Non-double-couple earthquakes of 1997 January in West Bohemia, Czech Republic: Evidence of tensile faulting

A mixed family of double-couple (DC) and non-double-couple (non-DC) earthquakes was observed during the 1997 January earthquake swarm in West Bohemia, which is a geothermal region typical for the occurrence of earthquakes swarms. The DC and non-DC earthquakes occurred within a small area of less than 1 km(3) on faults of different orientations. The stress in the area is determined from focal mechanisms and attains values (plunge/azimuth): sigma(1) = 33degrees/156degrees, sigma(2) = 48degrees/20degrees, sigma(3) = 23degrees/262degrees, with a shape ratio of R = 0.69 +/- 0.15. The error in the stress directions (plunge/azimuth) is up to similar to5degrees/10degrees. The absolute stress values were estimated to be sigma(1) = 350 MPa, sigma(2) = 220 MPa and sigma(3) = 162 MPa. The lithostatic stress is 250 MPa. The stress analysis shows that the DC earthquakes occurred on optimally oriented faults (with high shear traction) but the non-DC earthquakes occurred on misoriented faults (with low shear traction). The percentage of the DC in earthquakes clearly correlates with the magnitude of shear traction: high/low DC is associated with high/low shear traction. This indicates that the type of faulting is probably controlled by tectonic stress and that the non-DC earthquakes are not related to pure shear faulting but may combine shear and tensile faulting. The tensile faulting is manifested by positive isotropic and compensated linear vector dipole components. The tensile faulting is conditioned by high pore fluid pressure, which was estimated to be approximately 244 MPa. The fluid pressure is less than the lithostatic stress by similar to5 MPa. The ratio of the fluid pressure to the maximum compressive stress is 0.7.