Damage coalescence controls slow and fast faulting: Insights from dynamic X-ray microtomography experiments

<p>During fast and slow earthquakes deformation localizes along narrow and quasi-planar fault surfaces. However, processes controlling the localization process develop not only on the fault surface but also in the volume surrounding the fault zone. How these processes transition from a dispersed to more localized distribution of damage remains controversial. Moreover, to what degree the localization process controls the speed of coseismic slip is an open question. We perform a series of 4D X-ray microtomography experiments on crystalline rocks (granite, marble), with and without a pre-existing slip surface, and image the development of damage while each sample is loaded until system-size brittle failure. We image and deform the samples under in situ stress conditions of a few kilometers depth using the Hades deformation apparatus installed on the tomography beamline ID19 at the European Radiation Synchrotron Facility. By imaging all the microfractures that develop in the samples, we characterize their individual geometry and the geometry of the entire microfracture network. The results show that, when a pre-existing slip surface exists in the sample, slow earthquakes can generate damage in the volume around the fault, leading to catastrophic faulting. When no pre-existing fault is present, microfractures accumulate and can lead to two end-member types of earthquakes. One type is a catastrophic failure of the sample that occurs when the microfractures link into a macroscopic fault, producing a fast earthquake. Alternatively, the microfractures can grow without significant fracture coalescence, leading to the slow development of a fault network with a transient increase of macroscopic deformation rate that resembles that of a slow earthquake. We conclude that damage coalescence influences the slow and fast behaviours of earthquake slip.</p>

Interaction of a hydraulic fracture with parallel pre-existing fractures

<p>Formation and growth of hydraulic fractures can be strongly affected by pre-existing fractures in the rock mass. Until now the main attention was directed towards the investigation of the interaction between the hydraulic fracture and the pre-existing fractures intersecting its path, as they could significantly hamper its formation and growth, alter the geometry and produce additional leak-off. Less attention was paid to the interaction of the hydraulic fracture with parallel and coplanar pre-existing fractures, yet their interaction and coalescence can lead to unwelcome increase in the hydraulic fracture dimensions, change the direction of growth and in some cases result in undesirable effects such as environmental damage.  </p><p> </p><p>In order to investigate the hydraulic fracture interaction with parallel pre-existing fractures we conducted a series of tests on transparent rectangular samples with two artificial cracks. One of the crack was loaded with pressurised fluid. The types of interaction were classified and the conditions of fracture coalescence formulated. The results will contribute to the understanding of hydraulic fracture propagation in fractured rock masses and mitigating environmental damage.</p><p> </p><p><strong>Acknowledgements</strong>. Wang acknowledge support from the Natural Scinece Foundation of Jiangsu (BK20171130). The AVD and EP acknowledge support from the Australian Research Council through project DP190103260. AVD acknowledges the support from the School of Civil and Transportation, Faculty of Engineering, Beijing University of Civil Engineering and Architecture. Wang acknowledge support from the National Natural Science Fund (51409170,U1765204)</p>