John Rudnicki

This seminar presents results on the effects of imposed rate of pore pressure change on the stability of rate and state frictional slip. The work is motivated by recent experiments that impose pressure at different rates. These find that the slip velocity and shear stress drops of accelerated slip events correlated with pore pressure rate rather than the magnitude of the pore pressure. Additional motivation comes from field observations suggesting that injection rate is an important factor in the occurrence of induced earthquakes in mid-continental US. Numerical simulations for a simple spring – slider model, assuming sliding is governed by rate and state friction, show that the pressure rate can control the frequency of rapid slip events. Refinement of the model indicates how the features of slip events observed in the laboratory depend on frictional parameters, rate of loading, rate and magnitude of pore pressure increase, and diffusivity.

Despite limitations on size and time scales, laboratory experiments provide a more controlled environment for understanding fundamental physical processes. Comparing the results of numerical simulations with experimental observations is the basis for understanding more complex behavior of pore pressure interaction with frictional slip. Such comparisons provide a means of examining the effects of different parameters, correlating results from different experiments and suggesting new experiments. Because such simulations provide a way of generalizing results for a range of parameters not attainable in laboratory experiment they provide insight into field observations of induced seismicity, in particular, whether slip due to fluid injection occurs and, if it does, whether it is seismic or aseismic.