3D discrete element modeling for the simulation of seismic cycles on a strike-slip fault

Knowing about the geometry of both (i) ruptured zones during seismic events, and (ii) faults throughout seismic cycles, as well as the evolution of this geometry, is important to understand what is controlling the start and the ending of large earthquakes. In this study, we use 3D Discrete Element Modeling (DEM) in order to simulate a strike-slip fault, formed from an initially homogeneous, intact medium representing brittle rock that is submitted to tectonic loading. Indeed, this numerical method models the crust as an assembly of rigid spheres which are linked by user-defined interactions and reconfigurate very naturally when subjected to loading. Therefore, such approach is adapted to study the evolution of fault geometry through earthquake cycles, since it permits to simulate large displacements of the particles, while avoiding prescribing fault location and geometry, and letting such geometry evolve freely.

A 3D parallelepipedic model is designed and then indefinitely sheared by assigning periodic boundary conditions. The particular feature of our model is the implementation of a healing phenomenon, a key process which allows fractured zones to restrengthen after a slip event. During the simulation, the position of particles and the state of their bonds are recorded at regular time intervals; consequently, the shape and dimension of deformation are evaluated, the evolution of fault geometry is monitored, and the stresses in the domain can be measured. Results show a stick-slip behaviour which can be identified as earthquakes separated by locking periods. In addition, the amount of displacement and the rupture surface can be estimated and enable the computation of a magnitude-like quantity. Thus, earthquake-like events seem to follow a magnitude-frequency relationship, and earthquake-like surface deformations are comparable to observations of ground deformation after real size earthquakes. Eventually, the evolution of the fault geometry during the simulation is also scrutinized.