Near-real-time high-precision relocation of induced seismicity in the geothermal system below Sankt Gallen (Switzerland)
Abstract
In July 2013 a sequence of more than 600 detected events, of which more than 225 could be located, was triggered during stimulation tests in a planed geothermal system below the city of Sankt Gallen in eastern Switzerland. Seismicity initiated on July 14 and continued over a period of two weeks. The maximum local magnitude in the sequence was 3.5 comparable in size with the Ml 3.4 event induced by stimulation below Basel in 2006. A detailed understanding of the underlying mechanisms of this exceptional sequence requires the precise imaging of the active faults and their spatio-temporal distribution. Additional information on regional fault structures, their spatial extent, previous relevant local seismicity, the regional stress field and its relationship to the sequence are each crucial for an immediate hazard assessment. In this work we apply waveform cross-correlation and double-difference algorithms to improve the relative locations of the induced seismicity in the Sankt Gallen geothermal system. Semi-automated procedures allowed the precise relocation of seismicity shortly after the Ml 3.5 event. First results image a distinct NE-SW striking lineament in the relocated seismicity, consistent with a left-lateral fault plane derived from the analysis of first motion polarities and a regional moment tensor inversion. The seismicity is limited to a 0.8 km long band, in a depth of 4 to 5 km. The distribution of focal depths with respect to the basement is of particular interest. To resolve the rupture extent and to decide whether or not the rupture was limited to the sedimentary layers, to the basement, or affected both, the absolute position of the induced seismicity is further constrained. Local earthquake tomography is used to derive a minimum 1D model of the region around the geothermal system. The model is then used to improve the initial locations for the double-difference inversion and to image the velocity structure of the uppermost crust. Additional information on focal depths is derived from the analysis of waveforms at local and regional distances. To verify whether the fault structures associated with the 2013 sequence had been (spontaneously) active in the past, we re-evaluate instrumentally recorded seismicity and available focal mechanisms in the region around the geothermal system. High similarity of waveforms observed at common stations suggests that an Ml 3.2 earthquake in 1987 and an Ml 2.2 earthquake in 1993 occurred on a similar structure with a similar slip direction as the induced Ml 3.5 earthquake of July 2013. It appears that the fault zone targeted by the geothermal project is not only oriented favourably for rupture relative to the regional stress field, but is also close to failure. This suggests that even in regions of low seismic hazard, such as Sankt Gallen, differential stresses in the upper crust can be close to a critical level. For seismic monitoring of future geothermal systems we work towards fully automated double-difference procedures allowing precise relocation of seismicity in near-real-time.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.S33D2465D
- Keywords:
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- 7230 SEISMOLOGY Seismicity and tectonics;
- 7215 SEISMOLOGY Earthquake source observations;
- 7200 SEISMOLOGY;
- 8164 TECTONOPHYSICS Stresses: crust and lithosphere