Evidence for coseismic slip preserved in high-porosity sandstone at very shallow burial conditions (Crotone Basin, Italy)

The seismogenic zone is commonly defined as the portion of the Earth's upper crust where most earthquakes nucleate. According to seismological data, the seismogenic interval is typically located between 5 and 35 km depth. However, shallow seismicity, with earthquake hypocentral depths < 5 km, has been reported in several tectonic settings. Although less studied, such shallow seismic sources represent potential treats and deserve to be thoroughly investigated and included in seismic hazard evaluations.

For this purpose, we present the results of a multidisciplinary study focusing on faults affecting high-porosity fluvio-deltaic, sandstone-dominated deposits belonging to the Pliocene-Pleistocene succession of the Crotone Basin, South Italy. The studied fault zone is well exposed along the Vitravo Creek canyon, has a maximum displacement of ~50 m, and is characterized by an indurated, sharp master slip surface. The fault footwall displays an 8-10 m-wide deformation band-dominated damage zone with deformation bands occurring both as clusters and as single structures. The frequency of deformation bands increases towards the master slip surface. Approaching the master slip surface, a 1.5 m-thick mixing zone occurs, where strong tectonic mixing affected the sandstone strata with different grain size and thickness. The fault core consists of ~1 m-wide, calcite-cemented cataclastic volume and hosts a wealth of fault-parallel deformation bands and subsidiary slip surfaces. Due to its selective cementation, the fault core stems in strong positive relief compared to the host high-porosity sandstone. The hanging wall block is characterized by a dense network of thin deformation bands with diminishing frequency away from the fault surface. Along the master slip surface, at the top of the indurated fault core, a 1-2 cm-thick dark gouge layer is present. The gouge is persistent throughout all the fault exposure, and has been injected in the underlying, fractured cemented fault core. Microstructural analysis of the gouge reveals a strong cataclastic grain size reduction along thin (< 1 mm) slip zones alternated with portions showing lens-shaped (resembling S-C) fabric. XRD analysis of the < 2 µm grain-size fraction of the gouge layer displays short-ordered illite-smectite mixed layers which support deformation temperatures in the 100-120°C range. XRD analysis performed on clay fraction from the fault core, next to the dark gouge layer, indicates temperatures lower than 50-60°C, consistent with the expected shallow burial conditions. Following this, the anomalous temperature rise recorded within the dark gouge layer is suggested to be produced by frictional heating during coseismic deformation. We conclude that the microstructural observations, grain size, and XRD data provide a line of evidence supporting the occurrence of coseismic deformation affecting high-porosity granular materials at near surface conditions and could help in better evaluation and risk assessment of seismically active faults.