Collettini, Cristiano

Fluid induced fault reactivation experiments will take place as part of the “Fault Activation and Earthquake Rupture” project (FEAR) at the BedrettoLab, an underground laboratory for geosciences and geo-energy excavated within the Rotondo massif (Swiss Alps). The aim of this publication is to characterize frictional properties and permeability of the main segment of the fault zone selected for limited fluid-induced fault reactivation experiments. Firstly, we characterized fault zone microstructures in the field and in thin sections. Secondly, we assessed fault gouge mineralogy by X-ray powder diffraction analysis, yielding a composition in agreement with similar fault gouges in the same area. Finally, we performed a detailed frictional and permeability characterization in laboratory, using BRAVA (Brittle Rock deformAtion Versatile Apparatus). We performed five frictional experiments, run at the actual in-situ conditions: four experiments for frictional properties characterization; and one further experiment where we stimulated the experimental fault by fluid pressurization applying a similar injection protocol designed for the in-situ hydraulic stimulation experiment. Additionally, we performed microstructural analysis on experimental samples to link frictional and permeability properties with fault fabric evolution. The integration of experimental results with field investigations suggests that the selected fault is potentially seismogenic and can be dynamically reactivated and controlled with hydraulic stimulation. This study highlights the importance of bridging the gap between laboratory and in-situ fault characterization, where experimental results become instrumental for the correct design of injection protocols such as those of FEAR project.

The Opalinus Clay (OPA) is a clay-rich formation considered as a potential host rock for radioactive waste repositories and as a caprock for carbon storage in Switzerland. Its very low permeability (10−19 to 10−21 m2) makes it a potential sealing horizon, however the presence of faults that may be activated during the lifetime of a repository project can compromise the long-term hydrological confinement, and lead to mechanical instability. Here, we have performed laboratory experiments to test the effect of relative humidity (RH), grain size (g.s.) and normal stress on rate-and-state frictional properties and stability of fault laboratory analogues corresponding to powders of OPA shaly facies. The sifted host rock powders at different grain size fractions (<63 μm and 63 < g.s. < 125 μm), at room (∼25 per cent) and 100 per cent humidity, were slid in double-direct shear configuration, under different normal stresses (5–70 MPa). We observe that peak friction, μpeak and steady-state friction, μss, depend on water vapour content and applied normal stress. Increasing relative humidity from ∼25 per cent RH (room humidity) to 100 per cent RH causes a decrease of frictional coefficient from 0.41 to 0.35. The analysis of velocity-steps in the light of rate-and-state friction framework shows that the stability parameter (a–b) is always positive (velocity-strengthening), and it increases with increasing sliding velocity and humidity. The dependence of (a–b) on slip rate is lost as normal stress increases, for each humidity condition. By monitoring the variations of the layer thickness during the velocity steps, we observe that dilation (Δh) is directly proportional to the sliding velocity, decreases with normal stress and is unaffected by humidity. Microstructural analysis shows that most of the deformation is accommodated within B-shear zones, and the increase of normal stress (σn) promotes the transition from strain localization and grain size reduction to distributed deformation on a well-developed phyllosilicate network. These results suggest that: (1) the progressive loss of velocity dependence of frictional stability parameter (a–b) at σn > 35 MPa is dictated by a transition from localized to distributed deformation and (2) water vapour content does not affect the deformation mechanisms and dilation, whereas it decreases steady-state friction (μss), and enhances fault stability.

Although rheological heterogeneities are invoked to explain differences in fault-slip be- havior, case studies where an interdisciplinary approach is adopted to capture their specific roles are still rare. In this work, we integrated geophysical, geological, and laboratory data to explain how rheological heterogeneities influence the earthquake activity at the roots of the seismogenic zone. During the 2016–2017 Central Italy sequence, following the major earthquakes, we observed a deepening of seismicity within the basement associated with a transient stress change. Part of this seismicity was organized in clusters of events, with similar sizes and waveforms. The structural study of exhumed basement rocks highlighted a heterogeneous fabric made of strong, quartz-rich lenses (up to 200 m) surrounded by a weak, interconnected phyllosilicate-rich matrix. Laboratory experiments simulating the main shock–induced increase in loading rate showed that the matrix lithology experienced an ac- celerating and self-decelerating aseismic creep, whereas the lens lithology showed dynamic instabilities. Our results suggest that the post–main shock loading rate increases favored accelerated creep within the matrix, which promoted, as a consequence, seismic instabilities within the lenses in the form of clustered seismicity. Our findings emphasize the strong con- nection between seismicity and the structural and frictional properties of the seismogenic zone.

