Can we observe a non-shear pattern during 2003 fluid injection at Soultz -sous- Fôrets?

Can we observe a non-shear Can we observe a non-shear pattern during 2003 fluid injection at pattern during 2003 fluid injection at

Soultz-sous-Fôrets? Soultz-sous-Fôrets?

Zuzana Jechumtálová, Jan ŠílenýZuzana Jechumtálová, Jan ŠílenýInstitute of Geophysics, PragueInstitute of Geophysics, Prague

MotivationMotivation

Larger microearthquakesLarger microearthquakes (ML ≥ 1.6) which were induced(ML ≥ 1.6) which were induced during and after the 2003 massive fluid injection areduring and after the 2003 massive fluid injection are occurring onoccurring on pre-existing faultspre-existing faults (Horálek et al., 2010).(Horálek et al., 2010).

nearlynearly pure shear slipspure shear slips clusteringclustering on two fault segmentson two fault segments injectioninjection fluid pressurefluid pressure remainedremained below the tensile strengthbelow the tensile strength of the materialof the material

It doesIt does not a priori excludenot a priori exclude the existence ofthe existence of tensiletensile fracturesfractures during injection.during injection.

However, to answer this issue there is necessary to investigateHowever, to answer this issue there is necessary to investigateweaker microearthquakesweaker microearthquakes, where, where openingopening newnew crackscracks due to injectiondue to injectionis moreis more relevantrelevant..

ObjectivesObjectives investigation of weak microearthquakesinvestigation of weak microearthquakes

significantlysignificantly contaminated by noise contaminated by noise not detected not detected by all stationsby all stations

MTsMTs of which need not always be of which need not always be well constrainedwell constrained and and may may contain spurious non-DC componentscontain spurious non-DC components

comparison of the source mechanisms comparison of the source mechanisms resulting fromresulting from the alternative approachesthe alternative approaches

moment tensormoment tensor model describingmodel describing a slip along the fault a slip along the fault with an off-plane slip componentwith an off-plane slip component

•

offers a clue to offers a clue to estimate the reliabilityestimate the reliability of the shear vs. of the shear vs. non-shear source componentsnon-shear source components

Geothermal HDR site at Soultz-sous-ForêtsGeothermal HDR site at Soultz-sous-Forêts characteristics ofcharacteristics of the fluids injection the fluids injection experiment experiment in 2003in 2003 Soultz seismic networkSoultz seismic network

Source mechanism retrievalSource mechanism retrieval previous analysisprevious analysis of mechanisms of 2003 injection of mechanisms of 2003 injection criteria of criteria of additional event selectionadditional event selection inversion methods using inversion methods using two alternative source modelstwo alternative source models resultantresultant source source mechanismsmechanisms and their comparison and their comparison

ConclusionsConclusions

OutlineOutline

Adapted from Dorbath et al. (2009)Adapted from Dorbath et al. (2009)

Adapted from CuAdapted from Cuénoténot et al. (200 et al. (20066))

massive fluid injection massive fluid injection GPK2 & GPK3GPK2 & GPK3 stimulatedstimulated durationduration 11 days11 days

seismicity durationseismicity duration : : 20 days20 days

microearthquakes recorded microearthquakes recorded 5 000 events M 5 000 events M - 0.9 - 0.9 only 240 events M > 1.0only 240 events M > 1.0 three largest events M =2.7, 2.8, 2.9three largest events M =2.7, 2.8, 2.9

totaltotal foci volumefoci volume : : 2km x 2km x1km2km x 2km x1km

The Soultz 2003 injectionThe Soultz 2003 injection

▼▼ 88 three-three-component stationscomponent stations

▲▲ 66 one-one-componentcomponent stationsstations

frequency rangefrequency range of seismometers :of seismometers : 1.0 – 40 Hz1.0 – 40 Hz

sampling frequencysampling frequency :: 150 Hz150 Hz

epicentral epicentral distancesdistances :: ≈≈ 100m to 7km 100m to 7km

FairlyFairly uniform distributionuniform distribution ofof 14 stations14 stations used for the MTused for the MTestimationsestimations on the focal sphereon the focal sphere..

Soultz seismic networkSoultz seismic network

criteriacriteria of eventof event selectionselection

thethe 1.6 ≤ ML ≤ 2.91.6 ≤ ML ≤ 2.9 eventsevents coveringcovering the whole experimentthe whole experiment seismograms havingseismograms having high signalhigh signal

to noise ratio to noise ratio

MTsMTs meeting these criteriameeting these criteria wellwell constrainedconstrained stable to noisestable to noise contaminationcontamination stable tostable to structurestructure mismodellingmismodelling

source mechanismssource mechanisms dominantly pure sheardominantly pure shear dip-slip, oblique normal and strike-slipdip-slip, oblique normal and strike-slip

Horálek Horálek et al.et al. (2010) (2010)

Previous analysis of mechanisms Previous analysis of mechanisms of 2003 injectionof 2003 injection

