The heterogeneous near-surface velocity structure of carbonate-hosted seismogenic fault zones investigated at different length scales: from ultrasonic measurements to subsurface seismic tomography

Field geological studies have revealed the heterogeneous structure of fault zones down to the sub-metric scale due to the juxtaposition of rocks presenting distinct deformation intensity and physical-transport properties. However, such internal variability is not generally resolved by most seismic tomography techniques due to spatial resolution limits. Quantifying the heterogeneous internal structure of fault zones is fundamental to understand their mechanical and hydrological characteristics. In this sense, determining seismic wave velocities and related physical properties (elastic moduli, porosity and fracture intensity) within fault zones, at different observational scales, is crucial.

Here, the near-surface velocity structure of two active seismogenic fault zones located in the Central Apennines of Italy was quantified at different length scales, from laboratory measurements of ultrasonic velocities (rock samples of few centimeters, 1 MHz source) to high-resolution first-arrival seismic tomography (spatial resolution of few meters). Detailed structural mapping was conducted within the Vado di Corno and Monte Marine fault zones, two NW-SE trending structures with length of ~ 15 km and up to 1.5 km of extensional displacement. Distinct structural units separated by fault strands were recognized in the fault zone footwall blocks cutting Mesozoic dolomitic carbonates: (i) fault core cataclastic units, (ii) breccia unit, (iii) high-strain damage zone, (iv) low-strain damage zone. The single units were systematically sampled along transects orthogonal to the average strike of the faults and characterized in the laboratory in terms of directional P and S ultrasonic wave velocities, porosity and microstructures. The fault core cataclastic units were significantly “slower” (V_P = 4.5±0.4 kms^-1, V_S = 2.7±0.2 kms^-1) compared to the damage zone units (V_P = 5.6±0.6 kms^-1, V_S = 3.2±0.3 kms^-1) at short length scales (i.e. few centimeters). A general negative correlation between ultrasonic velocity and porosity was observed, with some variability within the fault core mostly related to the textural maturity (clast/matrix volume ratio) of the fault rocks and the degree of pore space sealing by calcite cements.

Multiple P- and S-wave high-resolution seismic profiles (length 90-116 m, geophone spacing 1-1.5 m) were acquired across the two fault zones at different structural sites, moving from the principal fault surface into the outer damage zone. The derived first-arrival tomography models highlighted fault-bounded rock bodies with distinct velocities and characterized by geometries which well compared with those deduced from the structural mapping. At the larger length scale investigated by the active seismic survey, relatively “fast” fault core units (V_P ≤ 3.0 kms^-1, V_S ≤ 1.8 kms^-1) and very “slow” high-strain damage zones (V_P < 1.6 kms^-1, V_S < 1 kms^-1) were recognized. These velocity ranges were significantly different from those determined in the laboratory on small samples. This apparent discrepancy could be reconciled using an effective medium approach, considering the effect of mesoscale fractures density and size distributions affecting each structural unit.

This combined study highlighted the high petrophysical variability of carbonate-hosted fault zones, with structural units characterized by sharp contacts and different velocity scaling. In particular, the persistence of compliant high-strain damage zones at shallow depth might strongly affect near-surface deformation.