Dynamics of frictional healing of anhydrite bearing faults imaged by ultrasonic waves

The ability of faults to regain strength between seismic events (frictional healing) is crucial to understand the seismic cycle. In lithologically heterogeneous faults, differences in healing rates among various rock types can lead to the locking of specific fault patches. These locked segments may store significant amounts of elastic strain energy, which can be dynamically released during earthquakes. Anhydrite is a key component of the Triassic Evaporites, the seismogenic layer responsible for destructive earthquakes in Central Italy, e.g. 2016 Mw 6.5 Norcia mainshock. Its complex mechanical behavior, strongly influenced by boundary conditions, remains underexplored. Minor variations in effective pressure, humidity, loading rate, and temperature alter healing properties, rheology, and fault slip behavior of anhydrite. These mechanical characteristics are inherently tied to its elastic properties. Hence, the broad spectrum of mechanical behaviors observed should correspond to an equally wide variation in its elastic moduli.

We performed dry and wet friction experiments on anhydrite gouge using the BRAVA2 biaxial apparatus. These experiments include Slide-Hold-Slide (SHS) sequences to investigate the healing properties of anhydrite by varying temperature between 20 and 100 °C. To inform mechanical data with the microphysical evolution of the fault, we equipped the sample assembly with PZT sensors in transmission mode. These sensors record ultrasonic wave (UW) propagation through the sample during SHS tests. Finally, mechanical and ultrasonic measurements were accompanied by comprehensive microstructural analysis.

At room temperature, distinct mechanical features emerge between dry and wet samples. Wet experiments are characterized by lower friction (μ = 0.49) and higher healing rate (β = 0.015) with respect to dry ones (μ = 0.6 and β = 0.004). In both wet and dry tests fault healing follows a log-linear dependence with hold duration, however, in wet samples this relationship occurs only after a characteristic cut-off time, tc (s). We report a log-linear increase of UW amplitude with hold time. Microstructural analysis of wet samples reveals shear localization and grain size reduction within multiple Y, B, and R shear bands. Conversely, dry samples predominantly feature distributed deformation within R shear bands and local S-C structures.

The observed differences between dry and wet experiments suggest that water-activated processes play a major role in controlling shear strength and healing properties of anhydrite. This hypothesis is corroborated by the presence of a cut-off time for wet healing measurements. We interpret tc as a necessary time for water-activated deformation mechanisms to effectively operate by increasing the healing rate.
The evidence of a log-linear relationship between UW amplitude and hold duration testifies that fault restrengthening is intimately related to gouge porosity reduction and fault zone evolution.

Our approach aims to correlate reductions in shear modulus with shearing along specific slip planes, which are known to be active through microstructural analyses. To achieve this goal, the relationship between complex frictional healing and the elastic properties of anhydrite will be investigated via the extraction of elastic moduli from UW velocities.