- © 2009 by the Seismological Society of America
To stimulate the reservoir for a “hot dry rock” geothermal project initiated by a private/public consortium in the city of Basel, Switzerland, approximately 11,500 m3 of water were injected at high pressures between 2 December and 8 December 2006 into a 5-km-deep well below Kleinhüningen (Häring et al. 2008). A six-sensor borehole array, installed by the operators of the project at depths between 317 and 2,740 meters around the well to monitor the induced seismicity recorded more than 10,500 seismic events during the injection phase. Hypocentral locations could be calculated for more than 3,000 of these events. The gradual increase in flow rate and wellhead pressure was accompanied by a steady increase in seismicity, both in terms of event rates and magnitudes. In the early morning hours of 8 December, after water had been injected at maximum rates in excess of 50 l/s and at wellhead pressures of up to 29.6 MPa for about 16 hours (Häring et al. 2008), a magnitude ML 2.6 event occurred within the reservoir. This exceeded the safety threshold for continued stimulation, so that injection was stopped prematurely. In the afternoon and evening of the same day, two additional events of magnitude ML 2.7 and 3.4 occurred within the same source volume. As a consequence, the well was opened and the water allowed to flow back. In the following days about one third of the injected water volume flowed back out of the well (Häring et al. 2008). Though the seismic activity declined rapidly thereafter, even more than two years later sporadic microseismicity was being detected in the stimulated rock volume by the downhole-instruments.
The mainshock was felt distinctly in the urban area of Basel. People reported short, high-frequency shaking lasting 1–3 seconds, often accompanied by a loud bang similar to an …