The three‐year CO2CO2 injection period at the Illinois Basin ‐ Decatur Project site (Decatur, Illinois, United States) produced a number of microseismic events distributed in very distinct spatiotemporal clusters with different orientations.

Further characterization of the microseismicity encompasses the determination of the event source mechanisms. Initially, the microseismic monitoring network consisted solely of borehole sensors, but has been extended with surface sensors, thereby significantly improving the data coverage over the focal sphere. This article focuses on 23 events from the northernmost microseismic cluster (about 2 km from the injection point) and takes advantage of both, surface and downhole, recordings. The resulting strike‐slip east–west‐oriented focal planes are all consistent with the east–west orientation of the cluster in map view.

The injection‐related increase of pore pressure is far below the formation fracture pressure; however, small stress‐field changes associated with the pore‐pressure increase may reach as far as to the investigated cluster location. Monte Carlo modeling of the slip reactivation potential within this cluster showed that the observed maximum stress‐field orientation of N068° is the optimum orientation for fault reactivation of the east–west‐oriented cluster.

Our results suggest that the east–west orientation of the investigated cluster is the main reason for its activation, even though the cluster is about 2 km away from the low‐pressure injection point.