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

West-Bohemian earthquake swarm in 2008

Springs of mineral water and emanations of CO2

Seismicity in West Bohemia, Czech republic

Data and methods Data 249 selected micro-earthquakes from the 2008 swarm Magnitudes between 0.5 – 3.7, depth between 7 and 11 km 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

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

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

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

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

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

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

Focal mechanisms and non-DC components

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)

Diamond source-type plot Moment tensorsSource tensors

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

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

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)

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

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 Swarm triggering main active fault 4 km left-lateral right-lateral σ1σ1 Swarm 2008

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