The authors analyzed microseismic events with large magnitude collected during and after hydraulic stimulations at Cooper Basin, Australia in 2003 and at Basel, Switzerland in 2006. The characteristics of the large events and their similarity/difference to the neighboring events were evaluated by hypocentral distribution and source radius, similarity of waveforms, fault plane solution and volumetric strains induced by microseismic events.

We found that some of the large events at Cooper Basin brought clear extension of the seismic cloud into previously seismically silent zones suggesting that some kind of hydraulic barrier was broken by the large events. These events can be interpreted by slip of single asperity in existing fracture. Meanwhile, we revealed that most of the large events from middle and deeper parts of the stimulated zone at Basel, where many hypocenters of the other microseismic events can be seen, occurred from shear slip of multiple asperities. Shallow large events at Basel, of which hypocenters were estimated to be spatially independent from the seismic cloud, are very likely to occur in sub-parallel fracture to the stimulated zone. The mechanism of the large events is not universal and may be depend on site and the triggering mechanism of the large events is remaining unidentified.

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