Aseismic slip may occur during a long preparatory phase preceding
earthquakes, and what controls it remains poorly understood. In this
study, we explored the potential dependencies of the slow slip during
the preparatory stage prior to stick-slip instabilities on two main
factors, namely the loading rate and surface roughness. To that end, we
conducted shear stress-driven friction experiments by imposing varying
loading rates on sawcut granite samples with different surface roughness
at confining pressure of 35 MPa. We measured the average slip along the
fault using far-field displacements and strain changes, while using
acoustic emission sensors and local strain gages to capture local slip
variations. We found that the average aseismic slip during preparatory
stage increases with roughness, whereas its duration decreases with
increased loading rate. These results also evidence a complex slip
pattern on rough faults which leads to dynamic ruptures at high loading
rates.