Coexisting Slow-to-Fast Laboratory Earthquakes: Insights into Nucleation Processes in Fault Gouge Across Spatial Scales

Laboratory shear experiments provide valuable insights into the physical processes driving earthquakes. While the nucleation of lab earthquakes on bare rock surfaces has been extensively studied, the preparatory processes within fault gouge volumes remain poorly understood. To address this gap, we performed stick-slip experiments on a 5 × 5 cm² fault using granular quartz gouge under single and double direct shear configurations at normal stresses ranging from 40 to 50 MPa.

Our experiments reproduce the full spectrum of fault slip behaviors, from stable creep to slow and fast slip events. As strain accumulates and the internal structure of the gouge evolves, seismic cycles exhibit complex sequences, often with slow-slip foreshocks preceding rapid and energetic stress drops. We studied events with slip velocities spanning nearly two orders of magnitude (0.1 to 10 mm/s), highlighting the coexistence of slow and fast slip events on the same fault under identical boundary conditions. We used eddy-current sensors to track volumetric strain during the inter- pre- and co-seismic phases. Simultaneously, acoustic emissions (AEs) were recorded at 6.25 MHz using piezoelectric sensors embedded in the loading blocks. Event detection is performed with a custom-trained deep-learning model based on the PhaseNet model developed for seismic data. The temporal evolution of AEs, coupled with waveform similarity analysis, which serves as a proxy for the spatial progression of AEs, helps to constrain the preparation and nucleation processes of slip events characterized by different velocities. Despite the continuum between slow and fast slip modes revealed by mechanical and acoustic scaling, our results show that the acoustic behavior of slow and fast slip events in a well-developed gouge fault differs. Slow-slip events are characterized by longer durations and temporally distributed swarms of small AEs, while fast-slip events exhibit shorter durations and concentrated bursts of energetic AEs. Supported by seismological estimates of AE source parameters from calibrated piezoelectric sensors, we propose a micromechanical model in which the progressive failure of asperities, signaled by increasing AE rates, drives seismic slip to a critical nucleation point—reached only for fast-slip events and not for slow-slip events. Our results are framed within the rate-and-state framework, with nonlinear time-series analysis tools used to evaluate the predictability of laboratory seismic cycles.

Finally, to address the long lasting question of upscaling earthquake physical processes, we performed additional experiments on a larger 77 × 8 cm² fault at lower normal stresses with the same gouge material. High-resolution displacement and acoustic measurements in these tests provided detailed insights into the spatial evolution of slip, which can only be fully resolved with larger fault samples. This enables us to better constrain our previous results and investigate the impact of fault size on lab earthquake nucleation within the fault gouge volume, laying the foundation for upscaling laboratory observations to larger-scale experiments and, ultimately, to natural faults.