Elastohydrodynamic lubrication of faults

Abstract

The heat flow paradox provides evidence that a dynamic weakening mechanism may be important in understanding fault friction and rupture. We present here a specific model for dynamic velocity weakening that uses the mechanics of well-studied industrial bearings to explain fault zone processes. An elevated fluid pressure is generated in a thin film of viscous fluid that is sheared between nearly parallel surface. This lubrication pressure supports part of the load, therefore reducing the normal stress and associated friction across the gap. The pressure also elastically deforms the wall rock. The model is parameterized using the Sommerfeld number, which is a measure of the lubrication pressure normalized by the lithostatic load. For typical values of the material properties, slip distance and velocity, the Sommerfeld number suggests that lubrication is an important process. If the lubrication length scales as the slip distance in an earthquake, the frictional stress during dynamically lubricated large earthquakes is 30% less than the friction with only hydrostatic pore pressure. Elastohydrodynamic lubrication also predicts a decrease in high-frequency (>1 Hz) radiation above a critical slip distance of a few meters. This prediction is well matched by the strong motion data from the 1999 Taiwan earthquake. The observed 2 orders of magnitude variation in scaled radiated energy between small (M_w < 4) and large earthquakes (M_w > 6) is also predicted by the lubrication model.