Seismic Monitoring and Analysis of Deep Geothermal Projects in St Gallen and Basel, Switzerland

kaplan s. robert;

Monitoring and understanding induced seismicity is critical in order to estimate and mitigate seismic risk related to numerous existing and emerging techniques for natural resource exploitation in the shallow-crust. State of the art approaches for guiding decision making, such as traffic light systems, rely heavily on data such as earthquake location and magnitude that are provided to them. In this context we document the monitoring of a deep geothermal energy project in St Gallen, Switzerland. We focus on the issues of earthquake magnitude,
ground motion and macroseismic intensity which are important components of the seismic hazard associated to the project. We highlight the problems with attenuation corrections for magnitude estimation and site amplification that were observed when trying to apply practices used for monitoring regional seismicity to a small-scale monitoring network. Relying on the almost constant source-station distance for events in the geothermal ‘seismic cloud’ we developed a simple procedure, calibrated using several ML > 1.3 events, which allowed the unbiased calculation of ML using only stations of the local monitoring network. The approach determines station specific ML correction terms that account for both the bias of the attenuation correction in the near field and amplification at the site. Since the smallest events (ML < −1) were only observed on a single borehole instrument, a simple relation between the amplitude at the central borehole station of the monitoring network and ML was found. When
compared against magnitudes computed over the whole network this single station approach was shown to provide robust estimates (±0.17 units) for the events down to ML = −1. The relation could then be used to estimate the magnitude of even smaller events (ML < −1) only
recorded on the central borehole station. Using data from almost 2700 events in Switzerland, we then recalibrated the attenuation correction, extending its range of validity from a minimum source–station distance of 20 km down to 1 km. Based on this we could determine the component of the previously derived station specific ML corrections due to local amplification. We analysed ground-motion and detailed macroseismic reports resulting from the 2013 July 20 St
Gallen ML = 3.5 ± 0.1 (Mw = 3.3–3.5 ± 0.1) ‘main shock’ and compared it to a similar ML = 3.4 ± 0.1 event (Mw = 3.2 ± 0.1) that occurred in 2006 at another deep geothermal project in Basel, Switzerland. Differences in ground motion amplitudes between the Basel and St Gallen events and to an extent, the associated macroseismic observations, were investigated in terms of the different source terms: Mw for long-period motions and the source-corner frequency (related to the source rupture velocity and stress-drop) for short periods.