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.