Focal mechanisms of micro-earthquakes in the Dobrá Voda seismoactive area in the Malé Karpaty Mts. (Little Carpathians), Slovakia

We have analyzed 44 micro-earthquakes with magnitudes between 1.2 and 3.4, which occurred in the Dobrá Voda area, Slovakia, in the period 2001–2009. The epicentres of the micro-earthquakes form a cluster elongated in the ENE–WSW direction. This direction coincides with the orientation of the main fault systems in the area: Dobrá Voda and Brezová faults. The depths of the hypocentres vary from 1km to 14km. Three different methods were used to calculate the focal mechanisms: (a) a method using the polarities of Pg and Pn waves, (b) the P-wave amplitude inversion of moment tensors, and (c) the waveform inversion of moment tensors. The majority of the analyzed micro-earthquakes have a left-lateral strike-slip focal mechanism with weak normal or reverse components. The full moment tensors comprise significant non-double-couple (non-DC) components. The non-DC components are partly numerical errors of the inversion but might be also of a physical origin. The most accurate values of the non-DC components are obtained from the P-wave amplitude inversion. For this inversion, the isotropic component (ISO) and the compensated linear vector dipole component (CLVD) are mostly positive and well correlated. This might indicate tensile faulting. Adopting the model of tensile faulting, we estimated the mean ratio of P to S wave velocities in the focal area from the values of ISO and CLVD, vP/vS=1.5–1.6. The three different datasets of the focal mechanisms have been inverted for the present-day tectonic stress in the Dobrá Voda area. The slip shear stress component criterion was applied in the stress inversion. The results of the three inversions are well-consistent and point to a high reliability and good accuracy of the inverted stress. The orientations of the principal stresses are (azimuth/plunge): σ1=210–220°/5–25°, σ2=70–105°/55–75°, and σ3=305–315°/15–25°, and the shape ratio is R=0.45–0.60. The azimuth is measured clockwise from the north and the plunge downwards from the horizontal plane. The retrieved maximum compression lies along the belt of the Malé Karpaty Mts. The local tectonic stress reflects complex tectonic conditions in the area. The presence of tensile faulting might point to an extensional stress regime in the area.