhasesandPauseTable6liststhephasesandpausesoftheeruptionoftheSoufrièreHillsVolcano,upto30September2016,andfollowsthetraditionaldefinitionofaPhaseasaperiodwhenlavaisbeingextrudedatthesurface.

Phase # Start End Duration(days) Duration(months)

Seismic 0 01-Jan-92 17-Jul-95 1,294 42.5Phreatic 18-Jul-95 14-Nov-95 120 3.9Extrusion 1 15-Nov-95 10-Mar-98 847 27.8Pause 1 11-Mar-98 26-Nov-99 626 20.6

Extrusion 2 27-Nov-99 01-Aug-03 1,344 44.2Pause 2 02-Aug-03 14-Apr-05 622 20.4

Transition 15-Apr-05 31-Jul-05 108 3.5Extrusion 3 01-Aug-05 20-Apr-07 628 20.6Pause 3 21-Apr-07 04-May-08 380 12.5

Transition 05-May-08 07-Aug-08 95 3.1Extrusion 4a 08-Aug-08 08-Oct-08 62 2.0Pause 4a 09-Oct-08 01-Dec-08 54 1.8

Extrusion 4b 02-Dec-08 03-Jan-09 33 1.1Pause 4b 04-Jan-09 04-Oct-09 274 9.0

Transition 05-Oct-09 07-Oct-09 3 0.1Extrusion 5 08-Oct-09 11-Feb-10 127 4.2Pause 5 12-Feb-10 Ongoing 2,423 79.7

Table6: PhasesandpausesoftheeruptionoftheSoufriereHillsvolcano,withdurationscalculateduptoandinclusiveof30September2016.

3HazardsandRisks

3.1RiskAssessmentThepresentlavadome,estimatedat190Mm3(Stintonetal.,2014),isstillofconsiderablesize.SincethebeginningofPause5,nomorethanafewpercentofthedomehasbeenremovedbyrockfallsandpyroclasticflows.Consequently,withlittlematerialremovedovertheinterveningperiod,thedomestillrepresentsasignificanthazard.

Withsolittlematerialshedfromthedomeoverthepastsixmonths,thestabilityofthedomeandthequantityofmaterialavailableremainsvirtuallyidenticaltothatdiscussedinSACs17-20.Therefore,wecontinuetoholdtheviewthatlittlehaschangedinregardstotheHazardandRiskassociatedposedbytheSoufriereHillsvolcanosinceSAC20(November2015,Neubergetal.,2015).

3.2HazardLevelSystemTherehavebeennochangesmadetotheHazardLevelSystemsincethecurrentversionwasreleasedinAugust2014.TheHazardLevelremainsatLevel1.

3.3AccesstoZoneVMVOcontinuestoworkcloselywithDMCAandotherauthoritiestocontrolaccesstoZoneV.

Todate,accesshasbeenarrangedforthefollowingactivities:

• ExportofsandandgravelusingPlymouthJetty.• ReclamationofmetalsfromLoversLane.• GeothermalwellMON#3(seebelow).• ControlofferallivestockbyDepartmentoftheEnvironment.• TouristsvisittoPlymouth(seebelow).

NoaccesshasyetbeenallowedforanyactivitiessouthofPlymouthJetty.

3.4TouristVisitstoPlymouthBetweenAprilandSeptember2016,thenumberoftouristvisitstoPlymouthdecreasedsignificantly,primarilyduetotheendofvisitsbycruiseshipsinMarch2016.Oncehurricaneseasonisoverandcruiseshipvisitsbeginagain,thenumberofvisitsshouldincreaseagain.

Therehavebeenafewoccasionswhereagreedsafetyproceduresfortouristvisitswerenotproperlyadheredto.MVOisworkingcloselywithDMCAtoensurethatpropersafetyproceduresarefollowedatalltimes.

3.5GeothermalWellMON#3MVOhavebeenheavilyinvolvedinthesafetyproceduresassociatedwiththedrillingofgeothermalwellMON#3.

ThedrillingrigarrivedbyshipatPlymouthJettyinlateAugust2016.MVOhelpedwritethesafetyproceduresfordredgingatPlymouthJettyandforunloadingtherig.TheMVOOperationsRoomwasmannedcontinuouslyduringtheseoperations.

ThelocationofthedrillingsiteisjustinsidetheZoneVboundaryonthenorthernsideofStGeorgesHill.MVOhelpdevelopsafetyproceduresandevacuationplansfortherigsite.TheMVOOperationsRoomisnotcontinuouslymannedduringthedrilling,butcommunicationsprotocolshavebeenestablishedtoenablerapidnotificationoftheneedforanevacuationbythedutyscientist.

MVOhashostedorientationcoursesforalmostallofthecrewworkingonMON#3.ThesecoursesincludeawarenessofthevolcanichazardslikelyfromtheSoufrièreHillsVolcano.

4MVOMonitoringNetworks

4.1SeismicMonitoring TheMVOseismicnetworkhascontinuedtoperformadequatelyduringthisreportingperiod.

On14May2016aproblemwithabreakerboard/panelintheserverroomledtoalossofpowerintheserverroomwhichresultedincompletedatalossforallstationsforseveralhours.

On7September2016anunknownissuecausedasimultaneouslossofnetworkconnectivityforseveraloftheLantronixserial-to-LANconverterboxes,resultinginminordatalossforseveralstations.

Lossofexternalnetworkaccessduringpowercuts(duetoproblemsattheOlvestonrepeatersite)hasmeantfrequentdatalossfromthestationsroutedviaSilverHillsduringpoweroutages.

Also,otherongoingtelemetryproblems,particularlywiththeSpidersandstationsroutedviaSilverHills,haveagaincompromiseddataqualityatsomeofthestationsinthenetwork.

Alackoffunctioningspareequipmenthascontinuedtoimpactonmonitoringcapacity,notablyforsomeofthemoreproximalstations,suchasMBFRandMBLG,whichhavesufferedextendedoutages.Thishasmeantareducedtriggeringthreshold,andanoverrelianceonthetwomostproximalstationsMBLYandMSS1fordetectingsmallevents.

4.1.1SoftwareandacquisitionchangesTherewerenosoftwarechangestotheseismicacquisitionandprocessingsystemsduringthisreportingperiod.

4.1.2InstrumentalchangesatstationsTable7belowliststhechangestoinstrumentsthathavetakenplaceatindividualseismicstationsduringtheperiodcoveredbythisreport,aswellasspecificissuesthathavebeenresolved.

Station Date Detailsofchanges

MBBYBroderick’sYard 03-Oct-2016 Fixed3dayoutageduetolossofcommunications.

Ongoingtelemetryissues,causingregularsmalldatagaps

MBFRFergusRidge

18-Nov-2015 Nodataacquiredsincethisdate.

MBGHSt.George’sHill 15-Sep-2016

Sinceatleastthisdatetherehasbeenatimingerrorofuptoseveralseconds.

ProblemislikelyduetoafaultyGPSantennaorcable,currentlyunresolved

MBLGLongGround 05-Dec-2015 Nodataacquiredsincethisdate.

