Coseismic fault lubrication by viscous deformation

Abstract

Despite the hazard posed by earthquakes, we still lack fundamental understanding of the processes that control fault lubrication behind a propagating rupture front and enhance ground acceleration. Laboratory experiments show that fault materials dramatically weaken when sheared at seismic velocities (> 0.1 m s -1 ). Several mechanisms, triggered by shear heating, have been proposed to explain the coseismic weakening of faults, but none of these mechanisms can account for experimental and seismological evidence of weakening. Here we show that, in laboratory experiments, weakening correlates to local temperatures attained during seismic slip in simulated faults for diverse rockforming minerals. The fault strength evolves according to a simple, material-dependent Arrheniustype law. Microstructures support this observation by showing the development of a principal slip zone with textures typical of sub-solidus viscous flow. We show evidence that viscous deformation (either at sub-or super-solidus temperatures) is an important, widespread and quantifiable coseismic lubrication process. The operation of these highly effective fault lubrication processes means that more energy is then available for rupture propagation and the radiation of hazardous seismic waves.Earthquakes are amongst the deadliest natural disasters, with statistics showing a global death toll of > 50,000 per year, in the period 2000-2016 1 . Despite their impact on society, there is still a lack of fundamental understanding about earthquake constitutive behaviour. During seismic events, part of the mechanical energy stored in the stressed rocks is dissipated by frictional heating along the fault, causing the local temperatures to rise 2,3 . This promotes the onset of thermally-activated weakening mechanisms that help to reduce the shear strength 4-6 in the fast sliding portion of the fault, behind the rupture front 2,3 . Efficient lubrication means that more elastic energy can be transferred to the rupture tip,

Fault Friction During Simulated Seismic Slip Pulses

“…Where flash heating is utilized to model the fault strength of carbonate built faults, then final restrengthening is always over predicted. In the case that a creep constitutive law is used there are some significant differences between requisite grain sizes for accurate strength predictions and observed grain sizes from microstructural observations (De Paola et al, 2015;Pozzi et al, 2018Pozzi et al, , 2021. This discrepancy remains unresolved, and might be due to incorrect assumptions about our choice of deformation mechanism or the estimated temperature.…”

“…The nanometric grain size required to match fault strength is probably unrealistic (De Paola et al, 2015;Pozzi et al, 2018Pozzi et al, , 2021Violay et al, 2013). However, this could be remedied by using a modified, much larger value for the preexponential factor in Equation 4; here, we decided to use an empirical estimate from an exisiting data set obtained at low strain rate, but several physical phenomena might dramatically change that value.…”

Rate and State Friction as a Spatially Regularized Transient Viscous Flow Law

“…As noted in Section 1.2 of the Introduction, a variety of continuum theories have been developed to explain the general rate‐ and state‐like behavior of deformation in faults and shear zones (e.g., Alevizos et al., 2014; Barbot, 2019; Braeck & Podladchikov, 2007; Chen & Spiers, 2016; Daub & Carlson, 2008, 2009; Daub et al., 2008, 2010; Elbanna & Carlson, 2014; John et al., 2009; Kurzon et al., 2019, 2020; Lyakhovsky & Ben‐Zion, 2014a, 2014b; Lyakhovsky et al., 2011, 2016; Niemeijer & Spiers, 2007; Poulet et al., 2014; Pozzi et al., 2021; Rattez, Stefanou, & Sulem, 2018; Rattez, Stefanou, Sulem, Veveakis, & Poulet, 2018; Roubíček, 2014; Rozel et al., 2011; Sleep, 1997; Thielmann, 2018; Thielmann et al., 2015; Van den Ende et al., 2018; Veveakis et al., 2014). We note again that an internal length scale in these models typically arises from the inclusion of a diffusion process (of e.g., temperature, pore pressure) but that may not always be adjusted to meet the constraints imposed by scale and computational power without changing the outcome of the model.…”

“…This knowledge has spurred a search for intermediate-scale continuum homogenizations of distributed microscale processes that obey the rate and state framework in a somewhat generalized sense. Currently proposed models focus on rheological feedback mechanisms involving temperature (Braeck & Podladchikov, 2007;Garagash, 2012;John et al, 2009;Platt et al, 2014;Pozzi et al, 2021;Rice, 2006;Rice et al, 2014;Roubíček, 2014; PRANGER ET AL.…”

“…This knowledge has spurred a search for intermediate‐scale continuum homogenizations of distributed micro‐scale processes that obey the rate and state framework in a somewhat generalized sense. Currently proposed models focus on rheological feedback mechanisms involving temperature (Braeck & Podladchikov, 2007; Garagash, 2012; John et al., 2009; Platt et al., 2014; Pozzi et al., 2021; Rice, 2006; Rice et al., 2014; Roubíček, 2014; Thielmann, 2018; Thielmann et al., 2015), grain size (Barbot, 2019; Pozzi et al., 2021; Rozel et al., 2011; Thielmann, 2018; Thielmann et al., 2015), porosity (Chen & Spiers, 2016; Niemeijer & Spiers, 2007; Sleep, 1997; Van den Ende et al., 2018), fluid pressure (Alevizos et al., 2014; Barbot & Fialko, 2010; Garagash, 2012; Platt et al., 2014; Poulet et al., 2014; Rattez, Stefanou, & Sulem, 2018; Rattez, Stefanou, Sulem, Veveakis, & Poulet, 2018; Rice et al., 2014; Veveakis et al., 2014), damage (Kurzon et al., 2019, 2020; Lyakhovsky & Ben‐Zion, 2014a, 2014b; Lyakhovsky et al., 2011, 2016), granular physics (Daub & Carlson, 2008, 2009; Daub et al., 2008, 2010; Elbanna & Carlson, 2014; Hermundstad et al., 2010) or combinations thereof. Continuum models employing a rate and state formulation with the slip rate and tractions replaced by strain rate and stress invariants have been developed by Herrendörfer et al.…”