Laboratory friction datasets of high porosity sediments deformed along extensional faults in the Crotone Basin, Italy

Most earthquakes nucleate within the unstable seismogenic zone of the Earth’s Crust between 5 and 35 km of depth. However, shallow seismicity (< 5 km) is frequently observed in tectonically active regions, with moment magnitude up to 6.0-7.0. These earthquakes pose a strong seismic hazard and deserve more attention on both their mechanics and structural signature. In this project, we propose to integrate geologic information from a well-exposed natural fault core in the Crotone Basin, Southern Italy, with laboratory frictional experiments. The study fault zone formed under shallow burial-near surface conditions and bears evidence for Pleistocene earthquakes (Mw < 5.5) provided by dark gouge, which is being studied by field work and microstructural, mineralogical, and chemical analyses. These field and laboratory constraints will drive laboratory frictional experiments designed to better understand the mechanics of earthquake faulting at very shallow burial conditions. The main objective of the project is the understanding of the mechanics of natural dark gouges by frictional, low to high-velocity, experiments conducted on natural loose sand samples collected from the studied fault zone. Laboratory experiments will allow defining the mechanical behavior (strain-hardening vs softening) of faulted materials under very low confining stress conditions, and with different water contents (dry and water damped). During experiments, test sample temperatures will be measured to quantify friction-related heating. Deformed samples will be analyzed by detailed microstructural and mineralogical analysis, and microstructural-textural features will be compared with the natural counterparts. Joining field and laboratory methodologies is expected to return pivotal information regarding the earthquake process and mechanics involving sediments at near surface conditions.
This publication results from work conducted under the transnational access/national open access
action at Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione Roma 1, HPHT Laboratory of
experimental Volcanology and Geophysics, supported by WP3 ILGE - MEET project, PNRR - EU Next
Generation Europe program, MUR grant number D53C22001400005.

SHIVA frictional rotary apparatus was used to deform natural sand samples to investigate their mechanical behavior. The two sand samples employed (hereafter referred as VIT.S1 and VIT.S2, collected along the footwall and hanging wall blocks, respectively) were characterized by different textural features and mineralogical compositions. Prior to frictional experiments, the sample amounts suitable to be inserted in SHIVA sample holders were obtained by vibrating rifflers. For all the 13 frictional experiments, sample mass was between 6.94 and 7.77 g. The experimental setting was aimed at constraining the mechanical behavior of the sand samples under a broad spectrum of conditions. Both sand samples were deformed simulating coseismic events (slip rate of 1 m/s), and aseismic deformation (slip rate of 100 μm/s). Coseismic and aseismic slip rate experiments were run under dry (room humidity) and partial water saturation (water content equal to 25% of sample weight) conditions. Normal stress was set to 10 and 20 MPa to faithfully reproduce the minimum (~400 m) and maximum (~800 m) burial experienced by the deformed sandstone. Displacement achieved during the experiments spanned from 0.25 to 1 m, with typical value of 0.5 m, which could be considered as a reliable coseismic displacement estimate for moderate magnitude earthquakes.
The preparation phase of all experiments consisted of a progressive loading to the set normal stress and a pre-shear phase. The loading path was done by slowly increasing the normal stress up to the desired values (10 or 20 MPa), with incremental steps of 2 kN/min. The normal stress was kept constant at the set values for 15 minutes. Slow loading path granted the gentle compaction of the loose sand. After the completion of the loading phase, a pre-shear was applied by deforming the sample at a slip rate of 10 μm/s for a total displacement of 0.01 m. The pre-shear phase was necessary to achieve a complete coupling between the sample and the stationary portion of the frictional apparatus and to obtain a controlled initial microstructure in the gouge layers.
During the experiments, normal and shear stress together with temperature inside the sample holder were continuously measured. Results from the footwall sand (VIT.S1) showed a marked slip-hardening behavior (increase of frictional coefficient) with both coseismic and aseismic slip rates and under dry and water damped conditions at normal stress of 10 and 20 MPa. Frictional heating in coseismic slip rate tests resulted in measured temperatures in the 90-190 °C range (Tab. 1). The hanging wall sand (VIT.S2) displayed a slip-hardening behavior at coseismic and aseismic slip rates under dry conditions at normal stress of 10 MPa, while a subtle slip-weakening was achieved in fast slip rate and water damped conditions for displacement exceeding 0.4 m. By increasing the normal stress up to 20 MPa, the slip-weakening behavior was enhanced. Maximum temperatures by frictional heating were 100-180 °C. From the results it stems that the hanging wall sand was weaker compared to the footwall host sand, thus, in principle, it should facilitate the propagation of coseismic deformation in the former (Fig. 1).
Deformed gouge samples extracted from SHIVA will be used to analyze the microstructural features through thin sectioning (SEM and optical microscope), and to perform particle size and petrophysical analysis. The natural fault gouge will be compared with the experimentally deformed counterparts to check similarities and differences in microstructural features. Moreover, the mineralogical composition will be constrained by XRD analyses to be compared with the mineralogy of the natural dark gouge.