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

TL;DR

This study investigates the heterogeneity of near-surface velocity structures in a carbonate fault zone across multiple scales, revealing complex relationships between ultrasonic velocities, seismic tomography, and fault mechanics.

Contribution

It provides a multi-scale analysis combining laboratory, seismic, and field data to characterize fault zone heterogeneity and its implications for fault mechanics.

Findings

Fault core units are slower than damage zones in ultrasonic measurements.

Seismic tomography reveals distinct fault-bounded rock bodies matching structural units.

Heterogeneous velocity structure influences fault mechanics and deformation distribution.

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.