Effective heterogeneity controlling premonitory slip on laboratory faults

Abstract:

Using a direct shear friction apparatus, we characterize factors controlling rheological differences along a PMMA-PMMA fault in the laboratory. Rheological differences on natural faults are believed to be a controlling factor to aseismic transients (slow slip) in nature. Asperity sizes and locations were measured using a pressure sensitive film at high (σ_n = 0.8 MPa) and low (σ_n = 0.4 MPa) nominal normal stress levels. Faults confined to low and high normal stress experienced lower and higher ‘effective heterogeneity,’ quantified using a characteristic elastic correlation length λ_c [Braun and Peyrard, 2012]. The fault was sheared at a constant far-field velocity V_LP under constant normal stress σ_n. Non-uniform premonitory slip accumulated along the fault prior to rapid sliding. Slow events (SE), which were characterized as local increases in slip rate, were observed when the effective heterogeneity was increased. These events nucleated from the weaker central section of the fault propagating outwards at speeds between 0.84 mm/s to 26.5 mm/s over times 406 s and 11 s. The rupture growth rates were dependent on the load-point velocity V_LP driving the system to failure.

Slip rates, which increased within the SE, were also dependent on the load-point velocity. The evolution of slip rates versus time was similar to the 2013-2014 Boso slow slip event [Fukuda et al., 2014]. The slow event culminated with rapid slip rate deceleration sufficient to generate seismicity measured by an array of acoustic emission sensors. Deceleration of the SE (left panel of figure) shows a lower-frequency event (~ 60 – 350 Hz). In Detail A, we show the smaller, high-frequency events (~ 300 – 500 kHz) were superimposed on the larger, low-frequency signal. These events only occurred at higher levels of effective heterogeneity and demonstrate interactions between larger/slow and localized/fast slip.

References:

Braun, O. M. & M. Peyrard (2012), ‘Crack in the frictional interface as a solitary wave’, Phys. Rev. E 85, 026111.

Fukuda, J., A. Kato, K. Obara, S. Miura & T. Kato (2014), ‘Imaging of the early acceleration phase of the 2013-2014 Boso slow slip event’, Geophysical Research Letters 41(21), 7493–7500.