Editorial Article

Nanoparticles control and record (?) Earthquakes propagation at large scales

Dr. Elena Spagnuolo,
National Institute of Geophysics and Volcanology, Rome.

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.

*Corresponding author:

Elena Spagnuolo,
elena.spagnuolo@ingv.it

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.

SEarthquakes 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 regu- lated by the efficiency of frictional reduction (e.g. dynamic weakening) which has proven experimentally true for a large number of rock lithologies (Di Toro et al. 2011) 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 ac- tive fault systems retrieved from deep drilling projects of subduction zones (e.g. Ma et al. 2006; Hirono et al. 2014 and references therein) and exhumed faults (e.g. Chester et. al, 2005; Fondriest et al. 2013; Siman-Tov et al, 2011) but also appear insistently in a number of experiments that simulated earthquake nucleation and propagation conditions at depth (e.g. Han et al. 2007; 2011; Reches and Lockner, 2010; De Paola et al. 2015; Green at al. 2015).

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 (Holyoke et al., 2013, and references therein).

Authors suggest that nanoparticles lubricate the fault by nano-rolling (Han et al. 2010) or by forming a ‘third body’ that flows and separates the sliding components (Han et al. 2007; 2010; Reches & Lockner, 2010), a mechanism which is reminiscent of tribological studies in material science (e.g., Rapoport et al., 2003; Wornyoh et al., 2007). Recent high- speed friction experiments demonstrate that nanoparticles diffuse through thermally activated grain boundary sliding leading to a state of (almost) vanishing friction termed superplasticity (De Paola et al. 2015; Green at al. 2015).

What is widely accepted is that nanoparticles form by mechanical fragmentation and wear (Boneh, et al. 2013; Reches and Dewers, 2005) and depend from strain rates (Sammis and Ben-Zion, 2008) and applied stress. More intimately, the transformation of crystalline mineral phases into nanoparticles follows atomic principles of dislocation mechanics. Moving dislocations appear to regulate ther- mal softening and frictional recovery of faults (e.g. Hirth and Tullis, 1992) 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, 2008), resulting in grain size reduction to the nanoscale (Spagnuolo et al. 2015; 2016). 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 (Armstrong and Elban 1989; De Hosson et al. 2001).

Grain size reduction to the nanoscale enlarges the fracture energy and enhances the kinetics of chemical reac- tions whereas any further deformation induces mechanical amorphization (e.g. Balaz et al. 2013). 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 (Hirono et al., 2016).

The possible use of nanoparticle as a seismic indica- torexplains the up surging interests in advances in nano- technologies. This interest is not only academic since nanoparticles hide a potential for earthquake hazard improvement. Unfortunately, what has stunned the com- munity in this regard, is that experiments run so far on a large range of deformation conditions (from aseismic to seismic deformation rates, e.g. Fondriest at al. 2013, Yund et al., 1990; Verberne et al., 2014, 2015; Aretusini et al. 2017) 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 effective- ness 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 na- noparticles can co-exist in two frictional states, exhibiting frictional duality (Dietzel et al. 2008). 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 superlubricity of solid graphite, Dienwiebel et al., 2004). The breakdown of the superlubric state is explained by a model of partial interface contamination (He et al. 1999) though a scenario of nanoparticle lattice’s orientation relative to the substrate was also examined.

Some field and experimental observations have identi- fied light-reflective surfaces (so-called mirror like, Smith et al. 2013, Siman-Tov et al. 2013, and Kuo et al. 2016) 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 (Fondriest at al. 2013, Kuo et al. 2016). 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. 2016) or atomistic electronic rearrangement like sp3 → sp2 rehybridization of amorphous carbon (Ma et al., 2014). When heat local- izes 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 be- tween material science, rock deformation and geophysics is essential to reach this ambitious purpose.

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Published: 18 April 2017

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Copyright: © 2017 Elena Spagnuolo. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.