Fluid-assisted frictional healing revealed by ultrasonic waves

The seismic potential of a fault is controlled by its ability to regain strength between earthquakes (fault healing). Variations in healing rate among different rock types can cause local locking and elastic strain energy accumulation, potentially leading to earthquake nucleation. The 2016 Mw 6.5 Norcia mainshock nucleated in the Triassic Evaporites, the seismogenic layer of central Italy, composed of dolostones and anhydrites. Despite its relevance, the mechanical behavior of anhydrite remains relatively less constrained compared to other common crustal lithologies. Only a limited number of studies have systematically investigated the frictional properties of anhydrite, suggesting that its mechanical behavior is strongly sensitive to boundary conditions.

We conducted room humidity (RH) and water-saturated (WS) Slide-Hold-Slide (SHS) friction experiments on anhydrite gouge to assess and isolate the role of water on its healing properties. To inform mechanical data with the microphysical evolution of the fault, we used piezoelectric (PZT) sensors in transmission mode, which record ultrasonic S-wave (UW) propagation through the sample. Finally, mechanical and ultrasonic measurements were complemented by microstructural analyses of the post-mortem sample.

Our results show that fault healing follows a log-linear dependence on hold time under both RH and WS conditions, but with markedly different magnitudes. Water-saturated experiments exhibit a healing rate nearly three times larger (β ~ 0.024) than RH experiments (β ~ 0.009). Ultrasonic measurements reveal a systematic log-linear increase in S-wave velocity during hold periods. This growth is significantly more pronounced in WS samples, where S-wave velocity increases by more than ~2.8% per decade of hold time, compared to ~1% in RH conditions. Microstructural observations indicate that RH samples deform through distributed cataclastic processes accommodated by multiple R-shear bands, whereas WS samples exhibit extreme strain localization along B-shear zones characterized by intense grain-size reduction and the development of a compressive foliation, consistent with semi-brittle deformation.

These results demonstrate that water fundamentally alters the healing efficiency and deformation style of anhydrite faults. Moreover, they show that ultrasonic wave measurements provide a powerful, independent tool to track fault restrengthening during simulated interseismic periods. The observed increase in S-wave velocity can be directly linked to an increase in the shear modulus of the gouge, which appears to be greater in the presence of water, probably due to fluid-assisted healing processes. Together, the high healing rates and the mechanical stiffening of the microstructure inferred from S-wave velocity measurements suggest that anhydrite gouge may be capable of efficiently accumulating elastic strain energy during interseismic periods. Our findings suggest that fluid-assisted healing in anhydrite-bearing fault zones may play a critical role in controlling fault stability and seismic behavior in natural settings.