MBRYRoche’sYard 29-Jun-2016

Nodatawasreceivedfromthisstationforseveraldays,betweenapproximately25-Jun-201612:44UTCand15:37UTCon29-Jun.LinkrestoredfollowingsitevisitbyMF.

Ongoingtelemetryissues,causingregularsmalldatagaps

MBWHWindyHill

05-May-2016

FollowingsitevisitbyMFcommunicationswererestored-problemappearstohavebeenwaterinconnector

followingrecentheavyrainfall.Transmissionissuesfirstarisinginmid-Aprilappeartohavenowbeenresolved.

MSCPChance’sPeak 01-Oct-2015

Nodataacquiredsincethisdate.DamagetoSpiderduetocorrosion.Electronicsretrievedfortesting,framestill

awaitingretrieval.

MSUHUpperHermitage

11-May-201601-Aug-2016

Communicationsrestoredfollowingsitevisit.Nodatareceivedfromthisdate.Stationcurrentlyoffline

withongoingtelemetryissues

Table7: Changesmadetoinstrumentsatindividualseismicstations.

4.1.3Ongoingproblemsandknownissues4.1.3.1SeismometermassproblemsInstrumentT4344installedatMBLYhascontinuedtosufferfrommassproblems,withpersistentlong-periodnoiseontheN-Shorizontalcomponent.

4.1.3.2SpidersInOctober2015theSpideratChance’sPeak(MSCP)ceasedfunctioning.Theelectronicswererecoveredfortestingwhiletheframeisstillawaitingsuitableconditionsforretrieval.

MSUHatUpperHermitagehassufferedfromseveralprolongedcommunicationoutagesduringthisreportingperiod.

MSS1inthecollapsehasfunctionedwell,operatingmoreorlesscontinuouslythroughoutthisreportingperiodwithnomajorproblems.

StationMSNWdeployedneartotheSinkholeclosetoNorthWestBluff,alsofunctionednormallyduringthisreportingperiod,althoughsuffersfromhighnoiselevelsduetoitsproximitytothecoast.

4.1.3.3TelemetryissuesSeveralstationshavecontinuedtobeaffectedbytelemetryandcommunicationsproblemsduringthisreportingperiod.StationsontheEasternsideoftheislandinparticular(MBHAandMBRY)aswellasMBBYandtheSPIDERshavesufferedmostoftenwithdatagaps.Evensmalldatagapscanhaveaknock-oneffectintermsofeventtriggeringinearthworm,especiallyformoreproximalstations(e.g.MBFRorMBLG).

4.1.4Futuredevelopments4.1.4.1ShorttermgoalsThemainissuescurrentlyaffectingtheperformanceofthenetworkaretelemetryproblemsandalackoffunctioningspareseismometersanddigitiserstoreplacethosewithmassproblemsorotherissues.SeveralinstrumentsanddigitisersarestillawaitingreturntoGuralpforrepair.Thiswouldhelp,intheshort-term,toresolvetheinstrumentmassproblemsatsomestationsandalleviatepressureonresources,particularlywiththecurrentoutagestoimportantproximalstationssuchasMBFRandMBLG.Intheslightlylongerterm,issuessuchasthisshouldbesolvedbytheplannedseismicnetworkupgradewhichwillprovidemuchgreatersparecapacity.

4.1.4.2SpidersAnothermoreimmediateshort-termgoalistotryandretrievetheSpiderfromMSCPandalsotodeployoneofremaininginstruments,withthesiteatGalwaysincludingatiltmeterbeingthepriority;althoughthetimingofthisisheavilydependentonfavourableweatherconditionsandhelicopteravailabilitywhichhasyettooccur.

TheacquisitionsoftwareforthedatafromtheSpidersisalsoplannedtobemovedfromitscurrentpositiononalaptopandmigratedovertoadedicatedvirtualserverassoonasthisresourceismadeavailable.

4.1.4.3LongertermplansIntheirproposaltomanageMVOin2008,SRCandIPGPsuggestedthattheseismicmonitoringsystemshouldbereviewedbeforetheendofthefirstfive-yearcontract(April2013).TherecommendationsfromthisreviewformedpartoftheproposalsubmittedfortherenewalofthecurrentMVOmanagementcontract.ThisreportwascompletedandformedpartofthenewMVOmanagementcontractwhichwasfinallysignedinSeptember2016.Weareawaitingfinalcostingsforinstrumentsandadditionaltelemetryhardware,buttheequipmentfortheupgradeshouldbeorderedwithinthecomingweeks.

Theproposalsinclude:

• Standardisingequipmentandinstallations,includinginstrumentperiodsandgainsetctoalloweaseofswappingbetweensites.

• Replacingolder/brokenequipmentandincreasingstockofspareparts.• Upgradingverticalshort-periodstationstobroadbandwherepossible.Hopefully,dependingon

cost,toincludeatleastone120sultra-broadbandinstrumentfordetectionofVLPsignals.• InstallationoftwonewbroadbandstationsatSouthSoufriere(collocatedwithcGPS),andJack

BoyHill(re-occupyingexistingsite)• Establishingseveralnewnear-domestationstohelpwithdepthconstraintsanddetectionof

smallermagnitudeevents.CandidatesitesareHermitage,GalwaysandGagesMountain.ThequalityandusefulnessofdatafromtheadditionalSpidersthathavebeenorareplannedtobedeployedinsimilarlocationstothesemayinfluencethedecisiononfinallocationsandwhetherpermanentstationsinsuchlocationsarenecessaryordesirable.Currentresearchintotheimpactofnearfieldstationsontheprecisionoflocatedearthquakedepthssuggestedproximalhaveanimportantpositiveeffectonthequalityofthehypocentres.

• IntegrationoftwoSRC-fundedstations(atsitesinthenorthofMontserrat/SilverHillsandRedonda)andpotentiallyonemoreLeedsUniversitystation(boreholesensorinOlveston)

• Minorrevisionsandupgradestosoftwareandsomecomputerhardware.

4.2GroundDeformationMonitoring

4.2.1ContinuousGPS(cGPS)On-goingissueswiththecGPSnetwork:

• HERM:issuewithGPSantennahascausedlossofdatasince19thMay2015.• SGH1:receivernotworkingssincethe13thJan2015.OneNetR9receivergivenbySRC,andwill

beusedtoreplaceSGH1receiver,oncethesiteisready(thereceiverneedstobeinstalledinsideabuildingratherthanoutsideasitwas).

ThecGPSnetworkhasexperiencedotherissuesduringthereportingperiod;whichhavebeensolved:

• RCHY:damagedauxiliaryequipmentcausedlossofdatabetween11Februaryand25May2016.• NWBL:theantennaLNAhadtobereplacedwhichcausedlossofdatabetween30Julyand18

August2016.

TelemeteredaccesstomostoftheMVOcGPSstationshasremainedgoodthroughoutthereportingperiod.However,theaccesstoSSOUhasbeenparticularlydifficultinthelast2months.

