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Mechanical data of rotary shear experiments in bituminous dolostones

Mirror-like Surfaces (MSs) are ultra-polished fault surfaces widespread in carbonate seismic terrains, but their formation process is still debated. We deformed gouge samples from exposed fault surfaces hosted in bituminous dolostone rocks in a rotary shear apparatus (SHIVA) at seismic slip rates (1 m/s). By changing the water availability (water-pressurised and room-humidity conditions) and the organic matter/dolomite content (> 35%, dark gouge DG; < 30% bright gouge BG) we investigated the mechanical behaviour leading to MSs formation in fault gouges. We run tests at 15 MPa effective normal stress, 2 MPa confinement and 1 MPa pore pressure for the water-pressurised experiments and a total displacement of 0.13 m. Mirror-like fault surfaces were obtained in all successful experiments; mirrors were more developed under room-humidity conditions. Bituminous dolostones under room-humidity conditions had a slip neutral behaviour with a low friction (0.3). Bituminous dolostones under water-pressurised conditions showed a slip weakening behaviour with an initial peak effective friction μp = 0.65, followed by a drop to effective friction μss DG than in BG (i.e., μss of 0.25 vs 0.28). Future work will focus on the microstructural analysis of the experimental products and the investigation of the slip behaviour of bituminous dolostones at sub-seismic slip rates for a complete study of the slip behaviour spectra. This publication results from work conducted under the national open access action at SHIVA (Slow to High Velocity Apparatus) - HP-HT laboratory of experimental Volcanology and Geophysics (INGV, Roma 1 section) supported by WP3 ILGE - MEET project, PNRR - EU Next Generation Europe program, MUR grant number D53C22001400005.

Ring-shear test data of feldspar sand FS900S used in the Tectonic Modelling Laboratory at the University of Bern (Switzerland)

This dataset provides friction data from ring-shear tests on feldspar sand FS900S used for the simulation of brittle behaviour in crust- and lithosphere-scale analogue experiments at the Tectonic Modelling Laboratory of the University of Bern (Zwaan et al. in 2022, 2023; Richetti et al. 2023). The materials have been characterized by means of internal friction parameters as a remote service by the Helmholtz Laboratory for Tectonic Modelling (HelTec) at the GFZ German Research Centre for Geosciences in Potsdam (Germany). According to our analysis both materials show a Mohr-Coulomb behaviour characterized by a linear failure envelope. Peak, dynamic and reactivation friction coefficients of the feldspar sand are μP = 0.65, μD = 0.57, and μR = 0.62, respectively, and the Cohesion of the feldspar sand is in the order of 5-20 Pa. An insignificant rate-weakening of less than 1% per ten-fold rate change is registered for the feldspar sand. Granular healing is also minor.

Friction data of simulated fault gouges derived from the Groningen gas field

We investigated the frictional properties of simulated fault gouges derived from the main lithologies present in the seismogenic Groningen gas field (NE Netherlands), employing in-situ P-T conditions and varying pore fluid salinity. Direct shear experiments were performed on gouges prepared from the Carboniferous Shale/Siltstone underburden, the Upper Rotliegend Slochteren Sandstone reservoir, the overlying Ten Boer Claystone, and the Basal Zechstein anhydrite-carbonate caprock, at 100 ºC, 40 MPa effective normal stress, and sliding velocities of 0.1-10 µm/s. As pore fluids, we used pure water, 0.5-6.2 M NaCl solutions, and a 6.9 M mixed chloride brine mimicking the formation water. Our results show a mechanical stratigraphy, with a maximum friction coefficient (µ) of ~0.65 for the Basal Zechstein, a minimum of ~0.37 for the Ten Boer claystone, ~0.6 for the reservoir sandstone, ~0.5 for the Carboniferous, and µ-values between the end-members for mixed gouges. Pore fluid salinity had no effect on frictional strength. Most gouges showed velocity-strengthening behavior, with little effect of pore fluid salinity on (a-b). However, Basal Zechstein gouge showed velocity-weakening at low salinities and/or sliding velocities, as did 50:50 mixtures with sandstone gouges, tested with the 6.9 M reservoir brine. From a Rate-and-State-Friction viewpoint, our results imply that faults incorporating Basal Zechstein anhydrite-carbonate material at the top of the reservoir are the most prone to accelerating slip, i.e. have the highest seismogenic potential. The results are equally relevant to other Dutch Rotliegend fields and to similar sequences globally. The data is provided in a .zip folder with 29 subfolders for 29 experiments/samples. Detailed information about the files in these subfolders as well as information on how the data is processed is given in the explanatory file Hunfeld-et-al-2017-Data-Description.pdf

