Estimation of source parameter of microseismic events with large magnitude collected at Basel, Switzerland in 2006

Occurrence of microseismic events with large magnitude, which are felt on the surface, has been recently receiving much attention as one of the practical problems in stimulation/production of HDR/HFR/EGS geothermal reservoirs. The microseismic activities with large magnitude have been also reported from some of the hydrothermal geothermal fields. Microseismic events with moment magnitude Mw exceeded 2.0 occurred in deeper and middle part of the microseismic cloud during and just after a hydraulic stimulation at Basel, Switzerland, in 2006. Three more large events (Mw>2.0) occurred in shallower part of the seismic cloud by within two months from the bleeding-off. Because of the occurrence of the large events, the project at Basel has been terminated. The authors estimated some of the seismic source parameters (fault plane solution, seismic moment, size of ruptured area, average slip along fault, and stress drop) of the large events to understand the nature of the shear slip on fractures. Spatial distribution of the hypocenter and the fault plane solution (FPS) showed that most of the FPS of the large events had nearly N-S azimuth and those for small events had NW-SE azimuth. Most of the estimated stress drops were less than 1.0MPa, while some of the large events had larger stress drop. Events with higher stress drop (>1.0MPa) showed a linear relationship between the seismic moment and the ruptured area, suggesting that "scaling law" in natural seismology stands for this data set. Observational facts indicated that control factor of the magnitude is not simply either the size of the fault area or the stress drop.