Moment tensors of the January 1997 earthquake swarm in NW Bohemia (Czech Republic): double-couple vs. non-double-couple events

We investigated moment tensors (MTs) of 70 events of the earthquake swarm which occurred in January 1997 in NW Bohemia. A refined location using the master-event procedure shows that all the foci clustered in a volume of less than 0.5 km³ comprising two compact clusters—the southern and northern ones. The results of single-source, absolute-moment tensor inversion of the P- and SH-peak amplitudes reveal two types of the source mechanisms, A and B in our denotation, which dominated in the swarm. Type A implies an oblique normal faulting with a nearly pure double-couple (DC) source. For the B type, an oblique-thrust faulting and a combined source [double-couple combined with the isotropic (ISO) and compensated linear-vector dipole (CLVD) components] are typical. Magnitudes of the non-double-couple components of MT appear unrelated to the M_L magnitude of the event. The proximity of hypocentres of A and B events guarantees the non-double-couple source mechanisms of the B events not to be an artefact of a mismodelling of the medium. To exclude finiteness of the focus or station-site effects as possible causes of spurious non-double-couple components of MTs of the B events, the residuals of the peak amplitudes across the set of the B events were analysed and the jack-knife test was applied. The A and B events separate in time and space. Consequently, three major phases of swarm activity can be distinguished. In the first, only the southern cluster was active and A events prevailed, while B events dominated in the northern cluster in the third phase. Both A and B events occurred (the former in the southern cluster, the latter in the northern one) during the second phase. The initiation of the B events in the northern cluster are reflected in a pronounced increase in the non-double-couple components of the MTs, which points to tensile-source mechanisms as a consequence of a hypothesised fluid injection.