Crack-Like Processes Governing the Onset of Frictional Slip

TL;DR

This study reveals that the onset of frictional slip involves three distinct crack-like detachment fronts with different velocities and contact area reductions, which are crucial for understanding fault nucleation and earthquake processes.

Contribution

It provides the first real-time measurements of contact area reduction and identifies three types of detachment fronts governing the initiation of slip.

Findings

Intersonic fronts propagate at velocities up to the Rayleigh wave speed with minimal contact area reduction.

Slow fronts, much slower than wave speeds, cause the largest contact area reduction (~20%).

No slip occurs until a slow or sub-Rayleigh front traverses the entire interface.

Abstract

We perform real-time measurements of the net contact area between two blocks of like material at the onset of frictional slip. We show that the process of interface detachment, which immediately precedes the inception of frictional sliding, is governed by three different types of detachment fronts. These crack-like detachment fronts differ by both their propagation velocities and by the amount of net contact surface reduction caused by their passage. The most rapid fronts propagate at intersonic velocities but generate a negligible reduction in contact area across the interface. Sub-Rayleigh fronts are crack-like modes which propagate at velocities up to the Rayleigh wave speed, VR, and give rise to an approximate 10% reduction in net contact area. The most efficient contact area reduction (~20%) is precipitated by the passage of slow detachment fronts. These fronts propagate at anomalously slow velocities, which are over an order of magnitude lower than VR yet orders of magnitude higher than other characteristic velocity scales such as either slip or loading velocities. Slow fronts are generated, in conjunction with intersonic fronts, by the sudden arrest of sub-Rayleigh fronts. No overall sliding of the interface occurs until either of the slower two fronts traverses the entire interface, and motion at the leading edge of the interface is initiated. Slip at the trailing edge of the interface accompanies the motion of both the slow and sub-Rayleigh fronts. We might expect these modes to be important in both fault nucleation and earthquake dynamics.