InadditionofbeingincorporatedintheMVOdailyprocessing,datagatheredattheUNAVCO-andNASA-fundedsites(RDON,NWBL,RCHY),andatthefourstationsoperatedbytheCALIPSOproject(AIRS,GERD,OLVN,TRNT)arebeingtransferreddailytotheUNAVCOFTPsite,wheretheyaremadepubliclyavailable.

4.2.2CampaignGPS(eGPS)Eightbenchmarksarebeingoccupiedepisodically,foraweekapproximatelyeverysecondmonth.Ashortthreadedrodwasinstalledatfourstations(NorthWhiteRiver,LongGround,DryGhaut,Bransbypoint)tofacilitatedeploymentandimprovedataaccuracy.NochangestotheeGPSnetworkhavebeenmadeduringthereportingperiod.

OneNetR9receiverhasbeenborrowedfromSRCtoreplacetheNetRSreceiverswhichfailedinDec.2014.Thecampaignmeasurementshavebeenresumedonthe5May2016,howeverproblemswiththeauxiliaryequipment(battery,cable)hascausedlossoffewweeksofdata.Theseissuesarenowsolved.

4.2.3GPSArchivingandProcessingAfterbeingdownloaded,rawdataareconvertedwiththeUNAVCOteqcutilityintoRinexformat,tobeprocessedusingtheGAMIT/GLOBKsoftwaresuite.AseriesofMATLABscriptsthenreadtheGAMIT/GLOBKoutput,applycorrectionandremoveacommon‘tectonic’trend,rotateeachstationtime-seriesintoaradial,tangentialandverticallocalcoordinateframecenteredonthevolcanicvent,andproduceplotsassomeofthosepresentinthisreport(e.g.Figure13).

4.2.4EDMTherewerenochangesintheEDMnetworkmonitoringthevolcanicactivityduringthereportingperiod.

TwoEDMbaselineshavebeenestablishednexttothesinkholeinordertomeasureanypossiblesubsidencenearthesinkholeorlandslidedowntheslopefromthesinkhole.Nodeformationhasbeenobserved.

Thecoordinatesofthetotalstationandofthetworeflectorsarethefollowing:

North(m) East(m) Elevation(m)

Totalstation(SKTS) 1859990.584 585337.755 43.397

Reflector1(SKH1) Bysinkhole 1859841.173 585388.457 35.462

Reflector2(SKH2) Downslope 1859837.618 585366.120 25.941

TwoEDMbaselineshavebeenestablishedfromtheMVOhelipad(EDM2)tothegeothermalsite,inordertomonitoranypotentialmovements(inparticularlandslide)relatedtodrilling.TheEDMreflectorssitearealsomeasuredevery2ndweekusingthekinematicGPS.Nodeformationhasbeenobserved.

Thecoordinatesofthetworeflectorsarethefollowing:

North(m) East(m) Elevation(m)

Reflector1(GEO1) 1849784.403 584638.199 144.765

Reflector2(GEO2) 1849821.175 584581.988 135.936

4.2.5TiltMVOcurrentlyhasnooperationaltiltmeters.ItisplannedtodeployonetiltmeterwiththeSPIDERonGalwaysMountain,whichisprovingverydifficulttoaccessduetoweatherandlogisticalproblems.MVOhastwoothertiltmetersthancanbedeployedatseismicstations.Thedeploymentofthesewillbeconsideredintheseismicupgradescheduledfor2018.

4.2.6StrainStraindataaremeasuredatthefourCALIPSOstations(OLVN,AIRS,TRNT,GERD)andaretransferredtotheUNAVCOFTPsite,andtotheMVO.Asignificantamountofdataisatthemomentmissing,becauseofdatatransmissionissues.

4.2.7Spidersstations:single-frequencyGPS,EDMandTiltmeterInJuneandDecember2014,incollaborationwiththeUSGS,threeadditionalcontinuousGPSstations,so-called‘spiders’,weredeployedclosetothe2010collapsescar(MSS1andMSUH),andonChance’sPeak(MSCP).

Thespidershavebeenrecordingandtransferringdatadailysincedeployment.Duringthereportingperiod,therehavebeenissueswithMSUHandMSCP,withdeteriorateddataornodataatall,since28thNov.2015and02ndOct2015,forMSUHandMSCPrespectively.

TherawdatafromtheSpiderGPSstationsareprocessedwithasimilarmethodologytothatoftheotherGPSstations.TherawdataareconvertedintorinexformatusingtheteqcutilityandthenprocessedbyGAMIT/GLOBK.TheSpidersareprocessedusingGAMIT/GLOBKincombinationwiththerestofthenetwork,althoughthroughamorecomplexprocedure,duetohavingadifferenttypeofreceiver.

InFebruary2016,aspiderwasdeployedneartheNorthWestBluffsinkholetomonitorforanyfurthergroundfailures.ThespiderisalsoequippedwithaGPSanddatahavebeencollectedsince25February2016.UnliketherestoftheSpidernetwork,itsdataareprocessedusingakinematicapproachusingthecontinuousGPSstationNWBLasabasestation.ThespidercodeisMSNWandislocatedatNorth1859991.699m,East585202.883m,Up62.290m(UTM20).

Finally,itisexpectedthat,oncetheequipmentisavailableandtheweatherallows,atiltmeterwillbeincludedwithGPSandseismicinstrumentsonaSpidertobedeployedtoGalway’sMountain.

4.3GasMonitoring

4.3.1MultigasTheMVOmultigaswastakentoItalyforrepairsinFebruary2016.INGVrecentlybegandevelopinganew,morecompactversionofthemultigas.Consequently,itwasdecidedthatMVOacquiretwoinstruments;onewouldbearefurbishedversionoftheoldermodelusingthesameLICORspectrometerandtwonewsulphursensors.ThisinstrumentwouldbeputtogetherbyaprivatecompanyinItalyundercontractfromINGVPalermo.ThesecondinstrumentwouldbebuiltfromscratchbyINGVPalermo.AtpresentthereisachangeinthedirectorshipofINGVPalermothusthereisaslightdelayonthisnewinstrument.However,therefurbishedinstrumentfromtheprivatecompanyhasbeenshippedbyINGVandshouldarriveonMontserratbytheendofOctober.

4.3.2DOASUVspectrometernetworkSincethecurrentnetworkoftwoS2000spectrometersfailed,MVOhasbeenunabletocollectSO2fluxdataforapproximatelyoneyear.Priortothis,theSO2networkwasrunningononeinstrumentfor18months,thustheSO2fluxdatahasbeenoflimitedutilitysinceFebruary2013.ThemostrecentventuretoputinanewnetworkofnewUSB2000+instrumentswiththeCambridgegroupdidnotbearfruit.

FollowingavisitbyDrT.ChristophertotheUVgroupatINGVPalermo,MVOisintheprocessofpurchasingthreeboxestohousecalibratedspectrometersfromBoGalle.Preparationoftheboxestakes6months,afterwhichtheywillbedeliveredtoFabioVita,INGVPalermo.FabioVitaandothercolleaguesfromtheINGVgroupwillthenbringthecompletedinstrumentboxestoMVOtosetupthe

networkandtraintherelevantMVOstaffinmaintainingthenetworkandprocessingtherawdatatoobtainSO2flux.Thisislikelytotakeplaceduringthefirsthalfof2017.

