Exploring Transient Fault Slip Behaviors and ``Earthquake'' Distributions Using Discrete Element Models

Discrete Element Method (DEM) simulations have been used to analyze the frictional properties and deformation along faults, including the effects of fault gouge, grain shape and gouge evolution, rate and state friction, and 3-dimensional configurations. With some exceptions, the transient behavior of fault slip in this type of model has not been explored in detail, although DEM simulations allow both large- and small-scale details of the fault system to be monitored throughout the model evolution. Fundamental questions about fault slip processes can be examined in detail. For example, do modeled slip events correspond to natural earthquakes? Do parameters such as slip distance, rupture size, and stress drop agree with observations from earthquake catalogs? Do they exhibit well-known frequency-magnitude relationships, such as Omori and Gutenberg-Richter laws? Are other slip behaviors (e.g. slow slip) evident? We examine these questions in a series of simple model experiments. We construct 2-dimensional infinite-length faults (1 km long with periodic lateral boundaries), analogs for crustal-scale strike-slip faults. We can adjust the fault friction, material properties of the blocks on either side of the fault, clamping pressure, and loading rate. Fault displacement is induced by moving the far-field boundaries to produce right lateral shearing conditions. Slip localizes naturally onto the pre-defined fault, which undergoes episodes of sticking and intermittent rapid slip. In this configuration, we can identify “earthquakes”, which are used to create catalogs for statistical analysis. We can also examine the fault rupture properties and processes. We compare our results to real world observations, and query the simulations further to address pressing questions about the range of fault slip behaviors. The models show regular, but locally heterogeneous, stress cycling on the fault, where stress builds with strain and is released in a series rapid slip events. Repeating events are also evident, denoting rupture of persistent asperities that evolve only gradually with time. Stress accumulation and release on the fault is a function almost entirely of asperity size and distributions. The slip events show a remarkable variability in size and duration.