The spectrum of slip modes on gouge-filled faults spans a continuum from fast ruptures to slow slip events. The nucleation of a certain slip mode is governed by the frictional heterogeneity of fault interface and the rheological fault stiffness. Though the pattern of mechanical parameter variation and dynamic stability loss during a seismic cycle is quite clear, it is important to have a unified seismic-acoustic signature of slow or fast slip event nucleation. We present laboratory acoustic emission (AE) experiments on a slider-model with a precise control of mechanical and AE parameters. A comprehensive analysis of AE activity points to the presence of two AE subpopulations. One of them manifests as pulses with harsh onsets. The second one exhibits a gradual amplitude rise and tremor-like signal. The second AE subpopulation shows a longer failure duration and increased energy dissipation. Regularities of changing the frequency-amplitude characteristics of AE subpopulations during a laboratory seismic cycle differ. The first AE subpopulation retains parameters of frequency-amplitude distribution, but the second one exhibits a pronounced cyclic recurrence of the b-value. The latter decreases before slip events and recovers after them. The detected features of AE subpopulations are common for the entire spectrum of slip modes. Findings reveal a coexistence of slow and fast modes at the same fault at the micro-scale and point to the unity of underlying physical mechanisms of different slip mode nucleation.

The spectrum of slip modes on gouge-filled faults spans a continuum from fast ruptures to slow slip events. The nucleation of a certain slip mode is governed by the frictional heterogeneity of fault interface and the rheological fault stiffness. There is a mounting evidence that a single fault can host multiple slip modes. We present laboratory acoustic emission (AE) experiments on a slider-model. The entire spectrum of fault slip modes with a precise control of mechanical and AE parameters was reproduced and the unity of underlying mechanisms of slip mode nucleation was detected. A comprehensive analysis of AE activity allows revealing coexistence of two distinct subpopulations of acoustic pulses (APs) emitted during a seismic cycle and accompanying nucleation of different fault slip modes. One of them manifests as APs with harsh onsets. The second one exhibits a gradual amplitude rise and a tremor-like waveform. The second AP subpopulation shows longer failure duration and increased energy dissipation. During a seismic cycle, the first AP subpopulation retains parameters of frequency-amplitude distribution, while the second one exhibits a pronounced cyclic recurrence of the b-value. The b-value of the second subpopulation decreases before slip events and recovers after them. The detected features of AE evolution are common for the entire spectrum of fault slip modes.