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Václav Vavryčuk Institute of Geophysics, Prague

Origin of earthquake swarms in West Bohemia inferred from observations of non- doble -couple components in seismic moment tensors. Václav Vavryčuk Institute of Geophysics, Prague. West-Bohemia n earthquake swarm in 2008. Springs of mineral water and emanations of CO2.

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Václav Vavryčuk Institute of Geophysics, Prague

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  1. Origin of earthquake swarms in West Bohemia inferred from observations of non-doble-couple components in seismicmoment tensors Václav Vavryčuk Institute of Geophysics, Prague

  2. West-Bohemian earthquake swarmin 2008

  3. Springs of mineral water and emanations of CO2

  4. Seismicity in West Bohemia, Czech republic

  5. Data and methods • Data • 249 selected micro-earthquakes from the 2008 swarm • Magnitudes between 0.5 – 3.7, depth between 7 and 11 km • 18-22 local short-period seismic stations • Sampling rate 250 Hz • Method • Double-difference location method • P and S wave arrivals obtained using cross-correlation • Frequency-domain waveform inversion for moment tensors • P waves, 1-D smooth model, ray-theoretical Green’s functions

  6. Double-difference locations: map view main active fault main active fault 4 km 2 km Bouchaala, Vavryčuk, Fischer, J. Seismology, 2013 Vavryčuk Bouchaala, Fischer, Tectonophysics, 2013

  7. Examples of focal mechanisms Waveform inversion of P waves good focal sphere coverage, slightly non-DC mechanisms

  8. Variety of focal mechanisms 249 most accurate focal mechanisms Nodal lines P/T axes o P axis, + T axis three basic types of focal mechanisms

  9. Locations & focal mechanisms: map view main active fault main active fault 4 km most frequent focal mechanism 2 km

  10. Tectonic sketch and principal faults P T Mohr’s diagram . first principal fault T EGR P . second principal fault MLF maximum compression coincides with that for western and central Europe 20 km

  11. Focal mechanisms and non-DC components Nodal lines Non-DC components

  12. Focal mechanisms and non-DC components

  13. Shear-tensile source model Positive ISO and CLVD : positive slope angle, fault is opening Negative ISO and CLVD: negative slope angle, fault is closing fault is opening Moment tensors components: ISO – isotropic component CLVD – compensated linear vector dipole Decomposition into ISO, DC and CLVD is after Vavryčuk (2001)

  14. Diamond source-type plot Moment tensors Source tensors

  15. Non-DC components and shear-tensile faulting: 1997 Non-DC components Slip deviation from the fault crack closing crack opening ISO [% ] CLVD [% ] Slope angle [ º ] opening of a fault

  16. Non-DC components and shear-tensile faulting: 2008 Non-DC components Slip deviation from the fault crack closing crack opening closing of a fault, rock compaction

  17. Origins of rock compaction: fluid-rock interaction • Permanent fluid flow • in the Earth’s crust • Hydrothermal alteration • of rocks • Dissolution of minerals • Transport of dissolved • material to the surface • Fault erosion by fluids Borehole picture of open fluid-filled fracture at depth of 111 m The fracture width is 1-2 cm After Heinicke et al. (2009)

  18. Seismic cycles: alternative scenarios Stress accumulation (seismicity at margins of continental plates) Fault weakening (intraplate seismicity)

  19. Swarm triggering main active fault σ1 left-lateral • intraplate seismicity in a geothermal area • with numerous springs • repeated occurrence of earthquake swarms • at the same focal zone (1985/86, 1997, • 1997, 2000, 2008, 20011) • prevailing focal mechanisms are slightly • compressive (negative ISO and CLVD) • swarms are triggered by fault weakening • rather than by stress accumulation 4 km right-lateral Swarm 2008

  20. Karlovy Vary spa Thank you for your attention

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