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

Abstract

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.

Off‐Fault Damage Characterization During and After Experimental Quasi‐Static and Dynamic Rupture in Crustal Rock From Laboratory P Wave Tomography and Microstructures

“…The use of a normalized fracture density, such as the one presented here, has the advantage of direct applicability with other effective medium models, for instance, to predict hydraulic properties (Gavrilenko & Gueguen, 1989; Guéguen & Schubnel, 2003). We therefore propose that high‐resolution geophysical measurements of wave speeds from dense arrays (Ben‐Zion et al., 2015) combined with microstructural characteristics measured in the field (Rempe et al., 2013, 2018) or from borehole data (Jeppson et al., 2010) can reveal the physical properties around fault zones. Such data can be used to calibrate the findings of rupture simulations that allow for off‐fault energy dissipation (Bhat et al., 2012; Okubo et al., 2019; Thomas & Bhat, 2018), and can be compared to laboratory failure experiments such as those presented here.…”

Pseudotachylyte Alteration and the Rapid Fade of Earthquake Scars From the Geological Record

“…Ambient conditions of faulting were at 9–11 km depth and 250–300°C and the fault zone accommodated ~1.1 km of dextral strike slip over a fault thickness of ~600 m. Ancient seismicity is attested by the widespread occurrence of pseudotachylytes associated with green chlorite‐epidote bearing cataclasites to ultracataclastites (Di Toro & Pennacchioni, 2005) (Figures 1a and S1 in the supporting information). Black “fresh”‐looking pseudotachylyte veins with well‐preserved primary microstructures (e.g., microlites‐spherulites) and injections are abundant within the fault damage zone, while pseudotachylytes become rarer within an ~200 m‐thick central altered propylitic zone delimited by two ultracataclastic fault cores (~2 m thick) affected by higher intensity of healed and sealed microfractures (Rempe et al, 2018; Smith et al, 2013). Mineralogical and isotopic studies revealed the ingression of metamorphic fluids at the time of seismic faulting which were focused within the central portion of the fault zone (Mittempergher et al, 2014; Smith et al, 2013).…”

Elastic Properties and Seismic Anisotropy Across the Alpine Fault, New Zealand

“…Agreement of measurements obtained at different spatial and frequency scales indicates that factors responsible for low velocity in damaged rock are present at lab and log scales. As laboratory measurements are made using a high‐frequency, short‐wavelength signal and physical samples cannot contain macroscale features, the velocity reduction must therefore be related to small‐scale processes such as microfracturing (Jeanne et al, 2012; O'Connell & Budiansky, 1974; Rempe et al, 2018), disaggregation, cataclastic grain size reduction (Fortin et al, 2007; Hamilton & Bachman, 1982), and formation of authigenic clay minerals (Castagna et al, 1985; Han et al, 1986; Klimentos, 1991; Tosaya & Nur, 1982; Vanorio et al, 2003). At the longer wavelength of log observations, wave velocities are also influenced by larger‐scale fractures.…”