Healing of fault surfaces: a field vs. experimental perspective

“Fault healing” is the ability of fault rocks to recover strength after rupture, due to a combination of several physical processes that include cementation, compaction, asperity growth etc. Healing is fundamental in the earthquake physics because it allows for the repeated accumulation of energy along faults over multiple seismic cycles. Fault healing is commonly studied in the laboratory, through Slide-Hold-Slide (SHS) tests and cementation experiments. However, laboratory measurements and the microstructures of experimental fault rocks are difficult to compare with natural rocks, due to the difference in kinetics of physical mechanisms and the small spatio-temporal scale of experiments.Here, we review the field and microstructural evidence of various processes of fault healing along a carbonatic fault surface, taking advantage of an outstanding case study: the Pietrasanta Normal Fault (NW Tuscany, Italy). In the field, the most common evidence of fault healing is the occurrence of cohesive fault rocks (cataclasites) and veins, but other fault surface properties may influence the re-strengthening of fault surfaces: e.g. adhesion phenomena (sidewall ripouts and fault surface patches) and geometrical complexity.We compare these observations with frictional healing experiments carried out on carbonatic fault rocks, in which both fault gouges and cohesive slip surfaces were used. We propose that a fault surface composed by “patches” of cohesive fault rocks bounded by anastomosing slip zones are the result of complex cycles of gouge formation and healing, which modulate the interplay of adhesion and localization along the fault surface.