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

Abstract

12Laboratory and theoretical studies provide insight into the mechanisms that control earthquake 13 nucleation, when fault slip velocity is slow (<0.001 cm/s), and dynamic rupture when fault slip rates 14 exceed cm/s. The application of these results to tectonic faults requires information about fabric 15 evolution with shear and its affect on the mode of faulting. Here we report on laboratory 16 experiments that illuminate the evolution of shear fabric and its role in controlling the transition 17 from stable sliding (v~0.001 cm/s) to dynamic stick-slip (v>1 cm/s). The full range of fault slip 18 modes was achieved by controlling the ratio, K = k/kc, where k is the elastic loading stiffness, and 19 kc is the fault zone critical rheologic stiffness. We show that K controls the transition from slow-20 and-silent slip (K > 0.9) to fast-and-audible (K < 0.7, v = 3 cm/s, slip duration 0.003 s) slip events. 21Microstructural observations show that with accumulated strain, deformation concentrates in shear 22 zones containing sharp shear planes made of nano-scale grains, which favour the development of 23 frictional instabilities. Once this fabric is established, fault fabric does not change significantly with 24 slip velocity, and fault slip behaviour is mainly controlled by the interplay between the rheological 25 properties of the slipping planes and fault zone stiffness. As applied to tectonic faults, our results 26 suggest that a single fault segment can experience a spectrum of fault slip behaviour depending on 27 the evolution of fault rock frictional properties and viscoelastic properties of the wall rock 28 surrounding the fault. 30Manuscript Click here to download Manuscript Text_vf.docx INTRODUCTION 31Understanding the relationship between the evolution of fault fabric and fault slip behaviour 32 is a long-standing problem in fault mechanics. Tchalenko (1970) used shear box experiments to 33 document fault zone evolution, from Riedel shears to boundary parallel shears with increasing 34 strain. The similarities of the experimental fault zone structure with earthquake faulting were 35 interpreted as indicating similarities in the deformation mechanism (Tchalenko, 1970). Several 36 laboratory and field studies have expanded on the kinematic descriptions, with the aim of 37 developing an integrated understanding of the evolution of fault zone microstructure and friction 38 constitutive properties (e.g. Sibson, 1977;Logan et al, 1979;Yund, 1990; Marone and Kilgore, 39 1993;Beeler et al., 1996). For quartzo-feldspatic fault gouge, increasing strain causes an evolution 40 from distributed to localized deformation along fault parallel shear planes. This microstructural 41 evolution is accompanied by a transition from a velocity strengthening (i.e. aseismic creep) to 42 velocity weakening behavior, which is a necessary condition for frictional instability (e.g. Marone, 43 1998). 44In the last decade, high-velocity friction experiments have shown that at earthquake slip 45 velocities (≥10cm/s) significa...

Frictional State Evolution During Normal Stress Perturbations Probed With Ultrasonic Waves

“…For ease of comparison, we express stiffness in units of MPa/μm and normalize stiffness values by normal stress and report stiffness in units of 1/μm (Figure ). The normalized shear stiffness decreases systematically with increasing normal stress, consistent with previous results (Leeman et al, , ; Scuderi et al, ). Note that the stiffness is higher for the smaller samples, because we are measuring the composite stiffness of the testing machine and DDS configuration.…”

“…A caveat of all previous laboratory studies on normal stress perturbations is the assumption of a single, constant value of loading stiffness and that changes in slip rate associated with a change in normal stress can be ignored for the range of conditions explored. However, recent work demonstrates that changing normal stress has a nontrivial effect on loading stiffness (Leeman et al, , , ; Scuderi et al, , ). Here we measure the effect of normal stress on shear loading stiffnesses and account for it in our data analyses and numerical simulations.…”

Asthenosphere Flow Modulated by Megathrust Earthquake Cycles

“…A wide range of fault slip styles have now been identified downdip megathrusts, including all of creep, slow-slip events, very-low frequency earthquakes, tsunami earthquakes, typical megathrust earthquakes, and giant ruptures (Araki et al, 2017;Baba et al, 2018;Ide, 2012;A. The style of rupture is dictated by the evolution of the frictional resistance during slip (Collettini et al, 2011;Leeman et al, 2016;Mele 10.1029/2018GL078197 Veedu & Barbot, 2016;Scuderi et al, 2017), the largest earthquakes probably involving strong weakening (H. Noda & Lapusta, 2013;Sone & Shimamoto, 2009;Toro et al, 2004), but the rheology of country rocks may also play an important role (Brantut et al, 2016;Fagereng & Sibson, 2010;Goswami & Barbot, 2018;H. The style of rupture is dictated by the evolution of the frictional resistance during slip (Collettini et al, 2011;Leeman et al, 2016;Mele 10.1029/2018GL078197 Veedu & Barbot, 2016;Scuderi et al, 2017), the largest earthquakes probably involving strong weakening (H. Noda & Lapusta, 2013;Sone & Shimamoto, 2009;Toro et al, 2004), but the rheology of country rocks may also play an important role (Brantut et al, 2016;Fagereng & Sibson, 2010;Goswami & Barbot, 2018;H.…”

Slow-slip events in semi-brittle serpentinite fault zones

“…If K < K c , then perturbations from steady-state are unstable and the system's oscillations grow without bound in phase space. One can observe a continuum of behaviors between slow-slip events 21 and large elastodynamic ruptures indexed by the stiffness ratio K/K c , as also observed in laboratory experiments [58][59][60] . A larger ratio implies more stable and slower slip events, whereas a smaller ratio implies instability and fast slip.…”