Characterization of microseismic events with larger magnitude collected at Basel, Switzerland in 2006

The authors analyzed microseismic events with large magnitude collected during and after the hydraulic stimulation at Basel in 2006. In this study, we firstly investigated possible physical models that could have been responsible mechanisms for the large events from seismological and rock mechanical points of view, and we listed the expected observational phenomena of each. The characteristics of the large events and their similarity/difference to the neighboring events were then evaluated considering the models. From this analysis is appears that the most of the large events from the middle and deeper parts of the stimulated zone can be interpreted in terms of slip of multiple asperities inside the stimulated zone. Although there are uncertainties in the hypocentral locations for the large events from shallower parts of the stimulated zone, they also might be interpreted by slips of multiple asperities in neighboring subparallel fractures to the stimulated zone. The large events observed at Basel can be interpreted as a kind of induced microseismicity either inside or neighboring the stimulated zone, although the triggering mechanism of the large events after long interval from the bleeding-off should be investigated further.