4.4DomeVolumeTwomethodsareemployedformonitoringdomegrowthandgeologicmapping:aerialphotogrammetryandtheAVTIS3radarinstrument.

AerialphotographyforphotogrammetrystudiesiscurrentlycollectedusingaGoProHero3actioncamera.Whilethiscameraiseasytouseandprogramforhigh-framerateacquisition,theimagesacquiredareofmoderatequality.Thisisduetotheverywide(172°)fieldofviewofthecamera’slens.Ahigherqualitysystemiscurrentlyunderdevelopmentinhouse.InSeptember,followingdiscussionwithCaribbeanHelicopters,theprototypehousingunderwentatestflighttoensuretherewasnosignofthehousingbecomingdetachedfromthehelicopter.Thistestwassuccessful.Thenextstageistoacquireandtestasuitablecamera,afterwhichthewholesetupwillbetestedbeforebeingpressedintoservice,mappingthelavadomeandsurroundingflanks.

TheAVTIS3radarinstrumentiscurrentlynon-operational.BoththeradarinstrumentandtheremotePCusedtocontrolitareinstorageatMVOawaitingrefurbishmentbyDrDavMacFarlane,UniversityofStAndrews.DrAdamStintonandPyikoWilliamsareworkingcloselywithDrMacFarlaneontheAVTISinstrument,whichwillincludetwoplannedvisitsbyDrMacFarlaneinDecember2016andearly2017.

4.5FumaroleMonitoringCurrently,thetemperatureoffumarolesonandadjacenttothelavadomearemonitoredonanadhocbasis,principallywhenweatherconditionsallow,usingaeitherahandheldFLIRThermalIRcamerafromthehelicopter,orusingJ-typethermocouplesinsertedtoatleast50cmdepth.Onlylow-temperaturefumarolesaremeasuredonthegroundwiththermocouplesduetoaccessandsafety.TemperaturesarepresentedinTable5,Section1.5.4above.

InSeptember2016,MVOacquiredthreeGeminiTinytagPlus2temperatureloggersforthepurposeofdevelopingacontinuoustime-seriesoffumaroletemperatures.Theloggers,capableofrecordingtemperatureevery60secondsandstoring32,000readings,arecurrentlyundergoingtestingandwhencompleted,willbeinstalledonGalway’sMountainwherethreelow-temperaturefumaroles(~100°C)arelocatedincloseproximitytothelavadome.Shouldtheseprovetobesuccessful,additionalloggersmaybeacquiredandinstalledinhighertemperaturefumaroleslocatedontheSWflankofthelavadome.

4.6RemoteCamerasCurrently,MVOhasonlyoneremotecamerainoperation,locatedatHarrisLookout,inadditiontoanetworkIPcameraatMVO.

On1July,thenetworkcamerapreviouslyinstalledatGaribaldiHillsufferedafailureofthewallmountbracket,suchthatthecamerawaslefthangingfromtheroofbythepowercable.Themounthadsufferedseverecorrosionduetotheweatherandgas.Thecamerahasbeenremovedandthereareasyetnoplanstodeployanewcameratothesite.

TheHarbortronicstime-lapsecamerakitinstalledatHarrisLookouthascontinuedtoacquireimages.However,twomajorissueshaveaffectedthesetup.Firstly,inAugust2016,itwasnoticedthatsomedaytimeimageswereoverexposed,i.e.,toobright.TheproblemwastracedtothemeteringsysteminsidetheCanonDSLR.Itwasalsofoundthatthecamerahadclockedupmorethan220,000shutteractuations.Thecamerawasswappedforaspare,whichalsoappearstosufferfromthesamemeteringissues.BothCanonDSLRsaremorethan6yearsoldandhavefarsurpassedtheofficialratingofjust100,000shutteractuations.Asaresult,replacementcamerasarebeingsourced.

Asecondissuedevelopedshortlyafterthemeteringissue.InearlySeptember,thedigisnapcontrollerusedtotriggerthecamerabegantriggeringthecameraatrandomintervalsofupto150minutes,insteadoftheprogrammedevery15minutes.Similarissueshaveoccurredinthepast,andwereresolvedbysimplyrebootingthedigisnapcontroller.However,thishadlittleeffectonthetimingissue,whichappearstobecausedacommunicationserrorbetweenthelensandthecamerabody.Itisanticipatedthatthenewcameraandlenswillresolvethisissue.

ContinuousvideorecordingfromtheStardotnetworkIPcameraatMVOhasbeentestedusingsoftwarefromthecameramanufacturer.Thesetup,whichrecordsduringdaylighthoursonly,providesinvaluablevideocoverageintheeventofrenewedactivity.Dataiskeptforaperiodof21daysbeforebeingautomaticallydeleted.ThisgivesMVOstaffampletimetosavesuitablevideoclipsofactivity.

4.7ThermalCamerasMVOcontinuestooperatetwothermalcameras:afixedcameraatMVOandahandheldFLIRT650scwhichisusedduringobservationflights.Therehavebeennoissueswitheitherinstrumentduringthisreportingperiod.

4.8RemoteSensingDataAcquisitionofTerraSAR-XradarimageryceasedattheendofJuly2015whenthetwo-yearextensiontotheoriginalprojectended.DuetothelackofsurfaceactivityatSHV,therewillbenomoreacquisitionofthedataasithasprovedtobeoflimiteduseduringthecurrentperiodofquiescence.

4.9InfrasonicMonitoringTherehavebeennochangestotheMVOinfrasonicnetworkduringthereportingperiod.

TheinfrasoundarrayaroundMVOoperatedbytheUniversityofFlorencehasbeeninoperativesinceitwasdamagedbyalightningstrikeon17September2011.MVOisstillwaitingforavisitbystafffromtheUniversityofFlorencetorepairand/orreplacetheinstruments.

MVOstilloperatesandmaintainsitsownnetworkoffourinfrasoundsensorsatthefollowingseismicstations:MBFL(Flemmings),MBGB(GaribaldiHill),MBGH(St.George’sHill)andMBLG(LongGround).

4.10WEBOBSTheWEBOBSsystemcontinuestofunctionwellasthemainportaltoMVOmonitoringdata.ThereareoccasionalissueswithWEBOBS,oftenassociatedwithnetworkconnectivity.MVOnowmaintainsthisinstallationofWEBOBSwithoutsupportfromIPGP.

5MVOOperations

5.1StaffingKristofferHellbeganworkasSoftwareEngineerinAugust2016.OneofhismaintasksduringhisfirstmonthsatMVOisthedevelopmentofanewMVOwebsite.

