1 / 17

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

Can we observe a non-shear pattern during 2003 fluid injection at Soultz -sous- Fôrets?. Zuzana Jechumtálová, Jan Šílený Institute of Geophysics, Prague. Motivation. Larger microearthquakes (ML ≥ 1.6) which were induced during and after the 2003 massive fluid injection are

faxon
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Can we observe a non-shear pattern during 2003 fluid injection at Soultz-sous-Fôrets? Zuzana Jechumtálová, Jan Šílený Institute of Geophysics, Prague

  2. Motivation • Larger microearthquakes(ML ≥ 1.6) which were induced during and after the 2003 massive fluid injection are occurring onpre-existing faults(Horálek et al., 2010). • nearlypure shear slips • clusteringon two fault segments • injectionfluid pressureremainedbelow the tensile strength of the material • It doesnot a priori excludethe existence oftensile fracturesduring injection. • However, to answer this issue there is necessary to investigate • weaker microearthquakes, whereopeningnewcracksdue to injection • is morerelevant.

  3. Objectives • investigation of weak microearthquakes • significantly contaminated by noise • not detected by all stations • MTs of which need not always be well constrained and • may contain spurious non-DC components • comparison of the source mechanisms resulting from • the alternative approaches • moment tensor • model describing a slip along the fault • with an off-plane slip component • offers a clue to estimate the reliability of the shear vs. • non-shear source components

  4. Outline • Geothermal HDR site at Soultz-sous-Forêts • characteristics of the fluids injection experiment in 2003 • Soultz seismic network • Source mechanism retrieval • previous analysis of mechanisms of 2003 injection • criteria of additional event selection • inversion methods using two alternative source models • resultant source mechanisms and their comparison • Conclusions

  5. The Soultz 2003 injection • massive fluid injection • GPK2 & GPK3stimulated • duration11 days Adapted from Cuénot et al. (2006) • seismicity duration: 20 days • microearthquakes recorded • 5 000 events M  - 0.9 • only 240 events M > 1.0 • three largest events M =2.7, 2.8, 2.9 • totalfoci volume: 2km x 2km x1km Adapted from Dorbath et al. (2009)

  6. Soultz seismic network ▼8three-component stations ▲6one-componentstations frequency range of seismometers :1.0 – 40 Hz sampling frequency:150 Hz epicentraldistances:≈ 100m to 7km Fairlyuniform distributionof 14 stationsused for the MT estimationson the focal sphere.

  7. Previous analysis of mechanisms of 2003 injection Horáleket al. (2010) • criteriaof eventselection • the1.6 ≤ ML ≤ 2.9events • coveringthe whole experiment • seismograms havinghigh signal to noise ratio • MTsmeeting these criteria • wellconstrained • stable to noisecontamination • stable tostructuremismodelling • source mechanisms • dominantly pure shear • dip-slip, oblique normal and strike-slip

  8. Additional events processed • criteriaof eventselection • the magnitudeML ≥ 1.4events • from thefirst phaseof the injection in 2003 whenonlythe borehole • GPK3wasstimulated • weak events –not testedyet • consequencesof these criteria • seismograms havinglow signal to noise ratio • eventsnot detectedbyall 14 stationsof Soultz surface network  • MTs need notalways be wellconstrained • sparsedatacoverage,noisecontamination and structure • mismodellingmay producespurious non-DC components • inversion of13 eventsusingtwo alternative source models

  9. a + / Inversion methods Moment tensor (MT) • general dipole source Shear-tensile/implosion (STI) • physical source BUT toogeneral shear slip + tensile crack / cavity closure • includes unphysical sources Dufumier & Rivera 1997, Vavryčuk 2001 • 6inversionparameters: M11, M22, M33, M12, M13 a M23 • advantage: • linear inverse problem • disadvantage: • spurious non-DC components • decomposition of MT non-unique • 5inversionparameters: 4 angles (,,,), magnitude (M0) • advantage: • pure physical source • less parameters, i.e. more robust • disadvantage: • non-linear inverse problem

