Can grain size sensitive flow lubricate faults during the initial stages of earthquake propagation?

Abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract 16Recent friction experiments carried out under upper crustal P-T conditions have shown that 17 microstructures typical of high temperature creep develop in the slip zone of experimental 18 faults. These mechanisms are more commonly thought to control aseismic viscous flow and 19 shear zone strength in the lower crust/upper mantle. In this study, displacement-controlled 20 experiments have been performed on carbonate gouges at seismic slip rates (1 ms -1 ), to 21 investigate whether they may also control the frictional strength of seismic faults at the higher 22 strain rates attained in the brittle crust. At relatively low displacements (< 1cm) and 23 temperatures (≤ 100 °C), brittle fracturing and cataclasis produce shear localisation and grain 24 size reduction in a thin slip zone (150 µm). With increasing displacement (up to 15 cm) and 25 2 temperatures (T up to 600 °C), due to frictional heating, intracrystalline plasticity mechanisms 26 start to accommodate intragranular strain in the slip zone, and play a key role in producing 27 nanoscale subgrains (≤ 100 nm). With further displacement and temperature rise, the onset of 28 weakening coincides with the formation in the slip zone of equiaxial, nanograin aggregates 29 exhibiting polygonal grain boundaries, no shape or crystal preferred orientation and low 30 dislocation densities, possibly due to high temperature (> 900 °C) grain boundary sliding 31 (GBS) deformation mechanisms. The observed micro-textures are strikingly similar to those 32 predicted by theoretical studies, and those observed during experiments on metals and fine-33 grained carbonates, where superplastic behaviour has been inferred. To a first approximation, 34 the measured drop in strength is in agreement with our flow stress calculations, suggesting 35 that strain could be accommodated more efficiently by these mechanisms within the weaker 36 bulk slip zone, rather than by frictional sliding along the main slip surfaces in the slip zone. 37Frictionally induced, grainsize-sensitive GBS deformation mechanisms can thus account for 38 the self-lubrication and dynamic weakening of carbonate faults during earthquake propagation 39 in nature. 40

Dislocation Motion and the Microphysics of Flash Heating and Weakening of Faults during Earthquakes

“…The former tends to be the predominant deformation mode at lower stress level; both are typical deformation mechanisms of the viscous-plastic regime at crustal and mantle sub-seismic strain rates (10´1 4 -10´1 0 s´1, e.g., [76][77][78] and references therein). Little is known about micromechanics associated with the high velocity side (10 1 -10 4 s´1 or seismic conditions) of the natural strain rate spectrum [79]. Our observations seem to suggest that the high velocity spectrum is likely dominated by brittle mechanisms over ductile mechanisms.…”

Evolution of shear fabric in granular fault gouge from stable sliding to stick slip and implications for fault slip mode

“…Goldsby and Tullis 2002; Hirose and Shimamoto, 2005;Reches and Lockner 2010; Di Toro et 47 al., 2011;De Paola et al, 2015;Smith et al, 2015). These studies have been fundamental to 48 characterize fault rocks produced during the earthquake slip phase, however, these experiments are 49 conducted with imposed co-seismic velocities, whereas along natural faults the instability arises 50 spontaneously and is driven by the elastic interaction between the fault zone and the surrounding 51 (e.g.…”

From slow to fast faulting: recent challenges in earthquake fault mechanics

“…Then implicitly summing all possible i, j in γ ij σ ij for the off-fault mechanical work, we may write using (3.5) and (3.6) 8) and hence summing all terms and using stress symmetry we can re-write the anelastic strain dissipation as…”

“…Such knowledge has been established only recently, because the technical implementation of high-velocity, seismic-like conditions in laboratory experiments was achieved no earlier than the 1980s with the pioneering work of Shimamoto & Tsutsumi [55] and became widespread in the last decade (see [8,33,[56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72] among others). It is notable that such high-velocity weakening has been documented even on rocks which show rate-strengthening in traditional, slow frictional tests.…”