Pore Fluid Pressure in St. Gallen Geothermal Field (Switzerland) Based on Earthquake Focal Mechanisms

Induced seismicity represents a negative drawback during subsurface exploitation for geothermal energy production. Understanding the triggering mechanisms of induced earthquakes can help implement effective seismic hazard mitigation actions. Among the triggering mechanisms, the pore fluid pressure is of primary importance. Here we provide a static picture of the excess pore fluid pressure at the hypocenters of a seismic sequence induced at the deep geothermal field in St. Gallen, Switzerland, in July 2013. We find that in addition to the Coulomb static stress change, fluids play a key role in promoting the sequence. The estimated excess pore fluid pressure for approximately half of the earthquakes is higher than the injection pressure necessary during the well control phase to fight the unexpected gas kick, that accidently occurred during field operations when a trap of overpressured gas was broken. In July 2013, a sequence of more than 340 earthquakes was induced during the exploitation of the subsurface for energy production in St. Gallen geothermal field (Switzerland). To understand the mechanisms underlying the evolution of the sequence, we investigated the role of fluids and elastic stress transfer. The excess pore fluid pressure measurements suggest that the main triggering mechanism is related to high-pressure fluids. The high values of the pore fluid pressure may be due to an already existing in situ overpressure condition from which a documented unexpected gas kick occurred. The main earthquake triggering mechanism is the effect of high-pressure fluids The high pore fluid pressure values may be due to an already existing in situ overpressure condition