Geoscience Reference
In-Depth Information
BOX 3.3
Induced Seismic Activity in Basel, Switzerland
Basel, Switzerland, is in the southeastern region of the Upper Rhine Graben, a fault-bounded trough, and
was selected as the site of a planned geothermal cogeneration plant. Basel is known to be an area of potential
seismic risk but had not suffered a damaging earthquake since a M 6.2 earthquake in 1356 that destroyed
much of the city. Due to awareness of historical seismicity, the geothermal project operators and planners had
installed both borehole and surface seismic sensors that formed a network for monitoring any seismicity, whether
natural or induced. The monitoring efforts included the drilling of six monitoring wells, ranging in depth from
300 m (~980 feet) to 2,750 m (~9,000 feet) in addition to a surface array of both weak and strong motion
detectors. Recording of seismic activity began in early 2006 to record background seismicity.
The seismic monitoring arrays served several purposes. They recorded the background seismicity before
well stimulation began and they were used to monitor the fracturing of the geothermal reservoir (the objective
of the stimulation). Finally they could provide information (magnitude and location if possible) of any induced
seismicity that might occur as a result of the stimulation. All monitoring stations were connected so that real-time
data could be recorded and quickly analyzed.
The drilling of a deep geothermal well near the center of Basel (Figure 1) began in May 2006 and was
completed some months later. Stimulation of the well to induce fractures for heat exchange with the geothermal
source at 5,000 m (~16,400 feet) began on December 2 and was accompanied by a significant increase in
the number of small seismic events (Figure 2). In accordance with the traffic light procedure—a procedure
where increases in seismic activity beyond a certain, predetermined level trigger reactions by the operator to
Figure 1 Drilling activity in the middle of the city of Basel. SOURCE: KEYSTONE/Georgios Kefalas.
 
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