From slow to fast earthquakes: laboratory insights on acoustic and mechanical fault slip behavior

A critical aspect of studying earthquake mechanisms involves understanding why a single fault can exhibit various slip behaviors. Fault heterogeneity leads to different slip behaviors in different fault portions: some slip seismically, generating catastrophic earthquakes, while others slip a-seismically in a stable and silent manner. Additionally, some fault portions exhibit slow, intermittent slip that can persist for months. Unraveling the physical mechanism at the base of these different fault slip behaviors is crucial for understanding how fault portions that slip slowly interact with portions capable of producing earthquakes.

In a laboratory setting, we can replicate the entire spectrum of fault slip behaviors by changing the loading stiffness of our experimental apparatus. Tuning the loading stiffness, we are able to match the critical rheological stiffness of the fault (kc) and investigate conditions around the critical point where k/kc = 1. Moreover, monitoring acoustic emissions (AEs) during laboratory earthquakes allows us to capture the rupture processes throughout the seismic cycle.

To constrain the nucleation mechanisms and rupture processes of different slip behaviors, we conducted friction experiments using quartz powder (MinUSil, average grain size 10 µm) to simulate fault gouge. The experiments were carried out in a double direct shear configuration, using an array of calibrated piezoelectric sensors for continuous, high acquisition rate (6 MHz) AE recording. The experiments were conducted at a constant displacement rate of 10 µm/s. During each experiment we maintained a constant normal stress and changed three acrylic blocks of different areas to change the apparatus stiffness (k). This technique allows us to reproduce both fast (i.e., when the apparatus stiffness is lower than a critical stiffness, k<kc) and slow (i.e., k=kc) slip events under the same stress conditions and test if the same fault patch can host a variety of slip behaviors.

Continuous AE recording, that is a proxy for seismicity, allows us to relate mechanical and acoustic fault behaviors. Our results show that different slip behaviors produce distinct acoustic waveforms during slip, with impulsive (high amplitude, short duration) AEs for fast slip, and emergent (low amplitude, longer duration) and continuous acoustic signals for slow slip. The distribution of AEs throughout the seismic cycle is characterized by an accelerating phase with small emissions for slow slips. While, fast slips exhibit no clear pre-seismic activity, and only strong AE in the co-seismic phase produced by fault rupture. Analyzing the frequency content of the acoustic signals also provides insights into the size, duration and the evolution of the seismic source along the seismic cycle.

By changing the stiffness of the fault, and monitoring acoustic emissions, our experiments not only accurately show that the same fault patch can experience different slip behaviors under the same stress conditions but also gives important insights into the complex dynamics of fault slip and rupture processes.