Slip Localization versus Instability Nucleation Feedback Loop: A Laboratory Perspective from Gouge Deformation Experiments

Experimental observations on fault gouges suggest that shear localization is a prerequisite for the nucleation of instabilities. Synthetic gouges typically produce lab-quakes when the shear deformation is localized along sharp, knife-edge shear planes – a condition that satisfies the steady-state existence requirement of the rate-and-state friction framework. Similarly, exhumed fault cores exhibit shear planes so localized that they appear as mirror surfaces.

Yet, the scenario in nature is far more complex. Many faults contain multiple fault cores, multiple localized shear planes, cemented fault rocks, and evidence for fault core recycling. This suggests that deformation localized along a principal slip plane during an earthquake – unlike in controlled shear experiments – does not necessarily persist over the lifespan of a mature fault. While in laboratory gouge experiments the steady state corresponds to a microstructural fabric that does not evolve significantly over seismic cycles, fault rock fabrics in nature evolve significantly during the seismic cycle—remaining far from a simple localized steady-state fabric.

Here, we present a preliminary study aiming at reconciling these two perspectives: the laboratory-derived nucleation, which involves deviations from a pre-existing steady-state, and the field-derived nucleation, often occurring far away from any steady-state condition. To address this, we conducted double-direct shear experiments on quartz gouges at 30 MPa normal stress, reactivating faults via shear stress steps that allowed spontaneous fault acceleration or deceleration.

The novelty of our approach lies in (1) the different textural states imposed on the gouge before reactivation and (2) the integration of acoustic emissions monitored during fault acceleration with post-mortem microstructures. Specifically, we designed three textures: a pre-localized texture – with localized shear planes developed by prior shearing at constant velocity and normal stress of 30 MPa for a few millimeters; a homogeneous texture – compacted under a normal stress of 30 MPa without prior shearing before reactivation; and an overconsolidated homogeneous texture – compacted under normal stress of 60 MPa before reactivation at 30 MPa.

Preliminary results reveal clear correlations between acoustic emission rate, slip evolution, and the degree of localization in post-mortem microstructures. Pre-localized textures remain locked until a critical stress is reached, after which they abruptly accelerate. Homogeneous textures display slow, progressive acceleration, with increasing slip velocity at higher shear stresses. The overconsolidated texture exhibits intermediate behavior. The acoustic emission signature during low constant-velocity slip reflects the grain interactions typical of granular flow, while rapid acceleration produces impulsive lab-quake-like signals typical of localized rupture nucleation.

These preliminary observations suggest a feedback loop between the localization of deformation and instability growth. While this type of relationship between shear strain localization and slip rate is well-established for the co-seismic propagation phase, our observations indicate that it may play a role during the nucleation phase, challenging the steady-state assumption commonly derived from laboratory studies.