Present-day tectonic Present-day tectonic stress determinedstress determined f from rom
focal mechanismsfocal mechanisms in in the the
seismoactive areaseismoactive area DobrDobrá Vodaá Voda
V. Vavryčuk, V. Vavryčuk, L. FojtíkováL. Fojtíková
Inversion for stress – Inversion for stress – Angelier (2002) Angelier (2002)
AssumptionsAssumptions focal mechanisms for a set of earthquakesfocal mechanisms for a set of earthquakes variety of focal mechanismsvariety of focal mechanisms homogeneous stress in the area under studyhomogeneous stress in the area under study earthquakes occur on pre-existing faultsearthquakes occur on pre-existing faults
MethodMethod invariable of the fault plane identificationinvariable of the fault plane identification
(standard method needs an identification of the fault (standard method needs an identification of the fault plane)plane)
shear stress along the slip is maximized (criterion SSSC) shear stress along the slip is maximized (criterion SSSC) inversion method: grid searchinversion method: grid search
OutputOutput directions of principal stress axes and the shape ratiodirections of principal stress axes and the shape ratio
unable to determine the absolute values of stressunable to determine the absolute values of stress
31
21
R
SSSC criterionSSSC criterion
TT slip shear stress component (SSSC) slip shear stress component (SSSC)
TT is maximized during the inversion is maximized during the inversion
n
S
jiij snT
sn
n – fault normal
s – slip vector
τ – stress tensor
Input dataInput dataFocal mechanisms calculated using 3 Focal mechanisms calculated using 3
methods:methods:
FOCMEC codeFOCMEC code inversion of P-wave polarities of Pg and Pn wavesinversion of P-wave polarities of Pg and Pn waves
polarities measured at 8 – 20 short-period or broad-band stations polarities measured at 8 – 20 short-period or broad-band stations epicentral distance up to 200 kmepicentral 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 stationsamplitudes measured at 7-20 stations
high-frequency waveshigh-frequency waves epicentral distance up to 80 kmepicentral distance up to 80 km
ISOLA codeISOLA code inversion of S waves and local surface wavesinversion of S waves and local surface waveswaveforms of 3 – 5 local short-period stationswaveforms of 3 – 5 local short-period stationsfull waveforms filtered for low frequencies (0.8-1.6 Hz) full waveforms filtered for low frequencies (0.8-1.6 Hz) epicentral distance up to 20 kmepicentral distance up to 20 km
FFocal mechanismsocal mechanismsthe most accurate focal mechanisms are selected
FOCMEC N = 16
AMTN= 14
ISOLAN= 15
InInversion for stressversion for stressMethod of Method of Angelier (2002)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
Mohr’s diagramsMohr’s diagramsFOCMEC AMT ISOLA
σ1σ2σ3 σ1σ2σ3 σ1σ2σ3
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 mechanismsAMT focal mechanismsFault planes
friction = 0.5 cohesion = 0.2pore pressure = -0.4
σ1=210/25°, 2=78°/55°, σ3=311°/23°
R=0.61
Mohr’s diagram
σ1σ2σ3
Mohr-Coulomb failurecriterion
Synthetic focal Synthetic focal mechanismsmechanisms
shape ratio = 0.61friction = 0.5 cohesion = 0.2pore pressure = -0.4
shape ratio = 0.61friction = 0.5 cohesion = 0.2pore pressure = -0.83
Generic focal mechanisms
250 focal mechanisms satisfying the Mohr-Coulomb criterion
generic faults
Active versus generic Active versus generic faultsfaults
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 Comparison with tectonicstectonics
Generic and active faults
Simplified tectonic map
maximumcompression
Tectonic stress Tectonic stress measurementsmeasurements
(Heidbach et al., 2008)
Tectonic stress in EuropeTectonic stress in Europe
(Bada et al., 2007)
Pannonian basin area
Smoothed SH max Smoothed SH max directionsdirections
(Gerner et al., 1999; Bada et al. 1998)Pannonian basin
Stress measurements Stress measurements versus smoothed SH max versus smoothed SH max
directionsdirections
(Bada et al., 2007)
Recent stress trajectoriesRecent stress trajectories
(Cloethingh et al., 2002)
Recent stress trajectoriesRecent stress trajectories
(Cloethingh et al., 2002)
Correct direction
SummarySummary The most accurate focal mechanisms are The most accurate focal mechanisms are
obtained using the AMT codeobtained using the AMT code Stress inversion for different datasets is stable Stress inversion for different datasets is stable
and yields consistent resultsand yields consistent results The calculated fault planes concentrate in a The calculated fault planes concentrate in a
region of validity of Mohr-Coulomb criterionregion of validity of Mohr-Coulomb criterion Two differently oriented fault systems are activeTwo differently oriented fault systems are active DobrDobrá Voda fault and Brezová fault are generic á Voda fault and Brezová fault are generic
faults in the regionfaults in the region Maximum compression is along the strike of the Maximum compression is along the strike of the
Little Carpathians Mts.Little Carpathians Mts.
SummarySummary The most accurate focal mechanisms are The most accurate focal mechanisms are
obtained using the AMT codeobtained using the AMT code Stress inversion for different datasets is stable Stress inversion for different datasets is stable
and yields consistent resultsand yields consistent results The calculated fault planes concentrate in a The calculated fault planes concentrate in a
region of validity of Mohr-Coulomb criterionregion of validity of Mohr-Coulomb criterion Two differently oriented fault systems are activeTwo differently oriented fault systems are active DobrDobrá Voda fault and Brezová fault are generic á Voda fault and Brezová fault are generic
faults in the regionfaults in the region Maximum compression is along the strike of Maximum compression is along the strike of
the Little Carpathians Mts.(!)the Little Carpathians Mts.(!)
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