Physical controls of spontaneous and triggered slow-slip and stick-slip at the fault gouge scale

Behrooz Ferdowsi, & David L. Goldsby

Published August 15, 2016, SCEC Contribution #7005, 2016 SCEC Annual Meeting Poster #038 (PDF)

Fault slip modes span a continuum of behaviors from tremors and slow slips to earthquakes. Recent laboratory studies reveal that a spectrum of slow-slip responses emerge near the threshold between stable and unstable failure, governed by the complex interplay of frictional properties, effective normal stress and the elastic stiffness of the damaged and undamaged zones of the fault. Here we use simulations of particle dynamics at the micro- to meso-scales of fault gouge to explore these observations. The virtual experiments are carried out at a range of loading conditions, including confining pressures of 0.5 to 50 MPa and boundary shear rates of 10-6 to 10-1 m/s. Our granular system consists of grains with the mechanical properties of quartz. The grains interact with each other in the normal direction via a Hertzian contact law and in the shear direction via a Coulomb friction law. The shear velocity is applied through a spring attached to the rigid top boundary of the gouge, with values of the spring stiffness that vary from infinity (the stiff/rigid spring limit) to finite, and soft, values. In the limit of infinite spring stiffness, the dynamical behavior of the gouge transitions from stick-slip to continuous (steady) sliding as the shear rate is increased from low to high values. For a finite and soft spring stiffness, the relative stiffnesses of the spring and gouge, and the feedback between these two stiffnesses, set the internal gouge dynamics and control the slip modes. We use grain-scale forces and displacements obtained from numerical simulations to reconstruct the variation of local stress versus local slip displacement for the spectrum of slip modes. Previous observations by Lehman et al. (Nat. Commun., 2016) suggest that slow earthquakes are governed by the same instabilities in dynamical phase space (stiffness, k, velocity, V and pressure, Pn ) as normal earthquakes; however, this hypothesis needs to be tested at the local and grain scales. To accomplish this, we measure local stresses, deformation components and displacements in numerical simulations for different slip modes to compare and contrast the dynamical transitions that give rise to these modes. Our ultimate goal is to provide physically-based friction laws with predictive power across the length scales that can in turn describe the response of gouge-filled faults to static and dynamic stresses in slow-slip and fast-slip regimes.

Citation
Ferdowsi, B., & Goldsby, D. L. (2016, 08). Physical controls of spontaneous and triggered slow-slip and stick-slip at the fault gouge scale. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)