Sheared granular matter & the empirical relations of seismicity

TL;DR

This study uses numerical simulations of a spring-slider system with granular matter to explore seismic-like behavior, revealing scale-free features similar to real earthquakes but with different underlying mechanisms for aftershock correlations.

Contribution

It demonstrates that numerical models can replicate key seismic features and clarifies the fundamental differences in aftershock relations between lab models and tectonic seismicity.

Findings

Scale-free statistical features are robust across parameter ranges.

Tectonic aftershock correlations are absent in the model.

The origin of aftershock relations differs from real seismicity.

Abstract

The frictional instability associated with earthquake initiation and earthquake dynamics is believed to be mainly controlled by the dynamics of fragmented rocks within the fault gauge. Principal features of the emerging seismicity (e.g. intermittent dynamics and broad time and/or energy scales) have been replicated by simple experimental setups, which involve a slowly driven slider on top of granular matter, for example. Yet, these set-ups are often physically limited and might not allow one to determine the underlying nature of specific features and, hence, the universality and generality of the experimental observations. Here, we address this challenge by a numerical study of a spring-slider experiment based on two dimensional discrete element method simulations, which allows us to control the properties of the granular matter and of the surface of the slider, for example. Upon quasi-static loading, stick-slip-type behavior emerges which is contrasted by a stable sliding regime at finite driving rates, in agreement with experimental observations. Across large parameter ranges for damping, inter-particle friction, particle polydispersity etc. the earthquake-like dynamics associated with the former regime results in several robust scale-free statistical features also observed in experiments. At first sight these closely resemble the main empirical relations of tectonic seismicity at geological scales. Yet, we show that the correlations associated with tectonic aftershocks are absent such that the origin of the Omori-Utsu relation, the aftershock productivity relation, and B{\aa}th's relation in the simulations is fundamentally different from the case of tectonic seismicity. We argue that the same is true for previous lab experiments.