Porosity Evolution in Rate and State Friction.

This letter compares the predictions of two expressions proposed for the porosity evolution in the context of rate and state friction. One (Segall & Rice, 1995, https://doi.org/10.1029/95jb02403) depends only on the sliding velocity; the other (Sleep, 1995, https://doi.org/10.1029/94jb03340) depends only on the state variable. Simulations of both are similar for velocity stepping and slide‐hold‐slide experiments. They differ significantly for normal effective stress jumps at constant sliding velocity. Segall and Rice (1995, https://doi.org/10.1029/95jb02403) predicts no change in the porosity; Sleep (1995, https://doi.org/10.1029/94jb03340) does. Simulation with a spring‐block model indicates that the magnitude of rapid slip events is essentially the same for the two formulations. Variations of porosity and induced pore pressure near rapid slip events are similar and consistent with experimental observations. Predicted porosity variations during slow slip intervals and the time at which rapid slip events occur are significantly different. The simulation indicates that changes in friction stress due to pore pressure changes exceed those due to rate and state effects. Plain Language Summary: The interaction of pore fluid and deformation is important for many geological processes and for technological applications involving fluid injection, such as carbon sequestration, disposal of waste fluids from hydraulic fracturing, and geothermal stimulation. Often these applications have induced earthquakes that have raised cause for concern. An important element of the interaction of pore fluid and deformation is the evolution of porosity, that is, the ratio of volume of voids to the total volume, with the slip on a fault surface. If the porosity increases more rapidly than pore fluid can diffuse out of pores, the pore pressure decreases and increases the frictional resistance to slip; conversely, porosity decreases can increase the pore pressure and reduce the frictional resistance to slip. This paper compares two proposed descriptions for porosity evolution with slip. Both have similar predictions for standard laboratory experiments but differ significantly for changes of pore fluid pressure at constant sliding velocity. In simulations with a simple spring ‐ block model both predict changes in pore fluid pressure with time near rapid slip events that are consistent with experiments. The predicted changes in porosity differ in the intervals of slow slip between rapid slip events. Key Points: Two formulations that have been suggested for porosity evolution give similar results for simulated velocity stepping and slide‐hold‐slide testsThe two formulations give significantly different results for effective normal stress changes at constant slip velocityA spring‐block simulation shows that both formulations predict pore pressure near rapid slip events consistent with laboratory observations [ABSTRACT FROM AUTHOR]