Moment tensor inversion for two micro-earthquakes occurred inside the Háje gas storage facilities, Czech Republic.

Broadband data from the Příbram seismological network was used to investigate the source of two earthquakes, with magnitudes Mw=0.2 and Mw=0.4 respectively, occurred in the period October-November 2009 in the Háje natural gas storage area (Czech Republic). Both events were located inside the limits of the storage area and at depths similar to those of the underground caverns where the gas is stored. We applied an inversion technique using the software ISOLA for moment tensor retrieval in order to assess the source process of both events and recognize whether a significant isotropic component existed that could be interpreted as a possible cavern collapse. We also performed an uncertainty analysis so as to confirm the reliability of the focal mechanism solutions and we controlled the consistency between the inverted focal mechanisms and those calculated using the P-waves first-motions. Our results showed that the nodal plane orientation, the centroid depth and the magnitude remained stable. Furthermore, we calculated synthetic waveforms for collapse-type ruptures and compared them with the original records. The match between the synthetic and the original data was very poor supporting the interpretation of the shear character of the events. The combination of the inversion results, which indicated significant double-couple (DC) components, and of the synthetic tests, which supported the inexistence of an isotropic component at the source, led to the conclusion that the possibility of rocks falling from the ceiling of the caverns or a cavern collapse is highly unlikely.