ABSTRACT: Sheared granular layers undergoing stick-slip behavior are broadly employed to study the physics of earthquakes. Here, a two-dimensional implementation of the combined finite-discrete element method (FDEM) is used to explicitly simulate a sheared granular fault system including both gouge and plate, and to investigate the influence of different normal loads on macroscopic friction coefficient, kinetic energy, gouge layer thickness, and recurrence time between slips. In the FDEM model, the deformation of plates and particles is simulated using the FEM formulation while particle-particle and particle-plate interactions are modeled using DEM-derived techniques. The simulation results show that with increasing normal load, (i) the kinetic energy of the granular fault gouge system increases; (ii) the gouge layer thickness shows a decreasing trend; and (iii) the macroscopic friction coefficient does not experience much change. Analyses of the slip events reveal that, as the normal load increases, more slip events with large kinetic energy release and longer recurrence time occur, and the magnitude of gouge layer thickness decrease also tends to be larger. The simulations not only reveal the influence of normal loads on the dynamics of sheared granular fault gouge, but also demonstrate the capabilities of FDEM for studying stick-slip dynamic behavior of granular fault gouge systems.
1. INTRODUCTION
Earthquakes generally occur due to a sudden release of the elastic energy accumulated in fault gouge and tectonic plates when subjected to long-time shear (Dorostkar et al., 2017b; Marone et al., 1990). The fault gouge, an ensemble of solid granular particles created by fragmentation and wearing, plays a key role in the macroscopic sliding friction and the friction stability of the fault (Dorostkar et al., 2017b; Pica Ciamarra et al., 2011). Therefore, sheared granular layers undergoing stick-slip behavior are broadly employed to study the physics and dynamics of earthquakes and a number of laboratory and numerical experiments have been conducted in this regard (e.g. Annunziata et al., 2016; Dorostkar et al., 2017b; Dratt and Katterfeld, 2017; Ferdowsi, 2014; Ferdowsi et al., 2013; Geller et al., 2015; Griffa et al., 2011; Johnson and Jia, 2005; Marone, 1998; Passelègue et al., 2016; Pica Ciamarra et al., 2011; Scuderi et al., 2017). Particularly, in addition to the ease of implementation, numerical simulations of granular fault gouge, which allow for analysis of the mechanical behavior of the system at a level of spatial and temporal resolution not accessible experimentally (de Arcangelis et al., 2011), are widely used to investigate the stick-slip behavior in granular fault gouge (Dorostkar et al., 2017b).
Among the many numerical methods used in simulating the evolution of granular systems, the discrete element method (DEM) has demonstrated its value as a tool for such investigations (de Arcangelis et al., 2011; Dorostkar et al., 2017a, b; Ferdowsi, 2014; Ferdowsi et al., 2013; Griffa et al., 2011; Griffa et al., 2012). In classic DEM models, granular fault gouge is usually represented by a pack of rigid particles, and the normal and tangential contact forces between particles are described using Hertzian contact and Coulomb friction, respectively (Dorostkar et al., 2017a); the representation of the shearing plate is often either ignored or simplified by a set of bonded particles (Abe and Mair, 2005; Dorostkar et al., 2017a, b; Ferdowsi et al., 2013; Ferdowsi et al., 2014; Griffa et al., 2013; Mair and Abe, 2008). As a result, DEM is incapable of capturing real and detailed deformation and stress distributions within the particles and plates (Dratt and Katterfeld, 2017; Ma et al., 2016). Since the deformation of particles and plates provides the means for a wide spectrum of elastic energy storage and release (Griffa et al., 2013), appropriate modeling of the real deformations of a granular system is critical to conduct an accurate exploration of the faulting process dynamics.
