Abstract

To stimulate the reservoir for a proposed "hot dry rock" geothermal project in the city of Basel, approximately 11500 m3 of water were injected between December 2nd and 8th, 2006, at high pressures into a 5 km deep well. A six-sensor borehole array, installed at depths between 300 and 2700 meters around the well to monitor the induced seismicity, recorded more than 10500 events during the injection phase. Events with magnitudes as low as ML 0.7 were also recorded by regional networks in Switzerland, Germany and France, as well as by up to 30 strong-motion stations installed in the epicentral area. Due to excessive seismic activity, that included an ML 2.7 event, injection had already been stopped when a few hours later an ML 3.4 event jolted the city of Basel. A large amount of (non-structural) damage, corresponding to an intensity of V (EMS98), has been claimed by home owners. The unusual number of damage claims compared to past events in this magnitude range observed in Switzerland is partly due to the shallow hypocenter (5 km) beneath a densely populated city. In addition, ground-motion modelling shows that the radiation pattern of the earthquake source was oriented unfavourably for the city of Basel and that in numerous locations shaking was amplified significantly by local site conditions. After bleed-off, about one third of the injected water volume flowed back out of the well. Although seismic activity declined rapidly thereafter, three additional ML>3 events occurred over the following two months. From a statistical analysis it is expected that it may take on the order of a decade for the activity to decrease to the regional background level scaled to the size of the source region. The hypocenters of the located events (about 3500 to date) are restricted to a NW-SE oriented lens-shaped cloud, about 1 km in diameter and 200 m wide, with a single offsetting branch to the ESE. The orientation of the cloud and the focal mechanisms determined so far match the stress field derived from observations of borehole breakouts and natural seismicity. Due to the premature abortion of the stimulation process, the size of the stimulated volume is insufficient for a commercially viable exploitation of geothermal energy. At present the project is on hold, pending a comprehensive assessment of the seismic risk associated with a continuation.