Synchronized oscillations and acoustic fluidization in confined granular materials

Abstract

According to the acoustic fluidization hypothesis, elastic waves at a characteristic frequency form inside seismic faults even in absence of an external perturbation. These waves are able to generate a normal stress which contrasts the confining pressure and promotes failure. Here we study the mechanisms responsible for this wave activation via numerical simulations of a granular fault model. We observe the particles belonging to the percolating backbone, which sustains the stress, to perform synchronized oscillations over elliptic-like trajectories in the fault plane. These oscillations occur at the characteristic frequency of acoustic fluidization. As the applied shear stress increases, these oscillations become perpendicular to the fault plane just before the system fail, opposing to the confining pressure, consistently with the acoustic fluidization scenario. The same change of orientation can be induced by external perturbations at the acoustic fluidization frequency.PACS numbers: 45.70.Vn, 63.50.Lm, 91.30.Px Confined granular materials under shear display the typical stick-slip dynamics observed in real fault systems. In the last years this dynamics has been deeply investigated in several experimental settings as well as by means of molecular dynamics simulations [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. These studies mostly focus on two central questions: i) Why the stress responsible for seismic failure is usually orders of magnitude smaller than the value expected on the basis of rock fracture mechanics? ii) Why seismic faults are very susceptible to even small amplitude transient seismic waves? Indeed, the resistance to shear stress of seismic faults is typically much larger than the one obtained in experiments measuring the friction coefficient of sliding rocks [15]. Furthermore, remote triggering of earthquakes [16][17][18][19][20] at distances of thousand kilometers from the main shock epicenter indicates a high susceptibility of seismic faults to the passage of seismic waves. The hypothesis of Acoustic Fluidization (AF), formulated by Melosh [21,22], provides an answer to both questions. According to AF, the elastic waves produced by seismic fracture, at a characteristic frequency ω AF , diffuse and scatter inside the fault and then generate a normal stress which can contrast the confining pressure. In this way seismic failure is promoted. To investigate this scenario, experimental studies [1,2,6,7] have demonstrated that acoustic perturbations modify granular rheology and lead to auto-acoustic compaction [7]. Recently the AF scenario has been explored in 3D molecular dynamics simulations [23] which have shown that weak external perturbations, at the frequency ω AF , even if increasing the confining pressure or reducing the applied shear, induce slip instabilities. Interestingly simulations have also shown that oscillations at the frequency ω AF are activated immediately before each slip, even in the absence of an external perturbation. Nevertheless the mechanisms responsible for this ac...

Noise amplification in frictional systems: Oscillatory instabilities

“…To drive home the message even further, we now remove the requirement of a smooth tangential force at the Coulomb threshold, allowing the tangential force to reach the limit in an abrupt way. Now the oscillatory instability disappears, since the Hamiltonian (31) remains now valid all the way to the Coulomb threshold. The price is of course that the Jacobian matrix J does not exist since the tangential force is not derivable at the threshold.…”

“…These simulations have shown that a granular system may become unstable without any new contact reaching its Coulomb threshold [26]: only after the system becomes unstable and starts slipping new contacts are seen to reach their Coulomb limit. An indirect support towards the existence of collective failure mechanisms also stems from the existence of a linear regime in the response of granular systems to applied perturbations [27][28][29][30][31], which is not easily rationalized assuming failure to originate from an underlying discontinuous process such as the Coulomb failure of a contact.…”

Induced and endogenous acoustic oscillations in granular faults

“…In order to identify the mechanisms responsible for the activation of these oscillations, the trajectories of each grain inside the stick phase can be monitored in numerical simulations [55]. Because of the high granular density, the large majority of particles is always in contact with their neighbours forming an almost rigid structure, i.e.…”

“…In order to identify the mechanisms responsible for the activation of these oscillations, the trajectories of each grain inside the stick phase can be monitored in numerical simulations [55].…”