Seismic Velocities in West Bohemia: Analysis of the Nový Kostel Seismic Zone

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1 Seismic Velocities in West Bohemia: Analysis of the Nový Kostel Seismic Zone Catrina Alexandrakis 1,2 Marco Calò 3, and Vaclav Vavryčuk 2 1 Institute of Geophysics and Geoinformatics, TU BAF 2 Institute of Geophysics, CAS 3 EOST, University of Strasbourg, now at Berkeley Seismological Laboratory AIM fourth annual meeting September 11-14, 2013 Třešť Castle, CZ

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

3 Outline Introduction Methodology Double-Difference Tomography Weighted Average Mean Analysis Synthetic Tests and Resolution Analysis 2008 Swarm 2011 Swarm Interpretation Conclusion 3

4 West Bohemia Seismic Zone 50 40' Greiz Reichenbach WEBNET Stations Zwönitz Lößnitz Schneeberg 50 40' Aue Greiz Reichenbach Auerbach/Vogtland Plauen 50 30' City Zwönitz Schwarzenberg Lößnitz Schneeberg Town Aue Schwarzenberg Auerbach/Vogtland Plauen 50 30' 50 20' Springs Mofettes 2008 Earthquakes 2011 Earthquakes Ostrov 50 20' Chodov Marktredwitz Chodov Karlovy Vary Selb Marktredwitz 50 10' Ostrov Karlovy Vary Selb 50 10' Cheb Cheb 50 00' 50 00' 0 Mariánské Lázne km 0 10 Mariánské Lázne 10 km m 300 m ' 49 50' 12 00' 12 00'12 10' 12 10' 12 30' 12 20'12 20' 12 30' 12 40' 12 40' 12 50' 12 50' 13 00' 13 00' 4

5 West Bohemia Seismic Zone 50 40' Greiz Reichenbach WEBNET Stations Zwönitz Lößnitz Schneeberg 50 40' Aue Greiz Reichenbach Auerbach/Vogtland Plauen 50 30' City Zwönitz Schwarzenberg Lößnitz Schneeberg Town Aue Schwarzenberg Auerbach/Vogtland Plauen 50 30' Springs Mofettes 2008 Earthquakes 2011 Earthquakes C B 50 20' Ostrov 50 20' A Marktredwitz A 50 10' Ostrov Chodov Karlovy Vary Selb Marktredwitz 50 10' Chodov Karlovy Vary Selb Cheb Cheb 50 00' 50 00' 0 B km 0 10 Mariánské Lázne 10 km m 300 C Mariánské Lázne m ' 49 50' 12 00' 12 00'12 10' 12 10' 12 30' 12 20'12 20' 12 30' 12 40' 12 40' 12 50' 12 50' 13 00' 13 00' 4

6 Swarm Triggers Smrčiny (Fichtelgebirge) Pluton Geissler et al., Tectonics, 2005 Babuška and Plomerová, Stud. Geophys. Geod.,

7 Outline Introduction Methodology Double-Difference Tomography Weighted Average Mean Analysis Synthetic Tests and Resolution Analysis 2008 Swarm 2011 Swarm Interpretation Conclusion 6

8 Double-Difference Tomography Based on the double-difference earthquake location algorithm (Waldhauser and Ellsworth, BSSA 2000) dr ij k = (t i k - t j k) obs - (t i k - t j k) cal dr ij k => observed and calculated time residual for events i and j recorded at station k 7

9 Double-Difference Tomography

10 Double-Difference Tomography dr ij k = (t i k - t j k) obs - (t i k - t j k) cal ( Waldhauser and Ellsworth, BSSA 2000) k i j

11 TomoDD (Zhang and Thurber, BSSA, 2003) Input data can include: P and S times Catalog (absolute and differential) times Waveform cross-correlated times Shots, blasts and/or earthquakes Data-types can be weighted differently in each iterations Inverts velocities only in nodes with a defined Derivative Weight Sum (DWS; weighted raypath hit count) 9

12 Catalog and Cross-Correlated Times Without Cross-Correlated dvp (%) Times Vp/Vs With Cross-Correlated Times dvp (%) Vp/Vs

13 TomoDD (Zhang and Thurber, BSSA, 2003) Advantages: Relocates earthquake hypocenters 3D Vp and Vs model of focal zone Disadvantages: No error estimate for larger datasets Starting model parameterization introduces bias and artifacts 11

14 Weighted Average Mean (WAM) Analysis (Calò et al., GJI, 2011) Solution to parameterization artifacts Calculates the Weighted Standard Deviation (WSTD) for the final model Method 1. Define basic model parameters (e.g. velocity model, hypocenters, node locations) 2. Perturb the basic parameters 3. Average models together at each node using DWS as a weighting factor 4. Calculate the standard deviation at each node using DWS as a weighting factor 12

