Laboratory investigation on the spectrum of fault slip modes

This dissertation investigates fundamental aspects of earthquake mechanics through laboratory experiments, focusing on fault slip behavior, stress triggering mechanisms, and seismic radiation. The work is structured in three interconnected studies that combine innovative experimental techniques with detailed analysis of mechanical and acoustic data. The first study examines how faults respond to stress perturbations under critically stable conditions. Through sinusoidal variations in normal stress, we identify conditions that can trigger fault instability. Our results demonstrate that high-frequency, high- amplitude perturbations can weaken the fault and potentially trigger instabilities, while lower amplitudes tend to strengthen it. Rate-and-state friction modeling successfully reproduces these observations. The second study presents a novel calibration technique for laboratory acoustic emissions, utilizing controlled sources generated by steel ball impacts. By applying the empirical Green function method to these sources, we develop a comprehensive characterization of the instrument apparatus response that accounts for sensor response, instrumentation effects, and path effects. This calibration enables accurate determination of source parameters from laboratory acoustic emissions, revealing stress drops consistent with natural earthquakes and supporting scale-invariant properties in earthquake mechanics. The third study investigates the complete spectrum of fault slip behaviors by systematically modulating the stiffness ratio of the experimental fault system. Our results demonstrate a continuous spectrum of slip modes, from slow to fast events, accompanied by distinct seismic signatures. Slow slip events generate swarms of small seismic events, while fast slip events produce single, larger bursts of seismic energy. The consistent scaling between mechanical energy and seismic moment release across all slip modes suggests similar fundamental physical processes govern rupture regardless of slip velocity. These findings provide new insights into earthquake triggering mechanisms, establish improved methods for analyzing laboratory seismic signals, and demonstrate the mechanical continuity between slow and fast earthquakes. This work advances our understanding of fault mechanics and contributes to bridging the gap between laboratory observations and natural fault systems.