Seismic monitoring and analysis of deep geothermal projects in St Gallen and Basel, Switzerland

Abstract

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 M L > 1.3 events, which allowed the unbiased calculation of M L using only stations of the local monitoring network. The approach determines station specific M L 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 (M L < −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 M L 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 M L = −1. The relation could then be used to estimate the magnitude of even smaller events (M L < −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 M L corrections due to local amplification. We analysed ground-motion and detailed macroseismic reports resulting from the 2013 July 20 St Gallen M L = 3.5 ± 0.1 (M w = 3.3-3.5 ± 0.1) 'main shock' and compared it to a similar M L = 3.4 ± 0.1 event (M w = 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: M w for long-period motions and the source-corner frequency (related to the source rupture velocity and stress-drop) for short periods.

A Hybrid Empirical Green’s Function Technique for Predicting Ground Motion from Induced Seismicity: Application to the Basel Enhanced Geothermal System

“…The larger event has been studied in detail and was shown to have M = 3.2 [7]. The events have all been precisely relocated using cross-correlation approaches [51,52]; 54 small events (M L ≤ 2.7, Figure 1) occurring prior to the mainshock and located in the seismic cloud of induced earthquakes were used to provide an estimate of the shaking expected for the M = 3.2 event.…”

“…Deep geothermal techniques have been trialed across Europe, with notable examples in Switzerland (Basel and St. Gallen). However, induced seismicity has proven to be a significant challenge [7,8].…”

“…Geothermal energy, geological CO 2 storage, and shale hydrocarbon extraction have all led to induced seismicity, including cases that have been documented as being felt [7,9,10] and even damaging [8,11]. The process of shale hydrocarbon extraction (and the associated injection of waste water) and geothermal energy production has therefore come under intense scrutiny and opposition.…”

“…These motions depend not only on the earthquake source magnitude, but on how energetic it is (earthquakes of equal magnitude can be more, or less, energetic as a result of their rupture characteristics), wavefield propagation effects (focussing or defocussing of waves in particular directions), and site amplification effects (different soils can have significantly different effects on the shaking level [19]). Recent work has also shown how M L is typically overestimated at the very short distances typical in these applications [7,20]. In order to gain the public's trust, operators must therefore openly convey the expected ground shaking that can be expected, rather than simply source magnitudes.…”

Characterizing the complexity of seismic signals at slow-moving clay-rich debris slides: The Super-Sauze (Southeastern France) and Pechgraben (Upper Austria) case studies

“…The distance attenuation function of the ML scale has been calibrated empirically for many regions around the world (e.g. Bakun and Joyner, 1984;Hutton and Boore, 1987;Stange, 2006;Edwards et al, 2015); however, standard calibrated distances range within 10-1,000 km (Fig. 11a).…”