  10. Mechanisms : MT vs. STI histograms of slope angle  : focal spheres : 05/28 – 01:42 =0.25º P T DC59% V(I) 6% CLVD(P)35% 121º 38º -147º strike dip rake 112º 46º -147º P T N N ‘confidence zones’ : the NRMS remains below • 120% - dark • 150% - medium • 200% - light colour percentage of the best value traditional fault-plane solution plots : • nodal lines of DC part of MT • principal axes T, P and N • decomposition of the MT • principal axes T, P and N • source lines & the direction of slip / fault normal vector 05/29 – 04:28 =1.25º DC85% V(E)3% CLVD(T)12% 244º 74º -51º strike dip rake 243º 74º -52º T N T N P P Moment tensor Shear-tensile/implosion

  11. Mechanisms : MT vs. STI 05/29 – 03:44 =-1.25º DC96% V(I) 3% CLVD(P)1% 6º 31º -33º strike dip rake 8º 28º -30º 05/29 – 14:25 =3.75º 191º 29º -33º strike dip rake DC74% V(E)11% CLVD(T)15% 190º 29º -35º 05/30 – 18:46 =1.0º DC45% V(E)10% CLVD(P)45% 231º 66º -51º strike dip rake 246º 74º -46º 05/30 – 21:11 =2.25º 239º 72º -60º strike dip rake DC79% V(E)9% CLVD(P)12% 242º 74º -55º

  12. Mechanisms : MT vs. STI 05/31 – 02:23 =4.5º DC74% V(E) 13% CLVD(P)13% 186º 39º -52º strike dip rake 181º 40º -54º 05/31 – 03:29 =0.25º 2º 56º -35º strike dip rake DC52% V(E) 6% CLVD(T)42% 5º 58º -30º 05/31 – 11:33 =-0.25º DC70% V(I) 5% CLVD(P)25% 15º 38º -41º strike dip rake 7º 37º -47º 06/01 – 05:42 =-3.25º 145º 51º -144º strike dip rake DC32% V(I) 4% CLVD(P)64% 162º 41º -155º

  13. Mechanisms : MT vs. STI 06/01 – 11:57 =1.0º DC83% V(E) 9% CLVD(T) 8% 199º 51º -50º strike dip rake 220º 59º -46º 06/01 – 14:27 =-1.25º 138º 49º -173º strike dip rake DC67% V(E) 5% CLVD(P)28% 145º 49º -179º 06/02 – 07:44 =1.75º DC54% V(I) 2% CLVD(P)44% 125º 69º -131º strike dip rake 118º 68º -134º

  14. Mechanisms : MT vs. STI traditional fault-plane solution plots : • equal-area lower-hemisphere projection • nodal lines of DC part of MT source lines of STI • principal axes T - a triangle apex up P - a triangle apex right N - a triangle apex left • compression – colour area dilatation – white area • direction of slip / fault normal, off-plane angle – yellow circle Moment tensor Shear tensile/implosion

  15. Discussion • orientationof double-couple part of MTinvery good agreementwith fault orientation in STI • all13 MTscorrespond to shear-slipSTI models • MTs withnon-DC partsbetween 4% and 68% • STI withslope anglesbetween -3.25º and 4.5º which are not significant non-DC componentsof MTspuriouscaused by sparse data coverage, noise contamination and structure mismodelling • direct parametrizationof shear/non-shear displacement in the STI straightforwardquantitative assessmentof fracture modes

  16. Conclusions investigatingthe 13 inducedevents with ML ≥ 1.4 • source mechanisms • mechanismsdominantly pure shears dip-slip and oblique normal • orientation of T-axes and P-axes stable directions of T-axes (sub-horizontally in E-W direction) variations of P-axes (from vertical to horizontal in N-S direction) • comparisonwith45 largest events(1.6 ≤ ML ≤ 2.9) • all 58mechanisms in agreement • orientation of all T-axes and P-axesin agreementwith thestress pattern from in-situ measurements  Even weak microearthquakeswith ML ≥ 1.4 were pure shear slips on pre-existing faults.

  17. Thank youfor your attention

More Related