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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.

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The investigation of the induced fractures network around seals in drifts or shafts, and in particular its evolution, is a key issue for the performance assessment of an underground waste repository. Within this framework, a specific experiment was designed and implemented in the Meuse/Haute-Marne Underground Research Laboratory (URL). This experiment, called CDZ (Compression of the Damaged Zone), is dedicated to studying the effect of mechanical compression within the induced fractures zone of the Callovo-Oxfordian claystone (COx). An unequalled level of knowledge in the 3D structure of the fractures network has been attained. A multidisciplinary approach was applied to observe not only the initial state of the induced fracture zone but also its evolution during a loading cycle. The investigations show that the fracture network which composed the Excavation Damaged Zone (EDZ) was initially interconnected and open for flow and then partially closed progressively following the increasing mechanical stress applied on the drift wall. Moreover, the evolution of the EDZ after unloading indicates a self-sealing process.