Václav Vavryčuk, Fateh Bouchaala, Tomáš Fischer Institute of Geophysics, Prague High-resolution fault tomography from accurate locations and focal mechanisms of swarm earthquakes

West-Bohemian earthquake swarm in 2008

Seismicity in West Bohemia, Czech republic

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 Good focal coverage Sampling rate 250 Hz Epicentral distance up to 40 km 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 Data and methods

Locations: map view main active fault 2 km main active fault 4 km

event X1613A Waveform inversion of P waves

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

Fault segmentation: depth sections cross sectionin-plane section 2 km 4 km , 5

Foci clusterings & focal mechanisms cross section 2 km o fault normals from focal mechanisms, + fault normal from clustering of foci

Inversion for stress: Angelier method (2002) SSSC criterion is maximized Principal stress axes  1 and  2 are inclined Fit function σ2σ2 x Optimum stress: σ1σ1 σ3σ3 + o

Definition of the fault stability Mohr’s circle diagram

Fault instability & focal mechanisms σ1σ1 σ3σ3 + o σ2σ2 x 3 4 Fault instability optimally oriented faults misoriented faults 12 Fault instability& fault normals optimally oriented faults

Fault instability & Mohr’s diagrams low shear stress σ3σ3 σ2σ2 σ1σ1 optimally oriented faults optimally oriented faults misoriented faults σ3σ3 σ2σ2 σ1σ1 σ3σ3 σ2σ2 σ1σ1 high shear stress σ1σ1 σ3σ3 + o σ2σ2 x Fault instability Mohr’s diagrams

complex geometry of active faults – rough and curved surface – fault segments with a different orientation clustering of foci almost exactly coincides with focal mechanisms two fault systems are optimally oriented with respect to stress (principal faults) misoriented faults segments with low shear stress are also activated Conclusions main active fault 4 km left-lateral right-lateral σ1σ1