This letter compares the predictions of the two expressions that have been proposed for the porosity evolution in the context of rate and state friction. One depends only on the sliding velocity; the other depends only on the state variable. The predictions of the two expressions are similar for simulations of velocity stepping and slide-hold-slide experiments but differ
significantly for normal effective stress jumps at constant sliding velocity. The formulation that depends only on the velocity predicts no change in the porosity; the other does.
A simulation with a spring-block model indicates that the magnitude of rapid slip events is essentially the same for the two models. 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.