Fault roughness’s influence on experimental fault mechanical behavior

Fault slip behavior and earthquake nucleation are often framed within the context of velocity-dependent friction, ranging from velocity-strengthening to velocity-weakening behavior. Previous studies have shown that fault roughness controls the fault slip behavior, with rougher faults having a greater tendency to be velocity-strengthening. However, it is not clear how the spontaneous transition from velocity-strengthening to velocity-weakening occurs nor what the role of roughness is. Using the High Strain TEmperature Pressure Speed (HighSTEPS) low to high velocity biaxial friction apparatus located at the EPFL in Switzerland, this transition has been investigated with load-controlled experiments. Creep in biaxial apparatuses has rarely been investigated, but this approach allows for a development of slip which is spontaneous and a loading which is more readily compared to natural seismicity. The investigation concerns bare surface norite samples with customized roughnesses. The samples were loaded to a normal stress of 20 MPa and then a shear stress of 8 MPa. The applied shear stress was held for 1000 seconds before the shear stress was increased in a step-wise fashion by 0.4 MPa, until the sample achieved a shear displacement of 25 mm (typically around 18 MPa). Typical mechanical data, such as the horizontal and vertical displacements and forces, were collected. The samples/sample holder were also equipped with 12 P-wave acoustic sensors. Smoother samples exhibited larger and more frequent stress drops than rougher samples, with reactivation at lower strength. Slip velocity was also observed to be higher for smooth samples. Localized acoustic emissions were used to assess the degree of localization and provide insight into the relative magnitude distributions for each roughness. These results highlight how local microscopic heterogeneities associated with surface roughness can influence the macroscopic stability of faults. They demonstrate roughness’ ability to result in stress barriers which can halt rupture.