Abstract

Rupture fronts can cause fault displacement, reaching speeds up to several ms−1 within a few milliseconds, at any distance away from the earthquake nucleation area. In the case of silicate-bearing rocks the abrupt slip acceleration results in melting at asperity contacts causing a large reduction in fault frictional strength (i.e., flash weakening). Flash weakening is also observed in experiments performed in carbonate-bearing rocks but evidence for melting is lacking. To unravel the micro-physical mechanisms associated with flash weakening in carbonates, experiments were conducted on pre-cut Carrara marble cylinders using a rotary shear apparatus at conditions relevant to earthquakes propagation. In the first 5 mm of slip the shear stress was reduced up to 30% and CO2 was released. Focused ion beam, scanning and transmission electron microscopy investigations of the slipping zones reveal the presence of calcite nanograins and amorphous carbon. We interpret the CO2 release, the formation of nanograins and amorphous carbon to be the result of a shock-like stress release associated with the migration of fast-moving dislocations. Amorphous carbon, given its low friction coefficient, is responsible for flash weakening and promotes the propagation of the seismic rupture in carbonate-bearing fault patches.

Highlights

  • Evidence for thermal decomposition[15,16,17] suggests that frictional heating occurs to activate thermochemical pressurization of fluids (CO2) leading to frictional strength reduction[15,16]

  • The experiments replicate the effect of an abrupt acceleration to seismic slip rates, which occurs when a part of the fault is subjected to a strong modification of the stress field

  • We suggest that fast-moving dislocations within carbonate-bearing rocks cause the abrupt temperature increase, rock nano-fragmentation and the formations of patches of amorphous carbon that, in analogy with flash melting in the case of silicate-bearing rocks, trigger fault weakening at the initiation of seismic slip

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Summary

Introduction

Evidence for thermal decomposition[15,16,17] (i.e., decarbonation vesicles) suggests that frictional heating occurs to activate thermochemical pressurization of fluids (CO2) leading to frictional strength reduction[15,16]. With progressive slip the transition to frictional weakening can be explained by nanoparticle and powder lubrication[18,19,20,21], grain boundary sliding, nanometric flow and phase transformations[22,23,24] or dynamic recrystallization[25,26,27]. All these proposed mechanisms rely on the presence of nanograins, crystal-plastic microstructures and mirror-like surfaces observed both in experimental and natural faults[26,27,28]. We suggest that fast-moving dislocations within carbonate-bearing rocks cause the abrupt temperature increase, rock nano-fragmentation and the formations of patches of amorphous carbon that, in analogy with flash melting in the case of silicate-bearing rocks, trigger fault weakening at the initiation of seismic slip

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