Acoustic signatures of slow and fast earthquake: insights from laboratory experiments on simulated fault gouge

In the last decades, it has been observed that faults can slip both by slow aseismic creep and seismic events (i.e., earthquakes). Between these two slip modes, a wide variety of fault slip behavior can be observed, including low-frequency earthquakes, slow slip events and tremors. This wide variety of slip modes can radiate seismic energy at different frequencies whose content may be linked to the physical mechanisms at play. In the laboratory, it is possible to reproduce the entire spectrum of fault slip modes by modulating the loading stiffness of the apparatus depending on the critical fault rheologic stiffness (i.e. k/kc). This technique allows us to study, under controlled laboratory conditions, the acoustic signature of different fault slip modes to infer the physical mechanisms at their origin. To shed light on the nucleation mechanisms and seek for reliable precursors to failure of different slip modes, we performed friction experiments on powders that differ for granulometry and grain shape (i.e., glass beads with a grainsize < 150 µm; and quartz powders MinUSil with an average grain size of 10.5 µm), to simulate fault gouge. The experiments were conducted in a double direct shear configuration, instrumented with an array of piezoelectric sensors to record continuously Acoustic Emissions (AEs) at high recording rate (~10MHz). The experiments are performed at a constant displacement rate of 10 µm/s and using a spring to reduce the apparatus stiffness k, to match the critical fault rheological stiffness (kc). Following this procedure we obtain slow slip events (i.e., k = kc) and fast events (i.e. k