A Simple Model for Probabilistic Seismic Hazard Analysis of Induced Seismicity Associated with Deep Geothermal Systems

Abstract In the research project MAGS (Microseismic Activity of Geothermal Systems) a simple model was developed to determine seismic hazard as the annual frequency of the exceedance of ground motion of a certain size. Such estimates of the annual frequency of exceedance of prescriptive limits for the effects of vibrations on buildings and people are needed for the planning and licensing, but likewise for the development and operation of deep geothermal systems. For the development of the proposed model well established probabilistic seismic hazard analysis methods for the estimation of the hazard for the case of natural seismicity were adapted to the case of induced seismicity. Important differences between induced and natural seismicity had to be considered. These included significantly smaller magnitudes, depths and source to site distances of the seismic events. Hence, different ground motion prediction equations (GMPE) had to be incorporated to account for the seismic amplitude attenuation with distance, as well as for differences in the stationarity of the underlying tectonic and induced processes. Appropriate GMPEs in terms of peak ground velocity (PGV) were tested and selected from the literature. In the paper we present probabilistic seismic hazard analysis (PSHA) results for ground motion which can be linked to engineering regulations (e.g. German DIN 4150). It is thus possible to specify the probability of exceedance of prescriptive standard values and to decide (e.g. by the regulator responsible for commissioning) whether the annual number of exceedances for a site at a given level is acceptable or not. Additionally, hazard curves for induced and natural seismicity are compared to study the different impact at a site. Preliminary results for the circulation period (operation phase of the plant) at a geothermal site in Bavarian Molasse, Germany - for stiff soil, ignoring site effects - indicate higher frequencies of exceedance for induced seismicity than for natural seismicity only for low PGV values.