Serpentinites are polymineralic rocks distributed almost ubiquitously across the globe in active tectonic regions. Magnetite-rich serpentinites are found in the low-strain domains of serpen- tinite shear zones, which act as potential sites of nucleation of unstable slip. To assess the potential of earthquake nucleation in these materials, we investigate the link between me- chanical properties and fabric of these rocks through a suite of laboratory shear experiments. Our experiments were done at room temperature and cover a range of normal stress and slip velocity from 25 to 100 MPa and 0.3 to 300 μm s −1 , respecti vel y. We show that magnetite-rich serpentinites are ideal materials since they display strong sensitivity to the loading rate and are susceptible to nucleation of unstable slip, especially at low forcing slip velocities. We also aim at the integration of mechanical and microstructural results to describe the underlying mechanisms that produce the macroscopic behaviour. We show that mineralogical composi- tion and mineral structure dictates the coexistence of two deformation mechanisms leading to stable and unstable slip. The weakness of phyllosilicates allows for creep during the interseis- mic phase of the laboratory seismic cycle while favouring the restoration of a load-bearing granular framework, responsible of the nucleation of unstable events. During dynamic slip, fault zone shear fabric determines the mode of slip, producing either asymmetric or Gaussian slip time functions for either fast or slow events. We report rate/state friction parameters and integrate our mechanical data with microstructural observations to shed light on the mech- anisms dictating the complexity of laborator y ear thquakes. We show that mineralogical and fabric heterogeneities control fault slip behaviour.

Analysis of seismicity can illuminate active fault zone structures but also deformation within large volumes of the seismogenic zone. For the Mw 6.5 2016-2017 Central Italy seismic sequence, seismicity not only localizes along the major structures hosting the mainshocks (on-fault seismicity), but also occurs within volumes of Triassic Evaporites, TE, composed of alternated anhydrites and dolostones. These volumes of distributed microseismicity show a different frequency-magnitude distribution than on-fault seismicity. We interpret that, during the sequence, shear strain-rate increase, and fluid overpressure promoted widespread ductile deformation within TE that light-up with distributed microseismicity. This interpretation is supported by field and laboratory observations showing that TE background ductile deformation is complex and dominated by distributed failure and folding of the anhydrites associated with boudinage hydro-fracturing and faulting of dolostones. Our results indicate that ductile crustal deformation can cause distributed microseismicity, which obeys to different scaling laws than on-fault seismicity occurring on structures characterized by elasto-frictional stick-slip behaviour.

Mineralogy, fabric, and frictional properties are fundamental aspects of faults. Despite the extensive effort spent in the characterization of such fault properties, the description of fabric elements is not always univocal and nomenclatures such as the Y-B-P-R and the S-C-C′ are at times used interchangeably. This work presents a sys- tematic mineralogical, microstructural, and frictional characterization of natural gouges designed to constrain a criterion for the distinction between the Y-B-P-R and S-C-C′ fabric. For this purpose, we tested four representative natural mixtures of granular minerals (quartz) with increasing amount of phyllosilicates (muscovite). 24 fric- tional experiments were performed at constant normal stresses of 25, 50, 75 and 100 MPa, at both room dry and water saturated condition. We document that Y-B-P-R fabric typically develops in frictionally strong, granular- rich experimental faults. This fabric is associated to strain localization in narrow shear zones characterized by intense grain size reduction and dominant cataclastic processes. Conversely, S-C-C′ fabric is observed in phyllosilicate-rich experimental faults, which are characterized by distributed deformation and pervasive foli- ation. Deformation is mainly accommodated by frictional sliding along the well-oriented phyllosilicate foliae. The transition from Y-B-P-R to S-C-C′ is observed for phyllosilicates content >30% and is facilitated by secondary mechanical processes as networking of phyllosilicates and grain mantling. The evolution from Y-B-P-R to S-C-C′ fabric is also associated with a marked reduction in friction, in healing rate and changes in the rate and state friction parameters. Despite their geometrical similarities, we show that Y-B-P-R and S-C-C′ represent distinct fabrics reflecting the dichotomy that exists between frictionally strong, granular-rich, and frictionally weak, phyllosilicate-rich faults.

Crustal seismicity is in general confined within the seismogenic layer, which is bounded at depth by processes related to the brittle-ductile transition (BDT) and in the shallow region by fault zone consolidation state and mineralogy. In the past 10-15 years, high resolution seismological and geodetic data have shown that faulting within and around the traditional seismogenic zone occurs in a large variety of slip modes. Frictional and structural heterogeneities have been invoked to explain such differences in fault slip mode and behaviour. However, an integrated and comprehensive picture remains extremely challenging because of difficulties to properly characterize fault rocks at seismogenic depths. Thus, the central-northern Apennines provide a unique opportunity because of the integration of deep-borehole stratigraphy and seismic reflection profiles with high resolution seismological data and outcrop studies. These works show that seismic sequences are limited within the sedimentary cover (depth < 9-10 km), suggesting that the underlaying basement plays a key-role in dictating the lower boundary of the seismogenic zone. Here we integrate structural data on exhumed outcrops of basement rocks with laboratory friction data to shed light on the mechanics of the Apenninic basement. Structural data highlight heterogeneous and pervasive deformation where foliated and phyllosilicate-rich rocks surround more competent quartz-rich lenses up to hundreds of meters in thickness. Phyllosilicate horizons deform predominantly by folding and foliation-parallel frictional sliding whereas quartz-rich lenses are characterized by brittle signatures represented by extensive fracturing and minor faulting. Laboratory experiments revealed that quartz-rich lithologies have relatively high friction, μ ≈ 0.51, velocity-strengthening to neutral behaviour, and elevated healing rates. On the contrary, phyllosilicate-rich (muscovite and chlorite) lithologies show low friction, 0.23 < μ < 0.31, a marked velocity strengthening behaviour that increases with increasing sliding velocity and negligible rates of frictional healing. Our integrated approach suggests that in the Apenninic basement deformation occurs along shear zones distributed on thickness up-to several kilometres, where the frictionally stable, foliated, and phyllosilicate-rich horizons favour aseismic deformation and therefore confine the depth of major earthquake ruptures and the seismogenic zone.

