Microseismic monitoring of hydraulic fracture stimulations is used to determine the extent of fractured rock resulting from the treatment by mapping the locations of induced microseismic events. Usually the geometry of the event locations is used to infer fracture orientations; e.g. trends of microseismic events concentrated along a particular azimuth (or with a planar distribution in 3D) can indicate fracturing along a plane with that orientation. In this study we use additional parameters extracted from induced microseismic events (source mechanisms) to determine the specific fracturing behavior and compare them with independent observations from an FMI log in the treatment well. In a horizontal well located in the mid-Continental USA, we present the results of source mechanism analysis for the best signal-to-noise events triggered by the fracture stimulation treatment. The microseismic events with source mechanisms have failure planes with very similar orientations to natural fractures in the image log. Our results are consistent with the reactivation of natural fractures during the stimulation treatment, suggesting that it is possible to determine natural fracture orientations in the reservoir in cases where image logs are not available. In addition, the microseismic event source mechanisms allow fracture characterization away from the wellbore, providing critical constraints for building fractured reservoir models.