Frictional behaviour of halite/muscovite fault gouge analogues: Effects of sliding velocity and of stepwise acceleration from sub- to co-seismic slip rates

Investigating the (transient) frictional behaviour of fault rock from low to high velocities is required in experimental research on the stability of faults and earthquake behaviour. Fault stability is an important factor controlling earthquake nucleaction, and it may also control the subsequent propagation of rupture over the fault zone. Rupture may propagate easily in unstable fault rock, but may be inhibited by a stably deforming region (eg.a region deforming by creep). Also phyllosilicates are ubiquitous in natural fault rock and may influence deformation behaviour. Experiments on analogue gouge were conducted in the low-to-high-velocity rotary shear apparatus (HV2 machine) at Hiroshima University. Both pure halite and halite + muscovite were sheared from 0.1 µms-1 up to 1 ms-1 at room-dry conditions and a low normal stress (5MPa). Mechanical data and microstructural evidence reveal velocity weakening from 1 µms-1 to ~0.01 ms-1, deformation being brittle and very localized, velocity strengthening from 0.01-0.1 ms-1 with deformation by an interplay of the formation of strong dense aggregates by healing and breaking up this material in smaller fragments in a distributed manner, and strong velocity weakening from 0.1-1 ms-1, where strong localization and possibly melting occur. Pressure solution occurs to some extent in the room-dry gouges, presumably causing observed stick-slip behaviour and healing. The addition of muscovite causes a lower frictional strength and more unstable behaviour from 0.1-10 µms-1, and is slightly higher velocity strengthening at 0.1 ms-1. Wet halite + muscovite experiments conducted by Niemeijer & Spiers (2005) were reproduced in the HV2 machine from 0.01 µms-1 and extended to higher velocities, up to 0.1 ms-1 where pore water evaporates. At low velocities (< 1 µms-1) the material is velocity strengthening, and sigmoidal halite clasts with the beginning of a muscovite foliation wrapping around them were observed. Velocity weakening was observed at velocities >1 µms-1, with the microstructure revealing a cataclasic, well mixed structure with a zone where fine grained material has accumulated. The frictional behaviour was also found to depend on the surface roughness of the wall-rock; smoother wall-rock yields more loaclization and unstable behaviour. To simulate rupture coming in on the stably sliding zone, large velocity steps were conducted from this regime to higher velocities. These steps could be modelled with RSF. The velocity strengthening regime found in the room-dry experiments at 0.01-0.1 ms-1 may act as a barrier to earthquake rupture. To investigate this, several large velocity steps were made from the velocity strengthening regime into the high velocity regime, revealing an initial slip strengthening stage over a significant displacement, before a transition to rapid slip weakening occurred. This strengthening stage is not recognized in traditional rate-and-state friction (RSF), implying frictional behaviour is governed by different constitutive properties and extrapolation from RSF to the high-velocity regime may not be straightforward