Unravelling kinematic source parameters of induced earthquakes at St Gallen geothermal field, Switzerland

On 14 July 2013, a deep geothermal drilling project close to the city of St Gallen, in Switzerland was initiated with the aim of producing electricity and heating. The target of the project was a fractured carbonate aquifer at a depth of 4 km b.s.l.. The project started with a stimulation phase on 14 July 2013 that produced only microseismicity. From 14 July 2013 through 19 July 2013 acid stimulations involving about 290 m³ of fluids broke the seal to a gas reservoir and caused a gas kick. Afterwards, well control operations were done by injecting ~700 m³ of water and heavier liquids, which probably induced the largest event in the sequence with M_L 3.5. Well-control operations ended on 24 July 2013. In September 2013 fishing operations (that is removing lost or stuck objects from the wellbore) were done together with a cleaning of the well. A total of 346 earthquakes were recorded from July 2013 to October 2013 at the seismic network managed by the Swiss Seismological Service (SED). The events occurred at depths up to 4.2 km with M_L between -1.2 and 3.5.

We infer seismic source parameters by using both a single-event approach and an empirical Green’s function (EGF) approach. This latter method helps to limit the spectral source parameters trade-off and to infer source parameters for the smaller earthquakes (M_L ≤ 2.0). In fact, performing the ratio between the spectra of co-located events recorded at the same stations allows to remove propagation and site effects. For the larger events (M_L>2.0), we use an iterative single-event approach where Q is fixed to avoid the trade-off with f_c. Specifically, the Q_P and Q_S values are obtained from the slope of the displacement spectra - at frequencies lower than the expected corner frequency - of the smaller events. In order to apply the EGF approach, for each of the smaller earthquakes, we select among the larger events those for which the cross-correlation in the time domain is higher than a given threshold. Next, for the selected couples we compute the spectral ratio and infer the seismic moments' ratio and the corner frequency ratios by using a grid-searching technique. We find self-similar parameters scaling down to Mo=10¹⁰ Nm and an average static stress drop of 0.1 MPa. The mean value of seismic efficiency, estimated from the average apparent stress to static stress drop ratios is 0.05 suggesting an overshoot dynamic weakening mechanism.

This work has been supported by PRIN-2017 MATISSE project (No. 20177EPPN2), funded by Italian Ministry of Education and Research.