Additional events processedAdditional events processed criteriacriteria of eventof event selectionselection

the magnitudethe magnitude ML ML ≥≥ 1. 1.44 eventsevents from thefrom the first phasefirst phase of the injection in 2003 whenof the injection in 2003 when onlyonly the boreholethe borehole

GPK3GPK3 waswas stimulatedstimulated weak events –weak events – not testednot tested yetyet

consequencesconsequences of these criteriaof these criteria

seismograms havingseismograms having low signal to noise ratiolow signal to noise ratio eventsevents not detectednot detected byby all all 14 stations14 stations of Soultz surface networkof Soultz surface network

MTs need notMTs need not always be wellalways be well constrainedconstrained sparsesparse datadata coveragecoverage,, noisenoise contamination and structurecontamination and structure mismodellingmismodelling may producemay produce spurious non-DC componentsspurious non-DC components

inversion ofinversion of 13 events13 events usingusing two alternative source modelstwo alternative source models

Inversion methodsInversion methodsShear-tensile/implosion (STI)Shear-tensile/implosion (STI)

physical sourcephysical source

66 inversioninversion parametersparameters :: MM1111, M, M2222, M, M3333, M, M1212, M, M1313 a M a M2323

advantageadvantage ::• linear inverse problemlinear inverse problem

disadvantagedisadvantage ::• spurious non-DC componentsspurious non-DC components• decomposition of MT decomposition of MT

non-uniquenon-unique

Moment tensor (MT)Moment tensor (MT)

general dipole sourcegeneral dipole source

++ //

shear slip +shear slip + tensile crack /tensile crack / cavity closurecavity closure

BUTBUT tootoo generalgeneral

includesincludesunphysical unphysical sources sources

Dufumier & Rivera 1997,Dufumier & Rivera 1997,Vavryčuk 2001 Vavryčuk 2001

55 inversioninversion parametersparameters :: 4 angles (4 angles (,,,,,,), magnitude (M), magnitude (M00))

advantageadvantage ::• pure physical sourcepure physical source• less parameters, i.e. more robustless parameters, i.e. more robust

disadvantagedisadvantage ::• non-linear inverse problemnon-linear inverse problem

strikestrikedipdip

rakerake

121º121º38º38º

-147º-147º

112º112º46º46º-147º-147º

05/28 – 01:4205/28 – 01:42DCDC 59%59%V(I) V(I) 6%6%CLVD(P)CLVD(P) 35%35%

strikestrikedipdip

rakerake

244º244º74º74º

--5511ºº

243º243º74º74º-52º-52º

05/29 – 04:2805/29 – 04:28DCDC 85%85%V(E)V(E) 3%3%CLVD(T)CLVD(T) 12%12%

== 1.25º1.25º

Mechanisms : MT vs. STIMechanisms : MT vs. STI

== 0.25º0.25º

Shear-tensile/implosionShear-tensile/implosionMoment tensorMoment tensor

traditional traditional fault-plane fault-plane solutionsolution plots : plots : nodal lines of DC part of MTnodal lines of DC part of MT principal axes principal axes TT, , PP and N and N decomposition of the MTdecomposition of the MT

principal axes principal axes TT, , PP and Nand N source lines & thesource lines & the direction of slip / direction of slip / fault normal vectorfault normal vector

histograms of histograms of slope angle slope angle ::

‘‘confidence zonesconfidence zones’ :’ :the NRMS remains belowthe NRMS remains below 1120% 20% - - darkdark 150% 150% - - medium medium 200% 200% - - light colourlight colourpercentage of the best valuepercentage of the best value

focal spheresfocal spheres : :

