Microseismicity observed at a non-pressure-stimulated geothermal power plant

• We observe tightly clustered seismicity at a non-pressure stimulated geothermal plant.
• Hypocenters and focal mechanisms associate to a known fault penetrated by the well.
• No obvious correlation of seismicity and operational parameters could be identified.
• Seismic monitoring needs to be realized even in low hazard and hydrothermal settings.

In the North Alpine Foreland Basin, geothermal plants exploit a deep natural aquifer. Considering the low seismic hazard in this region, the non-pressure-stimulated, hydrothermal usage of this aquifer was generally assumed to be unproblematic with regard to induced seismicity. However, a series of five, partly felt shallow magnitude Ml > 2 earthquakes at a geothermal power plant near Munich, Germany, lead us to conduct a detailed study of the local microseismicity.Here, we present results from two years of data acquired with a local five station seismic network. More than 130 events have been detected with local magnitudes ranging from −0.8 to 2.4 and a magnitude of completeness of around 0. Absolute locations are calculated in a 3D velocity model and are improved by double-difference and Master-Event techniques. Hypocenters are located in basement rock underlying the geothermal target horizon, and in combination with estimates of focal mechanisms, they can be associated to existing fault zones pierced by the geothermal well.This study shows that even non-pressure-stimulated geothermal plants with good hydraulic parameters operated hydrothermally at low injection pressures are able to generate seismicity even in low seismic hazard settings, and has led to a change in paradigm for the demand for seismic monitoring at upcoming geothermal facilities in this area.