Dilation or compaction? Laboratory insights into the role of fault roughness

The geometrical roughness of faults results in significant stress heterogeneity across various length scales, thus affecting rupture and sliding behavior during earthquakes. The dilatancy behavior on rough joints also becomes much more complicated than on planar faults. Whether dilation or compaction will occur on rough faults, especially those with large asperity heights, during interseismic and seismic slips is still an open question. 

Here, we perform laboratory shear tests on rough faults with millimeter-scale asperity heights and analyze the four types of dilation or compaction behavior observed during stick-slip cycles. In the stick phases, dilatancy behavior inferred from the asperity contacts agrees well with the variation of normal displacement. The locations of acoustic emission (AE) events are also consistent with the potential surface damage regions estimated from the evolution of asperity contacts at various shearing displacements. Stick-slip events with compaction-dominant interseismic slip usually occur at large shear displacements on interlocking faults when overriding high asperities. In those stick-slip events, the proportion of large-magnitude AEs is lower, resulting in higher Gutenberg–Richter b values. A generalized schematic model is also proposed for the complex dilatancy behavior during stick-slip cycles.

The experimental results provide new insights into the effects of fault roughness on shear-induced dilatancy behavior and serve as valuable benchmarks for our numerical simulations considering visible contact evolutions during shear sliding on rough faults.