Stress inversion from induced seismicity: The 2013 St. Gallen (Switzerland) case.

In 2013 a geothermal drilling project started in an area close to the city of St. Gallen (NE Switzerland). Due to the different phases of the project, a sequence of more than 340 earthquakes was induced with a maximum magnitude ML 3.5 and hypocentral depths ranging between 4.1 and 4.6 km b.s.l. We processed these earthquakes to obtain a dataset of Fault Plane Solutions (FPSs) by jointly inverting the long-period spectral-level $P/S$ ratios and the $P$-wave polarities following a Bayesian approach. The resulting FPSs were processed using the Bayesian Right Trihedra Method in order to derive the stress field variations associated to the different well stimulation phases (from July to October 2013). The results derived for the period July to August 2013, highlights an active stress field dominated by a well-constrained NE low-plunging sigma-3 and a corresponding NW low-plunging sigma-1. The corresponding Bishop ratio confirms the stability of the retrieved attitudes. In September 2013, along with a well cleaning procedure, the stress field seems to change drastically. We discuss the impact of the retrieved stress field on the regional tectonic setting.