Effects of physical fault properties on frictional instabilities produced on simulated faults

Abstract

Laboratory studies of large-scale simulated faults show that physical properties of the fault, specifically normal stress and fault roughness, significantly influence the unstable shear failure behavior of the fault. In addition, the experiments provide insights into important length-scaling effects that are useful for assessing concepts such as critical crack length or rupture nucleation dimension. Stick-slip shear failures have been generated along a 2-m-long simulated fault in a block of Sierra white granite. Experiments were conducted at normal stresses between 0.6 and 4 MPa with two different prepared roughnesses of the simulated fault: a smooth fault with a profilometer measured roughness of 0.2 #m and a rough fault with a measured roughness of •80 #m. High-speed records of shear strains and slip velocities clearly show both the propagation of the stick-slip failures and details of the local deformations and motions from which slip weakening of the fault at the onset of sliding can be resolved. Both the dynamic stress drops of the stick-slip events and the apparent fracture energies of the events determined from the records of the fault slip weakening increase with increasing normal stress. Although critical slipweakening displacements are insensitive to normal stress, they increase with increased fault roughness such that the apparent fracture energies are also larger on the rough fault than on the smooth fault. Slip velocities in these experiments are influenced by roughness and are proportional to stress drop. Although the onset of slip is sharply defined, the termination of slip is more gradual without a clear stopping phase. Measured rupture velocities on the rough fault are lower than those on the smooth fault. When a length scale based on rupture nucleation dimensions, directly proportional to critical displacements, is introduced, the data suggest that at fault lengths close to the nucleation lengths, rupture velocity and slip velocity increase with increasing fault length.

Preslip and cascade processes initiating laboratory stick slip

“…Previous work on meter-sized laboratory samples at 5 MPa normal stress indicated that the critical nucleation size R C~1 m [Okubo and Dieterich, 1984;Beeler et al, 2012;McLaskey and Kilgore, 2013]. The current experiments were conducted at about 20 times higher stress levels (~100 MPa) so we estimate that R C is about 20 times smaller.…”

“…The current experiments were conducted at about 20 times higher stress levels (~100 MPa) so we estimate that R C is about 20 times smaller. This is approximate since nucleation size is also known to be a function of fault roughness [Okubo and Dieterich, 1984] and loading history [Fang et al, 2010;Kaneko and Lapusta, 2008].…”

Analogue earthquakes and seismic cycles: experimental modelling across timescales

“…Acoustic sensors have been widely applied to study analogue earthquakes: Johnson et al (1973), Wu et al (1972), and Okubo and Dieterich (1984) used piezoelectric transducers to estimate slip rate and rupture speed in stick-slip experiments on precut rock and rock analogue materials. Lockner et al (1991), Zang et al (2000), and Thompson et al (2005, (2008) and Zigone et al (2011) used acoustic emission to characterize the stick-slip process in a spring-slider and salt-slider set-up (Fig.…”

“…It was studied experimentally using fault block models using precut rock (e.g. Dieterich, 1978a;Okubo and Dieterich, 1984;Ohnaka and Shen, 1999;McLaskey and Kilgore, 2013;McLaskey and Glaser, 2011;McLaskey et al, 2012) as well as rock analogues, e.g. polycarbonate (e.g.…”

Apparent stress scaling for tectonic and induced seismicity: Model and observations

“…Large overshoot has been suggested in the case the Cajon Pass borehole earthquakes (Beeler et al, 2003). Evidence from laboratory stick-slip friction experiments (Lockner and Okubo, 1983;Okubo and Dieterich, 1984) and observations from mining-induced events (McGarr, 1994) also indicate small or moderate overshoot.…”