Evidence for a low-permeability fluid trap in the Nový Kostel Seismic Zone from double-difference tomography 3rd Annual AIM Workshop I October 10 – 12, 2012 | Smolenice Castle, Slovakia Catrina Alexandrakis 1,3, Marco Calò 2, Fateh Bouchaala 1 and Vaclav Vavryčuk 1 1 Institute of Geophysics, CAS 2 EOST, University of Strasbourg 3 Institute of Geophysics and Geoinformatics, TU BAF

2 Acknowledgements Data: –J. Horálek, A. Boušková and other members of the WEBNET group Funding: –European Union Research Project AIM ‘Advanced Industrial microseismic Monitoring‘ - Marie Curie Actions

3 Outline Introduction Methodology –Double-Difference Tomography –Weighted Average Mean Analysis Results and Interpretation Conclusions

4 West Bohemia Seismic Zone

5 Swarm Triggers Smrčiny Pluton Babuška and Plomerová, 2008 Geissler et al., 2005

6 Outline Introduction Methodology –Double-Difference Tomography –Weighted Average Mean Analysis Results and Interpretation Conclusion and Future Work

7 Double-Difference Tomography TomoDD (Zhang and Thurber, 2003)

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9 Double-Difference Tomography Advantages: –Relocates hypocenter locations –3D Vp and Vs model of focal zone –Gives the Derivative Weight Sum (DWS) at each node Disadvantages: –No error estimate for the velocity models –Starting model parameterization introduces bias and artifacts

10 Weighted Average Mean (WAM) Analysis (Calò et al., 2011) Solution to parameterization artifacts Calculates the Weighted Standard Deviation (WSTD) for the final model Steps 1.Define basic model parameters (e.g. Velocity model, node locations, hypocenters) 2.Perturb the basic parameters 3.Average models together using tomoDD’s DWS 4.Calculate the standard deviation using DWS as a weighting factor

11 Single Inversions Weighted Average Mean Model Weighted Standard Deviation

12 Input Data Absolute P and S arrival times -- WEBNET Differential Times (two events, single station) –Catalog differential arrival times –Cross-correlated arrival times Event Locations -- WEBNET –474 events –Magnitude –Initial hypocenter locations range from 7 to 12 km depth –HypoDD - relocated events 3D Velocity Model –Initial Vp model and Vp/Vs (1.70) -- Malek et al., 2000

13 HRC A‘ A All Stations A‘ A HRED A‘ A A VAC A‘ A

14 Outline Introduction Methodology –Double-Difference Tomography –Weighted Average Mean Analysis Results and Interpretation Conclusions

15 Checkerboard Test

16 WAM Model

17 WAM Model

18 Average Velocities Base Model Base Model Base Model Average Model Average Model Average Model

19 P-Velocity –Expect a decrease in fluid-filled and fractured materials –Overpressured conditions may produce a velocity increase (Ito et al., 1979; Popp and Kern, 1993) Vp/Vs ratio: –Sensitive to the presence of fluids –Increases in fractured and fluid-filled materials Wave speeds and fluids

20 Average Velocities Base Model Base Model Base Model Average Model Average Model Average Model

Weise et al., 2001

23 Outline Introduction Methodology –Double-Difference Tomography –Weighted Average Mean Analysis Results and Interpretation Conclusions

24 3D velocity analysis reveals: –Layer of low Vp/Vs ratio values corresponds with the Smrčiny Pluton –May act as a low-permeability fluid trap –High Vp/Vs and P-velocities occur along the fault plane –Correspond with previously identified principal faults –High Vp/Vs values extend to the surface and may reflect fluid pathways

25 Future Work… North – South Principal FaultAcross-Strike

27 Anomaly Restoration

28 Starting Model Tests Slow ModelBase ModelFast Model