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Authors: Spagnuolo, Elena 
Title: Nanoparticles control and record (?)earthquakes propagation at large scales
Issue Date: Apr-2017
Series/Report no.: Editorial Article
Abstract: Frictional motion between sliding contact surfaces is critical to artificial and natural mechanical systems rang- ing from the nanoscale to the kilometres length scale of a seismogenic fault. Earthquakes result from frictional instabilities along either pre-existing or newly formed faults, when the energy accumulated by the slow tectonic plate motion is abruptly released in mechanical waves and largely dissipated into heat. The generation of small to large earthquakes is regulated by the efficiency of frictional reduction (e.g. dynamic weakening) which has proven experimentally true for a large number of rock lithologies [1] through a variety of lubrication schemes. In the framework of earthquake mechanics one of the most intriguing and debated concepts is referred to as “nanoparticle lubrication”. Nanoparticles are found in active fault systems retrieved from deep drilling projects of subduction zones [2,3] and exhumed faults [4–6] but also appear insistently in a number of experiments that simulated earthquake nucleation and propagation conditions at depth [7–11]. The mechanism of nanoparticle lubrication is still debated and complicated by the fact that nanostructural textures of experimental samples are often short living and blurred by post-deformation re-crystallization and sintering processes [12]. Authors suggest that nanoparticles lubricate the fault by nano-rolling [13] or by forming a ‘third body’ that flows and separates the sliding components [7–9], a mechanism which is reminiscent of tribological studies in material science [14,15]. Recent high-speed friction experiments demonstrate that nanoparticles diffuse through thermally activated grain boundary sliding leading to a state of (almost) vanishing friction termed super plasticity [10,11]. What is widely accepted is that nanoparticles form by mechanical fragmentation and wear [16,17] and depend from strain rates (Sammis and Ben-Zion) and applied stress. More intimately, the transformation of crystalline mineral phases into nanoparticles follows atomic principles of dislocation mechanics. Moving dislocations appear to regulate thermal softening and frictional recovery of faults [18] whereas fast moving dislocations are responsible for instantaneous embrittlement of the contact bodies, consistent with a shock-like loading (e.g. Summis and Ben-Zion), resulting in grain size reduction to the nanoscale [19,20]. Dislocation motion dissipates the vast amount of power exerted by the extreme seismic deformation conditions (1–10 MW/m2) by transforming their kinetic motion into frictional heat [21,22]. Grain size reduction to the nanoscale enlarges the fracture energy and enhances the kinetics of chemical reactions whereas any further deformation induces mechanical amorphization [23]. The survivability of nanoparticles is temperature sensitive and their presence in exhumed fault zone is questionable, given the significant heat produced during an earthquake. Because of the short life of these nanomaterials, dissolution reaction kinetics of amorphous ultrafine material was proposed as a proxy to mark fault systems that underwent recent seismicity [24]. The possible use of nanoparticle as a seismic indicator explains the up surging interests in advances in nanotechnologies. This interest is not only academic since nanoparticles hide a potential for earthquake hazard improvement. Unfortunately, what has stunned the community in this regard, is that experiments run so far on a large range of deformation conditions (from aseismic to seismic deformation rates) [5,23–27] all discovered nano–and amorphous material at the contact surface. This evidence revealed that the presence of nanoparticles is a necessary but not sufficient condition for fault lubricity posing a question mark on the effectiveness of “nanoparticle lubrication”. Moreover, the dualities of nanoparticle behaviour in mesoscopic friction make questionable their use as potential seismic indicator which remains, in a sense, transparent to fault activity. From an atomistic point of view it is inspiring a recent result, achieved by exploiting a novel implementation of the Friction Force Microscope (FFM), showing that nanoparticles can co-exist in two frictional states, exhibiting frictional duality [28]. Some particles show a linear scaling with contact area reinforcing the Bowden and Tabor theory of friction at the nanoscale. Other particles remain in the super low friction state exhibiting almost vanishing friction because of a mechanism of atomistic mismatch (i.e., structural lubricity, which is responsible for example of super lubricity of solid graphite) [29]. The breakdown of the superlubric state is explained by a model of partial interface contamination [30] though a scenario of nanoparticle lattice’s orientation relative to the substrate was also examined. Some field and experimental observations have identified light-reflective surfaces (so-called mirror like) [6,31,32] that can be framed in the context of mesoscopic friction regulated by atomic mismatch. The mirror-like surfaces appear mechanically weak, ultra smooth (nanoscale to microscale roughness) at the white light interferometry, and with no clear grain boundaries [5,32] Mirror-like surfaces seems to support the key role played by the contact surface roughness and the possible coexistence of two frictional states where the disproportion, or the breakdown of one of the two, finally dominates the mesoscopic friction behaviour. The breakdown is likely ruled by heat or rather, by the way the vast amount of heat is dissipated through the shear zone and localizes into shear-bands. If the heat production is faster than it is diffused away from the bulk, heat localization is efficient in feeding nanoparticle lubrication mechanisms (e.g. MgO nanoparticles on bare surface of Carrara marble in Yao et al. [33]) or atomistic electronic rearrangement like sp3 → sp2 rehybridization of amorphous carbon [34]. When heat localizes a large amount of energy is then transferred into the atomic structure in a narrow zone of the contact frictional surface to allow electronic jumps, atomistic mismatch and, in general, thermal runaway. The duality in friction behaviour of nanoparticles is relevant to earthquake mechanics since it could justify the duality of frictional behaviour observed in natural seismic sequences, field observations and experimental faults. Though, the upscaling of frictional motion over more than ten orders of magnitudes in length remains partially hidden in the depth of the earth, reconciling all these observations at different scale is one of the main goals in on-going researches. A close-up connection between material science, rock deformation and geophysics is essential to reach this ambitious purpose.
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