Recent Developments in 4D X-ray Tomography for Real-Time Observation of Fault Slip and Gouge Evolution

The properties of fault slip surfaces, gouge characteristics, and fluid-rock reactions are tightly coupled and control earthquake mechanics. To visualise and quantify the role of this coupling, we have developed a new operando imaging approach that allows the documentation of fast direct-shear deformation experiments in time-resolved 2- and 3-dimensional image data at low single-digit micrometer resolution. A direct-shear inset developed for the X-ray transparent Heitt Mjölnir triaxial deformation apparatus enables experiments at 20 MPa normal stress under fluid-pressurised conditions and allows real-time permeability measurements.

We apply this platform to three fault systems: 1) Slip surfaces in basaltic rocks, imaged while sliding at 1 mm.s^-1, reveal how asperities, phenocrysts, and surface roughness control stick-slip behavior and damage localization during fast slip. 2) Reactive quartz-gypsum gouges imaged during velocity stepping and healing experiments, enable the direct linking of evolving frictional properties to microphysical developments. 3) A shearing, dehydrating gypsum gouge provides insights into transient rheologies and the resulting strain distributions.

These datasets demonstrate that 4D imaging resolves coupled mechanical, chemical, and hydraulic fault evolution in real time. Our approach allows documenting microphysical processes underlying the frictional properties of faults and thereby constitutes a potent tool for studying faults in a variety of tectonic settings.