Heterogeneous Stress–Driven Shear Localization Governing Fault Slip in Decimeter-Scale Quartz Gouge under Variable Surrounding Stiffness

Fault slip stability is governed by the competition between the elastic energy stored in the surrounding medium and the rate of frictional weakening of the fault, which depends on its constitutive frictional properties. Laboratory shear experiments validate this framework under simplified conditions and homogeneous boundary conditions. In contrast, natural faults are heterogeneous across multiple scales, with variations in stress, frictional properties, fault-zone structure, and elastic properties as documented in laboratory, numerical, and field studies. In gouge-filled faults,  heterogeneities are dynamically coupled: stress concentrations promote shear localization, which modifies gouge fabric that controls the frictional properties, ultimately dictating fault stability and rupture dynamics. Resolving how spatially heterogeneous stress and evolving fault-zone structure interact to control rupture nucleation and propagation therefore requires experimental approaches that move beyond homogeneous assumptions.

Here we present results from large-scale biaxial shear experiments on a quartz gouge–filled fault (75 cm × 8 cm) using two forcing-block materials—Nylon 6 and PMMA—with different elastic stiffnesses. The fault was densely instrumented to investigate how stress heterogeneity and evolving shear fabric control slip behavior under different nominal normal stresses from 3 to 10 MPa and a constant loading rate of 10 µm/s. Far-field stress and displacement were monitored using load cells and LVDTs (1 kHz), while forcing blocks deformation was measured using Digital Image Correlation (DIC, 2–30 Hz). During laboratory earthquakes, local fault slip and volumetric deformation were recorded using eddy-current displacement sensors (1.25 MHz) and high-speed DIC (10 kHz). Emitted acoustic waves were recorded at 3.125 MHz using an array of 33 piezoelectric sensors calibrated through ball-drop tests, active-ultrasonic survey, laser vibrometry, and spectral-element waveform modeling.

The experiments produce a broad spectrum of slip behaviors, from stable creep, slow ruptures to fast, dynamic events. Transitions from slow to fast slip are promoted by increasing normal stress and decreasing elastic stiffness. Co-seismic slip, peak slip velocity, and high-frequency acoustic energy increase systematically with cumulative fault slip, increasing normal stress, and decreasing loading stiffness. Direct measurements of slip enable estimation of the critical nucleation length, which decreases with increasing cumulative slip and normal stress, in agreement with theoretical predictions. Finite-element modeling shows that the experimental geometry induces heterogeneous stress distributions promoting the development of heterogeneous shear fabrics and spatially variable frictional responses. When shear fabric is well developed, normal stress is low, and the nucleation lengths are correspondingly large, stress heterogeneities have little impact  on slip dynamics, which is dominated by system spanning events with regular, periodic seismic cycles. Conversely, at higher normal stress—conditions associated with smaller nucleation lengths—and/or poorly developed shear fabric, stress heterogeneity drives complex slip behavior, including partial and full ruptures and rupture cascades characterized by strongly spatially variable stress drops. These results highlight how the coupled evolution of stress, shear fabric, and frictional heterogeneities controls slip dynamics in gouge-filled faults.