Characteristics of large-magnitude microseismic events recorded during and after stimulation of a geothermal reservoir at Basel, Switzerland

► We investigated physical characteristics of seismic events with Mw>2.0 at Basel. ► Characteristics of large events were different from their location and origin time. ► Existence of 4 type of significant fracture planes in the reservoir were revealed. ► We found shear slip of the large events was not an unusual phenomenon. ► Critical pore pressure does not correlate with the magnitude of microseismic events. Induced seismicity with large events occurred during and after a hydraulic stimulation at Basel, Switzerland, in 2006. This paper describes a study of the characteristics of the large events (those of moment magnitude greater than 2.0) to understand their origin. The large events during the stimulation and just after bleeding off had hypocenters within the seismic cloud while the large events that occurred several weeks after shut-in were located outside of the seismic cloud. We found no evidence that either local stress concentration or increased pore pressure caused the increase of event magnitudes as no shear slip with extremely high stress drop, or a significant correlation between pore pressure and large event magnitude were identified. Our integrated analysis of the fault plane solution and rock failure mechanism showed unbalanced seismic activity and seismic energy release in the pre-existing fracture system. From these observations we conclude that the large events did not originate from the rupture of rigid asperities triggered by increased pore pressure. Our observations suggest instead that critical changes of the stress state or coefficient of friction on fracture planes during stimulation triggered the unstable shear slip of large events. We also conclude that the characteristics of the large events are dependent on their occurrence times and hypocentral locations.