The Roles of Shear Displacement and Normal Stress on Earthquake Nucleation in Meter-Scale Laboratory Faults

Earthquake nucleation is a fundamental problem in earthquake science and has practical implications for forecasting seismic hazards. Laboratory experiments performed on large, meter-scale fault systems offer unique insights into the nucleation process because the migration and expansion of the nucleation zone can be precisely detected, measured, and characterized using arrays of local strain and slip measurements. We report on a series of laboratory experiments conducted on a 1-m direct shear machine. We sheared layers of quartz gouge between roughened acrylic forcing blocks over a range of normal stresses between 3 and 12 MPa, generating a spectrum of slip modes, ranging from aseismic creep to fast-dynamic rupture. Co-seismic slip, peak slip velocity, and high-frequency acoustic energy content of laboratory earthquakes increases systematically with both cumulative fault slip and normal stress. Slower and smaller laboratory earthquake sequences have larger nucleation zones, creep more during their inter-seismic period, and are deficient in high-frequency energy compared to larger and faster rupture sequences. We find that the critical nucleation length scale, H*, scales inversely with cumulative fault slip and normal stress. A reduction in H* and an increase in event size can be explained by a decrease in the critical slip distance, D_c, or an increase in the frictional rate parameter b–a and is likely driven by shear localization. Together, our results indicate that homogeneous, mature fault zones that have undergone more cumulative fault slip are expected to have smaller H* and can more easily host dynamic instabilities, relative to immature faults.