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We explore the role of Coulomb stress transfer in the fault reactivation in Woodward, Oklahoma-a wastewater injection area. We address this issue by first defining fault segments from earthquake spatiotemporal clustering then parameterizing the geometries of each segment by combining seismicity and focal mechanisms. Finally, we calculate Coulomb stress transfer along each fault segment. Our results reveal a fault system characterized by a flower structure with strike-slip fault at deeper depth and distributed normal faults at shallower depth. Further, Coulomb stress analysis reveals that the fault reactivation initiates at the fault bend and sequentially migrates to northeast and southwest due to interevent stress interaction. The amplitude of Coulomb stress transfer is at least comparable to pore pressure and poroelastic stress changes estimated from fluid injection. Overall, our observations suggest that fault structure and Coulomb stress transfer constitute important factors in seismogenic fault reactivation within areas of wastewater injection.

Plain Language Summary The earthquakes in wastewater injection areas have been mainly linked to fluid injection, which increases the pore pressure or poroelastic stress and promotes fault failure. Only limited studies have explored another possible driving mechanism-stress interactions between the earthquakes during the fault reactivation in those areas. In this study, we focus on an isolated earthquake cluster in the northwest Oklahoma, a wastewater injection area, and study how earthquake interactions influence the step-by-step reactivation of the fault system. The calculated stress interactions from small earthquakes on the fault planes are larger than the pore pressure change and at least comparable to the poroelastic stress change from fluid injection. Our results suggest that the fluid injection is not the only driving mechanism of seismicity in wastewater injection areas, and earthquake interactions should also be considered for mitigating induced seismicity.