Multi-scale velocity structure of an active seismogenic normal fault zone (Central Apennines, Italy)

The petrophysical characterization of fault zones (e.g., ultrasonic velocities, porosity and fracture intensity of the fault zone rocks) is a relevant topic in reservoir geology (exploration and exploitation) and fault mechanics (for both long-term quasi-static and fast dynamic fault evolution). Here we characterized the shallow subsurface velocity structure of the active Vado di Corno normal fault zone (Campo Imperatore, Central Apennines, Italy). Based on a detailed structural mapping of the fault footwall block, four main structural units separated by principal fault strands were recognized: (i) cataclastic unit, (ii) breccia unit, (iii) high-strain damage zone, (iv) low-strain damage zone. The single units were systematically sampled along a transect ( 200 m) orthogonal to the average strike of the fault and characterized in the laboratory in terms of petrophysical properties (i.e., Vp, Vs and He-porosity). The cataclastic and breccia units (Vp = 4.68±0.43 kms-1, Vs = 2.68±0.24 kms-1) were significantly "slower" compared to the damage zone units (Vp = 5.43±0.53 kms-1, Vs = 3.20±0.29 kms-1). A general negative correlation between ultrasonic velocity and porosity values was reported; moreover measured acoustic anisotropies were related to deformation fabrics (i.e., open fractures, veins) observed at the sample scale. A Vp - Vsseismic refraction tomography was performed in the field along a profile ( 90 m) across the fault zone. The tomographic results clearly illuminated fault-bounded rock bodies characterized by different velocities (i.e., elastic properties) and geometries which match with the ones deduced from the structural analysis of the fault zone exposures. Fracture intensity measurements (both at the sample and outcrop scale) were performed to investigate the scaling relation between laboratory and field measurements. These results were then coupled with ultrasonic velocity vs. confining pressure (0-30 MPa) profiles measured in the laboratory to extrapolate the subsurface velocity structure of the fault zone to larger depths (up to 1 km). Such observations are potentially relevant to better understand the petrophysical evolution and geophysical expression of active fault zones during the seismic cycle and will be integrated in both static and dynamic fault mechanics models.