The influence of fault slip rate on temporal variations in frequency-magnitude statistics of acoustic emissions throughout the laboratory seismic cycle

Abstract:

Laboratory experiments have routinely documented an increase in the number and size of acoustic emissions (AEs), and a systematic decrease in the Gutenberg-Richter, b-value, throughout the laboratory seismic cycle. However, the physical processes allowing seismic events (AEs) to become bigger closer to failure are poorly constrained. We seek to illuminate these processes by carefully documenting the temporal variation in AE statistics in tandem with high resolution measurements of fault zone properties. We report on a series of laboratory experiments conducted on simulated fault-gouge over a wide range of far-field shearing velocities. Acoustic emission data were recorded continuously throughout the experiment at 4 MHz from an array of piezoceramic sensors. We analyze frequency-magnitude (F/M) statistics of AEs using event catalogs and continuous acoustic emission records. Our data show that b-value decreases systematically as failure approaches and is consistent with previous works. B-value scales inversely with the far-field shearing velocity and larger AEs nucleate prior to failure at faster shearing rates. In addition, the rate at which b-value decreases scales systematically with the far-field shearing velocity. This suggests that the background loading rate plays an important role in modulating temporal variations in F/M statistics. Shear stress oscillation experiments were conducted to decouple the effects of shear stress and slip rate on the temporal variations in b-value. Data from these experiments further indicate that fault slip rate plays a key role in modulating the size of AEs and temporal variations in b-value. We conclude that, in our experiments, the temporal variations in b-value throughout the seismic cycle are driven primarily by fault slip rate and not shear stress. Our work has important implications for understanding temporal variations in foreshocks and b-value in tectonic fault zones. Ultimately, our results indicate that foreshock activity could be correlated to fault slip rate and high seismicity rates and/or low b-values could be indicative of a high-slip rate associated with an earthquake nucleation phase.