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...