Source parameters of injection-induced microseismicity

We analyze source parameters of microseismic events (M< - 1) associated wit h high how-rate water injections in a shale formation at a depth of 220 m. Two types of events were observed: several hundred impulsive events with cl ear P- and S-wave arrivals, and continuous emissions with peaked spectra de tected well into the experiment. For a representative collection of impulsi ve events, an omega(-2) model provided satisfactory fits to displacement sp ectra corrected for attenuation, and average quality factors of 34 and 15 w ere obtained for P and S waves. P wave first motion analysis and E-S/E-P ra tios indicated the existence of a non-double-couple component in some event s, particularly early in the experiment. A clear difference was observed fo r estimates of stress release parameters as non-double-couple events had sm aller stress drops and apparent stresses. The seismic efficiency of double- couple and non-double-couple events was limited to 0.9% and 0.05% respectiv ely, with average values being 0.25% and 0.02%. A comparison of our results with those reported for a similar magnitude range in a hard-rock formation indicates considerably smaller estimates of stress drop and apparent stres s in our case while seismic efficiencies are comparable.