Velocity-dependent slip weakening by the combined operation of pressure solution and foliation development

Phyllosilicate-bearing faults are characterized by an anastomosing foliation with intervening hard clasts and are believed to be long-term weak structures. Here, I present results of sliding experiments on gouges of 80 wt% quartz and 20 wt% muscovite, sheared under hydrothermal conditions at constant velocity. The results show that significant strengthening occurs over a narrow range of sliding velocities (0.03-1* m-6/s). At the lowest velocity investigated, weakness is achieved after a considerable sliding distance of over 20 mm with friction reaching a value of 0.3. Microstructural observations and the application of existing models point to the operation of frictional-viscous flow (FVF), through the serial operation of frictional sliding over a weak foliation and pressure solution of intervening clasts, resulting in low frictional strength and pronounced velocity-strengthening. At higher velocities, grain size reduction becomes dominant in a localized zone, which results in disruption of the foliation and the cessation of the FVF mechanism. In natural settings, earthquakes originating elsewhere on the fault would be rapidly arrested when encountering a foliated part of the fault deforming via FVF. Furthermore, pulses of elevated slip velocity would lead to grain size reduction which would destroy the foliation and cause a long-term strengthening of the fault.

Mechanical data of simulated basalt-built faults from rotary shear and direct shear experiments

Here we report the raw data of the friction experiments carried out on basalt-built simulated faults defined by rock-on-rock contacts and powdered gouge. The experiments were specifically designed to investigate the role of fault microstructure on the frictional properties of basalts and the fault slip stability, and were conducted with the rotary-shear apparatus (SHIVA) and the biaxial deformation apparatus (BRAVA), hosted at the National Institute of Geophysics and Volcanology (INGV) in Rome. Simulated faults were sheared at constant normal stress from 4 to 30 MPa. In SHIVA experiments, we deformed samples at constant slip velocity of 10 μm/s up to 56 mm net slip. In BRAVA tests we performed a sequence of velocity steps (0.1 to 300 μm/s), followed by slide-hold-slide tests (30-3000 s holds; V=10 μm/s slides). Our main results highlight the frictionally strong nature of basalt faults and show opposite friction velocity dependence upon the velocity upsteps: while fault gouges exhibit velocity weakening behavior with increasing normal stress and sliding velocity, bare rock surfaces transition to velocity strengthening behavior as we approach higher slip velocities. The experiments setup and data are further described in the manuscript “Frictional properties of basalt experimental faults and implications for volcano-tectonic settings and geo-energy sites” to which these data are supplementary material.

Frictional and microanalytical data of basalts sheared with pressurized H2O- and CO2- rich fluids

Here we report the raw data of the friction experiments performed on basalt-built faults pressurized with de-ionized H2O, pure CO2, pure Ar, and H2O+CO2 mixtures, respectively (Dataset_friction_basalts.zip). The experiments were designed to assess the effects of the fluid chemistry on fault reactivation in faults juxtaposing basalts with different state of hydrothermal alteration. The experiments setup and data are further described in Giacomel et al (2018) to which these data are supplementary material. Laboratory faults were deformed at a constant normal stress over the range from 10 to 20 MPa, at a shear stress of 5 MPa and a starting fluid pressure from 0.5 to 5 MPa. Fluid pressure was increased stepwise of 0.1 MPa/100 s up to induce macroscopic frictional instability, i.e. the equivalent to a main shock in nature. Our mechanical data point to the paucity of any significant chemical weakening due to fluid-rock interation, regardless of the composition of the injected fluid and the degree of hydrothermal alteration of basalts. Moreover, microRaman investigation evidenced a few carbonate patches at the end of the tests performed in H2O+CO2 mixtures (Dataset_raman_calcite_s1018.txt and Dataset_raman_dolomite_s1018.txt).