15 Starting Earthquake Locations FastHypo locations provided by WEBNET HypoDD relocations from Bouchaala et al., (J Seismol, 2013) 13

16 Single model compared with WAM model A A A A Weighted Standard Deviation A A 14

17 A A A A HRED All Stations 50 20' VAC HRC 50 10' A A A Cheb km 0 10 m ' 12 10' 12 20' ' 12 40' A

18 Outline Introduction Methodology Double-Difference Tomography Weighted Average Mean Analysis Synthetic Tests and Resolution Analysis 2008 Swarm 2011 Swarm Interpretation Conclusion 16

19 P-Velocity Anomaly Restoration (5%) Input Output A A A A A A A A A A A A 17

20 Vp/Vs Ratio Restoration (~5%) Input Output A A A A A A A A A A A A 18

21 Checkerboard Test 19

22 Overall Resolution Starting Model Dependence - Minor Vp and Vp/Vs Anomalies Vp is influenced by large Vp/Vs perturbations Well recovered with minor smearing WAM checkerboard Focal zone are well recovered with minor smearing Shallow areas (less than 5 km) are not recovered WSTD Values less than 0.02 are well recovered Values greater than 0.03 are not stable 20

23 Outline Introduction Methodology Double-Difference Tomography Weighted Average Mean Analysis Synthetic Tests and Resolution Analysis 2008 Swarm 2011 Swarm Interpretation Conclusion 21

24 WAM Model Across Strike dvp (%) A A Along Strike B B dvp (%) A A Vp/Vs B B Vp/Vs 22

25 WAM Model - Depth Slices 7 km 8 km 9 km dvp (%) 7 km 8 km 9 km Vp/Vs 23

26 Outline Introduction Methodology Double-Difference Tomography Weighted Average Mean Analysis Synthetic Tests and Resolution Analysis 2008 Swarm 2011 Swarm Interpretation Conclusion 24

27 2011 Swarm Data 509 Events - HypoDD relocations (Bouchaala et al., 2013) Travel times - Absolute and differential catalog data P and S arrival times 22 stations 25

28 Tomography Results Along Strike B B Across Strike A A B B A A 26

29 Outline Introduction Methodology Double-Difference Tomography Weighted Average Mean Analysis Synthetic Tests and Resolution Analysis 2008 WAM Results 2011 Swarm - Preliminary Results Interpretation Conclusion 27

30 B B Along Strike B B A A Across Strike A A 28

31 Wave Velocities and Fluids P-Velocity Expect a decrease in fluid-filled and fractured materials However, saturated rocks under overpressured conditions may have a P-velocity increase (e.g. Popp and Kern, Surveys in Geophysics, 1993) Vp/Vs ratio Sensitive to the presence of fluids Increases in fractured and fluid-filled materials 37

32 Comparison with 9HR Profile (N. Mullick, TU Freiberg) Nový Kostel C C Krušné hory and Smrčiny Crystalline Unit Metasediments and Metabasites Granites Weise et al., Tectonophysics,

33 Comparison with 9HR Profile (N. Mullick, TU Freiberg) Nový Kostel C C Krušné hory and Smrčiny Crystalline Unit Metasediments and Metabasites Granites Weise et al., Tectonophysics,

34 Comparison with 9HR Profile (N. Mullick, TU Freiberg) Nový Kostel C C Krušné hory and Smrčiny Crystalline Unit Metasediments and Metabasites Granites Weise et al., Tectonophysics,

35 Outline Introduction Methodology Double-Difference Tomography Weighted Average Mean Analysis Synthetic Tests and Resolution Analysis 2008 WAM Results 2011 Swarm - Preliminary Results Interpretation Conclusion 31

36 Conclusions I Double-difference tomography successfully produces a high-resolution velocity model of the Nový Kostel Seismic Zone Synthetic tests show the focal zone is well resolved WAM method reduces artifacts due to the starting parameterization WSTD analysis is useful for data QC and resolution analysis 32

37 Conclusions II High Vp/Vs occur along the fault plane and correspond with previously identified principal faults May indicate fluid-saturated fractures under overpressured conditions 3D velocity analysis reveals a layer of low Vp/Vs ratio correlate with modelled lithology Lack of ruptures in the low Vp/Vs layer May act as a cap rock over the focal zone 33

Which physics for full-wavefield seismic inversion?

Which physics for full-wavefield seismic inversion? M. Warner* (Imperial College London), J. Morgan (Imperial College London), A. Umpleby (Imperial College London), I. Stekl (Imperial College London) &