The contribution of pore fluid pressure to earthquakes induced in St. Gallen geothermal field, Switzerland

In July 2013, a sequence of more than 340 earthquakes was induced during the deep geothermal drilling project close to the city of St. Gallen in Switzerland. Induced seismicity represents a disadvantage during sub-surface geoenergy operations, so understanding the underlying triggering mechanisms is crucial for mitigating the seismic hazard.  To this end, we investigate the role of fluids and elastic stress transfer as driving mechanisms of the St. Gallen seismic sequence. Following the underlying idea of the Focal Mechanism Tomography technique, we estimate the excess pore fluid pressure at the hypocenters of earthquakes from the analysis of their focal mechanisms. The uncertainties on the focal mechanism parameters, friction coefficient and rock density are taken into account using a Monte Carlo approach to calculate the effect on the estimated excess pore pressure. The results indicate that, in addition to Coulomb static stress change, high-pressure fluids had a primary role in the earthquake triggering. Unlike what is observed in other geothermal fields, the value of the calculated excess pore fluid pressure is higher than the injection pressure for approximately half of the earthquakes. This can likely be attributed to the accidental release of overpressured gas (gas kick) that occurred during field operations when the seal to a gas reservoir was broken.   This work has been supported by PRIN-2017 MATISSE project (No. 20177EPPN2), funded by Italian Ministry of Education and Research.