Processing of combined microseismic data from two independent sparse seismic monitoring networks deployed at the (near-)surface around the Illinois Basin – Decatur Project (IBDP) injection site was done to characterize the detected seismicity by locations and inverted source mechanisms. By employing waveform similarity analyses, consistent processing was achieved for similar events, making it possible to distinguish between different clusters of events. The seismicity response was observed to vary with injection location. Different clusters and trends of locations in space and time support an interpretation of activated (mostly previously unknown) faults. Magnitudes of the observed seismic events range from -2.1 to 1.2, no felt event was recorded. Source mechanisms of the stronger events inverted from body-wave arrival amplitudes show that the activated faults are dominated by strike-slip and oblique-slip type of failure. Faults are interpretable in the recently reprocessed 3D reflection seismic data volume, which helped to confirm the existence of faulting in some of the locations where microseismicity was detected. This study shows that proper continuous microseismic monitoring, and updated processing of 3D seismic volume with a special focus on areas with observed microseismicity helps to better understand the response of the reservoir to CO2 injection.
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