Near-surface earthquake rupturing in high-porosity sandstone documented by a combined meso-microstructural, mineralogical, and experimental approach (Crotone forearc Basin, Italy)

The seismogenic zone is the locus of most earthquakes occurring in the Earth’s crust and is located in a depth interval from 5 to 35 km. The shallow portion (< 5 km) of the seismogenic zone is considered stable, as crosscuts low cohesion, water saturated, rocks and sediments. Nevertheless, many earthquakes have been documented at depths shallower than 5 km in different geodynamic settings. Such shallow, and still poorly understood, seismicity could represent an additional threat to be accounted for in seismically active regions.

To provide new hints on this subject, we present the results of a multidisciplinary study dealing with near-surface earthquake deformation recorded along an extensional fault zone affecting high porosity, Plio-Pleistocene age, sandstone. The studied fault zone is exposed along the Vitravo Creek canyon, in the Crotone Basin, South Italy. The cumulated displacement reaches ~50 m, and deformation is accommodated by the development of deformation bands and secondary faults, both in the footwall and hanging wall blocks. Within the fault core, where most of the displacement is accommodated, a 2-3 cm-thick dark gouge layer can be found. The gouge is continuous along the entire outcrop exposure and locally has been injected into the joints affecting the calcite cemented fault core. Secondary, thinner (~1 mm-thick), gouge layers are present a few cm away from the main one in the hanging wall block. Microstructural and particle size analyses conducted on the dark gouge allowed to document a severe cataclastic grain size reduction and a marked gradient in comminution from the footwall towards the hanging wall side. XRD mineralogical analysis performed on the < 2 µm size fraction of the dark gouge, revealed up to 60% of illite in the illite-smectite short-ordered mixed layers, suggesting deformation temperature up to 100-120 °C. XRD analyses conducted on control samples collected along the entire fault zone returned estimated deformation temperatures of < 50 °C, compatible with the maximum sediment overburden (< 800 m). The anomalous and localized increase in temperature within the dark gouge has been linked with flash-frictional heating processes during coseismic deformation under shallow burial conditions. Frictional laboratory experiments run on natural host sand samples collected along the fault zone allowed to constrain their mechanical behavior at aseismic (100 µm/s) and coseismic (1 m/s) slip rates, under different water contents (dry vs water saturated) and at different normal loading-burial conditions (10-20 MPa). The experimental gouge displayed similar micro-textural characteristics compared to their natural counterparts. The multidisciplinary approach combining field-structural survey and mapping, microstructural-textural and mineralogical analysis with rock mechanics experiments could be useful to the study of shallow coseismic deformation of sediments and high porosity rocks. The systematic implementation of such approach to several fault zones and fault systems could enhance and improve the earthquake risk and hazard assessment in seismically active regions, unveiling shallow, previously unknown, seismic sources.