5.2EducationandOutreachEducationandOutreachCoordinatorNatalieEdgecombehadathree-monthworksecondmenttotheIncorporatedResearchInstituteforSeismology(IRIS)inWashingtonDC.ItprovidedanopportunitytoobservemanagementoftheIRISUndergraduateInternshipProgramandalsoonlinecoursedevelopment.TheexperienceandcontactsgainedatIRISwillhelpfacilitatethedevelopmentofsimilarprogramsandcoursesatMVO.ThenextstepinthisprocesswillbecollaboratingwiththeprincipalandstaffatMontserratSecondarySchoolandMontserratCommunityCollegetocreateMVObasedprogrammesformathsandscienceeducation.SRCisexpectedtoprovidevaluablesupportinestablishingtheseeducationalprograms.

Workisalmostcompleteonanillustratedchildren'sstorybookfeaturingwinningstoriesfromthe2013MVOCreativeWritingCompetition.Thelaunchisanticipatedtobebeforetheendof2016.

WhiletheMVOCreativeWritingCompetitionwillnotbeheldthisyearduetoNatalie'sworkintheUS,MVOwillpartnerwiththeMinistryofEducationintheplanningandexecutionofMathandScienceWeek,scheduledforNovember28-December2.MathandScienceWeekisoneofMVO’smostvisibleeducationandoutreachevents.

5.3HelicopterHelicoptersupportcontinuestobeprovidedonceperweekbyCaribbeanHelicopters,Antigua.ThecurrentcontractendsinApril2017and,attherequestoftheGovernmentofMontserrat,MVOwillbecallingforbidsforanewmulti-yearcontract.Thecontractingprocessiscurrentlyunderway.

5.4Volunteers/PhDstudents/VisitorsOswenCartyandMarissaAllen,GeologystudentsatKingstonUniversity,UK,andtheUniversityoftheWestIndiesrespectively,carriedouttheirfinal-yearprojectworkonMontserratduringthesummer,withassistancefromDrA.Stinton.

ProfessorDaleWebber,Pro-ViceChancellorforGraduateStudiesandResearchattheUniversityoftheWestIndiesvisitedMVOinSeptember2016.HeisthemostseniorUWIstaffmembertovisitMVO.ProfessorWebberwasveryimpressedbyworkofthestaffatMVOandwillbelookingatwaystobringmoreUWIactivitiestoMontserratandtoinvolveMVOstaffmoreinUWI.

ProfessorJurgenNeuberg(Leeds,UK)andJ.Saltzer(GFZ,Germany),visitedMVOfrom16-20MaytodiscusspotentialcollaborationsonInSARandphotogrammetry.

5.5InternshipsTheMVOinternshipprogrammehasbeensuspendedtemporarily.Thiswasduetotheneedtoprovidepositionsforwork-experiencestudentsfromMontserratSecondarySchoolandforstudentsontheirsummerbreakfromscholarshipstostudyGeologyatuniversity.

Theprogrammewillre-startinOctober,butMVOmayhavetolimitthenumbers.

5.6CollaborationandExternalProjectsMVOcontinuestoworkwithexternalresearchersonawidevarietyofprojects.Atthelastcount,therewereover40suchprojectseitherfundedorwithfundingappliedfor.Someofthesearecollaborationsandsomesimplyinvolvetheprovisionofdataunderadatalicence.

Highlightsfromthecurrentlistofcollaborationsaregivenbelow.

On-goingcollaborationsforK.Pascal:

• WithProfessorJ.NeubergandDrA.Collinson(UniversityofLeeds):modellingofalternativesourceofdeformation(tectonic,secondboiling)whichcouldcontributetotheMontserratdeformationfield

• WithDrK.Palamartchouk(UniversityofNewcastle):combinedprocessingofSpidersingle-frequencydataandMVOdualfrequencydata

• WithDrH.Odbert(UniversityofBristol):modellingofthedeformationinducedbyloading/unloadingoftheedifice.

• WithDrS.Hautmann(UniversityofZurich):modellingofgroundwaterflowtriggeredbypressurizedmagmaticsources,usingdeformationandgravitydata.

NewcollaborationsforK.Pascal:

• WithDrGrahamRyan(UniversityoftheWestIndies):IntegrationofInSAR(ALOS,ALOS2)dataandoftheGPSdataonMontserrat.

• WithProf.GuyNason(UniversityofBristol),DrSouravNason(UniversityofBristol),DrHenryOdbert,DrPatrickSmith(MVO):investigatingpatternsofnon-stationarityinvolcanologicaltimeseries

DrAdamStintonisinvolvedwithtwopendingprojects(bothawaitingfundingconfirmation):

• AnalysisofcorematerialfromGeothermalWellMON#3,withDrSteveSparks(Bristol,UK)andDrGrahamRyan(SRC/UWI,Trinidad);

• ModellinglaharhazardandlandscapedisturbancefollowingavolcaniceruptionontheislandofMontserrat–proposedPhDprojectwithDrGeorginaBennett(Supervisor,UEA,UK),DrJennyBarclay(Co-supervisor,UEA)andDrAdamStinton(Co-Supervisor).

Projectsunassignedtoanystaffmember:

• STREVA• VUELCO

• DocumentingtheculturalmemoryoftheeruptionsonMontserratandStVincent(withUniversitiesofEastAngliaandOxford).

• CoringandanalysisofgeothermalwellMON#3(GrahamRyan,SteveSparks).• ProvisionofdatatoWOVOaspartofWOVOdat.

5.7MVOArchiveWorkontheMVOArchivehasslowedrecently.Itshouldpickupatthestartof2017withnewresourcesandthesupportoftheMVOSoftwareEngineer.Thenextstageoftheprojectistodesignaportalthatcanbeusedbyresearchersandthegeneralpublic.

5.8MVOWebsiteMVOcontinuestomaintainaminimalwebsitefollowingaservercrashmorethantwoyearsagothatresultedinlossofboththeprimarywebsiteandbackupdata.WiththeappointmentofanewSoftwareEngineer,developmentofanewwebsitehasbegun.ThedesignisexpectedtobecompletedinNovember2016andthewebsiteshouldbepopulatedforreleaseenoughbytheendoftheyear.Intheinterim,MVOmakesextensiveuseofFacebookandthelocalradiostationZJBtopubliciseeventsanddistributeinformationaboutthevolcano.

ReferencesChristopher,T.,Edmonds,M.,Taisne,B.,Odbert,H.,Costa,A.,Hards,V.,Wadge,G.,2014.PeriodicsulphurdioxidedegassingfromtheSoufriereHillsVolcanorelatedtodeepmagmasupply.Geol.Soc.London,Spec.Publ.410,123–141.doi:10.1144/SP410.11

Neuberg,J.W.,Aspinall,W.P.,Barclay,J.,Calder,E.,Rivalta,E.,Sparks,S.,andStewart,R.,2015.20thReportoftheScientificAdvisoryCommitteeonMontserratVolcanicActivity,PartII:FullReport,10December2015,52pp.

Reasenberg,P.andOppenheimer,D.,1985.Fpfit,fpplot,andfppage:Fortrancomputerprogramsforcalculatinganddisplayingearthquakefaultplanesolutions.U.S.Geol.Surv.Open-FileReport85-739,109p.

