Shallow seismic damage and structural inheritance at the onset of faulting

Faults accommodate shear motion in the upper crust through brittle deformation like fracturing and cataclastic flow. The width of the embrittlement, the lateral extent of the fault, and the grain size distribution within the fault core seem to scale with shear displacement, though scaling laws have large uncertainties. Inherited structures may drive strain partitioning, restrict damage extension, and either favour or hinder seismic slip propagation. It is difficult to explore these features in natural faults since delicate structures such as crackle breccias and gouges are reworked during exhumation, overprinted during multiple deformation cycles, and altered by surface weathering. The Bedretto Underground Laboratory represents a unique chance to study the structure of immature faults at depth, whose pristine structures are hardly preserved elsewhere in the field. The exceptional exposure allowed for a detailed analysis of a fault zone named Waterfault, which exploited the pre-existing fabrics of a mylonitic shear zone in the Rotondo granite. The limited extension of the damage zone and the scarce presence of cataclastic products indicates that the fault’s activity was short-lived, possibly stemming from a single slip event. The presence of pulverised rocks, restricted to the hanging-wall side, suggests that faulting was produced during the propagation of a low magnitude earthquake. The pre-existing mylonitic fabrics provided a strong mechanical anisotropy, which confined the damage zone and favoured the lateral extension of the fault. We argue that the WF structures illustrate the onset of faulting along pre-existing anisotropies, revealing their role in the partitioning of energy and strain.