Present-day tectonic stress determined from focal mechanisms in the seismoactive area Dobrá Voda V. Vavryčuk, L. Fojtíková

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

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

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

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

Inversion for stress Method of Angelier (2002) SSSC is maximized σ 1 = 210–220°/5–25° σ 2 = 70–105°/55–75° σ 3 = 305–315°/15–25° R = 0.45–0.60

Mohr’s diagrams FOCMEC AMT ISOLA σ1σ1 σ2σ2 σ3σ3 σ1σ1 σ2σ2 σ3σ3 σ1σ1 σ2σ2 σ3σ3 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

AMT focal mechanisms Fault planes friction = 0.5 cohesion = 0.2 pore pressure = -0.4 σ 1 =210/25°, 2 =78°/55°, σ 3 =311°/23° R=0.61 Mohr’s diagram σ1σ1 σ2σ2 σ3σ3 Mohr-Coulomb failure criterion

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

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 Generic fault planes theoretical planes the most unstable fault planes under tectonic stress fault planes with the highest probability to be active N generic faults

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

Tectonic stress measurements (Heidbach et al., 2008)

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

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

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

Recent stress trajectories (Cloethingh et al., 2002)

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

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

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