MICROSEISMIC ACTIVITY IN LOW-HAZARD GEOTHERMAL SETTINGS IN SOUTHERN GERMANY

In the last few years several geothermal power plants have taken up production in southern Germany and many more are currently in exploration or construction stages. In addition to providing favorable production conditions, the Bavarian Molasse Basin is being considered as generally aseismic with very sparse and weak seismicity. The seismic hazard going along with production is therefore being considered negligible and in the past no particular efforts for seismic monitoring were made.

In 2008, however, an unexpected Ml 2.3 event south of Munich which was felt by local residents attracted public attention. The event was located in the general vicinity of a geothermal plant that took up production about half a year earlier. In the last two years a temporary network was set up that recorded more than 80 events with magnitudes mainly ranging from Ml -0.5 to 1.5.

Absolute hypocenters are located using the local network. A high resolution 3-D P-wave velocity model is constructed from data of a dense 3-D seismic survey. An S-wave velocity model is compiled from converted shear waves, an old survey with shear wave excitation and cluster analysis of Vp/Vs ratios. Results show the hypocenters close to the bottom of the injection well.

Based on event cluster analysis, hypocenters are relocated applying a nested master-slave relative location technique. The results are compared to structural information on the main fault systems derived from the 3-D seismics data. Source mechanisms are determined for selected events.

Still, the exact extent of the man-made influence on the seismicity remains arguable. Events below magnitude 1.5 could not be detected and located prior to the production stage of the geothermal plant in the main network of the local earthquake service Erdbebendienst Bayern. Questions also remain as to the unique features of this geothermal site in comparison with others not far away that do not produce notable induced seismicity.

Within the framework of the ongoing MAGS project, further field experiments are conducted to address these open questions and test some working hypotheses. At two geothermal projects in comparable settings pre-production data is acquired in local seismometer networks to facilitate a comparison of possible microseismicity during production stages with natural background microseismicity.