Injection‐Induced Earthquakes Near Milan, Kansas, Controlled by Karstic Networks

Induced earthquakes from waste disposal operations in otherwise tectonically stable regions significantly increases seismic hazard. It remains unclear why injections induce large earthquakes on non‐optimally oriented faults kilometers below the injection horizon, particularly since fluids are not injected under pressure, but rather poured, into the well as observed in the Milan, Kansas area. Here we propose a mechanism for induced earthquakes whereby the karstic lower Arbuckle provides the short‐circuit that establishes a tens of MPa stepwise fluid pressure increase within the basement upon arrival of the hydraulic connection to the free surface and ultimately induce slip on the deeper fault. We investigate this scenario through modeling and mechanical analysis and show that earthquakes near Milan are likely induced by large (and sudden) fluid pressure changes when the karst network links two previously isolated hydrological systems. Plan Language Summary We used numerical models to simulate the coupled hydrological and mechanical processes inducing seismicity in southern Kansas. We find that the presence of an extensive karst reservoir, the Arbuckle group, was a necessary condition to produce the M4.9 Milan earthquake, the largest earthquake to occur in over 100 years in Kansas. The results of these models also suggest that a significant percentage of the induced seismicity in Oklahoma would not have occurred without the presence of the Arbuckle. As demonstrated by this work, coupled hydromechanical models are critical to help understanding fluid behaviors in injection reservoirs and can be used to further understand the spatiotemporal distribution of induced earthquakes. Key Points Earthquakes near Milan are likely induced by large and sudden fluid pressure changes Hydrogeology of karst networks is the dominant factor controlling high pressure changes Large pore pressure changes are required to induce earthquakes on nonoptimally oriented faults