Kinematic and Dynamic Source Parameters of Induced Earthquakes at St. Gallen Geothermal Field, Switzerland

The exploitation of the St. Gallen geothermal reservoir in Switzerland induced a seismic sequence lasting from July 2013 to October 2013. The sequence was characterized by 346 earthquakes located at a depth ranging between 4.4 and 4.7 km, having magnitude ranging between (-1.2, 3.5). We study the seismic source properties of the induced earthquakes by implementing both the individual earthquake displacement spectral inversion and an empirical Green's function-based method that allows to reduce the trade-off between source parameters and anelastic attenuation. We find a scaling between corner frequency and seismic moment down to 5 x 10(10) Nm that differs from the generally assumed M-0 proportional to f(c)(-3). The retrieved average value of the static stress drop (3.8 MPa) is close to the median global value (4 MPa) observed for tectonic earthquakes and it is a small fraction (0.1%-20%) of the shear stress resolved on the fault planes. Moreover, we find that the analyzed events are characterized by low Savage-Wood efficiency indicating a positive dynamic overshoot suggesting that only a small fraction of the available strain energy is radiated seismically.