Fault surface evolution with slip: are faults really heterogeneous?

Abstract:

In nature, earthquakes occur when the stress accumulated on fault is released during slip. Throughout a seismic cycle, successive and various slip events lead to an intense and complex fault deformation, which is most of the time invisible because faults are at inaccessible depths. Fortunately, the evolution of fault surfaces can be investigated thanks to laboratory experiments.

Here, we reproduce earthquakes in the laboratory during triaxial experiments under natural in situ conditions. We track the complex microstructural evolution of lab-scale faults (including roughness and temperature evolution), by performing stick-slips on heterogeneous fault surfaces, prepared in advance and evolving with slip. The microstructural evolution of simulated faults is tracked on the same samples (several distinctive lithologies are chosen), over different and distinct seismic cycles (i.e. experiments after experiments), performed at the same confining pressure.

These experiments show that:

1. fault surface evolution can be tracked and imaged during stop-and-go experiments.
2. fault topography and temperature can be efficiently superposed even after several entire seismic cycles.
3. with increasing slip, starting fault heterogeneity disappears in favor of an homogenous stage of maturity during which the largest laboratory earthquakes occur.
4. fault temperature and topography changes (i.e. the amount of damage on the interface) increase with slip on all rock surfaces but vary in complex ways as a function of lithology and stress drops.