Migration-based microseismic event location in the Schlema-Alberoda mining area

Mining operations in the Schlema-Alberoda uranium mine located in south-west Saxony (Germany) stopped in 1990, and the mine was then allowed to flood. The region is seismically active and has been monitored for a long time. We have located some of the microseismic events which occurred between 2005 and 2012 in the mining area to investigate the nature of the seismicity, as well as the long-term role of mining-induced and triggered seismicity. In this study, we used pseudo-synthetic data obtained from the vertical component records of a local seismic network, to accurately locate the hypocenters. The location procedure is based on a Kirchhoff Prestack Depth Migration approach. In order to assess the accuracy of the migration results, a homogeneous and a 3-D velocity model are tested in the migration algorithm. Both of these velocity models were previously used to migrate data acquired by a 3-D reflection seismic survey in this area. The resulting images were comparable, with minor differences to the reflectors in our area of interest. In contrast, we found that the errors in the velocity model can strongly affect the migration-based earthquake localization. The located hypocenters are compared to the 3-D reflection seismic image of the area to determine possible correlations between the seismicity and reflectivity. This comparison shows that some of reflectors can be directly correlated with the induced and triggered seismicity of the area. The results also indicate that the main causes of the post-mining seismicity are changes in the mine flooding level and the consequent changes in the pore pressure of the subsurface structures. Finally, we confirm that the hypocenters are often bound to preexisting subsurface structures like faults and fractures in the area. In particular, the spatial pattern of the hypocenters proves the existence of a previously unmapped extension of the Ruhmvoll fault. This pattern also suggests a connection between this structure and the Roter Kamm fault, a major component of the larger Gera-Jàchymov fault zone.