Stinton,A.J.,Cole,P.D.,Stewart,R.C.,Odbert,H.M.,Smith,P.,2014.The11February2010partialdomecollapseatSoufriereHillsVolcano,Montserrat,in:Wadge,G.,Robertson,R.E.A.,Voight,B.(Eds.),TheEruptionofSoufriereHillsVolcano,Montserrat,from2000to2010.GeologicalSociety,London,Memoirs,39,pp.133–152.doi:10.1144/M39.7

AppendixA:25June2016RFeventP.Smith,A.Stinton,T.Christopher,K.Pascal

A1IntroductionOn27June2016,MVOreceivedreportsofrecentashfallintheUpperAmershamareafromGregScott,apilotwithCaribbeanHelicopters.Priortothis,anddespitetheabsenceofthermalimageryandvisualobservationduetocloudyconditions,MVOhadnotobservedorrecordedanyunusualactivitycapableofgeneratingashfall.

Followingthereportsofpossibleashfall,ashwasobservedonthegroundintheUpperAmershamareaduringafieldvisiton29June.Asearchoftheseismiceventsdatabsaealsoidentifiedalargerockfalleventon25June2016.

A2SeismicsAsearchoftheseismiccatalogueidentifiedalargetriggeredrockfallsignalrecordedataround01:59UTCon25Juneasalikelysourceoftheobservedash.ThiseventwasrecordedacrosstheMVOseismicnetwork,andthesignalwaveformsareshowninFigureA1.TherewasalsoacoincidentinfrasoundsignalrecordedbythepressuresensoratMBFL(Flemmings).

A2.1InfrasoundTherecordatstationMBFLshowsaninfrasonicsignalfollowingtheseismicsignal,withthepressuresignalonsetbeginningaround15secondsaftertheseismicP-wavearrival.

Usingthesearrivaltimes,anapproximatedistance,d,ofthesourcefromstationMBFLwascalculatedusingthefollowingformula:

𝑑 = 𝑡! − 𝑡! 𝑣!!! − 𝑣!!!!!

Where𝑡! istheinfrasoundarrivaltime,𝑡!theP-wavearrivaltime,vitheacousticvelocityandvptheP-wavevelocity.

Usingthepickedarrivaltimes(showninFigureA2)andvelocitiesvi=348m/sandvp=2.5km/s,resultedinanapproximatedistancebetweenthesourceandstationMBFLof5.9km.Unfortunately,duetostationoutages,noadditionalinfrasounddatafromotherstationswereavailabletocorroboratethesignalseenatMBFL,sonoisecannotbecompletelyruledout.However,thecoincidenttimingandthetimedelaybetweentheseismicandinfrasoundarrivalsstronglysuggestotherwise.

A2.2LowfrequencyonsetInadditiontotheinfrasonicsignal,spectralanalysisshowsthattheseismicwaveformsalsocontainsignificantlow-frequencyenergy(inthe0.1-1Hzrange)inthesignalonset(FigureA3),similartothat

seeninprevioussmallexplosionsatSHV(FigureA4).TheexamplesshownareforminorexplosiveeventsthatprecededtheonsetofextrusioninPhases3and4respectively(Coleetal.,2014).

FigureA1: Seismic(andinfrasonic)waveformsfortherockfalleventon25June2016.Therawverticalcomponentvelocityseismogramsareshownfromeachstation,alongwithaccompanyingamplitudespectra.Timeaxisisinsecondsafter01:59:27UTCandforthespectrathefrequencyaxisisfrom0to10Hz.NotethatthesignalamplitudeclippedonthetwoSPIDERs(MSS1andMSUH)

FigureA2: Seismicandinfrasonicwaveformsfortherockfalleventon25June2016recordedatstationMBFL.Thetoppanelshowstheseismicsignal,withtheP-waveonsetmarked.Thelowerplotsshowtherawandfilteredinfrasoundsignalsassociatedwiththeseismicsignal.Themarkedarrivaltimesandappropriateseismicandacousticvelocitieswereusedtoderiveanapproximatedistancetothesourceof5.9km

FigureA3: SeismicsignalanditsassociatedS-transformfortherockfalleventon25June2016,recordedatstationMBGH.Thetopplotshowstheverticalcomponentseismogram,andthelowerplottheassociatedS-transform–highlightingthelowfrequencysignalonsetwithenergyinthe0.1–1Hzrange.Notethelogfrequencyscaleinthetime-frequencytransform.

FigureA4: SeismicsignalsandassociatedspectrogramsforfourdifferentsmallexplosiveeventsatSHV,recordedatstationMBGH.TheexamplesshownarefromprecursoryexplosiveactivitytoPhases3and4(seeColeetal.,2014).Thetopplotsineachpanelshowtheverticalcomponentseismogram,andthelowerplotstheassociatedspectrograms,withlowfrequencysignalonsetswithenergyinthe0.1–1Hzrange.

FigureA5: Onehourofseismicandstraindatafortherockfall/explosiveeventon25June.ThetopplotshowstheverticalcomponentseismogramfromstationMBGH.Thelowertwoplotsshowtheavailable50Hzand1Hzstraindatarecordedat2CALIPSOstations:AIRS(AirStudios)andGERD(Gerald’s).Straindataamplitudeshavebeencorrectedtounitsofnanostrain.Nofilteringwasapplied,butalineartrendwasremovedfromthe24hrsofstraindatatoaccountfordrift,andthemeansubtractedfromthedatashownintheplots.Nosignificantstraintransientassociatedwiththeseismicsignalisseeninthestraindata.

A3FieldobservationsFollowingthereportsofashfallintheUpperAmershamarea,MVOstaffcarriedoutafieldinvestigationon27June.Onlanding,ashwasobservedblowingaroundandalsoontheleavesofsomeplants.Someofitwascakeduntotheplantssuggestingitmayhaverainedeitherduringoraftertheashwasdeposited.AsmallsampleoftheashwascollectedandbroughttoMVOforanalysis.

Observationsoftheashunderabinocularmicroscopeshowedthatitisveryfinegrainedandpalecoloured.Theashappearedtocontainahighproportionofalteredmaterial,aswellassomecrystalfragments(FigureA6).AsmallsubsamplewassenttoDrJenniBarclay,UEA,forfurtheranalysis,whichconfirmedthelackofjuvenilematerialintheash(J.Barclay,pers.comms.,2016).

FigureA6: PhotographoftheashfallsamplecollectedfromtheUpperAmershamareafollowingtherockfalleventon25June2016.Theashappearstocontainahighproportionofalteredmaterialandnosignofanyjuvenilefragments.

A4SummaryOn25June2016,at01:58UTC,alargerockfalloccurredonthelavadomeatSHV.Thepresenceofaninfrasonicsignal,alongwiththelow-frequencyonsettotheseismicsignal,stronglysuggestsatleastsome(small)explosivecomponenttotheeventandthatthiseventwasthereforetheprobablesource

oftheashfalldepositsobservedintheUpperAmershamarea.Aviewshedanalysis,combinedwiththe5.9kmdistancederivedfromtheseismicsignaltimingssuggeststhatthesourceoftheinfrasoundsignal,andthereforetheashandrockfall,islocatedsomewhereonthesummitorupperwesternflankofthelavadome(FigureA7).Anumberofhightemperaturefumaroles(temps>400°C)arelocatedatboththeeastern(2006-7dome)andwesternendsofthelavadomesummit,aswellasinsidethesummitcraterandtheupperpartoftherearheadwallofthe2010collapsescar.AllofthesefumaroleshavebeenpresentsincethestartofPause5andaresurroundedbyalteredmaterial.Thelackofvisualobservationsdirectlyaftertheeventhaspreventedtheidentificationofanexactlocationfortherockfall.

