Scaling Relationships of Slow and Fast Earthquakes in a Granular Mechanics Model

Why do fast earthquakes accelerate dramatically while slow ones are not able to? In this study, we explore the physical origins of various types of events in a particle-based simulation of fault slip. Our model simulates a vertical crustal strike-slip fault in two dimensions, viewed from the top. We use an assemblage of about 14,000 particles, bonded to impart cohesion at all contacts except along a pre-defined fault line of 50 km length. We characterize the scaling laws that emerge from analyses between strain, stress and slip rate during individual slip events within the fault zone. We examine 10 slow and fast earthquakes and identify several persistent behaviors and scaling relationships. First, we find that peak slip-velocity correlates with friction drop scaled by the preseismic rupture nucleation length. The best fit is a power law. Additionally, we find that during each rupture, shear stress decreases roughly linearly with slip. The weakening at an individual location persists throughout the event, even when the rupture tip has passed far beyond that location. Finally, we find that the initial absolute stress state correlates only weakly with the slip-velocity peak, although an overall trend cannot be ignored. We explore some of the physical origins of these scaling relationships and discuss the suitability of this model to investigate slow earthquake processes.