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V. Vavryčuk, L. Fojtíková

Present-day tectonic stress determined f rom focal mechanisms in the seismoactive area Dobr á Voda. V. Vavryčuk, L. Fojtíková. Inversion for stress – Angelier (2002). Assumptions focal mechanisms for a set of earthquakes variety of focal mechanisms

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V. Vavryčuk, L. Fojtíková

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  1. Present-day tectonic stress determined from focal mechanismsin the seismoactive areaDobrá Voda V. Vavryčuk, L. Fojtíková

  2. Inversion for stress – Angelier (2002) Assumptions • focal mechanisms for a set of earthquakes • variety of focal mechanisms • homogeneous stress in the area under study • earthquakes occur on pre-existing faults Method • invariable of the fault plane identification (standard method needs an identification of the fault plane) • shear stress along the slip is maximized (criterion SSSC) • inversion method: grid search Output • directions of principal stress axes and the shape ratio • unable to determine the absolute values of stress

  3. n S SSSC criterion T slip shear stress component (SSSC) T is maximized during the inversion n – fault normal s – slip vector τ– stress tensor

  4. Input data Focal mechanisms calculated using 3 methods: • FOCMEC code inversion of P-wave polarities of Pg and Pn waves polarities measured at 8 – 20 short-period or broad-band stations epicentral distance up to 200 km • AMT code inversion of vertical amplitudes of local P waves amplitudes measured at 7-20 stations high-frequency waves epicentral distance up to 80 km • ISOLA code inversion of S waves and local surface waves waveforms of 3 – 5 local short-period stations full waveforms filtered for low frequencies (0.8-1.6 Hz) epicentral distance up to 20 km

  5. Focal mechanisms the most accurate focal mechanisms are selected FOCMEC N = 16 AMT N= 14 ISOLA N= 15

  6. Inversion for stress Method of Angelier (2002) SSSC is maximized σ1=210–220°/5–25° σ2=70–105°/55–75° σ3=305–315°/15–25° R=0.45–0.60

  7. Mohr’s diagrams ISOLA FOCMEC AMT σ3 σ2 σ1 σ3 σ2 σ1 σ3 σ2 σ1 the instability criterion is used when selecting the fault plane from two alternatives • the majority of solutions concentrate in a region of validity of the • Mohr’s- Coulomb criterion! • the minimum scatter display the AMT focal mechanisms

  8. AMT focal mechanisms Mohr’s diagram Fault planes Mohr-Coulomb failure criterion σ3 σ2 σ1 friction = 0.5 cohesion = 0.2 pore pressure = -0.4 σ1=210/25°, 2=78°/55°, σ3=311°/23° R=0.61

  9. Synthetic focal mechanisms 250 focal mechanisms satisfying the Mohr-Coulomb criterion shape ratio = 0.61 friction = 0.5 cohesion = 0.2 pore pressure = -0.4 Generic focal mechanisms generic faults shape ratio = 0.61 friction = 0.5 cohesion = 0.2 pore pressure = -0.83

  10. Active versus generic faults • Active fault planes • focal mechanisms using P-wave amplitudes (program AMT) • calculation of the instability of the nodal planes • fault plane – the nodal plane with a higher instability N • Generic fault planes • theoretical planes • the most unstable fault planes under tectonic stress • fault planes with the highest probability to be active generic faults

  11. Comparison with tectonics Simplified tectonic map Generic and active faults maximum compression

  12. Tectonic stress measurements (Heidbach et al., 2008)

  13. Tectonic stress in Europe Pannonian basin area (Bada et al., 2007)

  14. Smoothed SH max directions (Gerner et al., 1999; Bada et al. 1998) Pannonian basin

  15. Stress measurements versus smoothed SH max directions (Bada et al., 2007)

  16. Recent stress trajectories (Cloethingh et al., 2002)

  17. Recent stress trajectories Correct direction (Cloethingh et al., 2002)

  18. Summary • The most accurate focal mechanisms are obtained using the AMT code • Stress inversion for different datasets is stable and yields consistent results • The calculated fault planes concentrate in a region of validity of Mohr-Coulomb criterion • Two differently oriented fault systems are active • Dobrá Voda fault and Brezová fault are generic faults in the region • Maximum compression is along the strike of the Little Carpathians Mts.

  19. Summary • The most accurate focal mechanisms are obtained using the AMT code • Stress inversion for different datasets is stable and yields consistent results • The calculated fault planes concentrate in a region of validity of Mohr-Coulomb criterion • Two differently oriented fault systems are active • Dobrá Voda fault and Brezová fault are generic faults in the region • Maximum compression is along the strike of the Little Carpathians Mts.(!)

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