Identification of Fracture Orientation for Large Induced Seismicity Recorded at Basel, Switzerland in 2006

摘要

One of the most critical problems in the development and operation of worldwide HDR/HFR/EGS geothermal reservoirs is the occurrence of induced seismicity with moment magnitude (Mw) > 2.0. Seismic activity with large magnitude has been also observed in the hydrothermal geothermal reservoirs. At Basel, Switzerland, events with moment magnitude (Mw) larger than 2.0 occurred in the deeper and middle part of the seismic cloud during and just after a hydraulic stimulation in 2006. Three more Mw>2.0 events occurred in the shallower part of the seismic cloud one to two months after the bleeding-off. Because of the occurrence of these events, the project at Basel was suspended for risk analysis and finally discontinued in 2009. We identified the orientation of fracture planes on which the Mw>2.0 events and other smaller events occurred. We found that there were four types of orientation of fracture planes around at the hypocenter of the Mw>2.0 events. The stress states along these fracture planes were evaluated considering the tectonic stress state around Basel. We found that most of the seismically activated fractures could be interpreted by Coulomb failure criterion for shear slip of pre-existing fractures and that the Mw>2.0 events occurred on one of the conjugate pair of fracture planes.