Fault stability is inherently linked to the frictional and healing properties of fault rocks and associated fabrics. Their complex interaction controls how the stored elastic energy is dissipated, that is, through creep or seismic motion. In this work, we focus on the relevance of fault fabrics in controlling the reactivation and slip behavior of dolomite-anhydrite analog faults. We designed a set of laboratory experiments where we first develop fault rocks characterized by different grain size reduction and localization at normal stresses of σN = 15, 35, 60, and 100 MPa and second, we reload and reactivate these fault rocks at the frictional stability transition, achieved at σN = 35 MPa by reducing the machine stiffness. If normal stress is lowered this way, reactivation occurs with relatively large stress drops and large peak-slip velocities. Subsequent unstable behavior produces slow stick-slip events with low stress drop and with either asymmetric or Gaussian slip velocity function depending on the inherited fault fabric. If normal stress is raised, deformation is accommodated within angular cataclasites promoting stable slip. The integration of microstructural data (showing brittle reworking of preexisting textures) with mechanical data (documenting restrengthening and dilation upon reactivation) suggests that frictional and chemically assisted healing, which is common in natural faults during the interseismic phase, can be a relevant process in developing large instabilities. We also conclude that fault rock heterogeneity (fault fabric) modulates the slip velocity function and thus the dynamics of repeating stick-slip cycles.

Many rock deformation experiments used to characterize the frictional properties of tectonic faults are performed on powdered fault rocks or on bare rock surfaces. These experiments have been fundamental to document the frictional properties of granular mineral phases and provide evidence for crustal faults characterized by high friction. However, they cannot entirely capture the frictional properties of faults rich in phyllosilicates. Numerous studies of natural faults have documented fluid-assisted reaction softening promoting the replacement of strong minerals with phyllosilicates that are distributed into continuous foliations. To study how these foliated fabrics influence the frictional properties of faults we have: 1) collected foliated phyllosilicate-rich rocks from natural faults; 2) cut the fault rock samples to obtain solid wafers 0.8-1.2 cm thick and 5 cm x 5 cm in area with the foliation parallel to the 5x5cm face of the wafer; 3) performed friction tests on both solid wafers sheared in their in situ geometry and powders, obtained by crushing and sieving and therefore disrupting the foliation of the same samples; 4) recovered the samples for microstructural studies from the post experiment rock samples; and 5) performed microstructural analyses via optical microscopy, scanning and transmission electron microscopy. Mechanical data show that the solid samples with well-developed foliation show significantly lower friction in comparison to their powdered equivalents. Micro- and nano-structural studies demonstrate that low friction results from sliding along the foliation surfaces composed of phyllosilicates. When the same rocks are powdered, frictional strength is high, because sliding is accommodated by fracturing, grain rotation, translation and associated dilation. Friction tests indicate that foliated fault rocks may have low friction even when phyllosilicates constitute only a small percentage of the total rock volume, implying that a significant number of crustal faults are weak.

The presence of weak phyllosilicates in mature carbonate fault zones has been invoked to explain weak faults. However, the relation between frictional strength, fault stability, mineralogical composition, and fabric of fault gouge, composed of strong and weak minerals, is poorly constrained. We used a biaxial apparatus to systematically shear different mixtures of shale (68% clay, 23% quartz and 4% plagioclase) and calcite, as powdered gouge, at room temperature, under constant normal stresses of 30, 50, 100 MPa and under room-dry and pore fluid-saturated conditions, i.e. CaCO3-equilibrated water. We performed 30 friction experiments during which velocity-stepping and slide-hold-slide tests were employed to assess frictional stability and to measure frictional healing, respectively. Our frictional data indicate that the mineralogical composition of fault gouges significantly affects frictional strength, stability, and healing as well as the presence of CaCO3-equilibrated water. Under room-dry condition, the increasing shale content determines a reduction in frictional strength, from μ = 0.71 to μ = 0.43, a lowering of the healing rates and a transition from velocity-weakening to velocity-strengthening behavior. Under wet condition, with increasing shale content we observe a more significant reduction in frictional strength (μ = 0.65–0.37), a near-zero healing and a velocity strengthening behavior. Microstructural investigations evidence a transition from localized deformation promoted by grain size reduction, in calcite-rich samples, to a more distributed deformation with frictional sliding along clay-enriched shear planes in samples with shale content greater than 50%. For faults cutting across sedimentary sequences composed of carbonates and clay-rich sediments, our results suggest that clay concentration and its ability to form foliated and interconnected networks promotes important heterogeneities in fault strength and slip behavior.