Low stress drops observed for aftershocks of the 2011 Mw 5.7 Prague, Oklahoma, earthquake

In November 2011, three M_w ≥ 4.8 earthquakes and thousands of aftershocks occurred along the structurally complex Wilzetta fault system near Prague, Oklahoma. Previous studies suggest that wastewater injection induced a M_w 4.8 foreshock, which subsequently triggered a M_w 5.7 mainshock. We examine source properties of aftershocks with a standard Brune-type spectral model and jointly solve for seismic moment (M₀), corner frequency (f₀), and kappa (κ) with an iterative Gauss-Newton global downhill optimization method. We examine 934 earthquakes with initial moment magnitudes (M_w) between 0.33 and 4.99 based on the pseudospectral acceleration and recover reasonable M₀, f₀, and κ for 87 earthquakes with M_w 1.83–3.51 determined by spectral fit. We use M₀ and f₀ to estimate the Brune-type stress drop, assuming a circular fault and shear-wave velocity at the hypocentral depth of the event. Our observations suggest that stress drops range between 0.005 and 4.8 MPa with a median of 0.2 MPa (0.03–26.4 MPa with a median of 1.1 MPa for Madariaga-type), which is significantly lower than typical eastern United States intraplate events (>10 MPa). We find that stress drops correlate weakly with hypocentral depth and magnitude. Additionally, we find the stress drops increase with time after the mainshock, although temporal variation in stress drop is difficult to separate from spatial heterogeneity and changing event locations. The overall low median stress drop suggests that the fault segments may have been primed to fail as a result of high pore fluid pressures, likely related to nearby wastewater injection.