Cross-effects of loading rate and cumulative fault slip on pre-seismic rupture and unstable slip rate of laboratory earthquakes

The fact that the same fault may host different rupture or slip modes was partly attributed to the change in strain rate of the fault in previous studies. However, contradictory observations were obtained in the previous experimental studies, that is, the loading rate can promote or inhibit the rapid instability of faults. Although the two opposite effects of the loading rate were separately explained in the previous studies, the contradiction remains unresolved. Based on an experiment conducted on a granodiorite sample, we use the cumulative fault slip to quantitatively describe the slip-driven changes in fault properties and study the cross-effects of loading rate and cumulative fault slip on the pre-seismic rupture speed, unstable slip rate, and friction coefficient drop of the laboratory earthquakes (stick-slip events). The experimental results show that under the same loading rate, the exponential propagating rupture speed, the unstable slip rate, and the friction coefficient drop of the stick-slip events increase with increasing cumulative fault slip. As a result, the loading rate has opposite effects on the exponential propagating rupture speed, the unstable slip rate, and the friction coefficient drop under different amounts of the cumulative fault slip. This provides an explanation for the observation contradiction that the loading rate can promote or inhibit the rapid instability of the fault. The experimental results also reveal that the steady rupture speed is approximately proportional to the loading rate but weakly depends on the cumulative fault slip. Finally, the significance of the experimental results for understanding the earthquake and faulting mechanics is discussed. •A slow rupture followed by an accelerated rupture are observed prior to stick-slip instabilities of a granodiorite sample.•The steady rupture speed is approximately proportional to the loading rate but weakly depends on the cumulative fault slip.•Cumulative fault slip can cause opposite effects of the loading rate on accelerated rupture speed and unstable slip rate.