FigureA7: Mapshowingacircleof5.9kmradiuscentredonstationMBFLatFlemmings,indicatingthatthis(approximate)distanceisconsistentwiththesourceofthesignalbeingfromsomewhereclosetothedome.Theredshadingshowsthelandscapethathasanuninterruptedlineofsightfromtheinfrasoundstation,indicatingthatthesourceofthesignalmayhavecomefromsomewhereonthesummitorupperreachesofthewestflankofthelavadome.

AnalysisoftheavailablestraindatafromtheCALIPSOboreholesshowsthatnosignificantshort-termstrainsignalwasobservedcoincidentwiththisseismicity(FigureA5),suggestingthatanyexplosivecomponenttothesignalwasprobablysmallinmagnitude.

AppendixB:TrendsinVTearthquakedepthssincePhase5P.Smith(updatedOctober2016)

B1IntroductionThisinvestigationwaspromptedbytheobservationthatVThypocentresatSHVappeartoshowatrendofincreasingdepthoverthelastfewyears.ThisobservationwasreportedinthepreviousMVOactivityreport,sothissectionservesasanupdatetothepreviousanalysisandtopromotefurtherdiscussionnowthatanadditionalfewmonthsofdataareavailable.

DepthsoflocalcrustalearthquakessuchasVTs,withfewstationspicksandapoorlyknownvelocitymodel,arenotoriouslybadlyconstrained;butidentifyingademonstrablyrobusttrendovertimemaywellprovidesomeusefulandimportantinformationaboutlongtermchangesinthestressconditionsandhencetheshallowvolcanicsystem.

B2DataanalysisFigureB1showsaplotofthedepthsofVTsrecordedsincePhase5,withtherighthandpanelzoomingintotheperiodsinceMarch2012.Despitesomescatterinthedata,thereappearstobeageneraltrendofshallowingVTdepthsfromFebruary2010untilMarch2012,followedbyincreasingVTdepthswithtimesinceMarch2012(verticaldashedblackline),gettingdeeperbyaround1kmoverthisperiod.ThistrendisobservedmoreclearlyinthemeanandmedianfilteredmovingaverageplotsinFigureB1,whichshowthedepthsfilteredwithasliding50-daytimewindow.

Inordertoassesshowrobustthistrendis,andwhetheritisanartefactofeitherthelocationprocessornetworkbias,FigureAX.1alsoshowstheerrorinthedepth(asgivenbytheoutputoftheHYPOCENTERprogram)andthenumberofstationpicks(P-waves)usedinobtainingthelocation.

Asexpected,asthenumberofstationpicksincreasestheerrorinthedepthsisreduced,particularlyfollowingtheintroductionoftheSPIDERStotheMVOseismicnetworkinJune2014,shownbythereddashedline.Animportantpointtonotehoweveristhatitisnotnecessarilyjustthenumberofstationsthatisrelevant,assomestations–suchasforexamplemoreproximalstationswithbettersignal-to-noiseratioforsmallerevents–willhaveagreaterimpactonthehypocenteruncertaintythanothers.However,thisissomethingthatisdifficulttoassessherewithoutamuchmoredetailedandindepthstudy,althoughthisispotentiallyanimportanteffectandshouldbeconsideredagoalforfutureworktoaddress.

FigureB1 PlotofVThypocentraldepthsvstime.Lefthandpanelshowspost-Phase5,right-handpanelshowspost-March2012.BlackdashedverticallinerepresentsthelargemagnitudeVTstringon23March2012,reddashedverticallinemarkstheintroductionoftheSpiderstotheMVOseismicnetwork.Topplotshowsthehypocenterdepths,followedby:theerrorinthedepth,thenumberofstation(P-wave)picksusedforthelocation,a50-daymovingaveragefilterofthedepths,minimumandmaximumdepthsinasliding50-daywindowandfinallythebottomplotshowsthe50-daystandarddeviationofthedepths.

FigureB2showsthefrequencydistributionsoftheVThypocenterdepthsandtheirassociatederrors,withthedepthsapproximatelynormallydistributedaroundameandepthofjustover1km.Thedeptherrorsaretypically1-2km,butcanbemuchhigherinafewcases.

Inordertoeliminateeventswithlargeuncertaintieswhenassessingthedepthtrends,earthquakehypocenterswitherrorsgreaterthanacut-offdeptherrorof±4.5kmwerediscarded.ThisthresholdwaschosenfromthedeptherrordistributiontoremoveoutliersandismarkedbythedashedredlineontheplotsinFigureB2.Ascanbeseeninthelower-rightpanelofFigureB2,theseweremostlyeventswith8picksorfewer.Thedatasetwiththeseearthquakeswithlargeuncertaintiesremovedisre-plottedinFigureB4,andshowsthatthetrendremainsclearwhilstsomeofthescatterhasbeenreduced–particularlybyremovingsomeoftheshallowestevents.

FigureB2: FrequencydistributionsofhypocenterdepthsandtheirassociatederrorsforVTearthquakesrecordedsinceMarch2012.Thelowerpanelsshowthedepthsandtheirerrorsasafunctionofthenumberofstationpicks(P-waves)usedinthelocations.Thereddashedline(s)inthetworight-handplotsmarkthechosencut-offof4.5kmforthemaximumdeptherrorinfurtheranalysis.

Foreachnumberofpossiblestationpicks(from4to14)themeanandmedianoftheearthquakedepthsandtheirassociateddepthswerecalculated,withtheresultsshowninFigureB3.Theerrorsdecreaseapproximatelyexponentiallyasthenumberofpicksincreases;whilstthedepthsappeartoincreasealmostlinearly(theendmembersof4and14probablycontaintoofeweventsforreliableestimates).However,asboththenumberofpicksandthedepthsaretime-dependent(wearelookingattemporaltrends)itisdifficulttoseparatetheseeffectsandconfidentlyattributethechangeindepthtoacausalresultofthenumberofstationpicksused.

Thedepthofthefirstinterfaceinthe1-Dseismicvelocitymodelusedbythelocationprogramisalsoplotted,markedbytheblackdashedlineatapproximately2.5kmdepth,togaugewhetherthedepthsaresimplyconvergingtowardsthedepthofthisinterface–althoughwithoutfurtherdataitisdifficulttoconcludeonewayortheother.

FigureB3: Hypocentraldepthsandassociatederrors,asafunctionofthenumberofstationpicksusedinthelocation.Foreachnumberofpicks,themean(left)andmedian(right)ofthedepthandassociatederrorfortheearthquakes(asshowninlowerpanelofFigureB1–includingallVTs)areplotted.Thedashedhorizontallineindicatesthedepthofthefirstinterfaceinthe1-Dvelocitymodelusedbythelocationprogram.

