The Relationship Between Microfracture Damage and the Physical Properties of Fault-Related Rocks: The Gole Larghe Fault Zone, Italian Southern Alps

Although geological, seismological, and geophysical evidence indicates that fracture damage and physical properties of fault-related rocks are intimately linked, their relationships remain poorly constrained. Here we correlate quantitative observations of microfracture damage within the exhumed Gole Larghe Fault Zone (Italian Southern Alps) with ultrasonic wave velocities and permeabilities measured on samples collected along a 1.5-km-long transect across the fault zone. Ultrasonic velocity and permeability correlate systematically with the measured microfracture intensity. In the center of the fault zone where microfractures were pervasively sealed, P wave velocities are the highest and permeability is relatively low. However, neither the crack porosity nor the permeability derived by modeling the velocity data using an effective-medium approach correlates well with the microstructural and permeability measurements, respectively. The applied model does not account for sealing of microfractures but assumes that all variations in elastic properties are due to microfracturing. Yet we find that sealing of microfractures affects velocities significantly in the more extensively altered samples. Based on the derived relationships between microfracture damage, elastic and hydraulic properties, and mineralization history, we (i) assess to what extent wave velocities can serve as a proxy for damage structure and (ii) use results on the present-day physical and microstructural properties to derive information about possible postseismic recovery processes. Our estimates of velocity changes associated with sealing of microfractures quantitatively agree with seismological observations of velocity recovery following earthquakes, which suggests that the recovery is at least in part due to the sealing of microfractures.