Frequency-Magnitude Statistics of Laboratory Foreshocks Vary With Shear Velocity, Fault Slip Rate, and Shear Stress

Understanding the temporal evolution of foreshocks and their relation to earthquake nucleation is important for earthquake early warning systems, earthquake hazard assessment, and earthquake physics. Laboratory experiments on intact rock and rough fractures have demonstrated that the number and size of acoustic emission (AE) events increase and that the Gutenberg-Richter b-value decreases prior to coseismic failure. However, for lab fault zones of finite width, where shear occurs within gouge, the physical processes that dictate temporal variations in frequency-magnitude (F/M) statistics of lab foreshocks are unclear. Here, we report on a series of laboratory experiments to illuminate the physical processes that govern temporal variations in b-value and AE size. We record AE data continuously for hundreds of lab seismic cycles and report F/M statistics. Our foreshock catalogs include cases where F/M data are not exponentially distributed, but we retain the concept of b-value for comparison with other works. We find that b-value decreases as the fault approaches failure, consistent with previous works. We also find that b-value scales inversely with shear velocity and fault slip rate, suggesting that fault slip acceleration during earthquake nucleation could impact foreshock F/M statistics. We propose that fault zone dilation and grain mobilization have a strong influence on foreshock magnitude. Fault dilation at higher shearing rates increases porosity and results in larger foreshocks and smaller b-values. Our observations suggest that lab earthquakes are preceded by a preparatory nucleation phase with systematic variations in AE and fault zone properties.