Brittle failure of intact rock and frictional sliding on faults are closely related. Much of my early career studying brittle failure using acoustic emission techniques was helpful in providing insight into processes associated with faulting and earthquakes. While I was focusing on failure processes with my colleague and mentor, Jim Byerlee, the basic tenants of what is referred to as rate- and state-dependent friction (RS) were being developed literally next door by Jim Dieterich with Andy Ruina and many others. It was a remarkable period in Menlo Park in the late ‘70s and ‘80s for which I had only limited appreciation at the time. While it is easy to reminisce, it is more useful to take stock of our current understanding of earthquake processes; what we have achieved and how very much farther we have to go. For example, one long-standing goal that remains elusive is earthquake prediction. While long term forecasting is clearly improving, prediction within hours to days remains out of reach. From a laboratory perspective, with tight control of fault roughness, stress, temperature, fluid pressure and other variables, prediction of timing and magnitude are possible, but with notable restrictions.
Rate- and state-dependent friction, for example, has been useful in the analysis of numerous earthquake-related phenomena including earthquake nucleation, earthquake triggering, slow slip, and repeating earthquakes. At the same time, it should be recognized that the RS model was developed using dry, planar laboratory faults at modest normal stress and limited total displacement. Along with the many successes of RS friction, it is useful to consider some of the limitations. Examples include (1) strain hardening- observed in most laboratory experiments as initial fault surfaces undergo rapid and irreversible changes in roughness and fault gouge properties; (2) melt formation or flash heating – where self-heating due to rapid sliding alters surface properties; and (3) hydro-mechanical coupling of low permeability faults where frictional heating increases pore fluid pressure or changes in porosity lead to transient dilatancy-strengthening or compaction-weakening.
I will present laboratory observations of fault strength evolution that are beyond the scope of standard RS formalism. Examples include constant loading rate tests near critical stiffness in which deformation mode spontaneously jumps between sequences of stable slow-slip oscillations and unstable stick-slip. In a second example with a hydraulically isolated, water-saturated fault gouge, incremental increases in slip rate lead to dilatancy, pore pressure decrease and fault stabilization. However, larger jumps in velocity lead to porosity collapse, fluid pressurization and fault instability. Hydrothermal slide-hold-slide tests at 200 °C, 10 MPa deionized water pressure and 30 MPa confining pressure produce the usual log-linear healing rate for hold times less than 5,000 s. Longer hold times, however, show increased weakening. Apparently, an overall time-dependent weakening of the fault surface occurs that dominates instantaneous fault strengthening for long hold periods and requires hundreds of microns of slip to be erased. These examples suggest that extrapolation of R/S models from idealized laboratory conditions to natural fault conditions may lead to erroneous predictions.