PP

PPPP

PP

TT

TTTT

TT

NN

NN

NN

NN

strikestrikedipdip

rakerake

6º6º31º31º

-33º-33º

8º8º28º28º-30º-30º

0055/29 – 03:44/29 – 03:44DCDC 996%6%V(I) V(I) 3%3%CLVD(P)CLVD(P) 11%%

strikestrikedipdip

rakerake

191º191º2299ºº

--33º33º

190º190º29º29º-3-355ºº

0055/29 – 14:25/29 – 14:25DCDC 7474%%V(V(EE)) 111%1%CLVD(T)CLVD(T) 115%5%

== 3.73.755ºº

strikestrikedipdip

rakerake

231º231º66º66º

--5511ºº

246º246º774º4º--446º6º

0055//330 – 0 – 118:468:46DCDC 45%45%V(E)V(E) 10%10%CLVD(P)CLVD(P) 4455%%

== 1.0º1.0º

Mechanisms : MT vs. STIMechanisms : MT vs. STI

== -1.25º-1.25º

strikestrikedipdip

rakerake

239º239º72º72º

--60º60º

242º242º774º4º-55º-55º

0055//330 – 21:110 – 21:11DCDC 779%9%V(E)V(E) 9%9%CLVD(P)CLVD(P) 12%12%

== 2.25º2.25º

strikestrikedipdip

rakerake

186º186º39º39º

-52º-52º

181º181º40º40º-54º-54º

0055/31 – 02:23/31 – 02:23DCDC 74%74%V(E) V(E) 13%13%CLVD(P)CLVD(P) 113%3%

strikestrikedipdip

rakerake

2º2º56º56º

--35º35º

5º5º58º58º-30º-30º

0055/31 – 03:29/31 – 03:29DCDC 52%52%V(V(EE)) 6% 6%CLVD(T)CLVD(T) 42%42%

== 0.20.255ºº

strikestrikedipdip

rakerake

15º15º38º38º

--4411ºº

7º7º3377ºº--447º7º

0055//331 – 1 – 111:331:33DCDC 70%70%V(I)V(I) 5% 5%CLVD(P)CLVD(P) 2255%%

== -0.25º-0.25º

Mechanisms : MT vs. STIMechanisms : MT vs. STI

== 4.5º4.5º

strikestrikedipdip

rakerake

145º145º51º51º

--144º144º

162º162º41º41º-155º-155º

06/01 – 05:4206/01 – 05:42DCDC 32%32%V(I)V(I) 4% 4%CLVD(P)CLVD(P) 64%64%

== -3.25º-3.25º

strikestrikedipdip

rakerake

199º199º51º51º

-50º-50º

220º220º59º59º-46º-46º

06/01 – 11:5706/01 – 11:57DCDC 83%83%V(E) V(E) 9% 9%CLVD(T)CLVD(T) 8% 8%

strikestrikedipdip

rakerake

138º138º49º49º

--173º173º

145º145º49º49º-179º-179º

06/01 – 14:2706/01 – 14:27DCDC 67%67%V(V(EE)) 5% 5%CLVD(P)CLVD(P) 28%28%

== -1.2-1.255ºº

strikestrikedipdip

rakerake

125º125º69º69º

--131311ºº

118º118º68º68º-134º-134º

06/02 – 07:4406/02 – 07:44DCDC 54%54%V(I)V(I) 2% 2%CLVD(P)CLVD(P) 44%44%

== 1.75º1.75º

Mechanisms : MT vs. STIMechanisms : MT vs. STI

== 1.0º1.0º

Mechanisms : MT vs. STIMechanisms : MT vs. STIM

omen

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Mom

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She

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traditional traditional fault-plane fault-plane solutionsolution plots : plots : equal-areaequal-area lower-hemisphere projectionlower-hemisphere projection

nodal lines of DC part of MTnodal lines of DC part of MT source lines of STIsource lines of STI

principal axes principal axes T - a triangle apex upT - a triangle apex up P - a triangle apex rightP - a triangle apex right N - a triangle apex leftN - a triangle apex left

compression – colour areacompression – colour area dilatation – white areadilatation – white area

direction of slip / fault normal, direction of slip / fault normal,

off-plane angle – yellow circleoff-plane angle – yellow circle

DiscussionDiscussion

orientationorientation of double-couple part of MTof double-couple part of MT inin very very goodgood agreementagreement with fault orientation in STIwith fault orientation in STI

allall 13 MTs13 MTs correspond to shear-slipcorrespond to shear-slip STI modelsSTI models MTs withMTs with non-DC partsnon-DC parts between 4% and 68%between 4% and 68% STI withSTI with slope anglesslope angles between -3.25º and 4.5º which arebetween -3.25º and 4.5º which are

not significantnot significant

non-DC componentsnon-DC components of MTof MT spuriousspurious caused by sparse datacaused by sparse data coverage, noise contamination and structure mismodellingcoverage, noise contamination and structure mismodelling

direct parametrizationdirect parametrization of shear/non-shear displacementof shear/non-shear displacement in the STIin the STI straightforwardstraightforward quantitative assessmentquantitative assessment of fracture modesof fracture modes

ConclusionsConclusionsinvestigatinginvestigating the 13 inducedthe 13 induced events with ML events with ML ≥≥ 1.4 1.4 source mechanismssource mechanisms

mechanismsmechanisms dominantly pure shearsdominantly pure shears

dip-slip and oblique normaldip-slip and oblique normal

orientation of T-axes and P-axesorientation of T-axes and P-axes

stable directions of T-axes (sub-horizontally in E-W direction)stable directions of T-axes (sub-horizontally in E-W direction)variations of P-axes (from vertical to horizontal in N-S variations of P-axes (from vertical to horizontal in N-S

direction)direction)

comparisoncomparison withwith 45 largest events45 largest events (1.(1.6 6 ≤ ML ≤ 2.9)≤ ML ≤ 2.9) all 58all 58 mechanisms in agreementmechanisms in agreement orientation of all T-axes and P-axesorientation of all T-axes and P-axes in agreementin agreement withwith

thethe stress pattern from in-situ measurementsstress pattern from in-situ measurements

Even weak microearthquakes with ML ≥ 1.4 were Even weak microearthquakes with ML ≥ 1.4 were pure shear slips on pre-existing faults.pure shear slips on pre-existing faults.

Thank youThank youfor your attentionfor your attention