Low stress drops observed for M1.5-5.6 Earthquakes During the 2011 Prague, Oklahoma sequence

Abstract:

In November 2011, three M≥4.8 earthquakes and thousands of aftershocks occurred along the structurally complex Wilzetta fault system near Prague, Oklahoma. Previous studies suggest that fluid injection in nearby wells was responsible for inducing the M4.8 foreshock [Keranen et al., 2013], which subsequently triggered the M5.6 mainshock [Sumy et al., 2014]. We examine earthquake source properties using waveforms collected by 47 temporary and permanent seismic stations installed within ~100 km of the events. Brune stress drops are calculated for a subset of 278 aftershocks of the Prague, Oklahoma sequence that range in magnitude from M1.0-4.9. We compute the horizontal amplitude spectra and smooth the spectra with a Konno and Ohmachi [1998] filter. We then assume a standard Brune source spectral model and iteratively solve for seismic moment (M_o), corner frequency (f_c) and kappa (κ) using a Gauss-Newton method. We are able to recover reasonably fit M_o, f_c, and κ for 172 earthquakes, but find the M_o determined by spectra fit are consistently overestimated for events with M<1.5. Therefore only stress drops for the 149 events with initial M_o > 1.5 are reported. The median stress drop is ~0.3 MPa (3 bars), which is significantly lower than typical values (10+ MPa) observed for CEUS events. We find minimal dependence of stress drop on focal depth for the depth range from 1-10 km. However, we observe some spatial variation of stress drops, with higher stress drops observed near the intersection of the mainshock rupture plane with the fault plane associated with the M4.8 aftershock. The low stress drops observed for events on both the foreshock and mainshock fault plane may indicate both of these segments of the Wilzetta fault system were influenced by local wastewater injection. Our results echo those of several recent studies [Hough, 2014; Sun and Hartzell, 2014], which conclude that induced earthquakes have lower stress drops than tectonic events.