'Burst-Like' Slow Slip Propagation on Frictional Faults in the Laboratory

We present laboratory findings on burst-like premonitory slip propagation that leads to fault rupture. The experiments take place on a PMMA-PMMA interface in a direct shear configuration, where the effective strength heterogeneity is controlled by the non-uniform distribution of asperities throughout the fault. A pressure sensitive film was used to locate, size and measure normal stress on individual asperities. Prior to rapid sliding, we observed slow premonitory slip which accumulated non-uniformly along the fault. Slow displacement was measured using slip sensors placed at seven locations along the fault strike and showed intermittent, 'burst-like' increases in spectral power between the frequencies of 60 to 150 Hz. Each burst event lasted between 5 to 12 seconds, and a local increase in the extended fault slip rate was observed after its cessation. The 'burst-like' features migrated along the fault at speeds between Vprop ~ 1.3 mm/s to 9.3 mm/s. Propagation speed of the 'burst-like' front Vprop was dependent on the normal stress confining the fault σn- increased normal stress caused the rupture to move slower by increasing 'effective fault strength'. Finally, foreshocks were recorded using absolutely calibrated acoustic emission sensors and occurred at the later stages of the slow slip phase. The source radii of the foreshocks [Brune, 1970] ranged from 0.21 to 1.09 mm and their ruptures occurred over timescales ~5-7 orders of magnitude faster than the 'burst-like' slow slip signals. Observations of similar variations in time scales have been made between regular earthquakes and aseismic transients in the field [Ide, 2007]. These results will help develop a mechanistic understanding of the effective fault strength heterogeneity necessary for the development of slow earthquakes and tremor-like shaking. References: Ide, S., D. R. Beroza, G. C. Shelly & T. Uchide (2007), 'A scaling law for slow earthquakes', Nature 447, 76-79. Brune, J. N. (1970), 'Tectonic stress and spectra of seismic shear waves from earthquakes', Journal of Geophysical Research 75(26), 4997-5009.