The heterogeneous near-surface velocity structure of a carbonate-hosted seismogenic fault zone and its dependence on the investigated length scale

Michele Fondriest; Thomas M. Mitchell; Maurizio Vassallo; Stephane Garambois; Giuseppe Di Giulio; Fabrizio Balsamo; Marta Pischiutta; Mai-Linh Doan

arXiv · 2601.04717

The heterogeneous near-surface velocity structure of a carbonate-hosted seismogenic fault zone and its dependence on the investigated length scale

Michele Fondriest·Thomas M. Mitchell·Maurizio Vassallo·Stephane Garambois·Giuseppe Di Giulio·Fabrizio Balsamo·Marta Pischiutta·Mai-Linh Doan

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Abstract

Field geological studies highlighted the heterogeneous structure of fault zones from the meter- to millimeter scale, but such internal variability is not generally resolved by seismological techniques due to spatial resolution limits. The near-surface velocity structure of the Vado di Corno seismogenic fault zone was quantified at different length scales, from laboratory measurements of ultrasonic velocities (few centimeters rock samples, 1 MHz source) to high-resolution first-arrival seismic tomography (spatial resolution to a few meters). The fault zone juxtaposed structural units with contrasting ultrasonic velocities. The fault core cataclastic units were slower compared to damage zone units. A negative correlation between ultrasonic velocity and porosity was observed, with dispersion in fault core units related to varying degree of textural maturity and pore space sealing by calcite. Low-velocity outliers in the damage zone were instead linked to microfracture networks with local cataclasis and partial calcite sealing. P-wave high-resolution seismic tomography imaged distinct fault-bounded rock bodies, matching the geometry and size of field-mapped structural units. At this length scale, relatively fast fault core units and low-strain damage zones contrasted with a very slow intensely fractured high-strain damage zone. The discrepancy between higher ultrasonic velocities and lower tomography-derived ones was reconciled through an effective medium approach considering the effect of meso-scale fractures in each unit. This revealed a heterogeneous fault zone velocity structure with different scaling among structural units. Lastly, the persistence of a thick compliant high-strain damage zone at shallow depth may significantly affect fault zone mechanics and the distribution of near-surface deformations.

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Michele Fondriest, Thomas M. Mitchell, Maurizio Vassallo, Stephane Garambois, Giuseppe Di Giulio, Fabrizio Balsamo, Marta Pischiutta, Mai-Linh Doan. 2026-01-08. The heterogeneous near-surface velocity structure of a carbonate-hosted seismogenic fault zone and its dependence on the investigated length scale. https://arxiv.org/abs/2601.04717

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