Dynamic rupture of a gouge layer in a meter-sized labquake: a coupled numerical model

Seismic waves originate from dynamic rupture propagation in faults. Seen from afar, faults are analogous to shear cracks, and their rupture can be analysed using the tools of fracture mechanics. However, a closer look reveals that faults can also be considered locally as a tribosystem, i.e. as a layered structure which accommodates deformation thought localized shearing in a thin granular layer of fault gouge. These two scales are equally important but are difficult to handle simultaneously in simulations.

In this communication, we propose a novel numerical model where this challenge is addressed. The gouge layer is represented using the Discrete Element Method, where each micrometric gouge grain (about 1 million of them in the present case) is explicitly represented and submitted to Newtonian dynamics, based on the forces it receives from its contacting neighbours. This layer is 2 mm-thick, and is confined between two continuum regions simulated using an explicit Meshfree Method. They receive the elastic properties of country rock, and are prestressed in the normal and tangential directions in order to bring the gouge layer just below its peak strength. The resulting fault system has a total length of 64 cm.

A labquake is then triggered from the central point of the fault, and the weakening rheology of the gouge layer allows it to propagate along two rupture fronts, which exhibit specific properties inherited from the frictional response and structure of the gouge. Inclined Riedel bands spontaneously develop at quasi-periodic intervals in the granular layer, and both rupture fronts propagate by leaps when successively activating slip in these structures. They both transition to a supershear regime after a certain sliding distance.

This model allows for the first time to observe the behaviour and response of the gouge layer as it endures the propagation of a rupture front. Localization patterns and granular complexity render the rupture irregular and heterogeneous, but a moving average in time in the frame of the crack tip allows to recover stress concentrations and slip velocity patterns which are consistent with the Linear Elastic Fracture Mechanics predictions. Il allows to relate gouge frictional response and rupture dynamics without the need to prescribe an arbitrary friction law or to rely on separation of scales.