FigureB4: PlotofVThypocentraldepthsvstime.PanelsareasinFigureB1,exceptwithearthquakeswithlargeuncertainties(deptherror>4.5km)removedfromthedataset.

AfurthercheckforbiasinthedatasetiswhethertheresultsareaffectedbytheoccurrenceoftheVTstringsduringthistimeperiod.Thesecanbeseenasshort-termincreasesindepthvariability–eitherduetophysicaleffectsorincreasedscatteranderrorduetothehigheventratesreducingthenumberandqualityofP-wavepicksandhencelocationquality.

FigureB5showsthesamedatasetasinFigureB4(outliersremoved),plusadditionallywithearthquakesoccurringaspartofVTstringsremoved–leaving‘normal’or‘background’VTs.Thisshowsthatthetrendremainslargelyunchanged,perhapsevenshowingagreaterincreaseindepthsinthelastyearorso.

OneofthesuggestionsinthepreviousreportwastoincludealongertimeseriesincludingVTseismicityoccurringpriortoPhase5,andthisisshowninFigureB6.Thisshowsthatdeptherrorsandscatterinthedatasetaremuchlargerpriortoaround2010,likelytobeduetothereducedstationcoverage.Henceitthismakesconfidentassessmentdifficult,althoughthereiscertainlynoclearortrendsuchasseensince2012,andthedepthscouldbearguedtohaveremainedrelativelystableovertime.

FigureB5: PlotofVThypocentraldepthsvstime.PanelsareasinFigureB1,exceptwith(1)earthquakesoccurringaspartofVTstrings,and(2)eventswithlargeuncertainties(deptherror>4.5km),removedfromthedataset.

FigureB6: PlotofVThypocentraldepthsvstime.PanelsareasinFigureB1,exceptthatthetimescalehasbeenextendedtoincludeallVTeventsrecordedatSHVsinceDecember2004(left)andJuly2008(right).Theblackdashedlineontheleft-handplotrepresentsthestartoftimeperiodshownintheright-handplot.

B3DiscussionRecentVTseismicityatSHVappearstoshowatrendofdeepeninghypocenters,withtheaveragedepthsincreasingbyaround1000m,from1to2km,inthe4.5yearsorsosinceMarch2012.ThissuggeststhelargemagnitudeVTstringandassociatedeventsinMarch2012mayhavehadasignificantimpactonthesystem.TheadditionaldatafromrecentVThypocentersacquiredsincethepreviousMVOreporthascontinuedtofollowthesametrendofincreasingdepths.

Overall,theresultsoftheanalysisandinvestigationintopossibleeffectsofnetworkbiasareinconclusive.Despitetheobviouscorrelationbetweenincreasingstationpicksandincreasingdepths(withdecreasederrors),itcanbearguedthatthetrendisphysicallyrealforthefollowingreasons:

• Thetrendremainspresentonceoutliersandeventswithlargeuncertaintiesareremoved

• ThepatternisnotafunctionofbeingbiasedbytheVTstrings(withhighereventratesandhigherlocationuncertainties)

• BetweenFebruary2010andMarch2012thereisapatternofshallowingdepthsthatoccurswhilethenumberofstationpicksremainsroughlyconstant(arguesagainstbias)

• Thetrendofincreasingdepthhascontinuedduringlast12-18month,despitethenumberofstationpicksavailablereducingduetosomeextendedstationoutages

Severalpossibleinterpretationsofthistrendare:coolingoftheedificeandupperconduitsystem;anincreased/moreestablishedhydrothermalsystemanddegassingpathways,aswellothertectonicornon-volcaniceffects.Howeverasyettheseexplanationsarefairlyspeculative,andthissuggestsfurtherinvestigationintopossiblecausesiswarranted.

AninterestingobservationisthatthereappearstobenoobviouschangefollowingtheVTstringon8March2014,despitethisbeingalmostaslargeintermsofenergyreleaseandmagnitudeasthatinMarch2012.

ItmayalsobeinstructivetolookatincludingthenumberofS-wavepicksratherthanjustP-waves,asthesearealsolikelytohaveasignificantimpactondepthresolution.Thehorizontalhypocentralcoordinatescouldalsobeinvestigatedforanycorrespondingspatialpatterns.

ThisanalysishighlightsthatthepatternanddistributionofVTseismicityhypocentersshouldalsocontinuetobemonitoredinthefuture,toseeifthetrendcontinues,asanychangesmaybeanindicatorofafurtherchangeinthevolcanicsystemandhencepossiblefuturebehaviour.

References

Reasenberg,P.andOppenheimer,D.,1985.Fpfit,fpplot,andfppage:Fortrancomputerprogramsforcalculatinganddisplayingearthquakefaultplanesolutions.Technicalreport,U.S.Geol.Survey.

AppendixC:AdditionalGPSData

FigureC1: TangentialdisplacementsrecordedatcampaignsitesduringPause5(February2010-September2016).Thereportingperiodisboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.HighenergyVTstringsareindicatedwithagreyline,andthestrongestevents(27October2010,9July2011,22March2012,8March2014)havebeenhighlighted(reddashedline).Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.SeeFigure19: Radial(top)andVertical(bottom)displacementsrecordedatcampaignsitesduringPause5(February2010-September2016).Thereportingperiodisboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.HighenergyVTstringsareindicatedwithagreyline,andthestrongestevents(27October2010,9July2011,22March2012,8March2014)havebeenhighlighted(reddashedline).Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.SeeAppendixC:AdditionalGPSDatafortangentialdisplacements.forradialandverticalcomponents.

FigureC2: ComparisonofthetangentialdisplacementsmeasuredatnineMVOcontinuousGPSstationsduringPause3,Pause4bandPause5(solidlines)andsubsequentextrusionphases(dashedlines).A60-daylow-passfilterwasappliedtothedata.SeealsoFigure19forradialandverticaldisplacements.

FigureC3: Radialextensionrelativetothevolcanicventsincethe1stJun.2015.Thereportingperiod(May-September2016)isboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.Displacementsduetoantennachangeshavebeencorrected.HigherenergyVTstringsareindicatedwithagreyline.Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.

FigureC4: Tangentialdisplacementsrelativetothevolcanicventsincethe1June2015.Thereportingperiod(May-September2016)isboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.Displacementsduetoantennachangeshavebeencorrected.HigherenergyVTstringsareindicatedwithagreyline.Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.

FigureC5: Verticaldisplacementsobservedsincethe1stofJun.2015.Thereportingperiod(May-September2016)isboundedbythetwobluelines.Thestation-volcanodistanceincreasesfrombottomtotopoffigure.Abackgroundplatevelocityof[E1.06,N1.40]cm/yrwasremovedfromthedata.Displacementsduetoantennachangeshavebeencorrected.HigherenergyVTstringsareindicatedwithagreyline.Thedistanceseparatingthegridlinescorrespondstoadisplacementof1cm.

mvo scientific report for volcanic activity between 1 may and 30

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