SIMULATION OF THE FRICTIONAL STICK-SLIP INSTABILITY

A lattice solid model capable of simulating rock friction, fracture an d the associated seismic wave radiation is developed in order to study the origin of the stick-slip instability that is responsible for eart hquakes. The model consists of a lattice of interacting particles. In order to study the effect of surface roughness on the frictional behav ior of elastic blocks being rubbed past one another, the simplest poss ible particle interactions were specified corresponding to radially de pendent elastic-brittle bonds. The model material can therefore be con sidered as round elastic grains with negligible friction between their surfaces. Although breaking of the bonds can occur, fracturing energy is not considered. Stick-slip behavior is observed in a numerical exp eriment involving 2D blocks with rough surfaces being rubbed past one another at a constant rate. Slip is initiated when two interlocking as perities push past one another exciting a slip pulse. The purse fronts propagate with speeds ranging from the Rayleigh wave speed up to a va lue between the shear and compressional wave speeds in agreement with field observations and theoretical analyses of mode-II rupture. Slip r ates are comparable to seismic rates in the initial part of one slip p ulse whose front propagates at the Rayleigh wave speed. However, the s lip rate is an order of magnitude higher in the main part of pulses, p ossibly because of the simplified model description that neglected int rinsic friction and the high rates at which the blocks were driven, or alternatively, uncertainty in slip rates obtained through the inversi on of seismograms. particle trajectories during slip have motions norm al to the fault, indicating that the fault surfaces jump apart during the passage of the dip pulse. Normal motion is expected as the asperit ies on the two surfaces ride over one another. The form of the particl e trajectories is similar to those observed in stick-slip experiments involving foam rubber blocks (BRUNE et al., 1993). Additional work is required to determine whether the slip pulses relate to the interface waves proposed by Brune and co-workers to explain the heat-flow parado x and whether they are capable of inducing a significant local reducti on in the normal stress. It is hoped that the progressive development of the lattice solid model will lead to realistic simulations of earth quake dynamics and ultimately, provide clues as to whether or not eart hquakes are predictable.