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The unjamming transition of granular systems is investigated in a seismic fault model via three dimensional Molecular Dynamics simulations. A two-time force-force correlation function, and a susceptibility related to the system response to pressure changes, allow to characterize the stick-slip dynamics, consisting in large slips and microslips leading to creep motion. The correlation function unveils the micromechanical changes occurring both during microslips and slips. The susceptibility encodes the magnitude of the incoming microslip.PACS numbers: 46.55.+d; 45.70.Ht; 91.30.Px In a number of industrial processes and natural phenomena, such as earthquakes or landslides, disordered solid granular systems start to flow. This solid-to-liquid transition, known as unjamming, occurs either on decreasing the confining pressure P , or increasing the applied shear stress σ. Understanding the properties of this transition is a big challenge due to the absence of an established theoretical framework for granular materials. A proposed analogy with the glass transition [1] of thermal systems has recently triggered the study of the jamming transition via numerical investigations of systems of soft frictionless particles at zero applied shear stress [2], where the only control parameter is the pressure (or the density). As the unjamming transition is approached by decreasing the confining pressure, the vibrational spectrum develops an excess of low frequency modes, known as soft-modes, leading to the identification of a length scale which diverges on unjamming [3]. This length scale is related to the emergence of an increasingly heterogeneous response as the system moves towards the transition [3]. A different approach to the study of the unjamming transition has been followed in a two dimensional numerical study [4] and in a number of experiments [5][6][7][8], where the applied shear stress is controlled via a spring mechanism, as the one in Fig. 1a. A stick-slip motion characterized by a complex slip size statistics [7] is recovered at high confining pressures P and small driving velocities V d . This stick-slip dynamics is altered by the presence of noise [9]. Analogous results have been found at fixed strain rate [10,11].

The unexpected weakness of some faults has been attributed to the emergence of acoustic waves that promote failure by reducing the confining pressure through a mechanism known as acoustic fluidization, also proposed to explain earthquake remote triggering. Here we validate this mechanism via the numerical investigation of a granular fault model system. We find that the stick-slip dynamics is affected only by perturbations applied at a characteristic frequency corresponding to oscillations normal to the fault, leading to gradual dynamical weakening as failure is approaching. Acoustic waves at the same frequency spontaneously emerge at the onset of failure in absence of perturbations, supporting the relevance of acoustic fluidization in earthquake triggering.

We investigate a recently introduced seismic fault model where granular particles simulate fault gouge, performing a detailed analysis of the size distribution of slipping events. We show that the model reproduces the Gutenberg-Richter law characterising real seismic occurrence, independently of model parameters. The effect of system size, elastic constant of the external drive, thickness of the gouge, frictional and mechanical properties of the particles are considered. The distribution is also characterised by a bump at large slips, whose characteristic size is solely controlled by the ratio of the drive elastic constant and the system size. Large slips become less probable in the absence of fault gouge and tend to disappear for stiff drives.